Mechanistic and Computational Studies of Exocyclic Stereocontrol in the Synthesis of Bryostatin-like Cis-2,6-Disubstituted 4-Alkylidenetetrahydropyrans by Prins Cyclization

Abstract

The Prins cyclization of syn-β-hydroxy allylsilanes and aldehydes gives cis-2,6-disubstituted 4-alkylidenetetrahydropyrans as sole products in excellent yields regardless of the aldehyde (R″) or syn-β-hydroxy allylsilane substituent (R') used. By reversing the R″ and R' groups, complementary exocyclic stereocontrol can be achieved. When the anti-β-hydroxy allylsilanes are used, the Prins cyclization gives predominantly cis-2,6-disubstituted 4-alkylidenetetrahydropyrans, now with the opposite olefin geometry in excellent yield. The proposed reaction mechanism and the observed stereoselectivity for these processes are supported by DFT calculations.