Formation of Heptaleno[1,2‐c]furans and Heptaleno[1,2‐c]furanones

Abstract

AbstractThe dehydrogenation reaction of the heptalene‐4,5‐dimethanols 4a and 4d, which do not undergo the double‐bond‐shift (DBS) process at ambient temperature, with basic MnO2 in CH2Cl2 at room temperature, leads to the formation of the corresponding heptaleno[1,2‐c]furans 6a and 6d, respectively, as well as to the corresponding heptaleno[1,2‐c]furan‐3‐ones 7a and 7d, respectively (cf. Scheme 2 and 8). The formation of both product types necessarily involves a DBS process (cf. Scheme 7). The dehydrogenation reaction of the DBS isomer of 4a, i.e., 5a, with MnO2 in CH2Cl2 at room temperature results, in addition to 6a and 7a, in the formation of the heptaleno[1,2‐c]‐furan‐1‐one 8a and, in small amounts, of the heptalene‐4,5‐dicarbaldehyde 9a (cf. Scheme 3). The benzo[a]heptalene‐6,7‐dimethanol 4c with a fixed position of the CC bonds of the heptalene skeleton, on dehydrogenation with MnO2 in CH2Cl2, gives only the corresponding furanone 11b (Scheme 4). By [2H2]‐labelling of the methanol function at C(7), it could be shown that the furanone formation takes place at the stage of the corresponding lactol [3‐2H2]‐15b (cf. Scheme 6). Heptalene‐1,2‐dimethanols 4c and 4e, which are, at room temperature, in thermal equilibrium with their corresponding DBS forms 5c and 5e, respectively, are dehydrogenated by MnO2 in CH2Cl2 to give the corresponding heptaleno[1,2‐c]furans 6c and 6e as well as the heptaleno[1,2‐c]furan‐3‐ones 7c and 7e and, again, in small amounts, the heptaleno[1,2‐c]furan‐1‐ones 8c and 8e, respectively (cf. Scheme 8). Therefore, it seems that the heptalene‐1,2‐dimethanols are responsible for the formation of the furan‐1‐ones (cf. Scheme 7). The methylenation of the furan‐3‐ones 7a and 7e with Tebbe's reagent leads to the formation of the 3‐methyl‐substituted heptaleno[1,2‐c]furans 23a and 23e, respectively (cf. Scheme 9). The heptaleno[1,2‐c]furans 6a, 6d, and 23a can be resolved into their antipodes on a Chiralcel OD column. The (P)‐configuration is assigned to the heptaleno[1,2‐c]furans showing a negative Cotton effect at ca. 320 nm in the CD spectrum in hexane (cf. Figs. 3–5 as well as Table 7). The (P)‐configuration of (–)‐6a is correlated with the established (P)‐configuration of the dimethanol (–)‐5a via dehydrogenation with MnO2. The degree of twisting of the heptalene skeleton of 6 and 23 is determined by the Me‐substitution pattern (cf. Table 9). The larger the heptalene gauche torsion angles are, the more hypsochromically shifted is the heptalene absorption band above 300 nm (cf. Table 7 and 8, as well as Figs. 6–9).