Bufadienolid.es. 2. 20-Hydroxy-21-nor-5α-cholanic Acid γ-Lactones (24å20)

Sodium borohydride reduction of both methyl 3j8,23-diacetoxy-20-oxo-21-nor-5-cholenate (la) and the corresponding 23-methyl ether lb was found to produce 3β,20-dihydroxy-21-nor-5-cholenic acid γ-lactone (24 å 20). Concomitant elimination of the 23 substituent (to yield lactone 2a) again occurred with methyl 3β,23-diacetoxy-20-oxo-21-nor-5a-cholanate (3) to give γ-lactone 4a, which was also obtained by palladium-catalyzed hydrogenation of olefin 2a. Hydrogenation of methyl 3l8-acetoxy-20-oxo-21-nor-5-trans-22-choladienate (5) gave 5a-cholanate 6a as a major product accompanied by small quantities of 3-deoxycholanate 6b and γ-lactone 4b. Sodium borohydride reduction of ketone 6a yielded γ-lactone 4b, thereby confirming structures assigned to lactones 2 and 4. Platinum-catalyzed hydrogenation of 23-methyl ether lb and of 23-acetate 3 led, respectively, to 23-substituted lactones 4c and 4d. A potentially useful lactone (3-oxo-4-ene, 8) for biological evaluation was obtained by Oppenauer oxidation of alcohol 2a.