Aldonolactones lactone group, reaction with

The application of lithiumtrimethylsilyl acetate for the C-l elongation of aldonolactones has been examined (73). Although the reagent had been successfully used for the alkenation of lactone carbonyl groups (74), in the case of aldonolactones 10b or 25b only insignificant yields of the alkenes, but 30 - 40% of the lactols 49a or 50a, were obtained (73). However, these lactols, alternatively prepared in good yields by a Reformatsky-type reaction (53,54), were readily eliminated to the desired alkenes by simple treatment with methanesulfonyl chloride-triethylamine at 0°. Thus, from 49a or 50a separable E,Z mixtures (76a and 76b, or 77a and 77b, respectively) were obtained in good yields (73). 

Selective tosylation or mesylation of unprotected aldonolactones, to give the primary sulfonate only, seemed not to be possible since the 2-hydroxy group has about the same reactivity in these reactions as the primary one [28,29]. Selective di-O-tosylation was possible by treatment of aldonolactones with 2.2 mole equivalents of tosyl chloride to give 2,5-di-O-tosylated pentono- or 2,6-di-O-tosylated hexono-l,4-lactones [28]. The yields of crystalline ditosylates were high, when the OH-2 and OH-3 groups were cis- oriented and also cis to the side chain, and somewhat lower when they were both trans to the side chain. By this... 

As pointed out above, aldonolactones/adds may isomerise at C-2 in the presence of strong base. No isomerisations were detected in the reaction of the 2,3-0-isopropylidene-protected lactones 3 and 5 with strong aqueous potassium hydroxide. Likewise, unprotected 5-bromo-5-deoxy-D-lyxono- and -D-ribono-lactones give rearrangements to L-ribonic and L-lyxonic acids, respectively [19]. In these cases isomerisation at C-2 was not observed either. It has thus been confirmed that aqueous base requires higher temperatures in order to epimerise at the carbon center a to the carboxylate group. 

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