Preparation from monosaccharides

Kinoshita, Nakata, and coworkers synthesized erythronolide A (20) (O Fig. 4) [14,15,16,17]. Erythronolide A (20) was divided into three segments, 22 (C1-C6), 23 (C7-C9), and 24 (CIO-C13), of these fragments 22 and 24 were prepared from monosaccharides as chiral starting materials. 

Kishi and coworkers have succeeded in the first total synthesis of halichondrin B (205) [111, 112,113,114,115,116,117,118,119]. They divided halichondrin B (205) into four segments, 206-209 (O Fig. 20), all of which were prepared from monosaccharides. 

The chiral precursors for this type of nucleoside can be prepared from monosaccharide derivatives. Thus, the synthetic sequence started with 2,3,5-tri-O-benzyl-D-arabinose 1 by conversion into the diethylthioacetal 2 followed by benzoylation of the 4-position to give 3. Decarbonylation of 4... 

Esters can be prepared from monosaccharides by standard techniques (Sections 19-9,20-2, and 20-3). Excess reagent will completely convert all hydroxy groups, including the h iacetal function. Eor example, acetic anhydride transforms jS-o-glucopyranose into the pentaacetate. 

Biopolymers are the naturally occurring macromolecular materials that are the components of all living systems. There are three principal categories of biopolymers, each of which is the topic of a separate article in the Eniyclopedia proteins (qv) nucleic acids (qv) and polysaccharides (see Carbohydrates Microbial polysaccharides). Biopolymers are formed through condensation of monomeric units ie, the corresponding monomers are amino acids (qv), nucleotides, and monosaccharides, for proteins, nucleic acids, and polysaccharides, respectively. The term biopolymers is also used to describe synthetic polymers prepared from the same or similar monomer units as are the natural molecules. 

Easily prepared from the appropriate monosaccharide, a glycal is an unsatu-rated sugar with a C1-C2 double bond. To ready it for use in potysaccharide synthesis, the primary -OH group of the glycal is first protected at its primary -OH group by formation of a silvl ether (Section 17.8) and at its two adjacent secondary - OH groups by formation of a cyclic carbonate ester. Then, the protected glycal is epoxidized. 

Functionalisation at OH-2 thus led to a series of 1,2-bisfunctionalised platforms prepared from two model lactones, one monosaccharidic (a-gluco) and one disaccharidic (ot-malto). Allylamine and propargylamine were used as model functional appendages due to the wide scope of their possible subsequent chemistry. Their addition on the lactones occurs in high yield and very mild conditions (room temperature, THF, no catalyst,... 

What are the products of oxidation and reduction of the monosaccharides Mucic add, of which the preparation from d-galactose has been described, is optically inactive and incapable of resolution, just like duldtol, the corresponding hexahydric alcohol. As the formula shows, the four asymmetric carbon atoms of galactose form two pairs (2, 5 and 3, 4) having the same substituents but opposite arrangement in space, and when the carbon atoms 1 and 6 become alike, inactive forms are produced by intramolecular compensation, as in mesotartaric acid. 

Synthesis. The synthases are present at the endomembrane system of the cell and have been isolated on membrane fractions prepared from the cells (5,6). The nucleoside diphosphate sugars which are used by the synthases are formed in the cytoplasm, and usually the epimerases and the other enzymes (e.g., dehydrogenases and decarboxylases) which interconvert them are also soluble and probably occur in the cytoplasm (14). Nevertheless some epimerases are membrane bound and this may be important for the regulation of the synthases which use the different epimers in a heteropolysaccharide. This is especially significant because the availability of the donor compounds at the site of the transglycosylases (the synthases) is of obvious importance for control of the synthesis. The synthases are located at the lumen side of the membrane and the nucleoside diphosphate sugars must therefore cross the membrane in order to take part in the reaction. Modulation of this transport mechanism is an obvious point for the control not only for the rate of synthesis but for the type of synthesis which occurs in the particular lumen of the membrane system. Obviously the synthase cannot function unless the donor molecule is transported to its active site and the transporters may only be present at certain regions within the endomembrane system. It has been observed that when intact cells are fed radioactive monosaccharides which will form and label polysaccharides, these cannot always be found at all the membrane sites within the cell where the synthase activities are known to occur (15). A possible reason for this difference may be the selection of precursors by the transport mechanism. 

RCM is now an important method for the building of oxepines. It should be noted that chiral functionalized precursors can be readily prepared from natural monosaccharides. Thus, glucofuranose derivatives can be transformed to tetrahydrooxepines (Scheme 21) in only three steps olefmation, O-allylation, and RCM <1998TL3025>. 

Preparation of Monosaccharides and Oligosaccharides from Bacterial Polysaccharides... 

Apart from sotolon, the other compounds in Fig. 5 can be explained as the products of a Maillard reaction, and their carbon skeletons simply originate from the active Amadori intermediate in other words, they still preserve the straight carbon chain structure of monosaccharides. In spite of being a simple Cg lactone, sotolon has a branched carbon skeleton, which implies another formation process in the Maillard reaction. Sulser e al.(6) reported that ethyl sotolon (ll) was prepared from threonine with sulfuric acid, and that 2-oxobutyric acid, a degradation product of threonine, was a better starting material to obtain II. This final reaction is a Claisen type of condensation, which would proceed more smoothly under alkaline conditions. As we(lO) obtained II from 2-oxobutyric acid (see figure 6) with a high yield in the presence of potassium carbonate in ethanol, a mixed condensation of 2-oxobutyric and 2-oxo-propanoic (pyruvic) acids was attempted under the same conditions, and a mixture of sotolon (22% yield) and II were obtained however, the... 

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