Bomyl fumarate

Control of absolute asymmetry is a relatively untouched area for [2 + 2] photochemical cycloaddition reactions despite the recent advances in the field of asymmetric synthesis. The first example of the use of a removable chiral auxiliary was reported by Tolbert, who obtained impressive enantioselectivity in the photocycloaddition of bomyl fumarate to stilbenes (equation 37). More recently, Lange has shown that menthyl cyclohexenonecarboxylates are useful in control of absolute stereochemistry (equation 38). Baldwin and Meyers have also obtained excellent facial selectivity in systems where the stereogenic center which controls the diastereoselectivity can be excised to afford products of high enantiomeric purity (equations 39,40). 

Another breakthrough came several years later, when the photoadduct of trans stilbene with chiral bomyl methyl fumarate 82 was obtained with a hi diastereomeric excess [60]. Here again, a model involving an approach of t reagents in parallel planes was proposed to explain the observed stereoselectivi (Scheme 19). In an attempt to increase the observed de, the cycloaddition reacti of dibomyl fumarates was examined, but a far lower selectivity was observe On this basis, a multistep process was proposed with control of the asymmet induction by the rate of cyclization of the 1,4-biradical intermediates. The natii of the substituents, however, the complexity of the reaction mixture, and the lc chemical yields of the chiral adducts are major limitations for synthetic applic tions [61]. 

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