Nov., 1950
A NEWSYNTHETIC ROUTETO THE
CORTICAL S I D E
CHAIN
5301
[CONTRIBUTION FROM THE WAITMORE LABORATORIES OF THE SCHOOL OF CHEMISTRY AND PHYSICS OF THE PENNSYLVANIA STATE COLLEGE AND THE RESEARCH LABORATORIES OF PARKE, DAVISAND Co.]
A New Synthetic Route to the Cortical Side Chain' BY R. B. WAGNERAND
JAMES
A. MOORE
I n the course of studies of steroidal bromo- 17-vinyl compoundsa or by treatment of 20ketones i t has been found2 that the 17-bromo- tosylates with ~ollidine.~ We have found that these pregnenol precursors and 17,21-dibromopregnanolones, readily available from sapogenins, are subject to rearrangement can be readily prepared from the 17-pregnen-21reactions, even under mild hydrolytic conditions, oic acids (111) by treatment of the acid esters and do not yield the highly desired compounds with lithium aluminum hydride. The reduction of the cortical series by direct metathesis. Thus leads to the desired carbinols in uniformly high 17,21-dibromopregnan-3/3CHzOR CH20Rz 01-20-one acetate (I)when I I treated with alcoholic R3O-kH potassium hydroxide or CHvI,,'OH aqueous methanolic potassium bicarbonate does not OS04 yield the 17,21-dihydroxy 111 LiAlH4 ____) on acetate ketone (11) but rather the 17-pregnen-21-oic acid ( a ~ ~ ~ ~ ~ (111). This unsaturated R d R d acid and others obtained IVa, R = H Va, RI = RZ = It = H b. R = Ac b.. RI. = Ac.. RI. = Rn = H in a similar manner have Ra = AC C, R1 = Rz now been further studied d, R1 = RP = Ac, R3 = €1 with the objective of utilizing them as starting materials in a practical syn- yields. This reaction has been used to prepare thetic route to compounds having a cortical side three unsaturated carbinols, two of which were hydroxylated to furnish the previously unreported chain. pregnan-3PI17a,20/3,21-tetrol (Va) and allopregCHzOH CHzBr nan-17a,20P,21-triol. I I The third pregnenol, 5,17-prenadien-3/3,21-diol was prepared in order to establish a basis for configurational assignment of the new triols. This diendiol has previously been obtained by a different m e t h ~ dand , ~ found to yield, upon treatment with osmium tetroxide, the 17a,20/3,21triol side chainss It has been shown by Fieser6 that the l7a-hydroxyl configuration is a conHO/ A c d I1 1 sequence of the rearward attack upon the 17,20double bond, and that further, by virtue of the COzH I cis addition of osmium tetroxide, the configuration of the 20-hydroxyl group is determined by the relative positions of the 20-hydrogen atom and the 21-carbon atom in the olefin. In the series of reactions III-+IV-+V, then, the configuration of the triol side-chain of V is fixed by that of the unsaturated acid 111. Thus the acid having the carboxyl group away from the ring system will furnish the 17a,2OP-glycol, whereas the other I11 geometrical isomer, with the carboxyl group Numerous cortical steroids contain the toward the ring system, would yield the 17a,20a17,20,21-troil side chain (V), which has been in- glycol on hydroxylation. Since steric factors troduced by osmium tetroxide hydroxylation greatly favor the formation, during the rearrangeof the corresponding 17-pregnen-21-01compounds ment of the dibromo ketone, of the acid with the (3) Ruzicka and Mdllcr, Relo. Chim, Acto, 01, 416 (1939). (IV). The latter have hitherto been obtained (4) Samtt, THISJOUIU~AL, 70, 1880 (1848). either by allylic rearrangement of 17-hydroxy(5) Koechlin and Reichstein, E r k Chim. Acla, 06, 1328 (1843). +
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(1) For a preliminary report, see Wagner and Moore, THIS (6) Firser and Fieaer, "Natural Products Related to Phen71,4160(1948). anthteet," 8rd cd.. Rcinhold Publishing Corp., N e w York, N. Y.. (2) Marker and Wagner, ibkl., U,016 (18411); Marker, Ctooks 1949, pp. 410-4SS; ma doo Gd&or and %itchrwky, Tan, and Wagner, Wd., 64,817 (1842). JOUENAL, I S , 882 (ISM). JOURNAL.
( I X a ) crystQUized in essentially Pure form. The course of this reaction is somewhat obscure. The presence of a free hydroxyl group in the product obviously indicates that some compound capable of contributing a t least a hydrogen atom enters into the process. This reactant has been assumed to be water, probably present in traces in the solvent, which, although of reagent grade, had not been rigorously dried. In one experiment, a sample of the black residue was vigorously digested with aqueous alcoholic sodium sulfite, but only a trace of organic material was obtained. However, when the residue was cautiously treated with concentrated nitric acid and immediately extracted with benzene, the nitric acid was reduced and the benzene solution was found to contain an additional quantity of the ketol diacetate. The ketol product was somewhat less pure than the material obtained directly from the original ethereal solution; undoubtedly the drastic treatment which was employed to free the compound resulted in some degradation. This hydroxylation has a counterpart in the cyanohydrin process which was employed by Sarett in the synthesis of COpCHa CIIBr> Kendall's substance E? In e---& l j this synthesis, the 20-cyanoc==o 17-pregnen-21-01 acetate side CH,I/I3' chain (X) is treated with osmium tetroxide in the presence 1. KOH-EtOH; H + of pyridine and the product 2. CH*N* obtained is the relatively stable cyclic osmate ester (XI). Sarett has attributed the stability of this ester to the presence of the negative cyano There is no evidence that such an ester is the primary product in the case of the 20-bromopregnenol diacePate. The fact that the ketol diacetate is present in the reaction mixture prior to any hydrolytic treatment indicates that this intermediate, if formed, is highly sensitive to hydrolysis despite the negative bromine atom. OAC, Rz = AC VIIIa, RI = OH, RZ = H IXa, R1 b, R1 OAc, Re = AC b, Rr = 0, R2 = AC This method of transformaC, R 1 OH, Rs AC tion of the 20-bromo-17-vre~d, Ri = 0, Rz = AC nene side chain t o a dihydrogy Hydroxylation of the vinyl bromide system acetone compound is highly sensitive to alterations presented some unusual features. The reaction CHeOAc CHzOAc was carried out by allowing an ethereal solution I I of the 20-brornopregnandiol diacetate (VIIIb) and an excess of osmium tetroxide to stand for at least one week, during which a heavy black deposit of osmium-containing materid formed. When the ethereal solution was decanted from this /r /-x XI precipitate and concentrated the keto1 diacetute 21-carboxyl group away from the nucleus, all of the triols obtained by this series of reactions may reasonably be assigned to the 17a,20/3-series. Many of the physiologically important cortical steroids are characterized by a 17,21-dihydroxy20-keto side chain rather than the 17,20,21-triol type. A practical synthesis of this dihydroxy ketone system has now been devised, in which the starting materials are the 20-bromo-17-pregnen21-oic acids obtained by rearrangement of 17,21,21-tribromopregnanolones (VI) .' This synthesis involves esterification of the 20-bromo acid, reduction of the ester (VII) with lithium aluminum hydride and hydroxylation of the 20-bromo17-pregnen-21-01 acetate ( \ X I b ) with osmium tetroxide. The 20-bromopregnendiol (VIIIa) obtained in the reduction could be converted to either the 3,21-diacetate (VI1Ib) or the 2 1-monoacetate (VIIIc) in excellent yields by the pyridine method. The 21-monoacetate was smoothly converted to the 3-keto derivative (VIIId) by treatment with chromium trioxide in acetic acid at room temperature, the unsaturated side chain being unaffected.
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i=
3
(7) Wageet and Mw% Tarn FoWr%A&, TS, 8866 (1060); 71, 4160 (10491.
E
(8) %rea,a d . , 70, 1484 (1948). (9) Rnrctt,
