PRACTICAL CATALYST FOR CYCLIC METATHESIS - SYNTHESIS OF FUNCTIONAL AND/OR ENANTIOPURE CYCLOALKENES

The oxo-tungsten complex trans-WOCl2(OAr)(2) (Ar = 2,6-dibromophenyl) is prepared by reaction of WOCl4 with 2 equiv of 2,6-dibromophenol. A variety of nonconjugated dienes are cleanly cyclized to the corresponding cycloalkenes using 2 mol % of this catalyst in combination with 4 mol % of tetraethyllead. All three components of the catalyst system are commercially available. The catalytic reactions are typically complete in 1 h at 90 degrees C and allow the synthesis of chiral cycloalkenes with little or no loss in optical activity. For example, (R)- (S)-citronellene have been cyclized to the corresponding (R)- or (S)-3-methylcyclopentenes in 97% enantiomeric excess. The cyclization is compatible with a variety of functional groups including some eater, amide, and ether derivatives. Tri- (but not tetra-) substituted cycloalkenes could be prepared using this catalyst.