Recent tracer studies 4 9 5 have shown that the p-phenylethylamine alkaloids are derived from tyrosine, by a generally accepted pathway.6 The tetrahydroisoquinolines are considered, on the basis of tracer studies* and i q z nitro experiments,' to be formed in nature by a Mannich reaction between an appropriately hydroxylated P-phenylethylamine and an aldehyde. However, acetate has recently been shown not to be incorporated directly into p e l l ~t i n e . ~ An acceptable biogenetic scheme for lophocerine is illustrated in Scheme 1. The intermediate 3-methylbutyraldehyde (111) may be

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