Photoactivated Tungsten Hexacarbonyl-Catalyzed Conversion of Alkynols to Glycals

The photoactivated W(CO)6/DABCO/THF system has been used for the formal endo-cyclization of
alkynes to pyran rings. We found that the regioselectivity of ring closure depends on the relative
configuration of the 3,5-dihydroxy-1-alkynes, as well as, more decisively, on the type of O-protective
group. Oxygen substitution at the propargylic carbon slows the rate of alkyne insertion and allows
for dihydrofuran formation through exo-cyclization. In contrast, the use of bulky silyl ethers or
carbon substituents leads to dihydropyrans through endo-cyclization. Substrates bearing leaving
groups such as esters, phenols, or thiophenols at the propargylic site eliminate and thus represent
a limitation to the cycloisomerization methodology. Propargyl vinyl ethers will rearrange to give
dienals instead of glycals. 1,2-Wittig rearrangement products of dihydropyrans are readily prepared
and converted to complex bicyclic building blocks for organic synthesis.