Synthesis and photocyclization of alpha-methylene-gamma-butyrolactone-thymine heterodimers

Two alpha-methylene-gamma-butyrolactone--thymine heterodimers 6 and 7 with 5 and 10 carbon alkyl chains, respectively, were prepared to study the photoreactivity of lactone rings toward thymine and the influence of spacer length on photoadduct geometry. Deoxygenated solutions (10(-3) M) of heterodimers 6 and 7 were irradiated at 365 nm in acetone. Irradiation of compound 6 (5 carbons alkyl chain) gave a single photoadduct 8 (70% yield), while irradiation of compound 7 (10 carbons alkyl chain) gave two photoadducts 9a and 9b in a 3/2 ratio (95% yield). Compound 8 was identified as a cis-syn-endo intramolecular [2 + 2] photoadduct involving the 5",6" double bond of the thymine moiety and the 3,6 exomethylenic group of the lactone. Compound 9a was identified as a cis-syn-exo intramolecular [2 + 2] photoadduct involving the 5",6" double bond of the thymine moiety and the 3,6 exomethylenic group of the lactone, and compound 9b as the corresponding cis-syn-endo intramolecular [2 + 2] photoadduct.