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.
Wipf
Peter
P
Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 15260, USA. pwipf+@pitt.edu
Graham
Thomas H
TH
D000438:Alcohols / Q000737:chemistry*; D002384:Catalysis; D010777:Photochemistry; D014414:Tungsten / Q000737:chemistry*
D000438:Alcohols; D014414:Tungsten