Science of Synthesis

Zeh, J.; Hiersemann, M., Science of Synthesis: Stereoselective Synthesis, (2011) 3, 374.

The 2-oxonia variant of the Cope rearrangement takes advantage of the equilibrium between two oxocarbenium ions, each representing a reactive intermediate, for a subsequent bond-forming event.[‌39‌] The oxonia-Cope rearrangement with a sequential Prins reaction was used in a synthesis of the C18–C25 segment of (−)-lasonolide A (‌93‌) (‌Scheme 30‌).[‌43‌] Treatment of the α-acetoxy ether ‌108‌ with trimethylsilyl trifluoromethanesulfonate results in the formation of an oxocarbenium ion, which rearranges via the chair-like transition-state structure ‌109‌ to provide the oxocarbenium ion ‌110‌ (equivalent to structure ‌111‌). The subsequent Prins reaction proceeds preferentially through 6-endo-trig cyclization to provide the oxocarbenium ion ‌112‌, which is reduced to provide the highly substituted tetrahydropyran ‌113‌ in 80% yield. This sequence of bond-forming events utilizes the absolute configuration of the single stereogenic center in the acetal ‌108‌ to establish four new stereogenic centers with modest diastereoselectivity.

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References