Thermal and photochemical reactions of biacetyl with 1,1-diethoxyethene. “Umpolung” of the reactivity of biacetyl by photochemical induced electron transfer.

The thermal and photochemical reactions of biacetyl 1 and 1,1-diethoxyethene 2 are totally complementary: 1 and 2 exclusively form the 2,2-diethoxyoxetane 4 in polar solvents at room temperature. Contrary to the literature this thermal cycloadditon proceeds uncatalyzed. 1 and 2 react photochemically under exclusive formation of the regioisomer oxetane 3 - preferably in nonpolar solvents. The inversion (“Umpolung”) of the reactivity of 1 is caused by the photoinduced electron transfer.