Indoloquinolizine alkaloids

The general method for the synthesis of different indoloquinolizine alkaloids via IV-tryptophyl 3-substituted pyridinium salts, developed by Wenkert and his collaborators, has been extended for the synthesis of yohimbine alkaloids (227-229). 

Another approach to the generation of an iminium cation suitable for cyclization is the protonation of dihydropyridine derivatives. One example can be found in Scheme 48, where treatment of compound 243 with acid induced its cyclization to indoloquinolizine 244, a precursor in the first total synthesis of (i)-tangutorine 245, an alkaloid isolated from a traditional Chinese medicinal plant <2001TL6593>. 

Pyridinium salts are reduced much more easily than pyridines by LAH. Reactions of quaternary salts have received much attention as they have found use in alkaloid synthesis. When the pyridinium salt (246 R = H) was treated with LAH the major product was the 1,2,3,6-tetrahydropyridine (247) rather than the desired indoloquinolizine (248). It was... 

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