Sakurai cyclisation

Highly substituted 4-methylenetetrahydropyrans can be obtained from the allyl alcohol via its carbamate 20. Reaction of the derived allyllithium with aldehydes affords the (Z)-enolcarbamates 21 which undergo an intramolecular Sakurai cyclisation to the pyran as a single diastereoisomer (Scheme 11) <00TL7225>. 

Substituted 4-methylenetetrahydropyrans are formed stereoselectively by a Bi(III)-promoted intramolecular Sakurai cyclisation (Scheme 7) <01TL8685> and Sn(ll) catalyses the reaction of acetals with the allylsilane 9 which also yields the 4-methylene compounds (Scheme 8) <01Ba569>. 

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