No abstract

Selenocysteine containing peptoids and peptide-peptoid conjugates were synthesized by combinatorial Ugi-MCRs (multicomponent reactions) in water: for the first time, an acetal (selenoacetal 2a) was used in Ugi-MCR to furnish selenocysteine peptoids in one step as model compounds for selenocysteine peptides and proteins.

On irradiation (! 350 nm) in the presence of 2,3-dimethylbut-2-ene or 2-methylpropene the title compound 2 undergoes cycloaddition at the C(9)0C(10) double bond selectively (b15:1) to afford the 8a,9,10,10a-tetrahydro-2H,8H-cyclobuta[c]pyrano[2,3-f ][1]benzopyran-2,8-diones 3.

, the head-to-head ( H H ) cis-cisoid-cis-cyclodimer of 2. X-Ray analysis of 2 confirms that this reaction proceeds according to the well-established topochemical principles. The same dimer 3 is obtained in low yields on irradiation of lo-' M solutions of 2 in either MeOH or MeCN, while no conversion at all is observed in benzene. On irradiation of 2 in MeCN in the presence of tetrachloroethene, the [2+2] photocycloadduct 4 is formed in good yield, the conversion 2 -+ 4 being efficiently quenched by naphthalene. In contrast, no reaction is observed .on irradiation of 2 in the presence of 2,3-dimethylbut-2-ene, neither in polar nor in apolar solvents.

Upon irradiation at wavelengths above 390 nm in the presence of 2,3-dimethylbut-2-ene, both the newly synthesized pyranoquinolinedione 8 and thiinobenzopyrandione 9 selectively afford one of two possible photocycloadducts, 23 and 24, respectively.Introduction. ± Crosslinking of thymine units within helical DNA strands can be achieved, for instance, by light-induced reactions of psoralens (2H-furo[3,2-g]benzopyran-2-ones), such as 1, via two consecutive [2 2] photocycloadditions, the first one occuring at the furan double bond [1]. In contrast, photocycloadditions in solution of both linear (1) [2] and angular tricycles (2 and 3) [3] to alkenes selectively occur at the double bond of the six-membered heterocycle. This difference in selectivity has been attributed to the observation that the photocycloaddition to a nucleic acid is preceded by intercalation of a psoralen, the smaller furan moiety fitting into the helix [1] (Scheme 1).