Bufadienolides. 9. Isobufalin

Isobufalin methyl ester (4a) was prepared by methanolysis of bufalin (3) in the presence of sodium methoxide, and saponification of the 30-acetoxy derivative 4b readily afforded isobufalin (4c). In each case, the configuration of the side-chain olefin was shown to be trans at positions 22 and 23 by proton magnetic resonance measurements. Isodigitoxigenin (7), acetal 8e, and dihydropyran 12a were prepared from digitoxin by way of digitoxigenin (6) as described in part 8. By a four-step reaction sequence via intermediates 12b-12d and 11a, both methyl esters 8e and 12a were converted into methyl 3/î-acetoxy-14β,21-epoxy-5β-chol-20 (21)-enate (11b). Dehydrogenation of methyl ester lib employing 2,3-dichloro-5,6-dicyanobenzoquinone completed total synthesis of 3β-acetoxyisobufalin methyl ester and therefore isobufalin.