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 ).