The intramolecular Diels-Alder reaction of 21, prepared from citraconic anhydride and sorbyl alcohol, gave 23, whereas the esters 29 and 32 produced trans-fused adducts 30 and 33, respectively. These were isomerized in base to 31 and 34. The (E)-citraconate 39 furnished 40 and 31 in the ratio 4.3: 1, respectively. The major isomer of this pair was converted by base to the ester 24, prepared from 23. The mesaconate diester 45 upon cycloaddition ...
