MO Analysis of Photocycloaddition Reactions of 2-Pyrones

Abstract The skeleton of the 2-pyrones (1) is a simple one composed of 2,4-pentadien-5-olides, but has many possibilities for photocycloadditions. To elucidate the peri-, site-, and regioselective photoadditions of the triplet 2-pyrones with some ethylenes, the factors were analyzed by PM3-CI-UHF methods, etc. The regioselective [4+2]cycloadducts (3) and site-selective [2+2]cycloadducts (4) across the C5–C6 double bonds of the 2-pyrones with electron-deficient ethylenes were deduced to be formed by their preferable HSOMO–LUMO interactions and via more stable biradicals by the C6-Cβ bondings. On the other hand, the other kinds of regioselective [4+2] (6) and [2+2]adducts (5) across the C3–C4 double bonds of the 2-pyrones with an electron-donating ethylene were inferred to mainly come from LSOMO–HOMO interactions and via biradicals by C3-Cβ bondings. Selectivities between 3 and 4, and 5 and 6 were also inferred to be caused by SOMOs of the biradicals.