The Oxidation of Primary Hydroxyl Groups

The oxidation of D-glucose to D-gluconic acid is also readily carried out by use of a platinum-on-carbon catalyst (a substantially more-active catalyst) in the presence of an equivalent of alkali. With the aid of the same catalysts, n-galactose, D-mannose, D-xylose, and L-arabinose can be converted to the corresponding aldonic acids. By this method, the pentoses are oxidized more rapidly than the hexoses. A reaction time of only 45 minutes is required at 22°, whereas the oxidation of D-glucose is complete only after five hours. 

The application of more-severe reaction-conditions, such as a reaction temperature of 50° with a platinum-on-carbon catalyst, results in attack on the primary hydroxyl group at C-6 thus, a 54% yield of D-glucaric acid is obtained from D-glucose. Aldoses are selectively attacked at the carbonyl group at room temperature, whereas, at higher temperatures, the hydroxyl group on C-6 is also oxidized, with the production of a glycaric acid. 

L-Ascorbic acid, the oxidation of which is catalyzed by such metal ions as iron or copper, is very readily oxidized under very mild conditions in the presence of a platinum-on-carbon catalyst. At 0°, it is transformed into dehydro-L-ascorbic acid by exposure to air for 80 minutes. The reaction is quantitative and affords no substantial amount of byproduct.  

Blocked L-sorbose derivatives can be catalytically oxidized in high yields to the corresponding L-xyZo-hexulosonic acid derivatives, using platinum-on-carbon. From the catalytic oxidation of 2,3 4,6-di-0-isopropylidene-L-sorbose, an almost quantitative jdeld of 2,3 4,6-di-0-isopropylidene-L-a /lo-hexulosonic acid has been reported. Similarly, methyl a-L-sorboside gives a quantitative yield of methyl a-L-xylo-hexulosidonic acid. Attempts to prepare L-xylo-hexulosonic acid from this product result in extensive 

The synthesis of n-glucuronic acid has already been reported in detail in this Series by Mehltretter. According to Mehltretter and Coldn, the most-favorable starting-material is 1,2-0-isopropylidene-a-D-glucofuranose 

---

For the oxidation of primary hydroxyl groups, three times the amount of nitrite is required when compared with nitrate, and three times as much of the toxic NO would be formed. This oxidation procedure has been applied to the glucans cellulose, amylose, and pullulan. A study of this system with cyclomaltoheptaose (/J-cyclodextrin) showed that the reaction is autocatalytic. 

Oxidation of carbohydrates The oxidation of isolated secondary hydroxyl groups of carbohydrates with CrOs complexed with pyridine is often unsatisfactory. This oxidation can be effected generally in 70-85% yield with PCC reaction in CH2CI2 is very slow, but proceeds readily in refluxing benzene. The various oxidants based on DMSO are not useful. PCC is also effective for oxidation of primary hydroxyl groups of carbohydrates Collins reagent is also effective in this case. 

Abstract Cellulose is the most important biopolymer in Nature and is used in preparation of new compounds. Molecular structure of cellulose is a repeating unit of p-D-glucopyranose molecules forming a linear chain that can have a crystallographic or an amorphous form. Cellulose is insoluble in water, but can dissolve in ionic liquids. Hemicelluloses are the second most abundant polysaccharides in Nature, in which xylan is one of the major constituents of this polymer. There are several sources of cellulose and hemicelluloses, but the most important source is wood. Typical chemical modifications are esterifications and etherifications of hydroxyl groups. TEMPO-mediated oxidation is a good method to promote oxidation of primary hydroxyl groups to aldehyde and carboxylic acids, selectively. Modified cellulose can be used in the pharmaceutical industry as a metal adsorbent. It is used in the preparation of cellulosic fibers and biocomposites such as nanofibrils and as biofuels. 

As stated, tertiary amines catalyze both the hydroxyl/isocyanate and the water/isocyanate reactions. One-shot foams utilizing primary hydroxyl-terminated polyesters as well as all types of prepolymer foams require tertiary amine catalysis only. Polypropylene ether one-shot foam formulations based on triols, in part, because of their low viscosity (about 300 cP versus 10000-30000 cP for polyesters or prepolymers) require the use of tertiary amine-metal catalyst combinations, even if the percentage of primary hydroxyl groups in the polyether is increased by capping with ethylene oxide. This is because of the relatively low polypropylene glycol activity. 

The formation of side product ester 10 in oxidation of diol 5 (Table 3.1, entry 4) can be explained with the intermediacy of aldehyde as well. Upon generation of aldehyde 11 from 5, intermolecular nucleophilic attack of the carbonyl group by the free hydroxyl group produced hemiacetal 12, oxidation of which led to carboxylic ester 10 (Scheme 3.2). Formation of similar by-products during oxidation of primary hydroxyl groups in carbohydrates has been observed previously. ... 

The aldehyde function at C-85 in 25 is unmasked by oxidative hydrolysis of the thioacetal group (I2, NaHCOs) (98 % yield), and the resulting aldehyde 26 is coupled to Z-iodoolefin 10 by a NiCh/CrCH-mediated process to afford a ca. 3 2 mixture of diaste-reoisomeric allylic alcohols 27, epimeric at C-85 (90 % yield). The low stereoselectivity of this coupling reaction is, of course, inconsequential, since the next operation involves oxidation [pyridinium dichromate (PDC)] to the corresponding enone and. olefination with methylene triphenylphosphorane to furnish the desired diene system (70-75% overall yield from dithioacetal 9). Deprotection of the C-77 primary hydroxyl group by mild acid hydrolysis (PPTS, MeOH-ClHhCh), followed by Swem oxidation, then leads to the C77-C115 aldehyde 28 in excellent overall yield. 

Bishop,141 interested in the relative reactivity of secondary hydroxyl groups without the complication of the veiy reactive primary hydroxyl group, condensed D-xylose under the conditions established by Ricketts and coworkers. A nondialyzable polyxylose having Pn 10-15 (by hypo-iodite oxidation) was isolated in 5.6% yield. Methylation and hydrolysis of a mole of q fraction having M + 55 2° gave 2,3,4-tri-O-methyl-D-xylose (3 moles), 2,3-di-O-methyl-D-xylose (7 moles), 2,4-di-O-methyl-D-xylose (1 mole), 3-O-methyl-D-xylose (5 moles), and D-xylose (1 mole). If, as was assumed, the D-xylose residues are in the pyranoid form, the ratio of (1—>4) (1— 2) (1— 3) bonds is 6.5 3 1. 

---