STUDIES ON THE SYNTHESIS OF MAVACURINE-TYPE INDOLE ALKALOIDS - 1ST TOTAL SYNTHESIS OF (PLUS-OR-MINUS)-2,7-DIHYDROPLEIOCARPAMINE

Closure of the six-membered C ring of pentacyclic mavacurine-type alka loids from suitably substituted tetracyclic substructures embodying ri ngs ABDE of these alkaloids, either by electrophilic cyclization upon the indole 3-position or by intramolecular alkylation of the piperidin e nitrogen, failed. In contrast, 6a-homopleiocarpamine (45) has been s ynthesized from dithioacetal 42 by an electrophilic cyclization involv ing the closure of the seven-membered C ring. The first total synthesi s of the alkaloid 2,7-dihydropleiocarpamine (68) has been achieved by photocyclization of the tetracyclic chloroacetamide 54 as the key step . The required tetracyclic ABDE ring systems were prepared by a straig htforward sequence consisting of nucleophilic addition of a 1-indoleac etatic ester enolate to the gamma position of a pyridinium salt, acid cyclization of the resulting 1,4-dihydropyridine, and final elaboratio n of the (E)-ethylidene substituent. An alternative synthesis of the t etracyclic alkaloid vinoxine (10a) is also reported.