Jervine—XV : Hydrogenation of the 13,17a-double bond

Abstract The α,β-unsaturated ketone XIa, produced from N-acetyl-tetrahydroisojervine (Xa) by alkali-catalyzed double bond shift, gave on catalytic hydrogenation with platinum in acetic acid and subsequent acetylation the 13β,17aα-dihydro compound VII rather than the product of cis (α, α)-hydrogen addition, VIIb, which is the more stable member of this pair of 13-epimers (VIII VII). 6 The formation of VIII as the kinetically favoured product is explained by the predominance of stereoelectronic over purely steric control of proton addition at C-13 in the re-ketonization of an enolic intermediate arising by 1,4-addition of hydrogen to the enone system of XIa. The observation 1 that the hydrogenation of jervine (Ia) under these conditions affords (in poor yield) tetrahydrojervine (IIIa) by cis (α, α)-addition has been confirmed. With O,N-diacetyljervine (Ib), however, the primary event is the hydrogenolysis of the 17,23-ether linkage resulting in the establishment of a free 23-hydroxyl group. This may be accompanied by de-conjugation of the enone grouping (leading to 3,N-diacetyl-tetrahydroisojervine, isolated as the triacetate Xb), or followed by hydrogenation of the double bond (leading by trans (13β,17aα)-addition to the 3,N-diacetate of compound IX, and to a configurationally as yet undefined new stereoisomer (XII) of the latter). These results are discussed in terms of their dependence of the ease of protonation of the 17,23-ether oxygen, and the direction of polarization of the enone system before and after cleavage of the 17-oxygen bond.