Sugars unsaturated, formation

Nitrone-Nitrile Oxide Cycloaddition. Unsaturated sugars have been used for the simultaneous formation of carbon-carbon and carbon-oxygen bonds in a cir-relation. One of the best ways to achieve this transformation is the cycloaddition of nitrone or nitrile oxides. The cycloaddition of nitrones with olefins has been reviewed [133]. The regioselec-tivity is almost complete when using activated double bonds, such as enone, enelactone (see compound 98, Scheme 35), or esters. 

Alternatively, Pd(0) adds oxidatively to the double bond of a glycal derivative resulting in the formation of a ir-allyl complex, which may react with carbon nucleophiles to give C-glycosides with a double bond between C(2) and C(3).26 A rt-allyl complex may also be formed starting from a Ferrier rearrangement product (2,3-unsaturated sugar derivative).22... 

Several examples of the addition of trifluoromethyl hypofluorite to an unsaturated sugar have been reported. " " " Some of these examples are illustrated by the formation of... 

The oxyamination of C-2 C-3 and C-3-C-4 unsaturated sugars gave mixtures of regioisomeric hydroxy amides, i.e., 12, 13 and 14, 158s-91. The cis dihydroxylation was often a competitive reaction, especially for C-2-C-3 unsaturated sugars, e.g., formation of 1888-91. The products were generally separated by column chromatography. 

The elimination of bis-sulfonates, described in the context of 2,3-unsaturated sugars (Scheme 6.40), was adapted to the formation of 3,4-unsaturated sugars [73]. In addition to this method, the direct conversion of vicinal hydroxyl groups was applied to a derivative of mannose. As shown in... 

SCHEME 6.41 2,3-Unsaturated sugars are available via formation of enamines. 

Formation of 2,3-unsaturated sugars was illustrated by the representative examples. Application of any other general methods which are described in Sect 2 should be considered when planning the synthesis of such derivatives. 

An example of chemically induced -elimination is shown in O Scheme 15. Treatment of the 3,4-0-isopropylidene-D-galactoside with a strong base induced the elimination of acetone with formation of the 3,4-unsaturated sugar [31]. 

An interesting methodology to highly oxygenated p)razolidines and indolizidines from 2,3-unsaturated sugar lactones was presented by Chmielewski [84]. The synthesis was initiated by a 1,4-addition of nitrogen nucleophiles to unsaturated lactones 93, which resulted in formation of appropriate heterocyclic derivative 94 (O Scheme 38). 

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