The N,O-bridged sesquinorbornadienes: a testing ground for establishing the superiority of N-Z pyrrole over furan as a dienofuge in retro-Diels-Alderreactions

N-R pyrroles (R=H, Z, COPh but not Bn, TMS) 6 add selectively at rt to the substituted pi -bond of 2,3-bis(trifluoromethyl)-7-oxanorbornadiene 7 under high pressure (14 kbar) to form exclusively the syn-facial N,O-sesquinorbo rnadienes 8; at higher temperatures the thermodynamic stereoisomers 10,11 a re produced by cycloreversion and reaction at the unsubslituted pi -bond. T he exclusive loss of N-Z pyrrole from the N-Z derivative of 8 demonstrated the superior dienofugacity of N-Z pyrrole over furan; kinetic studies showe d that the activation energy for this fragmentation was 34.7 kcal mol(-1). This selectivity is in accord with theory (AM1, ab initio). (C) 2001 Elsevi er Science Ltd. All rights reserved.