A new alkylation method for heptalene-4,5-dicarboxylates and of one of their pseudoester forms

Abstract

Dimethyl heptalene-4,5-dicarboxylates† undergo preferentially a Michael addition reaction at C(3) with α-lithiated alkyl phenyl sulfones at temperatures below −50°, leading to corresponding cis-configured 3,4-dihydroheptalene-4,5-dicarboxylates (cf. Table 1, Schemes 3 and 4). The corresponding heptalenofuran-1-one-type pseudoesters of dimethyl heptalene-4,5-dicarboxylates (Scheme 5) react with [(phenylsulfonyl)methyl]lithium almost exclusively at C(1) of the furanone group (Scheme 6). In contrast to this expected behavior, the uptake of 1-[phenylsulfonyl)ethyl]lithium occurs at C(5) of the heptalenofuran-1-ones as long as they carry a Me group at C(11) (Schemes 6 and 7). The 1,4- as well as the 1,6-addition products eliminate, on treatment with MeONa/MeOH in THF, benzenesulfinate, thus leading to 3- and 4-alkylated dimethyl heptalene-4,5-dicarboxylates, respectively (Schemes 8–13). The configuration of the addition reaction of the nucleophiles to the inherently chiral heptalenes is discussed in detail (cf. Schemes 14–19) on the basis of a number of X-ray crystal-structure determinations as well as by studies of the temperature-dependence of the 1H-NMR spectra of the addition products.