Benzo[a]heptalenes from Heptaleno[1,2‐c]furans. Part I

It is shown that heptaleno[1,2‐c]furans 1 , which are available in two steps from heptalene‐4,5‐dicarboxylates by reduction and oxidative dehydrogenation of the corresponding vicinal dimethanols 2 with MnO₂ or IBX (Scheme 4), react thermally in a Diels–Alder‐type [4+2] cycloaddition at the furan ring with a number of electron‐deficient dipolarophiles to yield the corresponding 1,4‐epoxybenzo[d]heptalenes (cf. Schemes 6, 15, 17, and 19). The thermal reaction between dimethyl acetylenedicarboxylate (ADM) and 1 leads, kinetically controlled, via a sterically less‐congested transition state (Fig. 4) to the formation of the (M*)‐configured 1,4‐dihydro‐1,4‐epoxybenzo[a]heptalenes, which undergo a cyclic double‐bond shift to the energetically more‐relaxed benzo[d]heptalenes 4 (Schemes 6 and 7). Most of the latter ones exhibit under thermal conditions epimerization at the axis of chirality, so that the (M*)‐ and (P*)‐stereoisomers are found in reaction mixtures. The (P*)‐configured forms of 4 are favored in thermal equilibration experiments, in agreement with AM1 calculations (Table 1). The relative (P*,1S*,4R*)‐ and (M*,1S*,4R*)‐configuration of the crystalline main stereoisomers of the benzo[d]heptalene‐2,3‐dicarboxylates 4a and 4f , respectively, was unequivocally established by an X‐ray crystal‐structure determination (Figs. 1 and 2). Acid‐induced rearrangement of 4 led to the formation of the corresponding 4‐hydroxybenzo[a]heptalene‐2,3‐dicarboxylates 5 in moderate‐to‐good yields (Schemes 8, 13, and 14). When the aromatization reaction is performed in the presence of trifluoroacetic acid (TFA), trifluoroacetates of type 6 and 13 (Schemes 8, 12, and 13) are also formed via deprotonation of the intermediate tropylium ions of type 7 (Scheme 11). Thermal reaction of 1 with dimethyl maleate gave the 2,3‐exo‐ and 2,3‐endo‐configured dicarboxylates 14 as mixtures of their (P*)‐ and (M*)‐epimers (Scheme 15). Treatment of these forms with lithium di(isopropyl)amide (LDA) at ?70° gave the expected benzo[a]heptalene‐2,3‐dicarboxylates 15 in good yields (Scheme 16). Fumaronitrile reacted thermally also with 1 to the corresponding 2‐exo,3‐endo‐ and 2‐endo,3‐exo‐configured adducts 17 , again as mixtures of their (P*)‐ and (M*)‐epimers (Scheme 17), which smoothly rearranged on heating in dimethoxyethane (DME) in the presence of Cs₂CO₃ to the benzo[a]heptalene‐2,3‐dicarbonitriles 18 (Scheme 18). Some cursory experiments demonstrated that hex‐3‐yne‐2,5‐dione and (E)/(Z)‐hexa‐3‐ene‐2,5‐dione undergo also the Diels–Alder‐type cycloaddition reaction with 1 (Scheme 19). The mixtures of the stereoisomers of the 2,3‐diacetyl‐1,4‐epoxytetrahydrobenzo[d]heptalenes 22 gave, on treatment with Cs₂CO₃ in DME at 80°, only mixtures of the regioisomeric inner aldol products 24 and 25 of the intermediately formed benzo[a]heptalenes 23 (Scheme 20).