PREPARATION OF UNSATURATED α,α-DICHLORO ACID CHLORIDES AND INTRAMOLECULAR CYCLOADDITIONS OF THE α-CHLOROKETENES REDUCTIVELY GENERATED FROM THEM. EFFECT OF DOUBLE BOND GEOMETRY ON THE CYCLOADDITION

Article abstract of DOI:10.1016/S0040-4020(01)80143-5

Reduction of unsaturated α,α-dichloracid chlorides with zinc dust in THF at reflux generates an unsaturated α-chloroketene which undergoes an intramolecular <2+2> cycloaddition in good yield.This reaction can be used with three carbon tethers to prepare 5-chlorobicyclo<3.2.0>heptan-6-ones and 1-chlorobicyclo<3.1.1>heptan-6-ones but fails with larger tethers.Unsaturated ketenes 18 and 28, with a trans-double bond, react stereospecifically to give bicyclo<3.1.1>heptanones 21 and 29 in good yield.Unsaturated ketene 12, with a cis-double bond, reacts with loss of stereochemistry to give a 2:1 mixture of 19 and 21 in poor yield.The greater reactivity of trans- than cis-double bonds in intramolecular <2+2> cycloadditions of ketenes contrasts to intermolecular cycloadditions in which cis-double bonds are more reactive.Adducts 41 and 42 containing both a chlorine and exomethylene group can be prepared readily.Reductive dechlorination of adducts can be achieved with either (n-Bu)3SnH or CrCl2.Ring contraction to give acids 48 and 49 occurs readily on base treatment.