New synthetic approaches to fused-ring carbocycles based on intramolecular photocycloadditions of 1,3-dione enol esters

Abstract

Irradiation of 3-acetoxy-2-pent-4-enylcyclohex-2-enone (12) leads to the tricyclic adduct (18), in high yield (>95%), which on hydrolytic cleavage in ethanolic potassium hydroxide gives rise to bicyclo[6.3.0]-undecane-2,6-dione (20).In contrast, irradiation of the isomeric enol acetate, 3-acetoxy-4-pent-4-enylcyclohex-2-enone (17) led to the two photoadducts (25) and (26) resulting from intramolecular photocycloaddition, and to the trione (24) produced via photo-Fries rearrangement of (17); the formation of (25) and (26) followed from the structures (29) and (27) produced after saponification-retroaldolisation of mixtures containing (25) and (26). Irradiation of the 6-pentenyl substituted enol ester (16) gave rise to only the photo-Fries rearrangement product (24).Irradiation of 1-acetoxy-2-pent-4-enoylcyclopentene(39) led (93%) to a 2:3 mixture of the photoadducts (41) and (42) whose structures followed from X-ray measurements. The corresponding cyclohexene (45), instead produced the product (46) of photo-Fries rearrangement on irradiation. Hydrolytic cleavage of (41) in KOH–EtOH leads to the bicyclo[5.3.0]decanone (47), whereas similar treatment of (42) instead gave directly the aldol (49) by way of the transient retro-aldol intermediate (48).