Thermal, photochemical, and acid-catalysed rearrangements of the spiro-dimer of α,α′-bis(methoxycarbonyl)-o-quinodimethane. X-Ray crystal structure of trans,trans-tetrakis(methoxycarbonyl)dibenzo[a,e]cyclo-octene

Abstract

The spiro-dimer of α,α′-bis(methoxycarbonyl)-o-quinodimethane is allocated the stereochemistry (1) by comparison of its properties with those of the isomers (6a) and (6b) obtained from it by photolysis. Under acidic conditions the isomers (1) and (6a) give mainly epimeric products [the epimeric tosylates (9) with toluene-p-sulphonic acid, and the epimeric dibenzocycloheptenes (8a) and (8b) and lactones (11) with boron trifluoride–diethyl ether] showing that the isomers (1) and (6a) differ in configuration at the benzylic carbon adjacent to the spiro-centre. A single-crystal X-ray structure determination shows that the major thermal rearrangement product of compound (1) has structure (4) rather than the structure (2) previously proposed; the minor rearrangement product is compound (7). The thermal rearrangements of compounds (1) to (4), (6a) to (7), and (6b) to (2) are formal 1,3-shifts following the forbidden suprafacial-retention geometry; rearrangement by a radical dissociation-recombination mechanism is likely.