A 'One-Pot' Anellation Method for the Transformation of Heptalene-4,5-dicarboxylates into Benzo[a]heptalenes

Article abstract of DOI:10.1002/hlca.19970800821

It has been found that dimethyl heptalene-4,5-dicarboxylates, when treated with 4 mol-equiv. of lithiated N,N-dialkylamino methyl sulfones or methyl phenyl sulfone, followed by 4 mol-equiv. of BuLi in THF in the temperature range of - 78 to 20°, give rise to the formation of 3-[(N,N-dialkylamino)sulfonyl]- or 3-(phenylsulfonyl)benzo[a]heptalene-2,4-diols (cf. Scheme 4, and Tables 2 and 3). Accompanying products are 2,4-bis{[(N,N-dialkylamino)sulfonyl]methyl}- or 2,4-bis[(phenylsulfonyl)methyl]-4,10a-dihydro-3H-heptaleno[1,10-bc]furan-3- carboxylates as mixtures of diastereoisomers (cf. Scheme 4, and Tables 2 and 3) which are the result of a Michael addition reaction of the lithiated methyl sulfones at C(3) of the heptalene-4,5-dicarboxylates, followed by (sulfonyl)methylation of the methoxycarbonyl group at C(5) and cyclization (cf. Scheme 5). It is assumed that the benzo[a]heptalene formation is due to (sulfonyl)methylation of both methoxycarbonyl groups of the heptalene4,5-dicarboxylates (cf. Schemes 6 and 8). The resulting bis-enolates 35 are deprotonated further. The thus formed tris-anions 36 can then cyclize to corresponding tris-anions 37 of cyclopenta[d]heptalenes which, after loss of N,N-dialkylamido sulfite or phenyl sulfinate, undergo a ring-enlargement reaction by 1,2-C migration finally leading to the observed benzo[a]heptalenes (cf. Schemes 8 and 9). The structures of the new product types have been finally established by X-ray crystal-structure analyses (cf. Figs. 1 and 2 as well as Exper. Part).