ENANTIOSELECTIVE CYCLOADDITION VIA A CHIRAL KETO-ESTER KETENE EQUIVALENT

The synthesis and cycloaddition of(+)-6-methoxy-1,3-benzoxathiolan-(Z)-2-carbomethoxypropenyl-3-oxide (1) with cyclopentadiene is described. This procedure allows the direct synthesis of useful enantiopure building blocks such as (+)-(1R,4R)-bicyclo[2.2.l]hept-5-en-2-one (2) and the precursor ketoester 11. Cycloaddition, at -78 degrees C in the presence of BCl3, proceeded with complete pi-facial preference syn to the sulfoxide lone pair of the dienophile 1.