Synthesis of Demethylgorgosterol and Its Stereoisomers

抄録

A stereoselective synthesis of demethylgorgosterol (2) is described. Alkylation at the C-22 position of the steroidal 23-aldehyde (5) was achieved by Claisen rearrangement of the piperidine enamine to give the (22S)-22-aldehyde (10a) predominantly. Compound 10a was transformed to the 22-hydroxymethyl-24-aldehyde mesylate (15a). When the mesylate was treated with potassium t-butoxide, the 22, 23-cyclopropyl 24-aldehyde (19a) was obtained in high yield. An isopropyl group was introduced at the C-24 position by means of the Grignard reaction and subsequently the hydroxy group was oxidized to provide the 24-ketone (25a). Wittig reaction of 25a followed by hydroboration and then LiAIH₄ reduction of the mesylate gave 2, which was identical with the natural compound in all physical properties. Three other epimers, the (22R, 23R, 24S)-isomer (32), (22S, 23S, 24R)-isomer (34) and (22S, 23S, 24S)-isomer (35), were prepared by the same procedure. These four isomers can be separated by gas-liquid chromatography on a glass capillary column coated with OV-17.