Tungsten-Mediated Syntheses of Fused alpha-Methylenebutyrolactones from Propargyl Bromides Containing Tethered Aldehydes and Ketones.

Abstract

The reaction of CpW(CO)(3)Na with a number of propargyl bromides with tethered aldehydes and ketones afforded eta(1)-propargyl species that were subsequently transformed into tungsten-eta(3)-2-(methoxycarbonyl)allyl compounds upon treatment with p-TSA/CH(3)OH; the overall yields exceeded 60%. Sequential treatment of these tungsten-eta(3)-allyl complexes with NOBF(4) and NaI in CH(3)CN led to intramolecular allyltungsten-carbonyl cyclization, yielding fused alpha-methylene butyrolactones of five-, six-, and seven-membered carbocyclic rings. All the reactions proceeded with high diastereoselectivities except for 9-methylene-7-oxabicyclo[4.3.0]nonan-8-one (22) and 10-methylene-8-oxabicyclo[5.3.0]decan-9-one (23). Modification of the metal center with a chloride ligand led to significant improvement of the trans-stereoselection of 22; the chloride modification did not significantly enhance stereoselection of 23. The stereochemical course of the reaction products is rationalized on the basis of a bicyclic transition-state mechanism.