Prins cyclizations in silyl additives with suppression of epimerization: versatile tool in the synthesis of the tetrahydropyran backbone of natural products.

[reactions: see text] A catalytic Prins cyclization reaction has been developed. The involvement of trimethylsilyl halides offers a versatile route to the formation of cis-4-halo-2,6-disubstituted tetrahydropyran rings. The problem of epimerization in Prins cyclization has also been addressed in the total synthesis of (-)-centrolobine using this methodology.