New approaches for the synthesis of erythrinan alkaloids

A concise asymmetric total synthesis of (+)-erysotramidine is described, using chiral base desymmetrisation of a meso-imide, N-acyliminium addition, retro-Diels-Alder cycloaddition and radical cyclisation as the key steps. A related route, starting from a cyclobutene-fused imide, was explored, and established a novel construction of the Erythrina alkaloid skeleton using a key ring-opening/ring-closing metathesis step. Completion of this synthesis was thwarted by problems with the removal of an unwanted vinylic side-chain. Complementary enantiospecific routes to Erythrina systems were explored, starting from (L)-malic acid. Some unexpected observations were made concerning the diastereocontrol in malic acid-derived N-acyliminium ion cyclisations, where changing the protecting group of the alcohol function from acetate to OTIPS resulted in a dramatic change in diastereocontrol. Products from these reactions could be transformed into known intermediates for natural alkaloids, and into (+)-demethoxyerythratidinone itself, by means of radical cyclisations or intramolecular aldol reactions as the key steps.