The rare trans-syn thymine photodimers: structures of polyoxyethylene-linked bisthymines and the derived trans-syn thymine photodimers. Comparison of the stereochemistry before and after photodimerization

Abstract

trans-syn Photodimer of polyoxyethylene bisthymine (1) [1,1′-(ethylenedioxydiethylene)dithymine], trans-syn-1,8-(ethylenedioxyethylene)hexahydro-4a,4b-dimethylcyclobuta[1,2-d:4,3-d′]dipyrimidine-2,4,5,7(3H,6H)-tetrone (2), C16H22N4O6, Mr = 366.4, triclinic P{\bar 1}, a = 7.900 (4), b = 10.061 (8), c = 12.048 (4) Å, α = 71.37 (5), β = 78.13 (13), γ = 72.17 (5)°, V = 857.7 (9) Å3, Z = 2, Dx = 1.418 g cm−3, μ = 0.103 mm−1, F(000) = 388, R = 0.0953 for 1482 observed reflections; 1,1′-(1,2-phenylenedioxydiethylene)dithymine (3), C20H22N4O6, Mr = 414.4, triclinic, P{\bar 1}, a = 8.558 (2), b = 11.077 (2), c = 11.437 (3) Å, α = 108.57 (2), β = 94.37 (2), γ = 99.77 (2)°, V = 1003.1 (4) Å3, Z = 2, Dx = 1.377 g cm−3, μ = 0.096 mm−1, F(000) = 436, R = 0.0443 for all reflections; trans-syn photodimer of (3), 1,8-(1,2-phenylenedioxydiethylene)hexahydro-4a,4b-dimethylcyclobuta[1,2-d:4,3-d′]dipyrimidine-2,4,5,7(3H,6H)-tetrone (4), C20H22N4O6, Mr = 414.4, monoclinic, P21/n, a = 10.357 (4), b = 14.578 (5), c = 12.676 (2) Å, β = 97.99 (3)°, V = 1895 (1) Å3, Z = 4, Dx = 1.453 g cm−3, μ = 0.102 mm−1, F(000) = 872, R = 0.0628 for 1077 observed reflections. The bisthymines (1) and (3) on photodimerization produce cyclobutane-linked trans-syn thymine dimers as the exclusive products. The planar thymines stack one above the other in (1), while they are approximately perpendicular in (3). Planarities of the thymines are altered on photodimerization. The stereochemistry around the cyclobutane ring in photoproducts (2) and (4) is highly strained. The distortions in the bond lengths and angles due to this strain are concentrated mainly on the C5 and C6 atoms, which form the interpyramidal bonds. This strain is comparatively less in the rigid catechol-linked dimer (4). The N3 atom of thymines forms hydrogen bonds in all structures. Photoproducts show closer packing compared with the parent compounds.