A [2 + 2] Photocycloaddition-Fragmentation Approach toward the Carbon Skeleton of cis-Fused Lycorine-type Alkaloids.

Starting from isoquinolones, the cis-selective annulation of six-membered rings was possible employing cyclobutenes as olefin components in a [2 + 2] photocycloaddition-fragmentation approach (nine examples, 54-80% yield). The developed sequence enables a conceptually new entry to cis-fused lycorine-type alkaloids of the Amaryllidaceae family with the complete carbon skeleton being successfully assembled. A subsequent von Braun-type reaction emphasized the biological relation between lycorine- and homolycorine-type alkaloids providing a synthetic tool to access this class of natural products.