The total synthesis of the Erythrina alkaloid (±)-3-demethoxyerythratidinone (20) has been accomplished in 39% overall yield from homoveratrylamine(1)by 8 chemical operations, using a tandem cationic cyclization of the Pummerer rearrangement intermediate derived from the sulfoxide 5 as the key step. Of particular interest is the observation that heating of 5 with p-toluenesulfonic acid provides the erythrinan 6 (and the deprotected derivative 7) as a single stereoisomer, whereas similar treatment of 5 in the presence of ethylene glycol gives initially the bicyclic lactam 8, which then cyclizes under the reaction conditions used to afford a mixture of two diastereomeric erythrinans 6 and 9.A possible explanation of these contrasting results is presented.
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Ishibashi et al. (1990) studied this question.