Hexopyranosyl derivatives

The contribution of the hydroxyl group at C-4" in substrate binding to the enzyme is not very large, as the K value for the 4-deoxy-a-D-xylo-hexopyranosyl derivative (100) is only twice that of the apparent Km value of uridine 5 -(a-D-galactopyranosyl pyrophosphate) with mung-bean enzyme.392... 

In the case of hexopyranosyl derivatives, it is generally accepted that the tendency of the side-chain to adopt an equatorial position is dominating in solution, implying that the compounds adopt the4 i(D) or C L) conformations. This effect is observed even when it means that four (9-acetyl groups must adopt axial positions, as in the case of the penta-D-acetyl-a-D-idopyranosyl chloride (5). In the case of a-compounds,the presence... 

Cyclization of Hexopyranosyl Derivatives Containing Reactive Substituents... 

The 3p-hydroxy triterpenes of the oleanane, ursane and lupane series have been converted to the 2-deoxy- and 2,6-dideoxy-a-L-arafcmo-hexopyranosyl derivatives by addition processes applied to L-glucal and L-rhamnal. p-Gluco-sides of diol 78 (R = R = H) have been made with the sugar bonded separately to each of the hydroxyl groups. ... 

Further examples have been provided of the formation of hexopyranosyl derivatives of barbituric acid from unprotected hexoses and barbituric acid (see Vol. 20, p.35).l5l... 

Simple analogs of an aminoglycoside antibiotic, 2,6-dideoxy-4-0- (671) and -5-0-(2,3-dideoxy-2-fluoro-o -D-r/Z>o-hexopyranosyl)streptamine (672) were prepared by coupling of tri-O-acetyl-2-fluoro-D-glucal (666) with cyclitol derivatives 668 or 667 (through 669 and 670) as shown. 

Compound 61 was derived from 3,4,6-tri-0-acetyl-2-deoxy-n-arafet no-hexopyranosyl bromide, which was produced by direct addition of hydrogen bromide to the double bond. The bromodeoxy sugar 62 was, presumably, formed by addition to a 2,3-unsaturated, rearrangement product of the type known118 to arise from tri-O-acetyl-D-glucal in the presence of acid. 

The strain of Agrobacterium tumefaciens capable of oxidizing D-glucose derivatives at C-3 was found to convert uridine 5 -(a-D-glucopyranosyl pyrophosphate) into the a-D-nfco-hexopyranosyl-3-ulose derivative264 (54). 

As shown in Fig. 2, two mechanisms involving an intermediate oxidation may be written for the epimerization at C-4". In the first one (A), the oxidation results in an a-D-xj/lo-hexopyranosyl-4-ulose derivative (96), which is then attacked by a hydride ion from the opposite side of the carbonyl group a change in conformation of the enzyme-intermediate complex seems necessary for such a process. The mechanism depicted under (B) postulates oxidation at C-3", and the resulting hexopyranosyl-3-ulose derivatives (54 and 97) then achieve equilibrium through the common enediol intermediate (98) before undergoing reduction at C-3". Compound 98 may also be formed from the hexopyranosyl-4-ulose ester 96, and in such a manner, both of the pathways may be linked. 

Similar 3,6-dideoxyhexosyl-4-ulose derivatives probably intervene in the conversion of guanosine 5 -(6-deoxy-a-D-lyxo-hexopyranosyl-4-ulose pyrophosphate) into the colitose ester, a reaction is catalyzed by an enzyme from Escherichia coli. 7... 

Deoxy -2,3 - O - isopropylidene - a - l - lyxo - hexopyranosyl - 4 -ulose)thymine (45) was synthesized30 by oxidation of the protected l-rhamnosylthymine 44 with the DCC-Me2SO system. The same reagent proved to be efficient in the synthesis of l-(6-deoxy-2,3-0-isopropyl-idene-a-L-h/xo-hexopyranosyl-4-ulose)-5-fluororouracil48 (47a) and of the 6-azauracil derivative 47b, respectively obtained from 46a and 46b. 

The Me2SO-oxalyl chloride method34 was successfully used13 to obtain2-(6-deoxy-2,3-0-isopropylidene-a-L-h/xo-hexopyranosyl-4-ulose)-8-nitro-t>-triazolo[l,5-fl]pyridine (49a) in 62% yield from the partially protected L-rhamnose nucleoside 48a. Treatment of 49a with formic acid gave the free ketonucleoside 50a. Hydrogenation over 10% Pd-C afforded the amino derivative 49b. 

---