Photochemistry of arylidene-beta-ionones: a highly efficient route to novel tricyclic ketones through intramolecular, exoselective photochemical (4 + 2) cycloadditions, occurring only in an aqueous-organic solvent

(E,E)-Arylidene-beta-ionones (1a-f) are converted to 1,7,7-trimethyl-3-(E-2'-arylethenyl)-2-oxabicyclo[4.4.0]deca-3,5-dienes (3a-f, approximately 90%) by irradiating in anhydrous solvents. Irradiation of (3a-f) in aqueous methanol results in Z,E-arylidene-beta-ionones (2), through retro-electrocyclization, which undergoes an intramolecular, exo-selective [4 + 2] photocycloaddition leading to 11-(exo)-aryl-1,7,7-trimethyl-tricyclo[4.4.0.1(2,4)]undec-5-ene-3-ones (8a-f, 60-80%). The latter rearrange over silica gel to afford, quantitatively, 5-aryl-7,11,11-trimethyl-tricyclo[5.4.0.0(3,6)]undec-1-ene-4-ones (5a-f). Irradiation of 1a-f in aqueous methanol leads to 8a-f, except in case of 1c,f wherein formation, respectively, of tricyclic ketones 9c (55%) and 9f (80%), derived from photodeconjugation in 2, followed by intramolecular [4 + 2] cycloaddition, is observed.