Synthesis and Diels‐Alder reaction of 5β‐methyl‐β‐dihydrothebaine and its 6‐demethoxy analogue (chemistry of opium alkaloids, Part XLII)

Abstract

AbstractThe scission of the 4,5α‐epoxy bridge of 5β‐methylthebaine (2) and 5β‐methyl‐6‐demethoxythebaine (5), yielded 5β‐methyl‐β‐dihydrothebaine (4) and 5β‐methyl‐6‐demethoxy‐β‐dihydrothebaine (6), respectively. The scission was carried out by zinc reduction in a potassium hydroxide solution. It took place with retention of the 6,8‐diene system and the configuration at the C‐5 atom. To perform the Diels‐Alder reaction on 4, the hydroxyl substituent at C‐4 had to be protected with a phenyl group, yielding 5β‐methyl‐4‐O‐phenyl‐β‐dihydrothebaine (7). The Diels‐Alder reaction of 7 was carried out in boiling ethyl propenoate for three weeks. Two 6β,14β‐ethenomorphinans (8 and 9) were obtained. For the Diels‐Alder reaction of 6 no protection of the hydroxyl substituent at C‐4 was necessary. Cycloaddition gave after one day two 6β,14β‐ethenomorphinans (10 and 11).