Glycals branched

The glycal epoxide method turned out to be useful for the construction of complex 2-branched [3-aryl glycosides, which are salient features of the potent antibiotic vancomycin [75a]. Glycal epoxide glycosylation with sodium salts of indoles pro-... 

C-1 Glycals, a relevant type of unsaturated branched sugar, are currently used in synthetic approaches towards C-glycosyl compounds and natural products. In this chapter we present a brief overview of the synthetic strategies employed for their preparation, which have been classified according to the type of starting material employed. Further modifications carried out on the C-1 glycals fall beyond the scope of this review and will not be discussed. 

Mithramycin (24) bears the terminal sugar unit -mycarose. By a similar preparation as for the methyl-branched glycals and the -enantiomer of namely the labile g-mycaral 40 w s obtained ( ). Its NIS condensation with the 3, 4 -Saol disaccharide proceeds smoothly, but the reaction turned out to be rather slow with respect to the stability of the g-mycaral. This results in only a modest yield (12%) of the trisaccharide which was hydrogenolyzed to compound 4. This in turn constitutes an isomer of the trisaccharide sequence in... 

This technique has been successfully used with 2,3- 3,4-, and 4,5- unsaturated sugars to form C-2, C-3, and C-4 branched-chain carbohydrates. Oxygen-substituted double bonds are also good radical acceptors [74,78]. Radicals also add to a glycal double bond to form C-2 branched-chain sugars [79]. 

The syntheses of these new C-branched sugars by regioselective and often stereoselective processes are obtained in better yields than with the precedingly mentioned glycals. The alkylation occurs at the less hindered side however, with methanol, some mixtures of isomeric C-branched sugars are obtained (Scheme 30). 

An interesting synthesis of a methyl-branched anthracycline has been reported [58], The methyl-branched 3-amino-ribo derivative 93 was opened by Hullar-Hanessian reaction to 94, transformed into the exocyclic glycal 95, inverted at C-5 and reacted to the glycal precursor 96. Its treatment with the aglycone and p-toluenesulfonic acid gave the derivative 97. 

Deoxy sugars." Branched 2-deoxy sugars can be prepared by solvomercuration of a glycal followed by the Giese reaction with an electrophilic alkene, with marked preference for formation of an equatorial substituent. 

Randolph, J T, Danishefsky, S J, Apphcation of the glycal assembly strategy to the synthesis of a branched oligosaccharide the first synthesis of a complex saponin, J. Am. Chem. Soc., 115, 8473-8474, 1993. 

The synthesis of the required compound 61 commenced with the D-glucal derivative 57 (Scheme 11.16), the cyclopropanation of which gave the (3-derivative 58 as the major compound [63]. Opening the latter gave the intermediate mercurio derivative 59, which was reduced to 60. Final mesylation at the anomeric center followed by spontaneous elimination yielded the branched-glycal 61. This compound was in turn debenzylated [62]. 

Other complementary methodologies include the preparation of substituted cyclopropanes from glycals using rhodium acetate carbenoid additions [65,66]. Additionally, acid catalyzed cyclopropane opening reactions in alcoholic solutions afford the 2-C-branched-glycosides. These combined reactions were used to prepare a key intermediate in marine diterpene norrisolide synthesis from D-mannose [67]. 

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