INTERNAL ACTIVATION OF ACRYLATE-TYPE DIENOPHILES FOR THE DIELS-ALDER REACTION - STEREOSELECTIVE SYNTHESIS OF CONFORMATIONALLY CONSTRAINED GLUTAMATE ANALOGS

The [4+2] cycloaddition reaction of the unreactive acrylate-type dieno philes such as 5a and 6a was accomplished by introducing an electroneg ative or electron-withdrawing group as the ester counterpart. Among th em, the pentafluorophenyl (PFP) group was found to be an excellent est er counterpart in view of its rate acceleration and chemical stability under the reaction conditions. The C-13 NMR spectral data of the dien ophiles with the PFP group suggested that the conjugated CC-double bon d was strongly polarized. The internal activation was found to be effe ctive for the related dienophile or other dienes, in particular, unsta ble to Lewis acid catalysts. A successful application of this method i s demonstrated by the syntheses of conformationally restricted analogs of L-glutamate, 3 and 4. (C) 1998 Elsevier Science Ltd. All rights re served.