Pentitols ring opening

This approach provides a new method for carbohydrate synthesis. In the synthesis of tetritols, pentitols, and hexitols, for example, titanium-catalyzed asymmetric epoxidation and the subsequent ring opening of the thus formed 2,3-epoxy alcohols can play an essential role. 

Another study pertinent to the present discussion is that of the acid-catalyzed dehydration of tetratols, pentitols, and hexitols, all of which yield tetrahydrofuran derivatives as the primary products. These reactions are first order with respect to the alcohol and the acidity function Hq, and the direct relationship shown from plots of versus Hq implies that the reaction proceeds via a protonated intermediate.In all cases except one, the major cyclic product is derived from HO-5 displacement of water from C-1. The exception is D-mannitol (213), where HO-3 participation leads to a slight preference for formation of 2,5-anhydro-D-glucitol (214) over 1,4-anhydro-D-mannitol (215). Inversion at C-2 occurs with HO-5 participation during ring opening of the epoxide (214). 

JV-Alditol-l-yl bases, e.g. the thymine derivative 86, were synthesized ftom the 6-0-tosyl-D-glucal derivative 84 (Scheme 8). The cu-double bond in 86 results from a Michael addition and elimination of methanol prior to reduction of the rrnns-enal 85. Reductive ring-opening of nucleosides to A-(l-deoxy-D-riho-pentitol-l-yl)purines is covoed in Chapter 20. 

Cyclization by Displacement of Leaving Groups in Carbohydrate Substrates by Amines. The synthesis of l,2,4-trideoxy-l,4-imino-D-erythro-pentitol (31) has been achieved in eight steps, in 42% overall yield, from 2,5-di-O-p-tol-uenesulfonyl-D-ribono-1,4-lactone according to Scheme 7. The correct stereochemistry in the product was achieved by epimerization at C-4 in a 5-0-tosylated 2-deoxy lactone intermediate base action on the lactone produced a 4,5-epoxide of an open-chain carboxylate derivative which subsequently ring-closed by intramolecular attack of the carboxylate anion on the epoxide to produce the diastereomeric lactone. ... 

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