5- and 6-Exocyclic Products, <i>cis</i>-2,3,5-Trisubstituted Tetrahydrofurans, and <i>cis</i>-2,3,6-Trisubstituted Tetrahydropyrans via Prins-Type Cyclization

Exocyclic products having <i>cis</i>-2,5 and <i>cis</i>-2,6 substitution were synthesized from terminally substituted alkynyl alcohols with various aldehydes via Prins-type cyclization in good yields. It is of interest that synthesized 5- and 6-exocyclic vinyl cations generated as a result of Prins-type cyclization could be trapped as a vinyl triflate in CH₂Cl₂ to give 3-furanylidenes and 3-pyranylidenes. Those 3-furanylidenes and 3-pyranylidenes underwent hydrolysis to give the corresponding 3-acyl-substituted products having <i>all</i>-<i>cis</i>-configured isomers, such as 2,3,5-trisubstituted tetrahydrofurans and 2,3,6-trisubstituted tetrahydropyrans.