Total Synthesis of the <i>N</i>,<i>C</i>-Coupled Naphthylisoquinoline Alkaloids Ancistrocladinium A and B and Related Analogues

The N,C-coupled naphthyldihydroisoquinoline alkaloids ancistrocladinium A (3) and B (4), which possess an unprecedented iminium-aryl axis and show high in vitro antileishmanial activities, have been synthesized via a short sequence of eight linear steps, without the need of protecting groups. Key steps were a Buchwald-Hartwig amination and a Bischler-Napieralski cyclization, preferentially leading to the naturally predominant M-atropo-diastereomer in the case of 3, while the N,C-axis is configurationally semistable in 4. The highly convergent first access to this type of alkaloids will now facilitate the preparation of structural analogues for structure-activity relationship studies. Its general applicability was shown by the preparation of the sterically even more congested, as yet unnatural N,3'- and N,1'-coupled analogues, ancistrocladinium C (5) and D (6).