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

Abstract

The 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 MnO, in CH,C1, at room temperature, leads to the formation of the corresponding heptaleno[l,2-c]furans 6a and 6d, respectively, as well as to the corresponding heptaleno[l,2-c]furan-3-ones 7a and 7d, respectively (cJ 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 MnO, in CH,CI, at room temperature results, in addition to 6a and 7a, in the formation of the heptaleno[l,2-c]-furan-Lone 8a and, in small amounts, of the heptalene-4,5-dicarbaldehyde 9a (cf. Scheme 3 ) . The benzo[a]heptalene-6,7-dimethanol4c with a fixed position of the C=C bonds of the heptalene skeleton, on dehydrogenation with MnO, in CH,Cl,, gives only the corresponding furanone l l b (Scheme 4). By [ZHz]-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-'H2]-15b ( c t 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 MnO, in CH2C12 to give the corresponding heptaleno[l,2-~]furans 6c and 6e as well as the heptaleno[l,2-c]furan-3-ones 7c and 7e and, again, in small amounts, the heptaleno[l,2-c]furan-l-ones 8c and Se, respectively (cf. Scheme 8). Therefore, it seems that the heptalene-1,2-dimethanols are responsible for the formation of the furan-l-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[l,2-c]furans 23a and 23e, respectively (cf. Scheme 9 ) . The heptaleno[l,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[l,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 MnO,. 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 ( c j Table 7 and 8, as well as Figs. 6-9).

Tetramethylheptalenes and Their Tricarbonylchromium Complexes: Synthesis, Structures, and Thermal Rearrangements

“…procedures developed during our earlier investigations of the formation of this new class of heptaleno compounds (see Scheme 6) [30]. Reduction of the heptalene-4,5-dicarboxylate 14b [12] with DIBAH gave the corresponding heptalene-4,5-dimethanol 15b, which, on treatment with MnO 2 in CH 2 Cl 2 , was converted to a mixture of 16, and the two isomeric heptaleno[1,2-c]furanones 17a and 18a 5 ).…”

Formation of 6‐Methyl‐7,11‐diphenylheptaleno[1,2‐c]furanones

“…Treatment of the heptalenedimethanols with activated MnO 2 in CH 2 Cl 2 led to the formation of the corresponding heptaleno[1,2-c]furans and heptaleno[1,2-c]furan-3-ones 1 and 1' (cf. [1] and lit. cit.…”

“…We suppose that the absorption at 292 nm reflects slight differences in the conjugation of the furanone C¼O groups with the heptalene chromophore of 6a and 7a, which can be attributed to small deviations in the torsion angles of the heptalene perimeter. Indeed, AM1 calculations (CS Chem3D Pro , 2001) of both structures indicate slightly smaller cisoid torsion angles at the central heptalene axis (C(6a)ÀC(11a)) for 7a in comparison with 6a (see also [1]) 1 ).…”

“…Both compounds exhibit for HÀC (4) (1) and C(3)¼O (cf. [1]). The DBS isomer of 6a, i.e., 6b, was not found in solutions of 6a (C 6 D 6 or CDCl 3 ) at room temperature.…”

“…When furanone 7a was treated in the established way with Tebbes reagent in toluene at À 25 to À 308 [1] (see also [4] [5]), a single product was formed. However, instead of the expected 3,6-dimethylheptaleno[1,2-c]furan 8, the ring-opened 2-isopropenyl-5-methylheptalene-1-methanol 9a was isolated (Scheme 3).…”