PHOTOISOMERIZATION OF (E)-IODOALKYLIDENE LACTONES - A ROUTE TO (Z)-IODOALKYLIDENE LACTONES

Photochemically induced free-radical cyclisation of iodo acetylenic esters provides low yields of mixtures from which both (E)- and (Z)-iodoalkylidene lactones may be isolated. However, photoisomerisation of (E)-iodolkylidene lactones, which have previously been obtained in good yields by dibenzoyl peroxide induced cyclisation of iodo acetylenic esters, provides (E)/(Z)-mixtures from which the previously inaccessible (Z)-isomers may be isolated by chromatography. Also reported is a collection of IR, UV and NMR spectral data, all of which provide useful information relating to the stereochemistry of the iodoalkylidene lactones. A comparison of the crystal structures of the (E)- and m-isomers of a trimethylsilyl iodomethylene lactone is also included.