Palladium-Catalyzed 2-Phenylethenylation of Codeine: 8-[(1E)-2-Phenylethenyl]codeinone Dimethyl Ketal as the Unexpected 'Masked' Diene for the Preparation of 19-Substituted Diels-Alder Adducts of Thebaine

In a search for starting materials for the preparation of 7,8‐fused morphine alkaloid derivatives, 8‐[(1E‐2‐phenylethenyl]codeinone dimethyl ketal (4) and 8‐[(1E‐2‐phenylethenyl]codeine (5) were prepared. These dienes were used as substrates in the Diels–Alder reactions. Compound 5 formed the ‘normal’ adduct 12 with N‐phenylmaleimide, while compound 4 behaved in reactions with dienophiles as the ‘masked’ diene 11, a 8‐[(1E)‐2‐phenylethenyl]‐substituted thebaine, yielding the corresponding 19‐substituted 6,14‐endo‐etheno‐6,7,8,14‐tetrahydrothebaines. Specifically, reaction of 4 with methyl vinyl ketone gave rise to 19‐[(1E)‐phenylethenyl]thevinone (14) whose structure was elucidated by an X‐ray diffraction analysis. The thebaine derivative 11 was also prepared from 4.