Guanacastepenes A and E
Sorensen and Shipe. JACS, 2006, ASAP, DOI: 10.1021/ja060847g
Furthering their interest in the synthesis of the Guanacastepenes, Sorensen et al have completed the synthesis of members A and D using an interesting fragmentation strategy. However, before this late-stage event, the A and C rings were constructed and united using a Stille cross-coupling. The required cyclobutane was then implaced via a photo-[2+2] cycloaddition:
With this in place, the cyclobutane was fragmented selectively to provide the seven-member ring, trapping with PhSeBr to facilitate elimination in the next step. A very reasonable explanation for this selectivity is explained in detail in the paper.
This left the core of the guanacastepene series, from which a few simple steps returned guanacastepene A. However, a slight flaw in the synthesis concerns the resolution of an advanced fragment, taken from a racemic model study.