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

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‐(phenylsul‐fonyl)benzo[a]heptalene‐2,4‐diols of. (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 of. 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 of. (cf. Scheme 5). It is assumed that the benzo[a]heptalene formation is due to (sulfonyl)methylation of both methoxycarbonyl groups of the heptalene‐4,5‐dicarboxylates of. (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 of. (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).