Hexopyranose nucleosides synthesis

The simultaneous protection of the 3 and S hydroxy groups of nucleosides and the 4,6- or 3,4-hydroxyls of hexopyranoses is a common problem in organic synthesis. In the case of hexopyranoses, we have already seen that benzylidene acetals and, in certain circumstances, isopropyiidene acetals can be used to good effect. An alternative silicon-based group would offer a wider repertoire of conditions for mild deprotection and such a group was devised by Markiewicz the 1,1,3,3-tetraisopropyldisiloxanylidene group (abbreviated TIPDS),22 229 TIPDS groups are stable to water, 0.3 M p-toluenesulfonic acid in dioxane, 10% tri-fluo mace tic acid in chloroform, 5 M ammonium hydroxide in dioxane-H20 (4 1), and tertiary amines in pyridine,... 

A number of -acyl- and thioureido-derivatives of 3 -amino-3 -deoxythymidine have been prepared. 1 0 Michael addition of phthalimide to enal (81) was used in the synthesis of the 3 -amino-2 ,3 -dideoxyhexo-furanose nucleosides (82) (all four isomers, R=H, Me, halogen),191 and a similar enal prepared by mercuric-ion catalysed hydrolysis of L-rhamnal was used in the same way to make the L-acosaminyl nucleoside (83) and its -anomer, and the L-ristosaminyl systems (epimers at C-3 ). 2 Michael addition of 1,2,4-triazole to (81) led, after base-sugar condensation, to the triazolyl-substituted nucleosides (84, B=T, U), together with the analogous hexopyranose systems. 93 xhe imidazolyl nucleosides (85) can be made by reaction of 3 -amino-3 -deoxythymidine with l,4-dinitroimidazoles, 9 whilst the unsaturated imidazole nucleoside (87) was made by treating the phenylselenone (86) (see Section 6) with imidazole. 195... 

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