IL35706A - Steroid enolates and esters derived therefrom and their preparation - Google Patents

Description

Steroid enolatea and esters derived therefrom and their preparation oJUDm π^κδ D'^a non onootrt D'l'tniDD o'otftisit This invention relates to the preparation of steroid enolates and of esters derived therefrom.

Steroid 1 , 4-diene- 3 , 11-diones are frequently encountered among the physiologically active steroids, especially the corticosteroids, and it is often found that the derivatives carrying substituents at the 6- or 9-position possess enhanced activity. Thus, for example, 9a-fluoro- l6 -methyl-prednisone (betamethasone) possesses greater anti- inflammatory activity than 16p-methyl prednisone and 6-methyl prednisone also exhibits greater activity than prednisone. In synthesising these compounds, the 6- or 9- substituent is usually introduced at a relative early stage, before the 1 , 4-diene-3 , 11-dione system is formed but considerable difficulty has been experienced in substituting at the sterically hindered 9-position.

We have now found that a 6- or a 9- substituent can be introduced into an existing steroid l, 4-diene-3 , 11-dione by selectively forming the 3- or the 9 ( 11 ) -enolate, converting this into an enol ester and reacting the latter with an electrophilic reagent to introduce the residue of an electrophile , for example, a halogen atom. In some instances the enolate itself 6- or 11- position without intermediary formation of an enol ester and the key step in our new method is, in fact, the selective enolisation predominantly either at the 3- or the 9(ll)-position. The new method provides a particularly effective method of introducing substituents at the hindered 9-position.

Attempts to prepare enol esters from steroid 1,4-diene-3,11-diones by treatment with a strong base to the enolate, followed by 'quenching' of the enolate anion with an acyl-ating reagent have previously yielded mixtures of the 3-and the 9(ll)-derivatives, with consequent difficulties of separation. Our researches have now succeeded in elucidating the mechanism of the enolisation reaction and have enabled us to formulate the conditions whereby either the 3-enolate or the 9(ll)-enolate is formed predominantly.

While wo do not wish to be bound by theoretical considerations, it is believed that on reaction of the 1,4-diene-3, 11-dione with a strong base, a proton is transferred from the 6-carbon atom to the base, yielding initially the 3-enolate, the removal of a proton from the 6-carbon atom to form the 9(ll)-enolate being strongly sterically hindered. The 3-lithium-enolates are stable, and do not enter into the subsequent reactions described below but other 3-alkali metal-enolates , in the presence of steroid 1 , -diene-3 , 11-dione , tend to rearrange to yield the 9(11) -enolate which appears to be more stable.

The rearrangement appears to be bimolecular, one molecule of 3-enolate reacting with one molecule of -3, 11-dione to yield one molecule of regenerated -3, 11-dione and one molecule of 9(ll)-enolate . Thus unreacted -3, 11-dione need only be present in a catalytic quantity since it is constantly regenerated. However, the bimolecular rearrangement is considerably slower than the enolisation and if an excess of base is added, the 3-enolate will be formed quantitatively before rearrangement can take place to a significant extent. Furthermore, at very low temperatures, for example, below -50°C, the bimolecular rearrangement is so slow as to be negligible and at such temperatures substantially only 3-enolate is apparently present (as demonstrated by forming an enol ester). It can thus be seen that if the reaction conditions are selected so that the initially formed 3-enolate cannot come in contact with the unreacted dione, or can only contact the dione under conditions in which rearrangement will not occur, the 3-enolate is obtained as reaction product, whereas if the reaction conditions are selected so that the initially formed 3-enolate can r n n r on ition in which rearrangement can occur, the 9(ll)-enolate is obtained.

If one equivalent of the dione is slowly added to a solution containing slightly more than one equivalent of the base, free dione is never present to a significant extent and only the 3-enolate is formed. On the other hand, if very slighly less than one equivalent of base is slowly added to a solution of one equivalent of dione, so that some unreacted dione is always present, predominantly 9(ll)-enolate is formed, provided, of course, that sufficient time is allowed for the bimolecular rearrangement to take place at the selected reaction temperature which, clearly, is desirably not unduly low.

As indicated above, if the reaction of base and dione is effected at a very low temperature, for example about -70°C to -80°C, predominantly the 3-enolate will be obtained. Our new method is to be contrasted with previously attempted enolisations of 1 , -diene-3 , 11-diones by the straight-forward addition of an excess of a base to the dione in a single batch whereby the rapid enolisation removed all the free dione before rearrangement was complete and a substantial quantity of 3-enolate always remained at the time of ester formation.

According to the present invention therefore we rovide a rocess for the selective 3- or 9 ll -enolisation of a sterod l,4-diene-3, 11-dione in which the 1,4-dione-3,11-dione is treated under substantially oxygen-free and anhydrous conditions in a non-hydroxylic solvent with an alkali metal base other than a lithium base soluble in the solvent to form the 3-enolate: when the 9(ll)-enolate is required, the dione in non-enolised form being allowed or caused either subsequently or simultaneously to react with the 3-enolate initially formed; when the 3-enolate is required the reaction being carried out at a temperature below -50°C or the reaction being carried out in a manner whereby contact between the 3-enolate and unreacted dione is reduced or avoided.

An alkali metal base, that is a sodium, potassium, rubidium or caesium base, acting as a strong nucleophile under anhydrous non-hydroxylic conditions is required to abstract the desired proton from the 6- and 9- position. Alkali metal hydrides; amides; alkylamides, e.g. sodium or potassium diisopropylamide ; triarylmethyls and acetyl-ides or other non-hydroxylic alkali metal derivatives which do not react with ketones may be used as base.

Particularly preferred bases' are the alkali metal bistertiary silylamides particularly alkali metal bistrialkylsilylamides such as bistrimethylsilylamides .

The reaction conditions, as stated above, should be oxygen-free and anhydrous : an atmosphere of a dry inert gas such as nitrogen or, preferably, argon is desirable. The non-hydroxylic solvent used should be dry and further should dissolve at least part of the reactant steroid dione and base.

Ethers are useful in this respect and it is particularly advantageous to use cyclic ethers such as tetrahydrof uran, which may conveniently be used with aromatic hydrocarbons such as benzene or toluene.

The reaction temperature, where low temperatures are not required to avoid bimolecular rearrangement, is preferably from -50°C to 100°C, advantageously ambient temperature e.g. 10° to 25°C.

Where 9(ll)-enolate is required, it may be convenient to effect the reaction under conditions leading to formation of a 3-enolate for example a sodium 3-enolate, and to add to the initial reaction products a quantity of the dione sufficient to effect the bimolecular rearrangement to the 9(11 )-enolate .

The enolates formed are advantageously, as indicated above, converted directly into enol esters. Thus, for example, the reaction with the alkali metal base may be conveniently 'quenched' and the corresponding ester produced, by treating the reaction mixture in situ with an acylating agent, e.g. a reactive ester, for example, the acid halide, or more particularly, the anhydride, of an organic acid such as an aliphatic, araliphatic or aromatic carboxylic or sulphonic acid. It is particularly preferred to use the anhydride of an aromatic carboxylic acid and, for example, quenching the reaction with benzoic anhydride yields the desired 3- or 9(ll)-enol benzoate, in virtually quantitative yield. Acetic anhydride similarly yields the corresponding acetates This quenching procedure is particularly useful in working up reactions carried out at very low temperatures, the rapid acylation effectively preventing any loss of the 3-enol.

Any steroid 1 , 4-diene-3 , 11-dione can be enolised by the process according to the present invention, especially corticosteroids of the prednisone type and suitably substituted progesterone and androstane derivatives. Particularly desirable enolates are formed from prednisone BMD ( 17 , 20 : 20 , 21-bismethylenedioxyprednisone) and its 16a and 16 -methyl analogues ,j'll-ketoprogesterone -ethylene ketal and androstane-1 , 4-diene-3 , 11 , 17-trione-17-monoethylene ketal.

The 9(ll)-enol esters described above are novel compounds and comprise a feature of the invention. The benzoate esters are preferred and among the compounds especially useful are included the 9(ll)-enol benzoates of corticosteroids of the prednisone type, particuarly 17 , 20 : 20 , 21-bismethylenedioxyprednisone-9(ll)-enol benzoate .

The 3- and 9(ll)-enol esters or the parent 3- and 9(ll)-enolates are, as indicated previously, particularly useful as starting compounds for electrophilic substitution reactions at the 6- or 9- positions. Thus, for example, reaction with sources of positive halogen such as molecular chlorine or bromide, introduces a halogen atom at the 6- or 9- position. It is particularly useful to introduce fluorine, especially at the 9oc-position by reacting an enolate or enol ester with an electrophilic fluorinating agent, such as perchloryl fluoride, or a hypofluorite reagent such as trifluoromethyl hypofluorite . The method according to the invention thus allows 9a-fluoro prednisone or a 16 -methyl or 16β -methyl derivative to be prepared directly from prednisone or its 16-methyl derivatives.

By the term steroid as used herein we mean substances structure and which may carry various substituents .

Apart from the 3- and 11-oxo groups and the double bonds in the 1,2- and 4,5- positions, the steroid may carry substituents at the 10- and 13-positions , e.g. alkyl groups such as the methyl groups usually present in the androstanes and pregnanes; at the 16a- or 16β-position, for example, alkyl, e.g. methyl, or alkylene, e.g. methylene, groups, or halogen atoms and at the 17-position, for example, a hydroxy or acyloxy group which may be present together with or as an alternative to an aliphatic group such as an acyl group e.g. the acetyl or β-hydroxyacetyl group. Where an enolisable oxo group is present such as in the 20-oxo pregnanes of the corticoid type, this should be protected prior to the reaction, for example, by formation of a ketal, or orthoester derivative. In particular, the 17a-hydroxy- 20-oxo-21-hydroxy-pregnane structure which occures in the corticosteroids of the prednisone and betamethasone type, may be protected by formation of bis-methylenedioxide derivatives .

Where the steroid 1 , -diene-3 , 11-dione used as starting material contains groups which may be hydrolysed under the basic conditions present in the reaction according to the invention, it is necessary to ensure a sufficient excess of base to complete the enolisation, the hydrolysed group, if desired, being subsequently restored to its original form. For example, acyloxy groups such as esterified hydroxy groups, may be hydrolysed under the reaction conditions and should then, if desired, be re-esterified .

The following Examples are given by way of illustration: - A : Preparation of 3-enolates characterised as their 3-benzoates Example 1 .: Reaction at low temperature Prednisone BMD (17 , 20: 20 , 21-bismethylenedioxyprednisone) (200 mg, 0.5 m.mole) was dissolved in freshly purified THF (tetrahydrofuran) (6 ml.) and sealed in a septum bottle under argon. The steroid solution was then cooled to -78°C. Sodium bistrimethylsilyamide (549.5 mg, 3.0. m.mole) in THF (5 ml.) at 78 °C was added to the steroid solution and the reaction allowed to proceed for 3 hours. Benzoic anhydride (crystallised from hexane : benzene; 452 mg, 2.0. m.mole) was added to the reaction mixture which was then allowed to warm up to room temperature. The mixture was then washed with water, dried and evaporated to dryness in vacuo and the residue taken up in chloroform. The crude mixture was purified by preparative thin layer chromatography on eluting with chloroform: ether (60:40 vol/vol) to yield the 3-enol benzoate (203 mg, 0.4 m.mole; 80%; m.p. 226 -228'C; [cc] HC13 = -182).

Example 2 : Slow addition of steroid to base Prednisone BMD (200 mg, 0.5 m.mole) in dry THF (10 ml.) was added dropwise over 30 minutes with vigorous stirring to sodium bistrimethylsilyamide (120.3 mg, 0.55 m.mole) in THF (5 ml.) under argon, the mixture remaining at room temperature throughout the addition.

The mixture was then stirred a further 2 minutes and benzoic anhydride (452 mg, 2.0 m.mole) added. The mixture was then worked up as before to yield the 3-enol benzoate (207 mg, 0.405 m.mole; 82%). Use of triphenylmethylsodium (3 mol equiv.) or sodium acetylide (5 mol. equiv.) yields the identical 3-enol benzoate.

Following this method, the 3-enol benzoates of 16a-methyl prednisone BMD, 16p-methylprednisone BMD, Δ^-11-keto-progesterone-20-ethylene ketal and androstane-1 ,4-diene-3 , 11 , 17-trione 17-raonoethylene ketal may be prepared. B : Preparation of 9(ll)-enolates characterised as their 9 (11)-benzoates .

Example 3: Slow addition of base to steroid in THF (5 ml.) was added dropwise at room temperature over prednisone BMD (200 mg, 0.5 m.mole) in THF (6 ml.) under argon. The reaction mixture was stirred a further 2 minutes and then benzoic anhydride (452 mg, 2.0 m.mole) added. The reaction mixture as then worked up as before to yield the 9(ll)-enol benzoate (203 mg, 0.4 m.mole, 80%; m.p. 290 - 293°C, [a] = +33.4°). Following this method, the 9(ll)-enol benzoates of 16a-methyl prednisone BMD, 16β -methyl prednisone BMD, A^-ll-keto-progesterone-20-ethylene ketal and androstrane-1 , 4-diene-3 , 11 , 17-monoethylene ketal. may be prepared. Use of triphenylmethylsodium (2 mol. equiv) or sodium acetylide (2 mol. equiv dispersed in xylene/ tetrahydrofuran) yields the identical 9(ll)-enol benzoate. Example 4 : Addition of unreacted steroid to a solution of the 3-enolate The reaction of Example 1 was repeated allowing a reaction time of 4¾ hours. The solution of 3-enolate was allowed to warm up to room temperature and more prednisone BMD (300 mg, 0.75 m.mole) in THF (9 ml.) was added. The reaction mixture was stirred at room temperature for a further 6 minutes and then benzoic anhydride (452 mg. 2.0. m.mole) was added. The reaction mixture was worked up as before to yield the 9(ll)-enol benzoate (192 mg, 0.38 m.mole, 76%; identical with the product of Example 3 (t.l.c)).

Example 5 : Reaction of prednisone BMP 9(ll)-enol benzoate with trifluoromethyl hypofluorite (CF^OF) Prednisone BMD 9(ll)-enol benzoate (500 mg.) was dissolved in a mixture of freon (100 ml.) and methylene chloride (50 ml.) and reacted with CF^OF in the presence of CaO (200 mg.) under nitrogen at about -78°C. The crude product (572 mg.) was chromatographed ove alumina (grade 3, 40 g.). Elution with benzene, followed by ether/benzene mixtures up to 10% ether, produced small amounts of several non polar products (total weight 103 mg.) which were not studied further. Elution with 20% ether/benzene yielded a series of fractions containing two main components, (A and B) resolved on t.l.c, of which the more polar (A) was the more abundant. Earlier fractions containing a greater abundance of (B) (total weight 127 mg.) were combined and absorbed onto a 20 x 20 x 0.1 cm. silica G thick layer chromatography plate. After elution thrice with 1% MeOH/Cl^C^, three zones were scraped from the plate and absorbed product was re-extracted using several aliquots of 10% MeOH/CH2Cl2. The next few fractions 97 re r ted in a simil r a Com arison of the I.R. spectra and t.l.c.s of the various zones indicated that A and B could be distinguished by their characteristic carbonyl absorptions. Combination of the appropriate fractions followed by crystallization from CI^Cl^/ eOH gave pure A (32 mg.), m.p. 276 - 84°C whose I.R. was superimposable on that of 9a-fluoro prednisone BMD and which did not depress a mixed melting point with an authentic sample. Combination of other fractions and crystallization from CH2Cl2/MeOH gave B (20 mg.).

Recrystallization from the same solvent gave heavy prisms (14 mg.), I.R. v * 1750 (s), 1670 (vs), 1635 (m) , 1610 (w), 1270 (vs), 1210 (vs) cm."1. The analytical specimen had m.p. 130-32°C [a] HC13 +44.7° (c = 1.1).

Hydrolysis of B with methanolic sodium hydroxide yielded compound A suggesting that B is 9cx-fluoro-lloc-trifluoromethoxy-ΙΙβ-benzoyl-prednisolone BMD.

Example 6 : 6 -Fluoro-prednisone BMD (9) Prednisone BMD 3-enol benzoate (250 mg.) was dissolved in a mixture of freon (40 ml.) and methylene chloride (20 ml.) in the presence of calcium oxide (100 mg.) and treated with CF3OF/ 2 at -78°C as in Example 5. After complete disappearance of starting material (t.l.c), CHC1 workup gave a single product (295 mg.), I.R. v 3 ΓΠ3.Χ . 1740 s 1260 s benzoate 1705 s ketone cm "1 Crystallization from methylene chloride/ether gave (9) (30 mg&. 1st cropt-),> I.R. vmCHaCx1#3 1705 (s) cm.'1 (ketone), 1670 (vs), 1625 (m) (dienone) cm. A second crop of 180 mg. was also obtained.

Claims (23)

WHAT IS CLAIMED IS:

1. A process for the selective 3- or 9(ll)-enolisation of a steroid l,4-diene-3, 11-dione in which the 1,4- diene-3, 11-dione is treated under substantially oxygen- free and anhydrous conditions in a non-hydroxylic solvent with an alkali metal base other than a lithium base soluble in the solvent, to form the 3-enolate : when the 9(ll)-eno- late is required, the dione in non-enolised form being allowed or caused either subsequently or simultaneously to react with the 3-enolate initially formed; when the 3-enolate is required, the reaction being carried out at a temperature below -50°C or the reaction being carried out in a manner whereby contact between the 3-enolate and unreacted dione is reduced or avoided? and where the 9(n) enolate is formed, it is optionally reacted with an electrophilic halogenating agent whereby the 9a-halogenosteroid 1 , -diene-3 , 11-dione is formed.

2. A process as claimed in claim 1 in which the base is a hydride, amide, alkylamide, triarylmethyl , acetylide or bis-tertiary-silylamide .

3. A process as claimed in claim 2 in which the base is a bistrimethylsilylamide .

4. A process as claimed in any of claims 1 to 3 in which the non-hydroxylic solvent is an ether.

5. A process as claimed in claim 4 in which the solvent is tetrahydrofuran.

6. A process as claimed in any of claims 1 to 5 in which the temperature, unless required to be below -50°C, is from -50°C to 100°C.

7. A process as claimed in claim 6 in which the temperature is ambient .

8. A process as claimed in any of claims 1 to 7 in which one equivalent of the dione is slowly added to a solution containing slightly more than one equivalent of the base whereby free dione is never present to a significant extent in the solution and only the 3-enolate is formed.

9. A process as claimed in any of claims 1 to 7 in which very slightly less than one equivalent of base is slowly added to a solution of one equivalent of dione whereby some unreacted dione is always present and predominantly 9(ll)-enolate is formed.

10. A process as claimed in any of claims 1 to 7 in which, where the 9(ll)-enolate is required, the reaction is effected by adding to a 3-enolate a quantity of the dione sufficient to effect the bimolecular rearrangement to the 9(ll)-enolate.

11. A process as claimed in any of claims 1 to 10 in which the enolate formed is converted directly into its enol ester by treatment of the reaction mixture in situ with an acylating agent.

12. . A process as claimed in claim 11 in which the acylating agent is the acid halide or anhydride of an aliphatic, araliphatic or aromatic carboxylic or sulphonic acid.

13. . A process as claimed in claim 12 in which the acylating agent is benzoic or acetic anhydride.

14. A process as claimed in any of claims 1 to 13 in which the 1 , 4-diene-3 , 11-dione starting material carries substituents selected from: an alkyl group at the 10- and/ or 13-positions ; an alkyl or alkylene group or halogen atom at the 16a- or 16 -positionl and at the 17-position a hydroxy or acyloxy group which may be present together with or as an alternative to an aliphatic group.

15. . A process as claimed in any of claims 1 to 14 in which the 1 , 4-diene-3 , 11-dione starting material carries other enolisable keto groups in a protected form.

16. . A process as claimed in claim 15 in which the other enolisable keto groups are protected as ketals or ortho ester derivatives.

17. . A process as claimed in claim 16 in which a 17a-hydroxy-20-oxo-21-hydroxypregnane structure is protected as a bismethylenedioxy derivative. 35706/2

18. A prooeie as claimed in any of claims 1 to 1? in which the 3- or 9(ll)-enolate initially prepared, or an ester thereof is subsequently reacted with a source of positive halogen whereby a halogen atom is introduced at the 6- or 9-poeition respectively*

19. · A process as claimed in claim 18 in which the source of poeltive halogen is perchloryl fluoride or a hypofluorite reagent*

20. A process as claimed in claim 19 in which the hypofluorite reagent is trlfluoromethyl hypofluorite*

21. A process as claimed in any of claims 1 to 20 substantially as herein described*

22. A process for the selective 3- or 9(11)»enolisation of steroid l,fe~dlene-3*ll dione substantially as herein described in any of Examples 1*4*

23. A process as olaimed in claim 1 in which the eleotrpphillc h&ogenating agent is trlfluoromethyl hypofluorite*? 2¾, A process as claimed in claim 23 substantially as herein described. 252,6.. A process as claimed in claim 23 ^ iiajM.2A substantially as herein described in Example 5. 6 2.1. 9(ll)-enol esters of steroid 1 ,4-diene-3, 11-diones . 27 «&. 9(ll)-enol esters of 17 , 20 : 21 , 21-bismethylenedioxy- prednisone and its l6o-methyl and 16p-methyl analogues, of ll-ketoprogesterone-20-ethylene ketal and of androstane-1, 4-diene-3 , 11 , 17-trione-17-monoethylene ketal . 28 29. 17,20:20,21-bismethylenedioxyprednisone-9(ll)enol benzoate and acetate . P. O. Box · ,i > , Tel-Aviv Attorneys f ur Applicant