2-Addition of Pyrroles to Dimethyl Acetylenedicarboxylate: Michael-Type Adducts and Diels-Alder Products1

Pyrroles 1 having an open 2-position react with dimethyl acetylenedicarboxylate (DMAD) in the presence of a source of active hydrogen, usually provided in the present cases by their own NH groups (if present) or by sufficient acetic acid, to give 1:1 Michael-type adducts at the 2-position (dimethyl 2-pyrrolyl-2-butenedioates), usually as both the Z (3) and E (4) isomers. Analogy with the data obtained permits assignment of stereochemistry to several other 1:1 adducts previously reported, including 3g and 4g from 2,3-dimethylpyrrole (lg) and 3h from 2,3,4-trimethylpyrrole (lh). On the basis of NMR and mass spectral data, the 2:1 adduct of If and DMAD (a coproduct of 3f and 4f) is reassigned the structure dimethyl 2,2-bis(3,5-dimethyl-2-pyrrolyl)butanedioate (9). Formation of 3 and 4 is usually competitive with Diels-Alder reactions in which the pyrrole acts as a diene. The Diels-Alder reaction tends to be relatively favored in the absence of weak acids such as acetic acid and at higher temperatures than in refluxing ether. The initial 1:1 Diels-Alder adducts (5) are unstable and were not isolated. They tend to react by one of four pathways: (1) they revert to 3 and 4; (2) they react further with DMAD, giving 1:2 adducts of type 6 (tetramethyl 3a,7a-dihydroindole-2,3,3a,4-tetracarboxylates); (3) at higher temperatures (63-160 °C) they undergo retro-Diels-Alder cleavage, giving the corresponding dimethyl N-substituted pyrrole- 3,4-dicarboxylates (7); (4) they eliminate the bridging nitrogen to give dimethyl 4-substituted phthalates (14) or to give (via5 and 7 and a further Diels-Alder reaction of 7 with DMAD) tetramethyl benzene-1,2,4,5- tetracarboxylate (24) from lp and lq. The 1:2 adducts of type 6 undergo further Diels-Alder reaction at their diene system with DMAD to give as coproducts trimethyl 5-substituted benzene-1,2,3-tricarboxylates (16) and trimethyl 1-substituted pyrrole-2,3,4-tricarboxylates (17, similar to 7). In a few instances the 2-vinylpyrroles 3 and/or 4 were observed to act as dienes in a Diels-Alder synthesis of indoles.