Stereoselective Synthesis of Substituted Halodihydropyrans

Significance

In Prins cyclizations, if the starting materials are chiral, the competing oxonia-Cope rearrangement often leads to partial or total razemization. As a potential solution to this problem, the authors report a stereoselective Prins-type cyclization between homopropargylic alcohols 1 and aldehydes 2 in the presence of the inexpensive and stable iron(III) chloride to obtain tetra- and pentasubstituted dihydropyrans 3. Using these conditions, no trace of the oxonia rearrangement was observed and the cis-six-membered ring products were obtained selectively. The presence of the TMS on the alkyne proved to be essential. Also, the retention of stereochemical information was demonstrated.