WO2008138389A1

WO2008138389A1 - A method of preparation of quaternary nitrogen compounds

Info

Publication number: WO2008138389A1
Application number: PCT/EP2007/054478
Authority: WO
Inventors: Peter Hermanus Gerardus Wiegerinck; Franciscus Michael Kaspersen; Pieter Westerduin; Herman Steven Overkleeft; Gijbert Arie Marel Van Der; Jacob Bastiaan Luijendijk
Original assignee: N.V. Organon
Priority date: 2007-05-09
Filing date: 2007-05-09
Publication date: 2008-11-20

Abstract

The invention relates to a method of preparation of a quaternary amine having formula 2, by reacting an epoxide having formula 3, Formula 3 with an aliphatic tertiairy amine group in the presence of a suitable Lewis acid.

Description

A METHOD OF PREPARATION OF QUATERNARY NITROGEN COMPOUNDS

The invention relates to a method of preparation of quaternary nitrogen compounds wherein the quaternary nitrogen is connected to a carbon atom, which is adjacent to a carbon atom carrying a secondary hydroxyl group.

Interest in the preparation of quaternary nitrogen compounds, wherein the quaternary nitrogen is connected to a sugar moiety, is increasing in view of the need to prepare N⁺-glucuronides of medicinal compounds with tertiary nitrogens for testing the biological activity of those compounds. The formation of a quaternary ammonium- linked glucuronide (see Formula 1 ), also referred to as ΛT-glucuronidation, plays a significant role in the metabolic elimination of many tertiary amine-containing therapeutic agents. It has become clear that this bio-transformation is an almost human-specific process. Classes of drugs containing a tertiary amine are a.o. H₁- antihistamines, neuroleptics, tri- and tetracyclic antidepressants.

Formula 1

There is a need for synthetic ΛT-glucuronides for use as authentic standards, amongst others, because regulatory governmental authorities are now demanding that drug metabolites should be tested thoroughly in their own right before acceptance of the parent drug. Therefore, there is interest in a generally applicable synthesis route towards quaternary ammonium-linked glucuronides.

In 1992, Luo et al (ό. of Pharmaceutical Sciences, VoI 81 , pp 1079-1083)) presented a general synthesis route reacting D-Glucopyranuronic acid, 1-bromo-1-deoxy-, methoxy ester, triacetate with the tertiary amine in a two-phase solvent system. In particular, coupling reactions with methyl (2,3,4-tri-O-acetyl-α-D-glucupyranosyl bromide) uronate were explored (Scheme 1 ):

Scheme 1 The reported yields vary between 2-55%. This route gave even worse results for a number of tetracyclic compounds. It was found that yields for e.g. mirtazapine never exceeded 1 %.

This invention provides for an improved method of preparation of a quaternary amine having a structure according to formula 2

comprising reacting an epoxide having a structure according to formula 3

Formula 3

with an aliphatic tertiairy amine in the presence of a suitable Lewis acid, wherein R¹, R², R³, R⁴ and R⁵ are groups having at least one carbon atom and are bound by this carbon atom to the structures. The groups represented by R¹-R⁵ can have interconnections between R⁴ and R⁵, and the groups represented by R¹ and R² can form a ring structure, whereby R³ is (1 C-6C)-alkyl, usually a methyl or ethyl. Such a ring structure formed by R⁴ and R⁵ can be a carbohydrate, or it can be a carbohydrate group, oxidized at its primary position. If Formula 3 represents a carbohydrate or a carbohydrate group, oxidized at its primary position, it is properly provided with protection groups to the hydroxyl groups to prevent side reactions with free hydroxyl groups. The reaction is apparently not hampered by a steric barrier in R⁵ so that the invention is particularly useful for coupling a tertiary amine to a compound of Formula 3 wherein R⁵ is having a structure according to Formula 4

Formula 4 wherein the connection to the structure of Formula 3 is indicated by the symbol *; R⁶ is H or (1 C-6C)alkyl; R⁷ is a group having at least one carbon atom and bound by this carbon atom to the structure of Formula 3 and R⁸ is (H₁H) or O. R⁷ in Formula 4 can be connected to R⁴ in Formula 3 to form a ring structure. In a more specific embodiment of the invention the reaction according to the invention is applied to a method of preparation of a quaternary amine wherein R⁴, R⁵, R⁶, R⁷ and R⁸ have a meaning such as to define a ring system characteristic of carbohydrates or glucuronic acids or esters of the latter. With the reaction according to the invention the quaternary glucuronides can be readily made. The invention also provides for a method of synthesis of the N+-glucuronide of mirtazapine.

A suitable Lewis acid is, for example, having the formula MX_n, wherein M is a metal, such as Zn, B, Sn, Sb, Fe, Yb, Sc or Ti and X can be a halogen or CF₃SO₃. Examples of suitable Lewis acids are ZnCI₂, BF₃, SnCI₄, SbF₅, FeCI₃, (CF₃SO₃)₃Yb, (CF₃SO₃)₃Sc and TiCI₄. In general, it can be tested whether a Lewis acid is suitable by testing the reaction with any Lewis acid of interest.

In view of the need for protecting groups on hydroxyl groups of the reactants during the coupling reaction it will be understood that the compound according to Formula 2 can be obtained after removal of protecting groups on the hydroxyl groups. For reacting, the epoxide and the tertiairy amine are dissolved in a solvent, preferably an organic solvent. The opening of the epoxide by a tertiary amine to the corresponding N+-glucuronide of the corresponding amine is catalyzed by addition of a Lewis acid. After the required reaction time, which can be 24 hours, protection groups are removed. For example, the addition of a weak basic aqueous solution, preferably a NaHCO₃ solution (aq), can be used to saponify a methyl ester to glucuronic acid. This step in the workup procedure needs some care, because if the reaction is worked up without addition of a weak base first or if directly a strong base is added, then the coupling product will probably be unstable and will degrade before it can be isolated. In the preparation of glucuronic acids of quaternary compounds, a strong basic aqueous solution, preferably a NaOH solution (aq), can be added after another 24 hours in order to remove all remaining protection groups (for example acetate functionalities) and thereby deprotect the hydroxyl groups of the glucuronic acid moiety. Such procedures, the coupling of the tertiary amine with the epoxide, followed by the subsequent removal of the protecting groups, can all be done in the same vessel without intermediate purification steps, which has advantages of efficiency. Subsequent purification and isolation will yield a completely deprotected N+-glucuronic acid of a tertiary amine. In a more specific embodiment the above described coupling reaction was performed with mirtazapine. Purification of the deprotected product was done by size exclusion chromatograpy over a Sephadex^® LH20 column. The overall yield was 50%. NMR spectroscopy showed a mixture of four β-isomers. This can be explained by the fact that mirtazapine is a racemic mixture and by the formation of two N⁺-isomers, caused by the new chiral center which is created with the quaternary nitrogen. One of the N⁺- isomers with the glucuronide in an axial position, isomer I, and another with the glucuronide in the equatorial position, isomer II.

Isomer I Isomer I

Comparison of the NMR spectra of our products with NMR spectra from the natural metabolites (Delbressine et al., Clinical Drug Investigation (1998), VoI 15(1 ), pp. 45-55.) revealed that isomer Il is a metabolite of mirtazapine which occurs naturally in humans treated with mirtazapine. This is the predominant isomer in the product of the reaction at 60°. Isolation of the separate diastereomers can be done with preparative column chromatography.

Further results of the method with different temperatures, reaction times and solvents, are shown in Table 2, show that the ratios are influenced by reaction temperature.

Table 1 of 2

Solvent reaction time yield isomer ratio (I: ")

20° C dichloromethane 24 h 51 % -1 :1.1

-20 °C dichloromethane 168 h 62% -1.5:1

60° C 1 ,2-dichloroethane 2O h 36% -1 :5

Protection of hydroxyl groups and acid functionalities before starting the coupling reaction according to this invention is preferably done with acetyl groups and to a methyl ester, respectively. This can be done with methods well-known in the art. For quaternary amines of glucuronic acid this can be done in several steps starting from D-glucurono-3,6-lactone (Scheme 2).

1 ) HBr/HOAc

2) Zn dust

(62 % over 2 steps)

Scheme 2

After protection a C1-C2 double bond can be introduced which can be transformed to an epoxide by a peroxide, preferably dimethyldi-oxirane (DMDO), to the corresponding epoxide. By using the epoxide in the α-position, it is possible to obtain exclusively the β-glucuronides.

Examples

Abbreviations used are: Me is methyl; Ac is acetate Epoxide; Et is ethyl; DCM is dichloromethane; DMDO is dimethyldi-oxirane; Glue is glucuronic acid; Ax is axial; Eq is equatorial

Methyl-(3,4-di-O-acetyl-1 ,2-anhydro)-D-glucuronate

A solution of 10 mmol (2.58 gr) methyl-(3,4-di-O-acetyl-1 ,2-anhydro)-D-glucuronate (as prepared according to Liebigs Ann.Chem.; 1987; 637-638) in 50 ml. CH₂Cb was added dropwise to a cooled (0°) solution of

Dimethyldi-oxirane in acetone (150 ml_, ~ 0.08 M). The reaction mixture was allowed to warm to room temperature and stirred overnight. The reaction mixture was concentrated in vacuo and purified by silica gel column chromatography (silicagel, ethyl acetate/petroleum ether 2/8) which yielded 894 mg, 65%. R_f 0.65 (silicagel, ethyl acetate/petroleum ether1/1 )

¹H-NMR (CDCI₃, 200 MHz): δ 5.30 (d, 1 H, H-3), 5.16-5.01 (m, 2H, H-2, H-4), 4.36 (d, 1 H, H-5, J_4-5 7.4 Hz) 3.78 (s, 3H, CH₃ COOMe), 3.09 (d, 1 H, H-1 , J_1-2 1.9 Hz), 2.10, 2.08 (s, 3H₁ CH₃ Ac, 2x).

¹³C-NMR (CDCI₃, 50.1 MHz): δ 169.3, 169.0 (C(O)Me), 167.7 (COOMe), 76.0 (C2), 68.8 (C5), 67.6 (C4), 67.2 (C3), 2.4 (COOCH₃), 50.9 (C1 ), 20.3 (COCH₃, 2x).

Synthesis ΛT-glucuronides

ΛT-glucuronide of racemic mirtazapine

Methyl-(3,4-di-O-acetyl-1 ,2-anhydro)-D-glucuronate (162 mg, 0.63 mmol) was dissolved in freshly distilled dichloromethane (0.2 M). Flame dried molsieves (4A) were added and the mixture stirred for 30 min. Rac-mirtazapine (334 mg, 1.26 mmol) was subsequently added and the mixture stirred for another 30 min, cooled to 0°C and a ZnCI₂-solution (1.0 M in diethylether, 630 μl_) was added dropwise. The mixture was stirred and followed by TLC-analysis (silicagel, ethyl acetate/petroleum ether, 1/1 and methanol/dichloromethane, 2/8).

After 24h, the reaction mixture was filtered over hyflo, and a 0.5 M NaHCO₃-solution was added (same volume as dichloromethane). After another 24 hr, the dichloromethane layer was discarded and the aqueous layer was washed with diethylether (20 ml_, 3x), basified to pH 12.5 with 10% aqueous NaOH and washed again with diethylether (2OmL, 3x). The aqueous solution was neutralized with 10% aqueous HCI and washed with diethyl ether (2OmL, 3x). The aqueous phase was concentrated in vacuo. The residue was suspended in methanol, filtered and the filtrate purified by gel-filtration over a Sephadex LH-20 column (eluent: methanol). The appropiate fractions were collected and concentrated in vacuo. The residue was dissolved in water and lyophilized. Yield, 134 mg (48%).

Claims

1. Method of preparation of a quaternary amine having formula 2

by reacting an epoxide having formula 3

with an aliphatic tertiairy amine group in the presence of a suitable Lewis acid whereby R¹, R², R³, R⁴ and R⁵ are groups having at least one carbon atom and are bound by this carbon atom to the structures.

2. The method according to claim 1 , characterized in that the groups R⁴ and R⁵ have interconnections to form a ring structure.

3. The method according to claim 1 or 2, characterized in that R⁵ is having a structure according to Formula 4

Formula 4

wherein the connection to the structure of Formula 3 is indicated by the symbol *;

R⁶ is H or (1 C-6C)alkyl;

R⁷ is a group having at least one carbon atom and bound by this carbon atom to the structure of Formula 4; R⁸ is (H₁H) or O.

4. The method according to claim 2 or 3, characterized in that the ring structure is a carbohydrate, or a carbohydrate group, oxidised at its primary position and wherein the free hydroxyl groups are provided with protection groups.

5. The method according to claim 4, characterized the carbohydrate is glucuronic acids or esters of the latter.

6. The method according to any one of claims claim 1 to 5, characterized in that the groups represented by R¹ and R² form a ring structure and R³ is (1 C-6C)-alkyl.

7. The compound having the formula