Pergamon Tetrahedron Letters 39 (1998) 1753-1754 TETRAHEDRON LETTERS Reaction of an Open-Chain Analogue of Reissert Compound Hydrofluoroborate Salt with Ethyl Acrylate. A Reinvestigation Sandrine Perrin, Karin Monnier, Bernard Laude* Laboratoire de Chimie et Electrochimie Mol6culaire, Universit6 de Franche-Comt6, 16 route de Gray, 25030 Besan~on (France) Marek M. Kubieki, Olivier Blacque Laboratoire de Synth6se et Electrosynth6se Organom6talliques, Universit6 de Bourgogne, 6 Bld Gabriel, 21000 Dijon (France). Received 10 December 1997; accepted 21 December 1997 Abstract : The reaction of an open-chain analogue of Reissert compound hydrofluoroborate salt with ethyl acrylate does not give a [4+2] cycloadduct as previously described, but a [3+2] cycloadduct which evolves to a 2-pyridone 15. © 1998 Published by Elsevier Science Ltd. All rights reserved. The reactions of isoquinoline Reissert compound hydrofluoroborate salts with alkynes and alkenes was described by McEwen et al.' and our group. 2 Moreover, McEwen et al. 3 reported such reactions of some open- chain analogues of Reissert compound hydrofluoroborate salts. These salts can exist as a mixture of tautomeric forms 2--4, of which 4 is the predominant one) They are also presumed to be in equilibrium with the original compound 1, the 1,3-dipolar species 5 and fluoroboric acid (scheme 1). R 2 R 2 BF4" R 2 BF4" R 2 BF4" R 2 R ~ ~ 'e '~ R ~ ~ R1 I~1. Ph +HBF4 RL-.~N'xK.ph Rl~/N~',,,~ph -HBF4 'H"~/CN~l'o ~ :N~--O2-~ P h = /~-(~ "H = ~-C~ ~ P h 1 HN 3 H2N 4 HN 5 Scheme 1 McEwen et al) claimed that olefins should react as dienophiles towards the heterodienes 4. Thus, with ethyl acrylate and the salt 4a (R' = R 2 = Ph), the presumed cycloadduct 6 should evolve via some complex condensation and rearrangement to give the ethyl 2-benzoyl-5-phenylpyrrole-3-carboxylate 10 (scheme 2). Ph BF4" '® Ph~./N~%.~Ph H2N 4a H2C=CH-CO2Et " Ph~.,j/~rPh . H + Ph-N CH-,....~ | 0 H - It / CH2 / L 6 CO2Et - H* EtO2c~]_ H [ EtO2CH~_.~%H 1 , Ph-N H 9 L- Ph-N 8 J Scheme 2 EtO2C H Ph-o 'ph ~ ~ -PhNH 2 O H 10 However, the structure 10 has never been proved. *Fax : + 33 03 81 66 62 30; e mail : bernard.laude@univ-fcomte.fr 0040-4039/98/$19.00 © 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S0040-4039(98)00029-X