Aldehydes aldonolactones

The hydroxyl groups of aldonolactones react with a variety of aldehydes and ketones to give the corresponding acetal derivatives. Treatment of the salts of aldonic acids with benzaldehyde and hydrochloric acid or zinc chloride as catalysts give benzylidene derivatives of aldonic acids or aldonolactones (3). 

The acid-catalyzed acetalation of aldonolactones with alkyl acetals of aldehydes or ketones takes place, in some instances, with esterification of the lactone group to give acetal derivatives of alkyl aldonates (11,22). 

Another method for the alkylidenation of aldonolactones uses addition of organ-ometallic reagents [78]. For example, Lin et al. [85] described an efficient route to conjugated anomeric dienes or aldehydes based on the reaction of aldono-1,4- and aldono-1,5-lactones (1, 62) with allylmagnesium chloride (Scheme 19), giving allyl hemiacetals (e.g., 63a, b). Hemiacetal can be dehydrated [e.g., with (CF3C0)20] to produce dienes 64a, b, or ozonolyzed (e.g., to give 65). 

There is no generally useful nonhydride method for the direct reduction of carboxylic acid esters to aldehydes. There are, however, procedures which are valuable under particular circumstances. An important example is the one-electron reduction of aldonolactones to aldoses. Two factors presumably contribute to the success of these reactions firstly the presence of electron-withdrawing substituents in the substrates, raising the reactivity of the carbonyl group, and secondly the ability of the products to form cyclic hemiacetals stable to further reduction. 

High yielding and highly stereoselective aldol condensations involving simple a,fi-unsaturated aldehydes (1), the silylenol ether (2), and the chiral catalyst (3), have been employed to prepare free sugars via aldonolactones. As illustrations, the syntheses of D-ribose and 6-deoxy-L-talose are given in Scheme 3. ... 

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