Enantioselective Formation of Bicyclic Lactones by Rhodium-Catalyzed Intramolecular CH-insertion reactions

The decomposition of cyclohexyl diazoacetate ( 5a ) in the presence of the chiral Rh₂{(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.