Sugar synthons

Synthesis of Natural and Unnatural Products from Sugar Synthons... 

As a final example to close this section, further advances in free radical chemistry will be explored. Unlike the examples presented earlier in this chapter, the radical acceptors used by Bimwala, et ai.,31 were derived from sugar synthons. Specifically, as shown in Scheme 8.9.5, treatment of the bicyclic ketone, shown, with tert-butyl-dimethylsilyltriflate followed by the Eschenmoser salt provided the desired conjugated ketone.32 34 The nature of the actual coupling reaction, which proceeded in a 48% yield, will be further elaborated upon in the next section. 

Furan was identified as a masked sugar synthon for the stereoselective synthesis of d-and L-C4-C5 linked disaccharides. The d- and L-furanyl saccharides (45a and 45b) (1.8 1) obtained from the aldehyde (44) on oxidative umnasking gave the enone system (46a and 46b). Stereoselective reduction of the enone followed by cw-hydroxylation offered the C—C linked d-(47) and L-(48a and 48b) disaccharides. Thus, the de novo construction of a sugar moiety employing furan as a masked sugar synthon led to the distereospecific... 

Amino-3 -deoxyadenosine has been prepared by coupling of the base to a sugar synthon derived from levoglucosenone (see Chapter 9). ... 

Sugars as chiral synthons in the preparation of fine chemicals 232... 

Besides a chiral remote induction for glycosidation, other glycosidic sugar-fused lactones have been also directly used as saccharidic delivery synthons. 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

Five-membered lactones (y-butyrolactones) fused to carbohydrates have proven to be convenient synthons towards branched-chain sugars through opening of the lactone unit. Velaskes et al. [208] described the synthesis of y-butyrolactones... 

The aim of this article is to focus on the diversity of aldonolactones as chiral synthons. The chemistry of aldonolactones was an almost unexplored area when, in 1979, we started our investigations on the reaction of aldonolactones with hydrogen bromide in acetic acid thereby obtaining bromodeoxyaldonolac-tones [1,2]. These compounds have over the years proven to be very versatile compounds for stereoselective synthesis, both in the carbohydrate field, giving access to otherwise less readily obtainable sugars, and as chiral, optically pure synthons in a broader sense within organic chemistry. 

With (2) and furan at 0°C a yield of 50% could be achieved with an exo-endo ratio close to 1 but a very high diastereoisomeric excess (>98 d.e.) for each adduct. The accompanying scheme shows how the endo adduct (4) led to the oxanorbornenone (5), a useful synthon in asymmetric synthesis of sugars. 

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