Sugars synthetic utility

The synthetic utility of the Henry reaction is shown in Scheme 3.1, where (3-nitro alcohols are converted into (3-amino alcohols, amino sugars, ketones and other important compounds. 

It is not as yet possible to assess the potential synthetic utility of this type of reaction properly, but, clearly, the apparent strength of the Grignard reagent-sugar complex suggests that detailed studies of the nature of this complex may prove rewarding. 

The reactions described clearly demonstrate the potential synthetic utility of asymmetric reductions with complexes of metal hydrides with sugars, but also indicate that much work is needed before the precise nature of the complexes is fully understood. 

The main interest in unsaturated sugars prepared by the Wittig reaction (and described in this Section) has concerned their synthetic utilization. The pathways of the latter depend on the structure of the unsaturated precursor. In the case of C-glycosylated alkenes, addition to the double bond (mainly hydration and hydrogenation) leads to a branched-chain or long-chain sugar, although correct choice of the reactant to be added may provide a variety of derivatives. 

Synthetic utilization of sugar epoxides with the use of known Wittig modifications has been shown to be very useful.92,99 Thus, the alkene 15a and its 3-O-benzyl analog were resynthesized,92 albeit in moderate yields, by the interaction of epoxides 236a and 236b, respectively, with triphenylphosphine in boiling N,N-di-methylformamide. 

Hanessian, S, Plessas, N R, Reaction of O-benzylidene sugars with iV-hromosuccinimide. II. Scope and synthetic utility in the methyl 4,6-O-benzylidenehexopyranoside series, J. Org. Chem., 34, 1035-1044, 1969. 

The incorporation of anomeric carbenes in C-glycoside technology has not received much attention. However, as shown in Figure 7.18, anomeric carbenes provide the only known method for the direct cyclopropanation of anomeric centers to date. Advantages to this method include general synthetic accessibility of the carbenes when benzylated sugars are utilized. 

Of aU the anhydro sugars, 1,6-anhydrohexopyranoses are the most studied and are of the greatest synthetic utility. 1,6-Anhydrohexopyranoses have been the subject of several comprehensive reviews, which the interested reader should consult for more details [3,4,5,6]. 1,6-Anhy-dro- 8-D-glucopyranose (1 0 Fig. 1) (also known as glucosan or levoglucosan on account... 

Dotz and coworkers prepared several interesting and novel glycosylidene carbenes (259-261) by reaction of lithiated glycal 258 with the appropriate metal carbonyl derivative. The synthetic utility of the formed carbenes was demonstrated by reaction with 3-hexyne to give a mixture of complexed and uncomplexed adducts 262 and 263, respectively. The anomeric chromium carhene 264 was converted to 265 hy exposure to ethoxy ethyne (Scheme 48) [70]. The same workers have carried out a similar chromium-mediated henzannulation (266 —> 268), this time with the chromium on the aromatic fragment [71]. Other sugar-hased carhenes have also been prepared [72]. 

The kinetics of the reaction of hydroxyl groups of monosaccharides and polyols with benzyl isothiocyanate has been checked by Augustin and Balaz using UV spectroscopy.289 However, because of the reversibility of the reaction, this procedure is not. of synthetic utility. O-Sugar thiocarbamates have been more conveniently prepared by aminolysis of thiocarbonate derivatives290,291 or by thiocarbamoylation of free hydroxy] groups with 1,1 -tbiocarbonyldiimidazoie.7 286 288... 

The addition of the Grignard reagents to intramolecular displacement of a halide appears to be a convenient, but challenging way leading to the five- and six-membered cyclic imines, compounds of high synthetic utility, e.g. in the context of total synthesis of imino sugars. However, this approach has not been yet extensively studied - only several reports can be found in the literature. 

As described above, a novel preparation of homochiral mono-, di-, as well as trifluoromethylated aldol structures starting from the readily available chiral pool compound, D-glucose, has been demonstrated. Further synthetic utilization of optically active cyclic materials as well as investigation of the similar route with different sugars is under way in our laboratory. 

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