Novel tunable CuX(2)-mediated cyclization reaction of cyclopropylideneacetic acids and esters for the facile synthesis of 4-halomethyl-2(5H)-furanones and 4-halo-5,6-dihydro-2H-pyran-2-ones

[reaction: see text] A mixture of cyclopropylideneacetic acids (or esters) and CuBr(2) (or CuI/I(2)) in aqueous acetonitrile afforded 4-substituted 2(5H)-furanones or 3,4-substituted 5,6-dihydro-2H-pyran-2-ones in moderate to good yields. The selectivity of the reaction greatly depended on the reaction temperature.