Synthesis of the hydroazulene portion of guanacastepene A using a [2.3]sigmatropic sulfoxide rearrangement: observations on silyl enol ether electrophilic chemistry for the introduction of the C-13 hydroxyl group

The intermediate 6 can be converted into enone 13 using a [2.3]sigmatropic sulfoxide rearrangement as the key transformation. The C-13 hydroxylation of 13 was studied, and found to give 14 (epimeric to guanacastepene A). Examination of silyl enol ethers of 13 demonstrated the ready isomerization of the kinetic silyl enol ether into the more stable thermodynamic silyl enol ether under mild electrophilic reaction conditions.