WO2008034598A1

WO2008034598A1 - Method for the synthesis of hexahydrofuro (2, 3-b) furan-3-ol compounds

Info

Publication number: WO2008034598A1
Application number: PCT/EP2007/008148
Authority: WO
Inventors: Peter Jan Leonard Mario Quaedflieg; Natascha Sereinig; Paulus Lambertus Alsters; Henricus Martinus Maria Gerardus Straatman; Martin Helmut Friedrich Hanbauer; Niek Johannes Ronde
Original assignee: Dsm Ip Assets B.V.
Priority date: 2006-09-19
Filing date: 2007-09-19
Publication date: 2008-03-27
Also published as: WO2008034598A8

Abstract

The present invention relates to a method for the synthesis of enantiomerically and diastereomerically enriched hexahydro-furo[2,3-b]furan-3-ol compounds by aldol coupling of two suitable O-protected hydroxyaldehydes and subsequent removal of the protecting groups and (optionally simultaneous) cyclization of the resulting aldol compound and subsequent isolation of the desired compounds. The resulting composition can be further diastereomerically enriched through the intermittent acylation or aroylation of the compound and further optionally using a stereoselective hydrolyzing enzyme.

Description

METHOD FOR THE SYNTHESIS OF HEXAHYDRO FURO[2,3-B]FURAN-3-OL

COMPOUNDS

The present invention relates to a method for the synthesis of hexahydro-furo[2,3-b]furan-3-ol compounds and in particular to the synthesis of diastereomerically pure (3R,3aS,6aR)-hexahydro-furo[2,3-b]furan-3-ol and its enantiomer (3S,3aR,6aS)-hexahydro-furo[2,3-b]furan-3-ol as well as novel intermediates for use in said methods.

The compound (3R,3aS,6aR)-hexahydro-furo[2,3-b]furan-3-ol (of formula [I])

is an important pharmacological moiety present in the structure of HIV protease inhibitors. Inhibition of the proteolytic HIV protease enzyme has proven to be an effective treatment against AIDS, especially in combination with the use of reverse transcriptase inhibitors as, for instance, described by Flexner in N. Engl. J. Med. 1998, 338, 1281 and by Cihlar et al. in Annυ. Rep. Med. Chem. 2000, 35, 177. Since drug resistance has emerged against the existing peptidic protease inhibitors, nonpeptidal congeners active against protease inhibitor-resistant mutants were recently developed. For instance, Ghosh et al. developed nonpeptidyl HIV protease inhibitors incorporating novel high-affinity so-called P2-ligands and (f?)-(hydroxy-ethylamino)-sulfonamide isosteres as described in Il Farm. 2001 , 56, 29 and in Bioorg. Med. Chem. Lett. 1998, 8, 687.

Of particular interest is the protease inhibitor TMC 114 of formula [VIII]

which was developed (see, for instance, Surleraux et al. in J. Med. Chem. 2005, 48, 1813) and recently launched by Tibotec Pharmaceuticals Ltd. The molecule possesses a bis-tetrahydrofuranyl moiety as the P2-ligand, which is introduced into TMC114 using a mixed carbonate of compound [I] as described by Ghosh et al. in J. Med. Chem.

1996, 39, 3278.

Several approaches for the synthesis of the building block of formula [I] are known. A first approach as described by Ghosh et al. in J. Med. Chem. 1996, 39, 3278, based on radical cyclization combined with an ozonolytic alkene cleavage, is cost-inefficient and not amenable to scale-up. Moreover, the enzymatic resolution of the resulting racemic mixture gives the desired enantiomer in only 95% ee. The same approach had been disclosed earlier by Pezechk et al. in Tet. Letters 1986, 27(32), 3715-3718 in which a radical cyclization is performed on a propargylic ether followed by a diastereoselective reduction. An improved version of this approach as recently disclosed by Davis et al. (WO 02/060905 A2), still requires expensive oxidation/reduction chemistry, as well as a racemate resolution in the final part of the synthesis. A second approach to prepare the compound of formula [I], described by Ghosh et al. in J. Med. Chem. 1996, 39, 3278, is based on the use of 3(R)-diethyl- malate as chiral pool and requires 4 oxidation and reduction steps. Therefore, it is economically very unattractive due to the expensive raw materials, the requisite of a low temperature in several steps and the environmental unfriendliness. A third approach to prepare compound [I] is described in WO

03/024974 A2 to Doan et al. This method is based on a photochemical cycloaddition reaction to give an oxetane intermediate which is subjected to a reduction, deprotection and rearrangement reaction. This process is cost-inefficient too due to the low yields, the requisite of expensive equipment for the photochemical step and the expensive oxidation/reduction chemistry, as well as a racemate resolution in the final part of the synthesis.

A fourth approach to prepare compound [I] as published by Ghosh et al. in J. Org. Chem. 2004, 69, 7822-7829 and in US 6919465 B2, is based on a highly diastereoselective photochemical addition of 1 ,3-dioxolane to an enantiopure furanone compound which is obtained via enzymatic resolution and ring closing methathesis.

The great disadvantage is that the wrong diastereomer is obtained, which is intrinsic for the chemistry. Therefore, 3 additional steps are required of which one is an environmentally unfriendly Mitsunobu inversion. This route is not cost-effective due to the many (10) steps, high raw material costs and the apparent necessity of chromatographic separation steps. The fifth approach published in WP 2004/002975 A1 to Doan et al., based on environmentally unfriendly reduction and halogenation chemistry, gives rise to the formation of a mixture of 4 diastereoisomers, from which the desired compound according to formula [I] is isolated at the end of the synthesis by enzymatic resolution. In a sixth approach to obtain the compound of formula [I]₁ as disclosed by Quaedflieg et al. in Org. Lett. 2005, 7(26), 5917-5920, S-2,3-0- isopropylidene-glyceraldehyde is used as the source of chirality. This aldehyde can be prepared from L-ascorbic acid in 3 steps (see EP 1673364 to Quaedflieg et al.) or from racemic solketal in 2 steps (see EP 1678158 to Quaedflieg et al.). In one variant of this approach (see also EP 1448567 to Kesteleyn et al.) the key step is a diastereoselective Michael addition of nitromethane to the dimethylmalonate adduct of S-2.3-O- isopropylidene-glyceraldehyde which is subsequently transformed to [I] in 5 steps. In an improved variant of this approach (see also WO 2005095410 to Quaedflieg et al.) the key step is a diastereoselective Michael addition of nitromethane to a enoate adduct of S-2,3-0-isopropylidene-glyceraldehyde which is subsequently transformed to [I] in 3 steps. Although the latter process variant can be conveniently scaled up and has relatively little raw material costs, it requires many synthetic steps so that the production costs are still considerable.

A seventh approach to prepare compound [I] is disclosed in US 2005 0256322 A1 to lkemoto et al. The key intermediate, an O-protected hydroxyacetyl-γ- butyrolactone, is asymmetrically hydrogenated and subsequently reduced, deprotected and cyclized to give the wrong diastereomer of [I] which needs to be inverted by oxidation/reduction chemistry. The large number of steps combined with moderate ee's as obtained in the asymmetric hydrogenation renders this approach highly cost- inefficient.

Hence, the approaches to prepare the compound according to formula [I] disclosed hitherto all have the disadvantage that they are unscalable or expensive on large scale or environmentally unfriendly. Since (3R,3aS,6aR)- hexahydro-furo[2,3-b]furan-3-ol is a pharmaceutical intermediate to be produced on large scale, there is a great need for a cheap production process consisting of a small number of steps, starting from readily available and non-chiral starting compounds and which is environmentally friendly. Furthermore, high diastereomeric ratios for (3R,3aS,6aR)-hexahydro-furo[2,3-b]furan-3-ol are required. High diastereomeric ratios are diastereomeric ratios of more than 4/1. - A -

In a first embodiment the present invention relates to a method for the synthesis of a compound of formula [I] or its enantiomeric form according to formula [II]

by aldol reaction of two O-protected hydroxyaldehydes of the general formula [III] and the general formula [IV], respectively,

wherein P₁ and P₂ each independently represent a hydroxyl-protecting group and wherein P₁ and P₂ may be identical, subsequent removal of the protecting groups and (optionally simultaneous) cyclization of the resulting aldol compound of formula [V]

and subsequent isolation of the compound of formula [I] or [M]. This compound [V] was found to be novel. Therefore the present invention also relates to a compound of the general formula [V], wherein P₁ and P₂ each independently represent a hydroxyl-protecting group and wherein P₁ and P₂ may be identical, and wherein the asterisks (^*) indicate chiral centers.

The term hydroxyl protecting group refers to a substituent which protects hydroxyl functions against undesirable reactions during synthetic procedures. Examples of hydroxyl protecting groups are disclosed in Greene and Muts, "Protective Groups In Organic Synthesis," (John Wiley & Sons, New York, 3^rd edition, 1999). Suitable hydroxyl protecting groups Pi and P₂ for the present invention comprise methyl and substituted methyl groups (such as methoxymethyl, 2- methoxyethoxymethyl, benzyloxymethyl, p-methoxybenzyloxymethyl, 2- (trimethylsilyl)ethoxymethyl, tetrahydropyranyl, tetrahydrofuranyl); ethyl and substituted ethyl groups (such as 1-ethoxyethyl, tert-butyl, allyl, propargyl); benzyl and substituted benzyl groups (such as p-methoxybenzyl, 3,4-dimethoxybenzyl, triphenylmethyl); silyl groups (such as trimethylsilyl, triethylsilyl, tert-butyldimethylsilyl, tert-butyldiphenylsilyl, tri-isopropylsilyl, diethylisopropylsilyl, thexyldimethylsilyl, triphenylsilyl, di-tert-butylmethylsilyl); acyl- or aroyl groups (such as formyl, acetyl, benzoyl, pivaloyl, methoxyacetyl, chloroacetyl, levulinoyl, 1-adamantoyl); alkoxycarbonyl or aryloxycarbonyl groups (such as benzyloxycarbonyl, p- nitrobenzyloxycarbonyl, tert-butyloxycarbonyl, 2,2,2-trichloroethyloxycarbonyl, 2- (trimethylsilyl)ethyloxycarbonyl, allyloxycarbonyl) and sulfate and sulfonyl groups (such as allylsulfonyl, methanesulfonyl, benzylsulfonyl and tosyl).

In a further embodiment of the present invention the yield and/or diastereomeric ratio is improved by selecting the hydroxyl protecting groups. The more electron donating or the larger hydroxyl protecting group Pi, the higher the yield and/or diastereomeric ratio. Preferably P₁ is an electron donating hydroxyl protecting group. More preferably Pi is an electron donating and bulky hydroxyl protecting group. Most preferably Pi is an electron donating, bulky and acid labile hydroxyl protecting group. Preferably P₂ is an acid labile hydroxyl protecting group.

In a preferred embodiment of the present invention P₁ and/or P₂ represent a silyl group.

In a further preferred embodiment P₁ represents a tert- butyldimethylsilyl group.

In a further preferred embodiment P₂ represents a trimethylsilyl group. Aldehydes of formula [III] may be obtained from commercial sources or synthesized by various methods described in the literature. For instance, aldehydes of formula [III] may be obtained by mono-O-protection of 1 ,2-ethanediol followed by oxidation of the remaining alcohol functionality. Suitable oxidation methods may be TEMPO based oxidation methods or Swern type oxidation methods like using pyridine. SO₃ as described for instance in March's Advanced Organic Chemistry, 5^th edition, Ed. M. B. Smith and J. March, John Wiley & Sons, 2001. Alternatively, aldehydes of formula [III] may be obtained by protection of both hydroxyl functionalities of 2-butene-1 ,4-diol followed by oxidative cleavage of the double bond for instance by ozonolysis. Aldehydes of formula [IV] may be obtained from commercial sources or synthesized by various methods described in literature. For instance, aldehydes of formula [IV] may be obtained by mono-O-protection of 1 ,4-butanediol followed by oxidation of the remaining alcohol functionality. Suitable oxidation methods may be TEMPO based oxidation methods or Swern type oxidation methods like using pyridine. SO₃ as described for instance in March's Advanced Organic Chemistry, 5^th edition, Ed. M. B. Smith and J. March, John Wiley & Sons, 2001.

In the present invention the aldehydes according to formula [III] and [IV] may be used without further purification but preferably they may be distilled prior to use in the aldol reaction. The aldol reaction between aldehydes of formula [III] and aldehydes of formula [IV] in the method according to the present invention may be carried out according to any suitable method known in the art. Suitable methods are e.g. subjecting a mixture of the two aldehydes to acidic or basic catalysts, preformation of enolates of the nucleophilic aldehyde prior to addition of the electrophilic aldehyde, formation of metal-enolates, preferably chiral metal-enolates of the nucleophilic aldehyde prior to addition of the electrophilic aldehyde. Metals comprised in said enolates can be boron, silicon, titanium, tin or the like.

In a preferred method the aldol reaction is carried out in the presence of a catalyst, preferably a chiral catalyst such as a chiral base, a chiral Lewis acid, a chiral metal containing homogeneous catalyst, an enzyme or a chiral organocatalyst. The term "organocatalyst", as used herein, refers to a catalyst consisting of an organic compound which does not contain a metal atom. Preferred catalysts are chiral organocatalysts, in particular chiral amines, such as L- or D-proline or L- or D-O- methyl-prolinol. The catalyst may also be employed on a solid support. For the production of the compound of formula [I] a suitable catalyst of the aldol reaction is L-proline, whereas for the production of a compound of formula [II] D-proline can be used. The use of chiral organocatalysts such as L-Pro for enantioselective and diastereoselective aldol reactions between two aldehydes is known in the art, e.g. in WO03/089396 (to D. MacMillan et al.) and as disclosed by D. MacMillan et al. in Angew. Chem. Int. Ed. 2004, 43, 2152-2154. However, according to the present invention it was found that this type of reaction surprisingly can also be applied to enantioselectively and diastereoselectively synthesize bis-tetrahydrofuranyl compounds as exemplified by the compound of the general formula [I]. Using enantiopure Pro as the organocatalyst, the enantiomeric excess (e.e.) of the aldol reaction products and, after deprotection/cyclization, of the compound of formula [I] or [II] is usually > 95%. On the other hand, the obtained diastereoselectivity is much lower. Surprisingly, the diastereomeric ratio (d.r.) depends largely on the choice of the protecting groups P₁ and P₂. If, for instance, P₁ an P₂ are both acetyl groups, the d.r. is only 1 :1 , i.e. the diastereomeric excess (d.e.) is 0%. If, on the other hand, P₁ is a tert- butyldimethylsilyl group, the d.r. is approximately 4:1 corresponding to a d.e. of 60%, seemingly irrespective of protecting group P₂. Additionally, by proper selection of the reaction conditions the d.r., yield and e.e. can surprisingly be improved significantly. For instance, if both P₁ and P₂ are tert-butyldimethylsilyl groups and the applied ratio of aldehyde [lll]/aldehyde [IV] is 1.5-2 and the temperature is reduced to 4 ⁰C the d.r. is 5: 1 , the e.e. > 98% and the yield of aldol product [V] based on aldehyde [IV] is 80%.

Aldehydes of formula [III] and formula [IV], respectively, may be applied in a molar ratio of at least 0.5:1, preferably in a molar ratio of at least 1:1, more preferably in a molar ratio of at least 1.2:1 and most preferably in a molar ratio of at least 1.3:1. An upper limit of the molar ratio of the aldehydes of formula [III] and formula [IV], respectively, is 10:1 , preferably 5:1 , more preferably 3:1 , most preferably 2:1.

The amount of catalyst is generally at least 0.001 mol equivalent, preferably at least 0.005 mol equivalent and most preferably at least 0.01 mol equivalent, and not more than 1 mol equivalent, preferably not more than 0.5 mol equivalent and more preferably not more than 0.2 mol equivalent, all based on the aldehyde which amongst the two aldehydes is present in the lowest molar amount.

The aldol reaction is typically carried out in an organic solvent like benzene, toluene, chloroform, ethyl acetate, dioxane, acetonitrile, dimethylsulfoxide, N- methylpyrrolidinone, N,N-dimethyl formamide or tetrahydrofuran. More preferably the organic solvent is dimethylsulfoxide, N-methylpyrrolidinone, N,N-dimethyl formamide or tetrahydrofuran. Most preferably the aldol reaction is carried out in tetrahydrofuran.

Suitably the aldol reaction should be carried out at temperatures above -8O⁰C, preferably above -1O⁰C and most preferably above 0⁰C. As an upper limit the temperature should be kept below 100⁰C, preferably below 30°C, most preferably below 20⁰C. The total weight % of aldehydes may be at least 0.1 weight %, preferably at least 1 weight % and most preferably at least 10 weight %. On the other hand the total weight % of aldehydes may not exceed 70 weight %, preferably not exceed 50 weight % and most preferably not exceed 30 weight %.

The reaction time is not critical and will be preferably chosen such that the aldehyde which amongst the two aldehydes is added in the lowest molar amount is almost completely or completely consumed.

The reaction product of formula [V] can be isolated using methods known to any person skilled in the art, such as aqueous work-up and extraction. Further purification of the reaction product of formula [V] can be performed by using methods known to any person skilled in the art, such as by chromatography.

Alternatively the reaction product of formula [V] can be further converted to compounds of formula [I] or [M]. Sequential or simultaneous removal of the protecting groups from [V] and cyclization of the resulting product gives a diastereoisomeric mixture of predominantly the compound of formula [Vl] or formula [VII], respectively.

Removal of the hydroxyl protecting groups can be accomplished by any suitable method known to a person skilled in the art. Preferably deprotection and cyclization are performed simultaneously, most preferably using an acid. The acid may be any protic acid, preferably an inorganic acid like hydrochloric acid, sulfuric acid or phosphoric acid. Most preferably the inorganic protic acid is aqueous hydrochloric acid.

The amount of the acid used for the conversion of the compound of formula [V] to the cyclized compound of formula [Vl] or [VII] is at least 0.01 molar equivalents based on the total amount of aldehydes, preferably it is at least 0.1 molar equivalents based on the total amount of aldehydes, and most preferably it is at least 0.25 molar equivalents based on the total amount of aldehydes. The upper limit of the amount of the acid used for the conversion of the compound of formula [V] to the cyclized compound of formula [Vl] or [VII] is at most 5 molar equivalents based on the total amount of aldehydes, preferably it is at at most 2 molar equivalents based on the total amount of aldehydes, and most preferably it is at most 1 molar equivalents based on the total amount of aldehydes.

Suitably the temperature for the reaction is at least -20⁰C, preferably at least -10⁰C, more preferably at least -5°C and most preferably at least O⁰C. As an upper limit the temperature for the reaction is at most 70°C, preferably at most 50°C, more preferably at most 30⁰C and most preferably at most 5°C.

In another embodiment of the invention, the aldol reaction, deprotection and cyclization are performed in a sequential one-pot synthesis. This sequential one-pot synthesis is characterized by the fact that the reactions occur sequentially, that there are variable reaction conditions and that there is no isolation of intermediate products (definition according to D. Ager, Handbook of Chiral Chemicals, 2006, p. 422). Most preferably the deprotection and cyclization part of this sequential one-pot synthesis is carried out in the presence of an acid.

The aldol reaction part of this sequential one-pot synthesis is typically carried out in an organic solvent like dioxane, acetonitrile, dimethylsulfoxide, N- methylpyrrolidinone or tetrahydrofuran. Most preferably the aldol reaction part of this sequential one-pot synthesis is carried out in tetrahydrofuran.

In order to isolate the compound of formula [Vl] or [VII] the reaction mixture after deprotection/cyclization may be treated with a base to neutralize the acid. The base may be an organic base like pyridine or triethylamine or an inorganic base like sodium hydroxide or sodium hydrogen carbonate. Purification of the diastereoisomeric mixture of the compound of formula [Vl] or [VII] may be performed by using techniques known to a person skilled in the art. For instance, the neutralized mixture may be extracted with a highly apolar solvent to remove certain apolar impurities. The resulting mixture may subsequently be subjected to a solvent switch which may give precipitation of the salts resulting from the neutralization and/or of the organocatalyst. The resulting slurry may subsequently be filtrated to remove the salts and/or the organocatalyst. The filtrate may subsequently be evaporated to obtain the diastereoisomeric mixture of compound [Vl] or [VII] in a purified form. In a further embodiment the invention relates to a method for the synthesis of enantiomerically and diastereomerically enriched compound of the formula [I] or its enantiomeric form [II] by aldol coupling of two O-protected hydroxyaldehydes of the general formula [III] and the general formula [IV], respectively, wherein P₁ and P₂ are hydroxyl protecting groups, and removal of the protecting groups from the resulting product of formula [V] and (optionally simultaneous) cyclization to yield a diastereomeric mixture of the compound of formula [Vl] or formula [VII], respectively, after which the diastereomeric purity of the compound is upgraded by a biocatalytic step using a hydrolytic enzyme. The hydrolytic enzyme may be pure, partially pure, immobilized or part of a more complex biological matrix such as a microorganism. In a further embodiment of the present invention the diastereomeric and optionally the enantiomeric purity of the compound of formula [I] or [II] is upgraded by a process comprising the following steps: a. Non-enzymatically acylating or aroylating the mixtures of the isomers of the compound [Vl] or [VII], respectively b. treatment of the acylated or aroylated product with a stereospecific hydrolytic enzyme which leaves the acylated or aroylated form of the desired diastereomer intact, but hydrolyses the other (undesired) diastereoisomer c. isolating the desired acylated or aroylated diastereomer, d. hydrolyzing the desired acylated or aroylated diastereomer and e. isolating the desired diastereomerically and optionally enantiomerically enriched compound of formula [I] or [II].

In a further embodiment of the present invention the diastereomeric and optionally the enantiomeric purity of the compound of formula [I] or [II] is upgraded by a process comprising the steps: a. acylating or aroylating the undesired diastereomers of the compound [Vl] or [VII], respectively, using a stereospecific hydrolytic enzyme which specifically acylates or aroylates the undesired diastereomers. b. isolating the desired diastereomerically and optionally enantiomerically enriched compound of formula [I] or [II].

In these methods the hydrolytic enzyme used may be an esterase or a lipase. Examples of suitable enzymes stem from Pseudomonas fluorescens, Mucor miehei, Mucor javanicus, Candida Antarctica. Preferably the enzyme is a lipase and more preferably a commercially available lipase like lipase PS-800 (Amano Enzyme Co., Japan) or Novozyme 525 (Novozymes A/S Denmark).

In a further embodiment of the present invention the diastereomeric and optionally the enantiomeric purity of the compound of formula [I] or [II] is upgraded by a process comprising the steps: a. acylating or aroylating the desired diastereomer of the compound [Vl] or [VII], respectively, using a stereospecific hydrolytic enzyme which specifically acylates or aroylates the desired diastereomer in the presence of a suitable acyl or aroyl donor, b. isolating the desired acylated or aroylated diastereomer, c. hydrolyzing the desired acylated or aroylated diastereomer and d. isolating the desired diastereomerically and optionally enantiomerically enriched compound of formula [I] or [II]. In a further embodiment of the present invention the diastereomeric and optionally the enantiomeric purity of the compound of formula [I] or [II] is upgraded by a process comprising the steps: a. non-enzymatically acylating or aroylating the mixtures of the isomers of the compound [Vl] or [VII], respectively b. treatment of the acylated or aroylated product with a stereospecific hydrolytic enzyme which specifically hydrolyzes the desired diastereoisomer and leaves the acylated or aroylated form of the undesired diastereomers intact c. isolating the desired diastereomerically and optionally enantiomerically enriched compound of formula [I] or [II].

Suitable acyl or aroyl donors are esters of formula R¹R²C=CR³-O- C(O)R, in which R¹, R², and R³ each independently represent hydrogen, an optionally substituted (hetero)alkyl group or an optionally substituted (hetero)aryl group with for example 1-50 C atoms and in which R is an (optionally substituted) alkyl or aryl group. Other suitable acyl or aroyl donors are esters of formula R⁴-O-C(O)R, in which R⁴ is a halogenated alkyl group of 2-50 carbon atoms in which at least the 2-position is substituted with one, two or three halogens and in which R is an (optionally substituted) alkyl or aryl group. Examples of heteroatoms in the optionally substituted (hetero)alkyl group and optionally substituted (hetero)aryl group with for example 1-50 C atoms which may be present in R, R¹, R² and R³ are N, O, P and S. Examples of substituents on the (hetero) alkyl group or (hetero) aryl group are a nitro group, a halogen, an alkyl group with 1-6 C atoms, and an alkoxy group with 1-6 C atoms. Examples of donor acyl esters of formula R¹R²C=CR³O-C(O)R are isopropenyl and vinyl esters, for example vinyl acetate, vinyl proprionate, vinyl butyrate, and isopropenyl acetate. Examples of donor acyl esters of formula R⁴-O-C(O)R are are 2,2,2-trichloroethylesters and 2,2,2- trifluoroethylesters.

Non-enzymatic acylation or aroylation of the mixture of the compound [Vl] or [VII] can be accomplished by a person skilled in the art. Preferably the ester is formed by reacting the free hydroxyl group of [Vl] or [VII] with the corresponding acid chloride or anhydride in a suitable solvent and in the presence of a base.

Non-enzymatic hydrolysis of the acylated or aroylated diastereomer can be accomplished by a person skilled in the art. In case acetyl esters are hydrolyzed the hydrolysis is preferably performed by reacting the compound in a suitable solvent with a base, for instance in methanol with potassium carbonate. It is an advantage of the current invention that performing the aldol coupling reaction, deprotection and cyclization in a sequential one-pot synthesis and further upgrading the diastereomeric purity of the compound by a biocatalytic step using a hydrolytic enzyme results in a diastereomeric excess of over 99%.

The invention is now illustrated by way of the following examples, without however being limited thereto.

EXAMPLES Abbreviations used: TMS = trimethylsilyl TBDMS = te/f-butyldimethylsiloxyl

Ac = acetyl

Bn = benzyl

Boc = di-tert-butyl-dicarbonate

Piv = pivaloyl

Materials and methods

Solvents and reagents were used as received without further purification. Tert-butyldimethylsilyloxy-acetaldehyde ([III], with P₁ = TBDMS) and benzyloxyacetaldehyde ([III], with P₁ = benzyl) were purchased from Aldrich and distilled under reduced pressure prior to use in order to remove the trimeric form of the aldehydes. ¹H and ¹³C NMR spectra were recorded at 300 MHz and 75 MHz, respectively, in CDCI₃ or DMSO-Cf₆ on a Bruker Avance Ultrashield™ 300 NMR spectrometer. Quantitative ¹H NMR spectroscopy was performed using p-nitrotoluene as internal standard. The course of reactions was monitored by ¹H NMR, TLC or GC. Enantiomeric and diastereomeric excesses of compounds of formula

[I] and [II] and their O-acetyl derivatives were determined with an HP 5890 GC and a Supelco 24305 Betadex column of 60 m length and with an internal diameter of 0.25 mm and a film thickness of 0.25 μm using a column head pressure of 30 psi, a column flow of 1.4 mL/min, a split flow of 37.5 mL/min and an injection temperature of 250 ⁰C. The used ramp was: initial temperature 80 ⁰C (1 min), rate 4 °C/min, final temperature 180 ⁰C (5 min). Detection was performed with an FID detector at a temperature of 250 ⁰C. Example 1. Synthesis of 4-terf-butyldimethylsilyloxy-π-butyraldehvde (FIVI. Po - TBDMS)

pyridine. S O₃ DMSO

To a stirred solution of 60 g (665 mmol) 1 ,4 butanediol and 25 g (248 mmol) triethylamine in 300 g dichloromethane was added dropwise a solution of 33.4 g (223 mmol) te/ϊ-butyldimethylsilylchloride in 30 g dichloromethane over 45 min at ambient temperature. After stirring for 1 h, 250 g saturated aq. sodium bicarbonate was added. The organic layer was washed with 250 ml_ water, dried (Na₂SO₄) and concentrated in vacuo to give 52 g of a yellowish residue which was purified by vacuum distillation (4-5 mbar, 82⁰C) giving 38 g of the mono-protected alcohol in 97% purity corresponding to 83% yield based on terf-butyldimethylsilylchloride.

To a cooled (-5⁰C) solution of 25 g of the mono-protected alcohol (119 mmol) in 100 mL toluene and 37 g (366 mmol) triethylamine was added a solution of pyridine.SO₃ (39 g, 245 mmol) in DMSO (175 g) during 1 h with the temperature being kept between 0°C and 10⁰C. After stirring for 30 min, TLC showed complete conversion. Water (100 mL) was added and after 10 min stirring the aqueous layer was extracted with 100 mL toluene. The combined toluene layers were washed with 150 mL water and concentrated to 41 g of a residue. Vacuum distillation (3-4 mbar, 72⁰C) of this residue yielded 18 g of 4-tert-butyldimethylsilyloxy-n-butyraldehyde (99% pure by GC) corresponding to a yield of 75% based on the mono-protected alcohol. The identity of the compound was confirmed by ¹H and ¹³C NMR spectroscopy. Example 2. Synthesis of 4-trimethvlsilvloxy-n-butvraldehvde (FIVl, P₇ = TMS)

pyridine.SO₃ DMSO

To a stirred solution of 160 g (1.77 mol) 1 ,4-butanediol and 15 g

(0.15 mol) triethylamine was added dropwise 15.5 g (0.14 mol) trimethylsilylchloride over 15 min at ambient temperature. After stirring for 2 h, 150 ml_ water and 100 mL toluene were added and the aq. layer was extracted with 200 ml_ toluene. The combined organic phase was washed with 50 ml_ brine and concentrated in vacuo to 150 g of a solution of the mono-protected alcohol, which was cooled to 15⁰C. After the addition of 36 g (356 mmol) triethylamine, a solution of pyridine.SO₃ (36.4 g, 229 mmol) in DMSO (150 g) was added during 30 min while the temperature was kept between 0⁰C and 5°C. After stirring for 4 h at ambient temperature, 250 ml_ water and 50 mL brine were added and the aq. layer was extracted with 200 ml_ toluene. The combined organic phase was washed with 300 mL water/brine 1/1 (v/v) and concentrated in vacuo to 14 g of a residue containing 47 weight% aldehyde (6.6 g), which was purified by vacuum distillation (5 mbar, 50⁰C) yielding 4.2 g of 4-trimethylsilyloxy-n- butyraldehyde (93% pure by GC) corresponding to a yield of 19% based on trimethylsilylchloride. The identity of the compound was confirmed by ¹H and ¹³C NMR spectroscopy.

Example 3. Synthesis of 4-acetoxy-π-butyraldehyde (TIVI. PT = AC)

To a cooled (0°C) solution of 40 g (444 mmol) 1 ,4-butanediol in 40 mL dichloromethane and 21.1 g (266 mmol) pyridine was added dropwise 21 g (266 mmol) acetylchloride over 30 min at O⁰C. After stirring for 18 h at ambient temperature, 100 mL water was added and the aq. layer was extracted with 400 mL diethylether. The organic phase was dried (Na₂SO₄) and concentrated in vacuo to 32

pyridine.SO₃ DMSO

g of a crude oil containing the mono-protected alcohol. To a cooled (-5⁰C) solution of 30 g of this crude oil in 100 ml_ toluene and 66.5 g (657 mmol) triethylamine was added dropwise a solution of pyridine.SO₃ (70 g, 438 mmol) in DMSO (390 ml_) over 2Vi h while the temperature was maintained between 0⁰C and 5°C. After stirring for VA h, 500 ml_ water was added over 30 min. The aq. layer was extracted with 800 ml_ dichloromethane and the combined organic phase was washed with 400 ml_ water, dried (Na₂SO₄) and concentrated in vacuo to give 49 g of a residue which predominantly consisted of 4-acetoxy-n-butyraldehyde. Part of this residue was purified by silica gel column chromatography using ethyl acetate/heptane 1/3 (v/v) as the eluent giving 4-acetoxy-n-butyraldehyde with > 98% purity. The identity of the compound was confirmed by ¹H and ¹³C NMR spectroscopy.

Example 4. Synthesis of pivaloyloxyacetaldehyde (NIII. Pi = Piv)

To a stirred solution of 250 g (4 mol) ethyleneglycol and 30 g (0.30 mol) triethylamine was added dropwise over 45 min 30 g (0.25 mol) pivaloylchloride at ambient temperature. After 1 h stirring, 150 ml_ dichloromethane and 250 ml_ water were added and the aq. layer was extracted with 200 ml. dichloromethane. The combined organic phase was washed with 300 ml. water, dried (Na₂SO₄) and concentrated in vacuo to give 33 g of a yellowish residue which was distilled (4-5 mbar, 74°C) to give 23 g of a colorless oil (mono-protected alcohol, 96% pure by GC), corresponding to a yield of 62% based on pivaloylchloride. To a cooled (-5⁰C) solution of 10 g of the mono-protected alcohol (66 mmol) in 75 g dichloromethane and 20 g (198 mmol) triethylamine was added a solution of pyridine. SO₃ (20 g, 132 mmol) in DMSO (100 g) over 15 min while keeping the temperature between 0⁰C and 10⁰C. After 1 h stirring, GC showed complete conversion of the mono-protected alcohol and 100 ml. water was added. The aq. phase was extracted with 200 ml_ dichloromethane and the combined organic phase was washed with 300 ml_ water, dried (Na₂SO₄) and concentrated in vacuo to give 12 g of a yellow oil which was purified by vacuum distillation (7 mbar, 56°C) to furnish 4.1 g pivaloyloxyacetal-dehyde, corresponding to a yield of 43% based on the mono-protected alcohol. The identity of the compound was confirmed by ¹H and ¹³C NMR spectroscopy.

Example 5. Synthesis of terf-butyloxycarbonyloxy-acetaldehyde (Mill. Pi = Boc)

pyridine.SO₃ DMSO

To a stirred solution of 50 g (806 mmol) ethyleneglycol and 0.5 g (4 mmol) 4-N,N-dimethylaminopyridine in 50 g dichloromethane was added dropwise a solution of 10.1 g (46 mmol) di-tert-butyl-dicarbonate (Boc-anhydride) in 10 g dichloromethane over 30 min at ambient temperature. After 24 h stirring 50 g water was added and the aq. phase extracted with 50 ml_ dichloromethane. The combined organic phase was washed with 50 mL 1 N aq. HCI and with 50 mL 5 wt% aq. sodium bicarbonate, subsequently dried (Na₂SO₄) and concentrated in vacuo to 6 g of a yellowish residue consisting of the mono-protected alcohol (83% pure by GC, yield 67% based on Boc-anhydride) which was used without further purification in the next step. To a cooled (-5⁰C) solution of 5.5 g crude mono-protected alcohol (28 mmol) in 50 ml. dichloromethane and 7.6 g (75 mmol) triethylamine was added a solution of pyridine. SO₃ (1O g, 63 mmol) in DMSO (50 g) during 20 min maintaining the temperature between 0⁰C and 10⁰C. After stirring for VA h, GC showed complete conversion of the mono-protected alcohol. After the addition of 50 mL water the aqueous phase was extracted with 50 mL dichloromethane and the combined organic phase was washed with 50 mL 1 N aq. HCI and 50 mL 5 wt% aq. sodium bicarbonate solution, dried (Na₂SO₄) and concentrated in vacuo to give 6 g of a yellow oil. Vacuum distillation (4 mbar, 54°C) of this residue gave 1.3 g tert-butyloxycarbonyloxy- acetaldehyde (85% pure by GC) corresponding to a yield of 29% based on the mono- protected alcohol. The identity of the compound was confirmed by ¹H and ¹³C NMR spectroscopy.

Example 6. Synthesis of acetoxy-acetaldehvde (NIII. P_L= Ac)

NMO cat. K₂OsO₄-H₂O

To a stirred solution of 20.5 mL (0.25 mol) but-2-ene-1 ,4-diol and 105 mL (0.75 mol) triethylamine in 400 mL tetrahydrofuran was added 60 mL (0.63 mol) acetic anhydride at ambient temperature. After 16 h stirring, 300 mL saturated aq. sodium bicarbonate solution and 300 mL diethylether were added. The aq. layer was extracted with 200 mL diethylether and the combined organic phase was washed with 300 mL brine, dried (Na₂SO₄) and concentrated in vacuo. The resulting residue was purified by vacuum distillation (3 mbar, 80-82⁰C) to give 39.3 g of the diacetate, corresponding to a yield of 91% based on but-2-ene-1 ,4-diol. To a stirred solution of 17.2 g diacetate (100 mmol) in a mixture of 150 mL acetone and 50 mL water were added 26 g N-methylmorpholine-N-oxide (NMO, 200 mmol) and 270 mg K₂OsO₄-H₂O (0.75 mmol) at ambient temperature. After 3 h stirring, TLC showed complete conversion of the diacetate and 300 ml. saturated aq. sodium hydrogen sulfite solution and 300 mL ethylacetate were added. The aq. layer was extracted with 600 mL ethylacetate and the combined organic phase was washed with 400 mL brine, dried (Na₂SO₄) and concentrated in vacuo to give 18.5 g of the diol as white crystals, corresponding to a yield of 90% yield based on the diacetate. To a stirred solution of 8.2 g diol (40 mmol) in 150 mL dichloromethane was added 80 g sodium periodate on silica (55 mmol) at ambient temperature. After 1 !4 h stirring, TLC showed complete conversion. The silica was removed by filtration and rinsed with 100 mL dichloromethane. The combined filtrates were concentrated in vacuo and the residue purified by careful distillation to give 4.1 g of the aldehyde, corresponding to 50% yield based on the diol. The identity of the compound was confirmed by ¹H and ¹³C NMR spectroscopy.

Example 7. Study of the L-Pro catalyzed aldol reaction between aldehydes FIIII and aldehydes NVI to give aldol products FVl

al

or

[V], minor

General procedure:

L-Proline (20 wt% based on aldehyde [IV]) and freshly distilled aldehydes [III] and [IV], in the molar ratio as indicated in Table 1 , were weighed into a sample flask. Subsequently, such an amount of tetrahydrofuran was added that the sum of the concentrations of aldehydes [III] and [IV] was 30 wt%. The flask was subsequently closed and the reaction mixture stirred at the temperature indicated in Table 1. The conversion was monitored by ¹H NMR analysis by taking samples of 150 mg from the reaction vessel and adding 20 mg p-nitrotoluene as the internal standard; the resulting mixture was concentrated in vacuo to dryness and redissolved in DMSO- d₆. The OH doublets of the desired R₁S-[V] aldol products (at approx. 5.1 ppm) were clearly distinguishable from the OH doublets of the undesired S₁S-[V] aldol products (at approx. 4.1 ppm). The diastereomeric ratio (d.r.) between R₁S-[V] and S₁S-[V] was determined by integration of these signals. The NMR yield in Table 1 corresponds to the total yield of both R₁S-[V] and S₁S-[V] based on aldehyde [IV] at maximum conversion of aldehyde [IV].

Table 1

From these results it can be concluded that: i) the d.r. is strongly influenced by the steric size and electronic properties of P₁ (entries 4-9); the more electron donating or the larger P₁, the higher the d.r.; ii) the nature of P₂ does not seem to influence the d.r. (entries 3-5); iii) increasing the ratio between aldehyde [III] and [IV] leads to higher yields (entries 2 and 3). Example 8. Synthesis of 1 ■5-di-(fe/t-butyldimethylsilvn-3-formyl-pentane-1.2.5-triol (N], Pi = P, = TBDMS)

cat. L-Pro THF

major minor

A mixture of 52 mg L-proline, 1.5 g (8.7 mmol) freshly distilled tert- butyldimethylsilyloxy-acetaldehyde ([III], P₁ = TBDMS), 760 mg (3.4 mmol) freshly distilled 4-terf-butyldimethylsilyloxy-n-butyraldehyde ([IV], P₂ = TBDMS) and 5 mL tetrahydrofuran were stirred in a closed flask at 4⁰C for 17 h and at 20⁰C for another 24 h. Subsequently, 5 mL of saturated aq. sodium bicarbonate and 20 mL methyl-tert- butylether were added and the aq. phase was extracted with 20 mL dichloromethane. The combined organic phase was concentrated in vacuo to give 2.4 g of a yellow oil which was purified by flash chromatography on silica gel using ethylacetate/n-heptane (12/88, v/v) as the eluent. This gave 480 mg of (2R.3S)- and (2S,3S)-1 ,5-di-(tert- butyldimethylsilyl)-3-formyl-pentane-1 ,2,5-triol in a diastereomeric ratio of 5:1 with a purity of 98% (as determined with quantitative ¹H NMR spectroscopy), corresponding to a yield of 37% based on tert-butyldimethylsilyloxy-acetaldehyde ([III], P₁ = TBDMS). ¹H NMR (300 MHz, DMSO-c/₆) of the (2R,3S)-diastereomer: δ = 9.60 (1 H, d, C(O)H), 5.06 (1H, d, OH), 3.85-3.78 (1H, m, CH-OH), 3.59-3.53 (4H, m, 2 x CH₂-O-TBDMS), 2.45-2.42 (1 H, m, CH-C(O)H), 1.94-1.81 (1 H, m, CH₂-CH₂-OSi), 1.69-1.57 (1 H, m, CH₂-CH₂-OSi), 0.82 (9 H, s, NBu), 0.81 (9 H, s, f-Bu), 0.0 (12 H, 2 x s, 4 x CH₃-Si). Example 9. Synthesis of 1-(te/t-butyldimethylsilvO-3-formyl-5-(trirnethylsilyl)- pentane-1.2.5-triol (M, Pi = TBDMS. P, = TMS)

cat. L-Pro THF

major minor

A mixture of 275 mg L-proline, 2.0 g (6.8 mmol, purity 60%) freshly distilled tert-butyldimethylsilyloxy-acetaldehyde ([III], P₁ = TBDMS), 4.0 g (17.5 mmol, purity 72%) freshly distilled 4-trimethylsilyloxy-n-butyraldehyde ([W], P₂ = TMS) and 50 ml_ tetrahydrofuran were stirred in a closed flask at 20⁰C for 24 h. Subsequently, 25 ml. saturated aq. sodium bicarbonate and 40 ml. methyl-terf-butylether were added and the aq. layer was extracted with 50 mL dichloromethane. The combined organic phase was concentrated in vacuo to give 5 g of a yellow oil which was purified by flash chromatography on silica gel using ethylacetate/n-heptane (14/84, v/v) as the eluent. This gave 0.6 g of (2R.3S)- and (2S,3S)-1-(tert-butyldimethylsilyl)-3-formyl-5- (trimethylsilyl)-pentane-1 ,2,5-triol in a diastereomeric ratio of 5:1 with a purity of 97% (as determined with quantitative ¹H NMR spectroscopy), corresponding to a yield of 26% based on tert-butyldimethylsilyloxy-acetaldehyde ([III], P₁ = TBDMS).

¹H NMR (300 MHz, DMSO-Cf₆) of the (2R,3S)-diastereomer: δ = 9.57 (1 H, d, C(O)H), 5.04 (1 H, d, OH), 3.82-3.76 (1 H, m, CH-OH), 3.57-3.48 (4H, m, 2 x CH₂-O-Si), 2.43-2.39 (1 H, m, CH-C(O)H), 1.91-1.77 (1 H, m, CH₂-CH₂-OSi), 1.65-1.55 (1 H, m, CH₂-CH₂-OSi), 0.80 (9H, s, f-Bu), 0.0 (9H, s, TMS), 0.0 (6H, s, 2 x Si-CH₃ TBDMS). Example 10. Synthesis of (3R,3aS,6aR)-hexahvdrofuror2,3-b1furan-3-ol using tert- butyldimethylsilyloxy-acetaldehvde (MIII. Pi = TBDMS) and 4-tert- butyldimethylsilyloxy-butyraldehvde (FIVI. P, = TBDMS)

cat. L-Pro THF

major minor

aq. HCI

(3R,3aS,6aR)-diastereomer (3S,3aS,6aR)-diastereomer (major) (minor)

A mixture of 1.57 g L-proline, 24 g (137 mmol) freshly distilled tert- butyldimethylsilyloxy-acetaldehyde ([III], P₁ = TBDMS), 14 g (68.4 mmol) freshly distilled 4-te/t-butyldimethylsilyloxy-n-butyraldehyde ([IV], P₂ = TBDMS) and 89 g tetrahydrofuran were stirred in a closed flask at 4°C. After 45 h stirring, when the conversion to the aldol products (as monitored by ¹H NMR spectroscopy) had reached its maximum, 20 g 3 wt% aq. HCI was added at 2-4°C. After 20 h stirring at that temperature, when the conversion to (3R,3aS,6aR)- and (3S,3aS,6aR)- hexahydrofuro[2,3-b]furan-3-ol (as monitored by GC analysis) had reached its maximum, 2.5 g pyridine, 90 g toluene and 40 ml_ water were added. After 10 min stirring, the mixture was filtered over a filter precoated with Decalite and the toluene layer was extracted with 4 x 40 mL water and the combined aq. phase was washed with 50 g toluene. The aq. phase was concentrated in vacuo giving 15 g of a yellowish residue. GC and ¹H NMR analysis and comparison with the physical data as disclosed by Quaedflieg et al. in Org. Lett. 2005, 7(26), 5917-5920, showed that this residue contained 36.7 wt% (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol and 7.3 wt% (3S,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol corresponding to 73% yield based on 4- terf-butyldimethylsilyloxy-n-butyraldehyde ([IV], P₂ = TBDMS) and a d.r. of 5:1. The e.e. of the (3R,3aS,6aR)-diastereomer was > 98%.

Example 11. Synthesis of (3R.3aS.6aR)-hexahydrofuror2.3-blfuran-3-ol using tert- butyloxycarbonyloxy-acetaldehyde (NIH. Pi_⁼ Boc) and 4-tert- butyldimethylsilyloxy-n-butyraldehvde (NVl. P₇ = TBDMS)

A mixture of 40 mg L-proline (0.35 mmol), 690 mg freshly distilled (3.66 mmol, 85% purity) terf-butyloxycarbonyloxy-acetaldehyde ([III], P₁ = Boc), 325 mg (1.59 mmol) freshly distilled 4-te/t-butyldimethylsilyloxy-n-butyraldehyde ([IV], P₂ = TBDMS) and 2.1 g tetrahydrofuran was stirred in a closed flask at 4°C. After 84 h stirring, when the conversion to the aldol products (as monitored by ¹H NMR spectroscopy) had reached its maximum, 0.5 g 18 wt% aq. HCI was added to the reaction mixture at 20⁰C. After 44 h stirring at that temperature, when the conversion to (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol and (3S,3aS,6aR)-hexahydrofuro[2,3- b]furan-3-ol (as monitored by GC analysis) had reached its maximum, triethylamine (0.3 g, 3.0 mmol) was added to neutralize the reaction mixture. The mixture was concentrated in vacuo and the volatiles coevaporated with 2 x 10 mL tetrahydrofuran giving (3R,3aS,6aR)- and (3S,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol in a d.r. of 2.3:1 and in 63% yield

cat. L-Pro THF

major minor aq. HCI

(3R,3aS,6aR)-diastereomer (3S,3aS,6aR)-diastereomer (major) (minor)

based on 4-tert-butyldimethylsilyloxy-/7-butyraldehyde ([IV], P₂ = TBDMS), as determined by quantitative ¹H NMR spectroscopy and by comparison with the physical data as disclosed by Quaedflieg et al. in Org. Lett. 2005, 7(26), 5917-5920. The e.e. of (3R,3aS,6aR)-hexahydro-furo[2,3-b]furan-3-ol was > 98%.

Example 12. Synthesis of (3R.3aS.6aR)-hexahvdrofurof2,3-b1furan-3-ol using tert- butyldimethylsilyloxy-acetaldehvde (FIIIl. Pi = TBDMS) and 4- trimethylsilyloxy-π-butyraldehvde (NVI. P? = TMS)

A mixture of 214 mg L-proline (1.86 mmol), 3.1 g (17.6 mmol, 99% purity) freshly distilled terf-butyldimethylsilyloxy-acetaldehyde ([III], P₁ = TBDMS), 1.55 g (9.7 mmol) freshly distilled 4-trimethylsilyloxy-n-butyraldehyde ([IV], P₂ = TMS) and 14 g tetrahydrofuran were stirred in a closed flask at 4°C. After 84 h stirring, when the conversion to the aldol products (as monitored by ¹H NMR spectroscopy) had reached its maximum, 2.6 g 18.5 wt% aq. HCI was added at 0^cC and the reaction mixture was further stirred for 18 h at 4°C. Subsequently, 3 g triethylamine was added and a solvent-switch to ethyl acetate (overall volume 25 ml.) resulted in

cat. L-Pro THF

(3R,3aS,6aR)-diastereomer (3S,3aS,6aR)-diastereomer (major) (minor)

precipitation of triethylamine.HCI and L-proline. After filtration and washing of the solids with 10 ml. ethyl acetate, the combined filtrates were concentrated in vacuo to 1.7 g of a yellow oil. GC and ¹H NMR analysis and comparison with the physical data as disclosed by Quaedflieg et al. in Org. Lett. 2005, 7(26), 5917-5920, showed that this residue contained 36.3 wt% (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol and 7.3 wt% (3S,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol corresponding to 63% yield based on 4- trimethylsilyloxy-n-butyraldehyde ([IV], P₂ = TMS) and a d.r. of 5:1. The e.e. of the (3R,3aS,6aR)-diastereomer was > 98%.

Example 13. Synthesis of diastereomericallv and enantiomericallv pure

(3R.3aS.6aR)-hexahydrofuror2,3-blfuran-3-ol via biocatalvtic enrichment Chemical acetylation

Of the residue obtained in Example 10, a portion of 7.5 g (containing 3.3 g (25 mmol) (3R,3aS,6aR)- and (3S,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol with a d.r. of 5:1 and of which the (3R,3aS,6aR)-diastereomer had an e.e. of > 98%) was dissolved in

d.e. = 67%

Lipase PS-800 pH = 7

d.e. = 99 (e.e. > 99.5%)

22 g dichloromethane. Subsequently, 6 g triethylamine (59.4 mmol) and 65 mg N.N-4- dimethylaminopyridine (0.53 mmol) were added and the mixture was stirred at 0⁰C. Acetic anhydride (5.6 g, 54.8 mmol) was added dropwise at 0⁰C and the reaction mixture was stirred for 3 h at 20°C. Methanol (1.8 g, 56.3 mmol) was added and after 30 min stirring at 20⁰C the mixture was washed with 10 mL water, 10 ml. 3 wt% aq. HCI and 10 mL brine. The organic phase was dried (Na₂SO₄), concentrated in vacuo and distilled by Kugelrohr (3-5 mbar, 120-130⁰C) furnishing 3.6 g of the acetylated compounds, having a purity of 86%, a d.r. of 5:1 , corresponding to a yield of 72% based on the non-acetylated isomers. The e.e. of the acetylated (3R,3aS,6aR)-isomer was >98%.

Enzymatic resolution with Lipase PS-800

Part of the mixture of acetylated compounds as obtained above (350 mg, 1.75 mmol, 86% purity, d.r. 5:1) was dissolved in 9.3 g 100 mM aq. potassium phosphate buffer (pH = 7.0) at 35°C. After the addition of 5 mg Lipase PS-800 (Amano), the pH of the reaction mixture was kept at 7.0 via automatic titration with 1 M aq. NaOH and the course of the deacetylation reaction was monitored by GC. After 24 h, 280 mg Celite and 20 mL dichloromethane were added and the mixture was stirred for 10 min. After filtration, the Celite was washed with 10 mL dichloromethane and the combined organic phase washed with 2 x 15 mL water and 10 mL brine, dried (Na₂SO₄) and concentrated in vacuo, giving 250 mg (1.45 mmol) (3R,3aS,6aR)-3-O- acetyl-hexahydrofuro[2,3-b]furan-3-ol, corresponding to a yield of 83% based on the mixture of acetylated compounds. GC indicated that the d.e. was 99% and the e.e. > 99.5%.

Enzymatic resolution with Novozvme 525

The mixture of acetylated compounds (1.0 g, 5.0 mmol, 86% purity, d.r. 5:1) was dissolved in 9.0 g 100 mM aq. potassium phosphate buffer (pH = 7.0) at 35°C. After the addition of 100 μl Novozyme 525, the pH of the reaction mixture was kept at 7.0 via automatic titration with 1 M aq. NaOH and the course of the deacetylation reaction was monitored by GC. After 47 h, 500 mg Celite and 40 mL dichloromethane were added and the mixture was stirred for 10 min. After filtration, the Celite was washed with 10 mL dichloromethane and the combined organic phase was washed with 2 x 30 mL water and 20 mL brine, dried (Na₂SO₄) and concentrated in vacuo, giving 820 mg (4.53 mmol) (3R,3aS,6aR)-3-O-acetyl-hexahydrofuro[2,3-b]furan- 3-ol, corresponding to a yield of 91% based on the mixture of acetylated compounds. GC indicated that the d.e. was 91% and the e.e. > 99%.

Chemical hydrolysis of (3R,3aS,6aR)-3-0-acetyl-hexahydrofuror2,3-b1furan-3-ol

The (3R,3aS,6aR)-3-O-acetyl-hexahydrofuro[2,3-b]furan-3-ol as obtained from the Lipase PS-800 resolution (250 mg, 1.45 mmol, with d.e. = 99% and e.e. > 99.5%) was dissolved in 5 mL anhydrous methanol. Potassium carbonate (5 mg) was added and the reaction mixture was stirred at ambient temperature until all starting compound had been converted (as indicated by GC). After filtration the methanol was evaporated giving (3R,3aS,6aR)-hexahydrofuro[2,3-b]furan-3-ol (189 mg, 1.45 mmol) corresponding to 100% yield based on the acetylated compound. GC indicated that the d.e. was 99% and the e.e. > 99.5%.

Claims

Method for the synthesis of a compound of formula [I] or its enantiomeric form

[II]

by aldol coupling of two O-protected hydroxyaldehydes of the general formula and the general formula [IV], respectively,

wherein P₁ and P₂ are protecting groups, and subsequent removal of the protecting groups and (optionally simultaneous) cyclization of the resulting aldol compound and optionally subsequent isolation of the compound of formula [I] or [II].

Method according to claim 1 , characterized in that the synthesis is carried out in a sequential one-pot synthesis.

Method according to any of claims 1-2, characterized in that the removal of the protection groups and cyclization are carried out simultaneously in the presence of an acid.

Method for the synthesis of diastereomerically enriched compound of the formula [I] or its enantiomeric form [II]

[H]

by aldol reaction of two O-protected hydroxyaldehydes of the general formula and the general formula [IV], respectively,

wherein P₁ and P₂ are protecting groups, and subsequent removal of the protecting groups and (optionally simultaneous) cyclization of the resulting aldol compound to yield a diastereomeric mixture of the compound of formula [Vl] or formula [VII], respectively,

after which the diastereomeric purity of the compound is upgraded by a biocatalytic step using a hydrolytic enzyme, and optionally subsequent isolation of the compound of formula [I] or [II].

5. Method according to claim 4, characterised in that the synthesis is carried out in a sequential one-pot synthesis.

6. Method according to any of claims 4-5, characterized in that the removal of the protection groups and cyclization are carried out simultaneously in the presence of an acid.

7. Method according to any of claims 4-6, wherein the diastereomeric purity of the compound is upgraded by a. acylating or arylating the mixture of the compound [Vl] or [VII], respectively b. treatment of the acylated or arylated product with a stereospecific hydrolytic enzyme which leaves the acylated or arylated form of the desired diastereomer intact, c. isolating the desired acylated or arylated diastereomer, d. hydrolyzing the desired acylated or arylated diastereomer and e. isolating the desired diastereomerically enriched compound of formula [I] or [M].

8. Method according to any of claims 4-6, wherein the diastereomeric purity of the compound is upgraded by a. acylating or aroylating the undesired diastereomers of the compound [Vl] or [VII]₁ respectively, using a stereospecific hydrolytic enzyme which specifically acylates or aroylates the undesired diastereomers. b. isolating the desired diastereomerically and optionally enantiomerically enriched compound of formula [I] or [II].

9. Method according to any of claims 7-8, wherein the hydrolytic enzyme is a I ipase.

10. Method according to claim 9, wherein the lipase is Lipase PS-800.

11. Method according to any of claims 1-10, wherein the aldol reaction is carried out in the presence of a catalyst.

12. Method according to claim 11 , wherein the aldol coupling is carried out in the presence of an organocatalyst.

13. Method according to claim 12, wherein the organocatalyst is a chiral amine.

14. Method according to claim 12, wherein the organocatalyst is L-proline or D- proline.

15. Method according to any of claims 1-14, wherein P₁ is an electron donating and/or bulky hydroxyl protecting group.

16. Method according to any of claims 1-15, wherein P₁ and/or P₂ is a silyl group.

17. Method according to any of claims 1-16, wherein P₁ is a tert-butyldimethylsilyl group.

18. Method according to any of claims 1-17, wherein the ratio of aldehydes with the formula [III] and [IV], respectively, is between 1:1 and 5:1.

19. Method according to any of claims 1-18, wherein the temperature of the aldol reaction is between - 10 ⁰C and 30 ⁰C. 20. Method according to any of claims 1-19, wherein the aldol reaction is performed in tetrahydrofuran. 21. A compound according to formula [V] wherein P₁ and P₂ each independently

represent a methyl, substituted methyl, ethyl, substituted ethyl, benzyl, substituted benzyl, silyl, acyl, aroyl, alkoxycarbonyl, aryloxycarbonyl, sulfate or sulfonyl group.