SUBSTITUENT EFFECTS IN THE INTRAMOLECULAR DIELS-ALDER REACTION OF 6-FURYLHEXENOATES

The series of furyl enone esters, 4a-c, were synthesized from furan or furfural by straightforward routes. Their thermal intramolecular Diel s-Alder reactions to give the tricyclic ketones 5a-c were studied in a cetonitrile and toluene at two temperatures and the kinetics of the re actions determined. A comparison of these data with that obtained for the corresponding esters 2 to give the lactones 3 indicates that the r ate enhancements seen for the esters (rate of dimethyl 310 times that of monomethyl) are much larger than those seen for the ketones (rate o f dimethyl 6.8 times that of monomethyl). Thus, this is additional evi dence for the earlier hypothesis that the presence of the oxygen atom in the tether is a factor responsible for the larger than normal rate enhancements.