A REITERATIVE SYNTHESIS OF TRANS-FUSED POLYPYRANS VIA TUNGSTEN PENTACARBONYL-PROMOTED ALKYNOL ENDOCYCLIZATION

Endocyclization of 1-alkyn-5-ols occurs in the presence of tungsten pe ntacarbonyl-THF complex to give cyclic six-membered tungsten oxacarben es which are easily converted to alpha-stannyl enol ethers by treatmen t with Bu3SnOTf and Et3N. These stannylated dihydropyrans undergo orga nocuprate-mediated alkylation to provide 6-(2-propynyl)-3,4-dihydro-2H -pyrans. Oxidation of these propargylated dihydropyrans with m-CPBA fo llowed by reduction with Et3SiH under Lewis acid conditions generates another trans-fused 1-alkyn-5-ol. Repetition of this process provides are iterative synthesis of trans-fused polypyrans.