Catalytic quantities of bismuth(III) triflate efficiently initiate the rearrangement of epoxides to aldehydes which subsequently react with (Z)-δ-hydroxyalkenylsilanes to afford 2,6-disubstituted-3,6-dihydro-2H-pyrans. Isolated yields of desired products using Bi(OTf)3 were compared with yields when the reactions were run with TfOH and TMSOTf in the presence and absence of several additives. These studies, as well as NMR spectroscopic analyses, indicate an initial Lewis acid/base interaction between Bi(OTf)3 and substrates providing TfOH in situ.

Reaction of 6-methyl-1-phenylhept-3-yne-2,6-diol with various aldehydes under Lewis acid conditions provides an atom economical, two-component cascade reaction sequence to novel 2,4-dihydro-1H-benzo[f]isochromene compounds. Aliphatic aldehydes as well as electron-deficient and electron-rich aromatic aldehydes can be used.

A three-step domino reaction between 1-aryl-3-hexyne-2,6-diol derivatives and aldehydes is used to construct tricyclic 1,4-dihydro-2H-benzo[f]isochromenes. The cascade is initiated by BF·OEt and involves alkynyl-Prins cyclization, Friedel-Crafts alkenylation, and dehydration/aromatization to create a new, central aromatic ring and eliminate 2 equiv. of water. Electron-donating substituents on the aryl ring of the 1-aryl-3-hexyne-2,6-diols significantly increase overall yields as do electron-rich aldehyde reaction partners. For 2,4-disubstituted 2H-benzo[f]isochromene products, diastereoselectivities in the alkynyl-Prins reaction are ∼1.4 : 1 in favor of the cis-diastereomer. The stereochemistry of one cis-product was verified by X-ray crystallographic analysis and a second structure was also verified by X-ray analysis.