A new directing mode for singlet oxygen ene reactions: the vinylogous gem effect enables a 1O2 domino ene/[4 + 2] process.

The singlet oxygen reactions of 4-methyl-2,4-hexadienoates E,E- and E,Z-4 proceed in a highly mode selective and regioselective domino process. The initial product is the allylic hydroperoxide 5 directed by a vinylogous gem effect. The subsequent (1)O2 [4 + 2] cycloaddition delivers a 3:2 diastereoisomeric mixture of 1,2-dioxanes 8 in a one-pot process. The identical protocol delivers from the more reactive α-methylated substrates E,E-10 and E,Z-10 with excellent primary regioselectivity the 1,2-dioxane 13.