ENANTIOSELECTIVE FORMATION OF BICYCLIC LACTONES BY RHODIUM-CATALYZED INTRAMOLECULAR CH-INSERTION REACTIONS

The decomposition of cyclohexyl diazoacetate (5a) in the presence of the chiral [Rh-2{(2S)-mepy}(4)] catalyst leads to a 3:1 cis/trans mixture of bicyclic lactone 6a with an enantiomeric excess of 95-97% (cis) and 90% (trans). The conformationally rigid tert-butyl derivatives 5b and 5c afford, in the presence of the same catalyst, 6b and 6c, respectively, via insertion into the equatorial C-H bonds exclusively, with ee's of ca. 95%. A remarkable degree of induction (92-95%) results in the lactone 6g upon decomposition of 1-isopropyl-2-methylpropyl diazoacetate (5g). The diazoacetates derived from 1-methylcyclohexanol, cyclopentanol and 1-methylcyclopentanol (5d-f) afford under similar conditions insertion products with higher diastereoselectivity, but significantly lower enantioselectivity. Other dirhodium catalysts are less efficient.