Abstract
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.