US2571889A - Selective oxidation of dihydroxy steroids - Google Patents

Description

Patented Oct. 16, 1951 SELECTIVE OXIDATION OF DIHYDROXY STEROIDS Robert E. Jones, Rahway, N. J assignor to Merck & 00., Inc., Rahway, N. J., a corporation of New Jersey No Drawing. Application September 16, 1950, Serial No. 185,320

' 9 Claims. 1 This invention is concerned generally with selective oxidation of hydroxyl groupings in steroid compounds. More particularly, it relates to the process of selectively oxidizing the 3-hydroxyl grouping of the steroid compound A -3,21-dihydroxy-ll-keto-ZO-cyano-pregnene without a1- fecting the hydroxy substituent in the 21-position, thereby forming A -3,11diketo-20-cyano- 21-hydroxy-pregnene. The latter compound is valuable as an intermediate in the synthetic manufacture of cortisone (A -3,1l,20-triketo- 17,2'1-dihydroxy-pregnene) The selective oxidation reaction, subject of the present invention, may be chemically represented as follows:

Heretofore, it was not considered possible to selectively oxidize a secondary alcohol grouping in the presence of a primary alcohol grouping as is required to achieve the reaction described hereinabove. Attempts have been made to accomplish this transformation by subjecting the A -3,21-dihydroxy-11-keto-20-cyano pregnene to a partial acetylation reaction whereby the primary 2l-hydroxy substituent is preferentially acetylated while leaving the secondary 3-hydroxy radical unchanged. The resulting A -3-hydroxy-ll-keto-ZO-cyano 21 acetoxy-pregnene may then be oxidized to the corresponding 3-keto-derivative without affecting the 21-acetoxy substituent. Alternatively, the A -3-hydroxy-11-keto-20-cyano 21 acetoxy-pregnene may be reacted with osmium tetroxide and the resulting 17,20-osmate ester of 3,17,20-trihydroxy-ll-keto-ZO-cyano 21 acetoxy-pregnane may then be oxidized to the corresponding 3-keto derivative utilizing chromic acid. The overall yield obtained according to the foregoing procedures, however, has ordinarily been in the neighborhood of 50% due in part to the fact that the yield of monoacetyl derivative, obtained utilizing the partial acetylation reaction, is ordinarily less than about 85% of theory.

It is now discovered, however, that the 3-hydroxy-grouping in A -3,2l-dihydroxy-ll-keto- CHzOH ZO-cyano-pregnene can be selectively oxidized to a 3-keto radical without affecting the hydroxy substituent in the 21-position by reacting said A -3,21-dihydroxy-11-keto-20-cyano pregnene with an oxidizing agent selected from the class which consists of N-bromoacetamide and N-bromosuccinimide. This discovery is indeed surprising in view of the fact that N-bromoacet amide has previously been used in place of hypobromous acid (HOBr) for the addition of hydroxyl and bromine groupings to a carbon to carbon double bond. Since A -3,2l-dihydroxy- 11-keto-20-cyano-pregnene possesses an unsaturated carbon to carbon linkage between the 17 and 20-carbon atoms. it might have been expected that addition of hypobromous acid to the double bond between the 17 and 20-carbon atoms of the A -3,21 dihydroxy-ll-keto-ZO-cyanopregnene would have occurred.

In carrying out my improved process, the A -3,21-dihydroxy-11-keto-20-cyano pregnene is dissolved in an organic solvent such as pyridine, tertiary butyl alcohol, methanol, and the like, and the oxidizing agent is added to the solution. As the oxidizing agent, it is ordinarily preferred to employ N-bromoacetamide, N-bromosuccinimide, and the like. The amount of oxidizing agent employed in the oxidation reaction may vary from 1.5 to 3 moles per mole of A -3,2l-dihydroxy-'11-keto-20-cyano prognene. It is ordinarily preferred to utilize approximately 2.75 moles of oxidizing agent per mole of 3,21- dihydroxy-pregnene starting material.

The preferred solvent for carrying out the reaction is methanol, and the addition of a small amount of pyridine to the reaction mixture appears to be advantageous. The oxidation is ordinarily carried out at a temperature between about 18 and 40 C. and for a period of about 6 to 30 hours. The optimum time and temperature for the reaction are respectively 6 to 8 hours and 25 C. The preferred pH range is between 6 and 7, although the reaction can be carried out if desired under more acid conditions or slightly more alkaline conditions than the preferred range. In this connection, it should be noted that at a pH greater than 8, the selectivity of the oxidation is lost, while at an acid pH less than about 2. bromination in the 4-position occurs.

It is also desired to point out that a certain amount of bromination occurs during the reaction even under the preferred conditions of pH, with the result that the A -3,11-diketo-20- cyano-21-hydroxy-pregnene compound is contaminated with bromine. In order to remove this bromine, which is usually present in amounts of 1 to 2%, it has been found advantageous to treat the crude A -3,1l-diketo-20-cyano-21- hydroxy-pregnene with zinc and acetic acid whereby the bromine is completely removed. The treatment with zinc is preferably carried out utilizing zinc dustiand glacial acetic acid at a temperature of approximately 40 C. Instead of treating the crude (hrominated) A 3,ll-diketo-20-cyano-21-hydroxy-pregnene with zinc, it has been found advantageous to react this compound with an acylating agent such-as acetic anhydride and pyridine to .produce the corresponding A -3,11 diketo 20 cyano 21 acetoxy-pregnene. This 21-acetate can then be debrominated by reaction with zinc dust and acetic acid to give pure A -3,1ldil eto=20-cyanozl-acetoxy-pregnene.

Alternatively, it has been found possible -to minimize the contamination of the B-keto- .pregnene product. by .brominated impurities by careful-controbof .the reaction conditions. This is-accomplished by :titrating aliquots of the reaction mixture. at inter-vals during the oxidation period utilizing-as the titrating agent. a stand- :ard thiosulfatesolution thereby measuring the amount of oxidizing agent which .is unconsumed .(the oxidizing agent-being usually present .in excess). .When the-thiosulfate titration indi- .cates that pno more-N-bromoacetamide or N- bromosuccinimide isabeing consumed, whichindicates .the-completiOn-Of- ;the reaction (ordinarily .oomplete-.at..the end of about 8 to. 12 .hours), ,allyl'alcoholiis added-to react withany excess .N-bromoacetamide 1-01' with any .tree eNrbTOmO- succinimidewhich-:may be present. The latter procedure hash-een foundto resultin the produc- .tion of ya- ..substantially more crystalline product. It has also been found :that brominationofcthe .3+ketone pr.oduct-.can :be :minimized *by 'conduct- .ingtheoxidationmeactionzin the absence of .light,,-.withr-strict temperatureeontrol (25 C.) and :by. maintaining vclose adjustment :of the pH (within the range of 6 .to '7) I as pointedout above. It isw-important' to add :allyl alcohol or other .unsaturated'compound z-prior to working up the :reaotionmixture since this treatment also.mini- .m-izes'bromination of: the 3+ketone.

The (following examples illustrate methods of carrying-v out the .presentxinvention but it is to :be. understood :that :these examples are'given by way of. illustration -,-and not of limitation.

"Example 1 v16.75 grams (flfl ilmole) of A -3,21-dihydroi y 11.+keto-ZO-cyanmpregnene was dissolved in .37 .cc. .of pyridine, .25 cc. .of water was added to the. resulting solution with swirling, and 16.34 grams .(Q.11 .mole) .of N-loromoacetamide (92% ,pure) .was .thenv added .to .the resulting solution. -=A few.minutes after .the addition of theN-bromoacetamide thecolorofthe solution turned to mleeporange. The temperature of the reaction mixture v wasmaintained Within the rangeof 20=25 ,C. ,for a period .of 2 /2 hours. The reac.tion .,mixture was thenallowed to stand at .room temperatureovernight, at the end of which .time .the -mixture was very .dark, and profuse crystallization had occurred. The crystalline mixture was filtered and the crystalline product thus recovered was slurried with water and then given twoadditional displacement washes. The resulting solid product was dried to constant weight to produce 11.20 grams of A -3,11-diketo 20rcyanoe21ehydroXy-pregnene (M. P. 221-225 25 then dissolvedindcc. (if-benzene.

.30 resulting solution,

45 tertiary butanol.

0.); yield 67.0% of theory. (This product contained much brominated impurities as shown by blackening and bubbling on melting.) The mixed melting point of this crude product with an authentic sample of A -3,11-diketo-20-cyano- -21.:11ydroxy-pregnene .of M.,P., 248-254! C. was

The product was more positively identified by acetylation followed by purification of the rem sulting A -3,11-diketo-20 cyano 21 acetoxy pregnene as follows: 2 grams of crude A -3,11- diketo-20-cyano21-hydroXy-pregnene was dis- ..so1ved,in.5.00 cc. of pyridine and the solution was warmed to 55 C. cc. of acetic anhydride was added to thedark brown solution and the resulting solution was maintained at 55 C. for a period of IO-minutes. The pyridine solution was poured (with stirring) into 250 cc. of water. The resulting mixture was stirred for a period of minutes to decompose excess acetic anhydride .and...the precipitated; material 1was"re- .coveredby. filtration. The producttwas washed withithree, 25 cc. portions vof water anddried-to produce 2.2 grams of crude acetate, 'which was The benzene solution was decolorizedby treatment Withabout 02.5,gram of activated charcoal .(Darco 6-60). The .charcoal ..was removedbyfiltration ;and .washedvvith 4 ccrof benzene. -,U.pon' cooling .the crystallization commenced and was brought to "completion by the 4 addition of about 2 volumes of petroleum ether-and. 2 .volumes of .ethyl ether. iThecc-rystallineama- @terial. which rprecipitated -.-was filtered and-idr-ied -.35 to produce 1.2 grams of substantially-pureA 3,1l-diketo-20-cyano-21-acetoXy-pregnene; M. P. 1942-1952 C.; mixed M. P. ;with an authentic sample of A -3,11-diketo -20-cyano-21-acetoxypregnene was 193-195 C.

Example? 2 then added 9.75 cc. of pyridine, 6.00 cc. of water and. 1.6.90,grams of (90%,.pure) .N-bromoeacetamide. [The mixture turned orange colored. after a few .minutesand .was. maintained at a tempera- .ture. oi 20-25? .C. overnight. Crystallization 1 was apparentafter- 2 to 3 .hoursand, at the endiof 15 hours reaction time,.,.crystallization of the productwasbrought to -completionby the addition .of..an excess of water (about. 3 volumes).

. The preoipitatedmaterialwas recovered by filtraabout-99% of theory.

18.69 grams of said crude A -3, 11-diketo120- cyanos2hhydroxy-pregnene was dissolved ill- :45 .cc. of..pyridine,.and '15 cc.:of acetic:anhydride wasadded to the resulting solut-ionxwhile main- 5 .taining the temperature'withinhthe range'of 45-55 C. The final mixture .washeated fora :period of 5 minutes at-a temperaturerof 45%55" .C.,- and :the crude acetate .was isolated -.-by "mixing the reaction mixture with-5 volumes of :ice :wa- -.ter, followed-bystirring 3101 a:peri0d of 1-2 hours.

Theprecipitated-material' was recovered by filtration, .washedand dried to-;produce 517;.31. grams of crude (partially brominated) -,A 3,11-diketo- .201cyano-21eacetoxy-pregnene; P. .186-188" .CJ. yield; 83% theory.

To the resulting solution-was 17.3 grams of crude A "-3,1l'-diketo-20-cyano- 21-acetoxy-pregnene was'dissolved in 115 cc. of glacial acetic acid by warming the solution to a temperature of approximately 40 C. The resulting solution was vigorously agitated while three 800 mg. portions of zinc dust were'added thereto at 45 minute intervals, while maintain ing the temperature of the reaction mixture at approximately 40 C. Two additional 1200 mg. portions of zinc dust were added to the resulting mixture at intervals of 30 minutes each. 500 mg. of activated charcoal (Darco G-60) was added together with the last portion of zinc dust, the reaction solution was filtered hot (temperature approximately 40 C.) and the crystallization of the product contained in the filtered solution was induced by a slow addition of three volumes of water. The precipitated material was recovered by filtration, washed free of acid and dried at a temperature of 50 C. to produce 16.01 grams -of substantially pure (debrominated) A" 3,11-diketo-20-cyano-2l acetoxy-pregnene; M. P. 193.7 194.6 0.; yield approximately 79.6% of theory.

Example 3 Six grams of A -3,21-dihydroxy-11-keto-20- cyano-pregnene was dissolved in a mixture of 50 cc. of methanol and 9 cc. of pyridine by warming the mixture to approximately 40-50 C. Two cc. of water and 5.53 grams of N-bromoacetamide (98.5% pure) was added to the warm solution thus obtained and, after 1 hour, a clear orange solution resulted. The resulting mixture was allowed to stand for a period of 3%; hours, at the end of which time 20 cc. of water was added to initiate crystallization whereupon the mixture was stirred for a period of 1 hour. An additional 125 cc. of water was added portionwise over a 1 hour period to the resulting mixture whereupon the remaining product in the solution crystallized and was recovered by filtration. The yellowishcolored crystals thus obtained were washed with water and dried at room temperature to produce 5.74 grams of crude (partially brominated) A" 3,11-diketo-20-cyano-2l-hydroxy-pregnene.

The crude material, prepared as described in the preceding paragraph, was reacted with zinc dust in acetic acid solution utilizing the reaction conditions set forth in Example 2 to produce 4.46

grams of purified A -3,11-diketo-20-cyano-21- hydroxy-pregnene; M. P. 2558-259 C.; yield approximately 74.3% of theory.

Example 4 6550 grams of acetamide was dissolved in 6550 cc. of water. 8850 grams of technical grade bromine was cautiously mixed with the acetamide solution, and the resulting mixture was cooled to 15 C. Meanwhile 7285 grams of 85% aqueous potassium hydroxide solution was mixed with 7230 cc. of water and the resulting solution was cooled to 0-5 C. This caustic solution was then added portionwise over a 2 to 3 hour period to the cold bromine-acetamide mixture while maintaining the temperature of the mixture within the range of -15" C.

The resultin slurry of N-bromoacetamide was stirred for V hour to complete the reaction and 37.9 liters of methanol was added to effect complete solution of the product and to cause much of the undesired potassium bromide to crystallize. The resulting slurry was stirred for an additional hour period and was then filtered. The deep orange solution was weighed, titrated to determine its oxidation equivalent and the speciflc gravity determined. One cubic centimeter of the solution (sp. g. 1.07) was found to contain 138 mg. of N-bromoacetamide, which showed that 41.5 kilograms of the N-bromoacetamide solution would be required for the oxidation of one hydroxyl in five kilograms of A -3,21-dihy-' droxy-l'l-keto-20cyano-pregnene.

5000 grams (14 'moles) of A -3,2l-dihydroxy- 11-keto-20-cyano-pregnene, 69.3 liters of methanol and 1045 cc. of pyridine were placed in a gallon vessel and the resultin mixture was cooled to 18 C. 41.5 kilograms (38.6 moles) of the N-bromoacetamide solution prepared hereinabove was added portionwise while vigorously agitating the reaction mixture. The temperature of the reaction mixture was maintained within the range of 18-20 C. for a period of 22 /2 hours, and during this time light was excluded from the mixture in the reaction vessel. At the end of the reaction period 1800 cc. of allyl alcohol was added to the reaction mixture to destroy the excess oxidizing agent. 1020 cc. of concentrated hydrochloric acid was then added to the result ing solution, and the solution was transferred to a gallon vessel and. cooled to 5-10 C. 210.8 liters of water was added portionwise to the cooled mixture over a 45 minute period and the crystalline slurry which resulted was stirred for a period of 3 hours while maintaining the temperature within the range of 510 C. The slurry was then filtered and the filter cake was washed thoroughly with four 10 gallon portions of water, and dried for a period of 3 days at a temperature of 50 C. to produce 4700 grams of A 3,11-diketo-20-cyano-2l-hydroxy-pregnene, M. P. 245.7 C.;yield 94.5% of theory.

The A 3,11 diketo 20 cyano 2'1 hydroxy-pregnene thus obtained was acetylated utilizing the same acetylation procedure described in Example 1, and the acylated product debrominated by reaction with zinc dust and glacial acetic acid utilizing the reaction conditions set forth in Example 2 thereby producing substantially pure A" 3,11 diketo 20 cyano 21-acetoxy-pregnene; M. P. 194-195 C. yield approximately 86% of theory.

Example 5 Two grams of A "-3,21-dihydroxy-11-keto-20- cyano-pregnene (0.0056 mole) were suspended in 25 cc. of methyl alcohol containing 1.1 cc. of pyridine and 0.6? cc. of water. 2 grams of N- bromosuccinimide (0.0112 mole) was added to this solution with stirring in the absence of light, and the resulting mixture was stirred at room temperature for a period of 20 hours. 0.6 cc. of allyl alcohol was then added to the solution, followed by a solution containing 1.2 cc. of concentrated hydrochloric acid mixed with 1.2 cc. of water. 75 cc. of water was then added portionwise to the resulting mixture whereupon the 3- keto compound crystallized, and was recovered by filtration and dried to produce 2.01 grams of A 3,11 diketo 20 cyano 21 hydroxypregnene; M. P. 2195-2225 C.

This product was then dissolved in chloroform and subjected to chromatographic purification utilizing a column of 20 grams of acidwashed alumina thereby producing 1.76 grams of substantially pure A -3,11-diketo-20-cyano-21- hydroxy-pregnene; M. P. 248.5-250.6 C.; yield 88.4% of theory.

made in carrying out the present invention without departing from the spirit and scopethereof. Insofar as these changes andmodifications are (within the purview of the annexed claims. they are to be considered as part of my invention.

1 claim:

' 1. The process of selectively oxidizing the 3- hydroxy grouping in A i-3,2l-dihydroxy-11- keto-ZO-cyano-pregnene without affecting the hydroxy substituent in the 2l-position which comprises reacting said A -3,21-dihydroxy-1l keto-20-cyano-pregnene.with an oxidizing agent selected from the class which consists of N- bromoacetamide and N -bromosuccinimide.

2. The process which comprises reacting A"- 3,21 dihydroxy ll keto 20 cyano pregnene with N-bromoacetamide to produce A" 3,11- diketo 20 cyano 21 hydroxy pregnene.

3.'The process which comprises reacting A 3,21 dihydroxy 11 keto 20 cyano pregnent with N-bromosuccinimide to produce A 3,11 diketo 20 cyano 21 hydroxy pregnene.

4. The process which comprises reacting A 3,21 dihydrcxy 11 keto 20 cyano pregnene with N-bromoacetamide in methanol to produce A" 3,11 diketo 20 cyano 21 hydroxy pregnene.

'5. The process which comprises reacting A 3,'2l dihydroxy l1 keto 20 cyano pregnene with N-bromoacetamide in tertiary butanol to produce A" 3,11 diketo 20 cyano 21.- hydroxy-pregnene.

' 6'. The process which comprises reacting A 3,21-dihydroxy-11-keto-20-cyano-pregnene with N-bromoacetamide in'pyridine to produce A 3,11 diketo 20 cyano 21 hydroxy pregnene.

'7. The process which comprises reacting 13"- 3,21 dihydroxy 11 keto 20 cyano pregnene with N-bromoacetamide, said reaction be- .presence of pyridine, thereby producing A -3,11- diketo-2-0-cyano-2l-hydroxy-pregnene.

8. The process for selectively oxidizing the 3- hydroxy grouping in A "-3,21-dihydroxy-11- keto-ZO-eyano-pregnene without aifecting the hydroxy substituent in the 2l-position,- which comprises reacting approximately 1 molecular equivalent of said A -3,2l-dihydroxy-ll-keto- 20-cyano-pregnene with approximately 2.75 molecular equivalents of an oxidizing agent selected from the class which consists of N-bromoacetamide and N-bromosuccinimide, said reaction being carried out at a temperature of about 25 C. for-a period of 6 to 8 hours in an organic solvent selectedfrom the class which consists of pyridine, tertiary butyl alcohol and methanol, the pH of said solution being adjusted within the range of 6 to 7.

9; The process of producing-A -3,11-diketo-20- cyano-2l-hydroxy-pregnene substantially free of brominated impurities which comprises reacting together, in solution in an organic solvent selected from the class which consists of pyridine, tertiary 'butyl alcohol and methanol, A -3,21-dihydroxy-ll-keto-20-cyano-pregnene and an oxidizing agent selected from the class which consists of N-bromoacetamide and N-bromosucsinimide thereby producing crude A -3,11-diketo-ZO-cyano-Zl-hydroxy pregnene contaminated with brominated impurities, and reacting said crude A -3,11-diketo-20-cyano-2 1hydroxypregnene withzinc and acetic acid.

ROBERT E. JONES.

REFERENCES CITED The following references are of record in the file of this patent:

Reich et al.: Helv. Chim. Acta 26, 562585 (1943).

Claims (1)

1. THE PROCESS OF SELECTIVELY OXIDIZING THE 3HYDROXY GROUPING IN $17-3,21-DIHYDROXYL-11KETO-20-CYANO-PREGNENE WITHOUT AFFECTING THE HYDROXY SUBSTITUENT IN THE 21-POSITION WHICH COMPRISES REACTING SAID $17-3,21-DIHYDROXY-11KETO-20-CYANO-PREGNENE WITH AN OXIDIZING AGENT SELECTED FROM THE CLASS WHICH CONSISTS OF NBROMOACETAMIDE AND N-BROMOSUCCINIMIDE.