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Tetrahydropapaverhe (VI), derived from (-)-nor-reticuline ( 11) and (-)-nororientaline (111), is the principal immediate precursor of papaverine ( V I I I ) in Papaver somniferum. The trimethyl ethers of papaveroline ( X I a ) and b) are incorporated less effectively into papaverine, the dimethyl ether ( X I c ) not a t all. Norprotosinomenine ( I V ) and noriso-orientaline(V) can also be biotransformed into papaverine in the plant, but this occurs by an aberrant route which does not involve tetrahydropapaverine as an intermediate.The stereoselectivity of various 0-and N-methylations and of dehydrogenation of tetrahydropapaverine is discussed. ' Unnatural ' (-) -1audanosine w a s biosynthesized from ' unnatural ' (+)-tetrahydropapaverine, but could not be obtained from ' unnatural ' (+)-nor-reticuline. Enzymic racemization of benzyltetrahydroisoquinolines, which is a prerequisite for thelbiosynthesis of morphine alkaloids. appears to be limited to reticuline (Xa). N-Methylations may occur at several stages along the biosynthetic routes, and there is evidence that enzymatic N-demethylations may take place to an appreciable extent. PAPAVERINE (VIII) had been shown to be biosynthesized in the opium poppy from tyrosine 1$ via 3,4-dihydroxyphenylalanine (DOPA) and norlaudanosoline (I) as proposed originally by Winterstein and Trier * in 1910.In an earlier paper we reported that nor-reticuline (11) was an efficient precursor of papaverine, and the question arose as to the sequence of O-methylation and the stage at which aromatization of ring B takes place.

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