Acetals furanoses

Notice that the eclipsed conformation of d ribose derived directly from the Fischer pro jection does not have its C 4 hydroxyl group properly oriented for furanose ring forma tion We must redraw it m a conformation that permits the five membered cyclic hemi acetal to form This is accomplished by rotation about the C(3)—C(4) bond taking care that the configuration at C 4 is not changed... 

A particular carbohydrate can mterconvert between furanose and pyra nose forms and between the a and (3 configuration of each form The change from one form to an equilibrium mixture of all the possible hemi acetals causes a change m optical rotation called mutarotation... 

Structurally, 0-glycosides are mixed acetals that involve the anomeric position of furanose and pyranose forms of carbohydrates. Recall the sequence of intermediates in acetal formation (Section 17.8) ... 

Acetyl-5-deoxy-5-iodo-l, 2-O-isopropylidene-p - l - arabinofuranose (38). Acetylation of 5-deoxy-5-iodo-l,2-0-isopropylidene-/ -L-arabino-furanose (37) (27) in pyridine as described above for the D-xylo-isomer afforded the 3-acetate (38) as a colorless sirup which did not crystallize. [ ]D24 + 13.7° (c, 3.5). Anal Calcd. for C10H15IO5 C, 35.1 H, 4.4. Found C, 35.0 H, 4.5. 

Online detection using 4H nuclear magnetic resonance (NMR) is a detection mode that has become increasingly practical. In a recent application, cell culture supernatant was monitored on-line with 1-dimensional NMR for trehalose, P-D-pyranose, P-D-furanose, succinate, acetate and uridine.33 In stopped-flow mode, column fractions can also be analyzed by 2-D NMR. Reaction products of the preparation of the neuromuscular blocking compound atracurium besylate were separated on chiral HPLC and detected by 4H NMR.34 Ten isomeric peaks were separated on a cellulose-based phase and identified by online NMR in stopped-flow mode. 

As conformational anomeric effects represent only a fraction (—1 to —3 kcal/mol) of the global enthalpic anomeric effect or gem-dioxy stabilizing effect (—6 to —17 kcal/mol), additional factors have to be taken into account. Depending on substitution, steric factors can affect the relative stability of acetal conformers. Dubois et al.2S have demonstrated that in furanose pyranose derivatives a bulky substituent at the furanose... 

Lewis-acid promoted C-allylations of furanose acetates 101 with allyl-(trimethyl)silane are a-selective and the selectivity is governed by the alkoxy group at C-3. The lowest energy conformers bear the 3-alkoxy group in a pseudoaxial orientation (Scheme 35). To a lesser extent, the 2-substituent... 

Monomeric carbohydrates in their cyclic form (furanoses and pyranoses) are hemiacetals, which, to become acetals, form 0-glycosyl conjugates. The C-atom C(l) that bears two O-atoms is the reactive, electrophilic center targeted by glycosidases. Nonenzymatic hydrolysis is also possible, although, as a rule, under physiological conditions of pH and temperature, the reaction is of limited significance. 

The insertion of a second oxygen atom in a sugar furanose ring in essence converts that moiety to an acetal. This modiftcation leads to another false substrate for viral reverse transcriptase. Glycosilation of the silylated purine (46-1) with chiral dioxolane (46-2) prepared in several steps from anhydromannose in the presence of ammonium nitrate affords the coupling product as a mixture of anomers (46-3). The mixture of products is then separated on a chromatographic column. The desired diastereomer (46-3) is reacted with ammonia to afford the product (46-4)... 

Only acetal C —OMe is hydrolyzed the other ether linkages are stable.) The unmethylated OH s of (A) are those involved in ring formation. They are C and C" if the ring is five-membered (furanose) or C and C for a six-membered ring (pyranose). See Fig. 22-13. (Wavy lines denote cleavages.)... 

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