A Bestmann reaction employing 3β-acetoxy-20-oxo-21-iodo-5-pregnene (2) and carbomethoxymethylenetriphenylphosphorane was used to complete a synthesis of methyl 3β-acetoxy-2-oxo-21-nor-5-imns-22-choladienate (1), which upon irradiation in sunlight gave cis isomer 5. Similarly, 3β-acetoxy-20-oxo-21-iodo-5,16-pregna-diene (6) was converted into trans side-chain olefin 7 and cis side-chain olefin 8. Palladium-catalyzed hydrogenation of triene 8 resulted in methyl 3-acetoxy-20-oxo-21-nor-5a-cholanate (9a) accompanied by small amounts of the corresponding 3-deoxy derivative 9b and 3β-acetoxy-20-hydroxy-21-nor-5a-cholanic acid γ-lactone (24 å 20). Subjecting triene 8 to sodium borohydride reduction in ethanol yielded isocardanolide 10. The principal objective, synthesis of isocardenolide 11, was realized by selective reduction of ketone 5 with sodium borohydride in dimethylformamide.
ASJC Scopus subject areas
- Organic Chemistry