Protopine alkaloids from tetrahydroprotoberberine

Scheme 73. Synthesis of protopine alkaloids from tetrahydroprotoberberine JV-oxides. Reagents a, K2Cr07 b, Mel.

Bentley and Murray (201) reported another method for synthesis of protopine alkaloids allocryptopine (392) and cryptopalmatine (395) from tetrahydroprotoberberine /V-oxides (35a and 400) through oxidative rearrangement with potassium chromate (Scheme 73). 

Another important route stems from the formation of the N-methylated moieties of the (S)-tetrahydroprotoberberines, which serve as precursors for the protopine, benzophenanthridine, tetrahydrobenzazepines (rhoeadines) and spirobenzylisoquinoline alkaloids (Kutchan and Zenk, 1993 Liscombe and Facchini, 2008 Ziegler and Facchini, 2008) (Fig. 2.7). 

Recently, attention has been paid to the photosynthesis of tetrahydroprotoberberine alkaloids (131, 282) from iV -acetyl-l-benzylidenetetra-hydroisoquinolines (a) and from bases of protopine type (b) which proceeds along the two pathways shown in Scheme 8. 

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