Synthesis of Benzo[a]heptalenes via Benzanellation–Aromatization Sequence

Abstract

A novel approach towards the synthesis of functionalized benzo[a]heptalenes 9 and 10 via a 6p-electrocyclic ring closure ± aromatization sequence of corresponding bis[prop-2-enoates] 5 and 6 has been developed (Scheme 1). The starting bis [prop-2-enoates] have been prepared from the corresponding dialdehydes 3a and 4a in a Wittig-Horner reaction, and their UV/VIS properties have also been investigated ( Fig. 1 and Table 1). The dehydrogenations of the corresponding diols 1 and 2 to dialdehydes with a number of oxidizing reagents, including MnO 2 in CH 2 Cl 2 , tetrapropylammonium perruthenate (TPAP), and activated DMSO, have been studied in detail. 1. Introduction. ± So far, benzo[a]heptalenes have been synthesized by degradation of colchicinoids [1], by application of Hafner×s heptalene synthesis to benz[a]azulenes [2] [3], or by benzene-ring-forming reaction of heptalene-4,5-dicarboxylates and their derivatives [4] [5]. The benzanellation procedures, which have already been tested, are the following: i) Bergman cyclization of corresponding bis(ethynyl)heptalenes [6] to give benzo[a]heptalenes in an overall yield of 6%; ii) Diels-Alder reaction with heptaleno[1,2-c]furans, followed by ring opening of the formed oxabicyclo[2.2.1]heptane substructure [5b] [7]; iii) −one-pot× benzanellation procedure, starting directly from heptalene-4,5-dicarboxylates and lithiated methyl sulfones, leading to 3-sulfonylsubstituted benzo[a]heptalene-2,4-diols in moderate-to-excellent yields [5].It has been shown that the thermal electrocyclic ring-closure reaction of azulenebis [prop-2-enoates] can successfully be applied to the synthesis of benz[a]azulenes [8]. We report here on the first results of the utilization of the later benzanellation concept to the synthesis of new functionalized benzo[a]heptalenes from the corresponding bis [prop-2-enoates]. A new approach to heptalene-4,5-dicarbaldehydes has also been developed.