Chemo- and stereoselective cobalt-mediated [2+2+2]cycloaddition of alkynes with uracil derivatives: a new synthesis route to modified nucleosides.
Uracils I [R = (CH2)3C.tplbond.CH, R1 = Bz, H] underwent cycloaddn. with R2C.tplbond.CR2 (R2 = SiMe3, CO2Me) and CpCo(CO)2 (Cp = cyclopentadienyl) to give the adducts II. I (R = protected ribose, deoxyribose, Me; R1 = Me, H) gave adducts III (n = 1-3) on treatment with HC.tplbond.C(CH2)n+2C.tplbond.CH and CpCoL2 (L = CO, C2H4). III (R = R1 = Me, n = 2) was demetalated with CuCl2-Et3N. The crystal structure of III (n = 2, R = 3,5-di-O-methyl-2-deoxyribosyl, R1 = Me) is reported.
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