Synthesis of 5β‐alkylthebaines, 5β‐alkyl‐10α‐ethylthebaines and 5β,10α, 10β‐triethylthebaine; Diels‐Alder reactions to 5β‐alkyl‐ and 5β‐alkyl‐10α‐ethyl‐6α, 14α‐ethenoisomorphinans (chemistry of opium alkaloids, Part XXXVI

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Abstract
5β‐Ethylthebaine (3), together with 5β‐butylthebaine (4), was obtained from thebaine (1) after reaction with butyllithium and ethyl iodide. With butyllithium and diethyl sulfate, thebaine yielded 5β‐ethylthebaine (3), 5β, 10α‐diethylthebaine (5), and 5β, 10α, 10β‐triethylthebaine (6). The ratio of the compounds (3:4 and 3:5:6) depended on the excess of reagents used. Employing the reversed sequence of addition of reagents, that is, first diethyl sulfate and then butyllithium, we prepared 5β, 10α‐diethylthebaine (5) and 5β‐methyl‐10α‐ethylthebaine (8) from 5β‐ethylthebaine (3) and 5β‐methylthebaine (7), respectively. These alkylation reactions are discussed in terms of Pearson's hard‐soft acid‐base theory.Diels‐Alder reaction of 3, 5 and 8 with ethyl acrylate gave exclusively 7α‐substituted 6α, 14α‐ethenoisomorphinans 9, 10 and 11, respectively. Esters of the cycloadducts 9 and 11 were converted into their tertiary alcohols 12 and 13 with methylmagnesium bromide. The 3‐methoxy group in 12 and 13 was hydrolyzed with a solution of potassium hydroxide in 1,2‐ethanediol to give phenols 14 and 15.

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