Spirans. Part 14. Regioisomeric quinone methides and spirodimers related to tocopherol

Abstract

Evidence is provided that the quinone methide (5), 3,4-dihydro-2,2,5,8-tetramethyl-7-methylene-2H-benzo[b]-pyra n-6(7H)-one, is more, not less, readily formed than its regioisomer, the corresponding 5-methylenebenzo-pyran-6-one (4), and consequently that ‘bond fixation’ and allied geometrical constraints (Mills–Nixon effects) cannot be responsible for the regioselective oxidative dimerisation of tocopherol. The quinone methide (5) obtained transitorily by treating 7-chloromethyl-3,4-dihydro-2,2,5,8-tetramethyl-2H-benzo[b]pyran -6-ol (11) with hydrogen carbonate ion rapidly forms the corresponding spirodimer (7), 3,3′,4,4′,8,9-hexahydro-2′,2′,5,5′-7,7,8′,10-octamethylbenzo[1,2-b:4,5-b′]dipyran-2-spiro-7′(6′H)-[1]benzopyran-6′-one and trimer (12), which is the main product. The spirodimer (7) reverts to quinone methide (5) near 80 °C and is, therefore, rapidly trans-formed into the trimer at this temperature, whereas the regioisomeric spirodimer (6), previously recognised as the product from the oxidations, is stable. The difference in reactivity between the known quinone methide (4) and the new one (5) is thought to be associated with the benzylic methylene groups of the terminal pyran rings.