Unprotected sugars

Extensive work has been done by using buta-l,3-dienyl-glycosides of unprotected sugar to study aqueous Diels-Alder reactions and to prepare optically active oligosaccharides [20]. 

Staudinger reaction lead to the preparation of unprotected sugar... 

K. Toshima, Y. Ushiki, G. Matsuo, and S. Matsumura, Environmentally benign aryl C-glycosidations of unprotected sugars using montmorillonite K-10 as a solid acid, Tetrahedron Lett., 38 (1997) 7375-7378. 

Hall, L.D., and Yalpani, M. (1980) Synthesis of luminescent probe-sugar conjugates of either protected or unprotected sugars. Carbohydr. Res. 78, C4. 

In carbohydrate chemistry, the preparation of ethers that are stable in the presence of acids, bases, and aqueous alkali is an important analytical and synthetic tool. The methods used for the etherification of hydroxyl groups51 generally employ reactions of unprotected sugars and glycosides with methyl, allyl, benzyl, triphenylmethyl, and alkylsilyl halides in the presence of a variety of aqueous and nonaqueous bases. 

Unprotected sugar Ac20, acid, arene thiols [21-23]... 

Table 1 Anomeric alkylation of OH-1 of acetylated or unprotected sugars with tert-butyl bromoacetate...

Sugar Yield of anomeric alkylation from acetylated sugars having OH-1 free (%) a f Yield of anomeric alkylation from unprotected sugars (%) a p... 

Amino-2-deoxy aldoses. The behaviour of O-unprotected sugars is exemplified in D-gluco series after basic hydrolysis of the starting 2-benzamidoglycoside followed by buffering the medium with carbon dioxide and treatment with thiophosgene, an intermediate isothiocyanate was obtained.320 However, NMR revealed a temperature-dependent equilibrium of this isothiocyanate with a trans-fused OZT (Scheme 5). 

OZTs from aldoses and ketoses. The second and the oldest-as well as the less studied—method is based on the condensation of O-unprotected sugars with thiocyanic acid, generated in situ from potassium thiocyanate and a protic acid. The reaction involves the free anomeric position and a y- or (5-hydroxyl group able to promote intramolecular cyclization of a transient open-chain isothiocyanate, to form the thermodynamically most stable OZT. The first results obtained by Zemplen in d-gluco and D-Fru series reported the formation of OZTs fused to pyran backbones (Scheme 20).42... 

Peri, F. Cipolla, L. Nicotra, F., Tin-mediated regioselective acylation of unprotected sugars on solid phase. Tetrahedron Lett 2000,41, 8587-8590. 

The present review is focused on the Knoevenagel reaction applied to unprotected sugars. It has been divided into two parts (acidic and basic conditions) in which the arguments are treated in chronological order. 

Many reagents known for the direct conversion of alcohols into halides have been used to achieve site-selective nucleophilic substitution in polyhydroxy compounds [44-46,62]. In some cases, it is possible to effect selective displacements of primary alcohols in the presence of secondary ones, and to achieve differential substitutions of secondary alcohols in unprotected sugars [44,45,64,65] (Scheme 4). 

The chemoselective character of this reaction permits the use of unprotected glycosyl thiols.92 With a similar strategy, an unprotected sugar derivative has been linked to 2-mercaptoacetyl-functionalized dendrimer forming a disulfide bond.93... 

Otherwise, unsaturation may be introduced by use of carbonyl-containing carbohydrate derivatives and carbon nucleophiles that contain alkene (or, if desired, alkyne) functionality, a notable illustration being the tin-or indium-mediated C-l allylation of unprotected sugars. As an illustration, D-arabinose, treated with allyl bromide in aqueous ethanol in the presence of tin gives, after acetylation, 278 in 85% yield.258 In this procedure aldoses react better than do ketoses, and pentoses better than hexoses. More usual is the use of Grignard reactions to give, for example, the octynes 279. 

K. Toshima, G. Matsuo, T. Ishizuka, Y. Ushiki, M. Nakata, and S. Matsumura, Aryl and allyl C-glycosidation methods using unprotected sugars, J. Org. Chem., 63 (1998) 2307-2313. 

Then the other hydroxy groups are benzylated. If (Bu3Sn)20 and BnBr are used for benzylation the 4-OH unprotected sugar 45 is formed, because of a stannyl complex which acts as in the case of 39. 

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