STUDIES IN THE DIRECTED CYCLOPROPANATION OF ALPHA-ALLENIC ALCOHOLS

A highly regioselective cyclopropanation of alpha-allenic alcohols usi ng samarium/dihalomethane provides a variety of methylene- and alkylid enecyclopropane carbinols in good yield. The nearby hydroxyl moiety an d the samarium reagent are essential for this unique conversion. A com parison with the more traditional Simmons-Smith conditions and its var iants is made. The diastereoselectivity varies from 1:1 to 50:1 and de pends on the substituents on the carbinol side chain (R group) and on the substitution of the allene. Unequivocal assignment of the relative stereochemistry of the prepared methylenecyclopropanes has been made on the basis of NMR studies, independent synthesis, and X-ray crystall ography. A facile one-pot, two-step enantioselective reduction of ynon es with the chiral LiAlH4/Darvon alcohol complex provides the starting allenic alcohols in 82-93% enantiomeric excess. Hydroboration of the methylene-cyclopropane carbinols results in the formation of (Z)-subst ituted cyclopropyl dicarbinols, which were cyclized to 3-oxabicyclo[3. 1.0]hexanes upon treatment with p-TsCl.