SET-induced photorearrangement of 2-phenylallyl phosphites. Stereochemistry at phosphorus. Application to cyclic nucleotide derivatives

The stereochemistry at phosphorus of the SET-induced photorearrangement of diastereomeric 4-tertbutyl-2-phenylallyl-1,3,2-dioxaphosphori (8) to the co rresponding 2-phenylallylphosphonates (9), which involves excited singlet 1 ,4-dicyanonaphthalene ((DCN)-D-1*) as one-electron oxidant, was investigate d. The rearrangement occurs with close to complete retention olf configurat ion at phosphorus. The previously postulated mechanism for this photorearra ngement is shown to be consistent with the stereochemical finding. Thus, on e-electron reduction by DCN- of the presumably stereospecifically formed di stonic cyclic 1,3-cation radical intermediate 15, generated from cis-8 (Sch eme 2), yields the thermodynamically stable diradical 16. beta scission of 16 forms phosphonate cis-g. An alternative mechanism involving beta scissio n of 15 to a styryl cation radical, prior to one-electron reduction to 15, is discounted on the basis of unpublished trapping studies using MeOH. The direct, kinetically controlled formation of diradical 16 rather than the th ermodynamically less stable 21 with CH2 bonded epically to phosphorus is ar gued to be consistent with the essentially equal values of the quantum yiel d for phosphonate formation (phi(P)) on SET-induced rearrangement of the ac yclic 2-phenylallyl phosphite 1 and phosphite 7 with phosphorus incorporate d in a six-membered (1,3,2-dioxaphosphorinane) ring. This mechanism is cont rasted to that for the previously reported triplet-sensitized photorearrang ements of phosphites 1 and 7, which have greatly different gp values. For t hese reactions, kinetic formation of the triplet analogue of 21, but withou t the tert-butyl substituent, requires a permutation of substituents for co nversion to diradical 16 prior to intersystem crossing and beta scission to form the phosphonate corresponding to 7. The preparative-scare SET-induced photorearrangement of the thymidine-based 2-phenylallyl 3',5'-phosphite 10 gave both diastereomers of phosphonate 11 that were separated by HPLC. The 8-phenylallyl functionality provides an opportunity for further functional ization. As reported elsewhere, 11 was not formed in useful amounts via tri plet-sensitized reaction of 10.