An improvement of our strategy for the stepwise synthesis of oligo 5'-deoxy-5'-thiodeoxyribonucleotides [Chladek and Nagyvary (1972) J. Amer. Chem. Soc. 94, 2079] involves the use of 5'-O-tosylthymidine 3'-S-2-cyanoethyl phosphorothioate. The displacement of the tosylate by thymidine 3'-phosphorothioate and subsequent alkaline deblocking afforded the dinucleotide (Tps)2. The process of displacement and deblocking was repeated three more times at an average yield of 30 percent per step. The corresponding bifunctional derivative of deoxyadenosine was found much less reactive and practically unsuitable for repeated chain elongation. The ORD and CD spectra of the analogs are similar to those of the natural oligonucleotides.
Two grams (0.011 mole) of 6-ethoxybenzothiophene, 2.0 g. (0.026 mole) of dimethylformamide and 3.0 g. of phosphorus oxychloride were dissolved in 10 ml. of dioxane and allowed to stand at room temperature for 2 hr. The mixture was heated at 70°for 3 hr. and poured into an aqueous solution of excess sodium acetate. After the mixture had been allowed to stand overnight, the precipitate was collected and recrystallized from dilute ethanol to give 0.8 g. (34%) of light strawcolored needles melting at 99°.Anal. Caled, for CuHmChS: S, 15.52. Found: S, 15.44. 6-Ethoxybenzothiophene-S-carboxaldehyde oxime. This derivative was found to melt at 171°.
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