Isopropylidenation, aldonolactones

Similar addition of other lithiated thio derivatives to substituted aldonolac-tones has been performed (69). Thus, reaction of tris(methylthio)methyl-lithium to benzylidenated or isopropylidenated aldonolactones, followed by mercury(II)-catalyzed desulfuration, led to derivatives of glyc-2-ulosonic acids. 

Addition of lithiated heterocycles to aldonolactones yields carbon-linked nucleosides (56). Thus, the reaction of 2,3 5,6-di-O-isopropylidene-L-gu-lono-1,4-lactone (9b) or 2,3-O-isopropylidene-D-ribono-l,4-lactone (16a) with various lithiated heterocycles gave gulofuranosyl derivatives 53a-g or ribofuranosyl derivatives 54b,c. Gulonolactols 53a-g and ribonolactols 54b,c were acetylated with acetic anhydride in pyridine to yield their acetyl derivatives. The stereochemistry of compounds 53a-g and 54b,c was discussed in terms of the Cotton effect of circular-dichroism curves of the ring-opened alcohols formed upon reduction by sodium borohydride. The configuration at C-l of 53g was proved by means of X-ray analysis (57,58). 

Condensation of the corresponding aldonolactone derivative with 1,1,1-tri-chlorotoluene and trimethyl phosphite gives the exo-alkenes 166 from which C-glycosides are available. Reaction of 0-substituted 1,2-0-isopropylidene-a-D-xylofuranose with allyltrimethylsilane and a Lewis add gives the 2-0-un-protected p-C-allyl C-glycosides. ... 

Aldol condensation of 2,3-0-ethylidene-D-erythrofuranose with formaldehyde gave the 2-C-hydroxymethyl-erythrose derivative (6), an isomer of apiose, which was characterized through aldonolactone and glycoside derivatives 2,3-O-isopropylidene-D-ribose similarly yielded D-hamamelose (2-C-hydroxymethyl-D-ribose) after deprotection. ... 

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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