Oxidation with Chromium Trioxide

Oxidation of Alcohols to Aldehydes and Ketones with Chromic Acid Adsorbed on Silica Gel [5J7] 

To a solution of 2.0 g (0.02 mol) of chromium trioxide in 50 mL of water, 20 g of silica gel is added with stirring. The mixture is evaporated in vacuo, and the yellow solid is dried overnight at 100 °C. If kept under vacuum in the dark, the reagent retains its activity for at least a week. 

The reagent (3,g) is suspended in 5 mL of ether, the mixture is stirred magnetically, and a solution of 0.158 g (1 mmol) of 1-decanol is added. After 5 min, the solid is filtered off and washed with three 10-mL portions of ether. The combined filtrates are evaporated to give 0.135 g (86%) of decanal, mp 102-104 C. 

Oxidation of Acetylenic Alcohols to Ketones by the Jones Reagent [578] 

A solution of 10.3 g (0.103 mol) of chromium trioxide in 30 mL of water and 8.7 mL of concentrated sulfuric acfd is added over a period of 2 h to a stirred solution of 15 g (0.126 mol) of 3-oclyiH2-.oI in 30 mL of acetone at 5-10 °C. After being stirred for an additional 30 min, the mixture is diluted with water to 250 mL, and the ketone is isolated by extraction with ether. Distillation of the ether solution gives 11.5 g (73.6%) of 3-octyn-2-one, bp 70.5-71.5 at 11 mm of Hg. 

This oxidation has been used to determine the anomeric nature of sugar residues in oligosaccharides.153 The oligosaccharide is reduced to the alditol, this is acetylated, and the ester is treated with chromium trioxide in acetic acid in the presence of an internal standard. From the sugar analysis of the product, the residues that have survived (and, consequendy, are a-D-linked) may be identified. The 

The method has also been applied to acetylated polysaccharides. Thus, whereas the D-mannopyranose residue in the acetylated lipo-polysaccharide from Salmonella typhi was found to be inert, that in Salmonella Strasbourg was oxidized, demonstrating that the former is a-D-linked and the latter /3-D-linked.154 The D-mannose residues in the acetylated lipopolysaccharide from Klebsiella O-group 3 were found resistant to oxidation, and are consequently a-D-linked.  

Similar results were obtained with the related, Salmonella newport lipopolysaccharide.155 

Oxidation of the carboxyl-reduced and acetylated Pneumococcus type 2 capsular polysaccharide revealed that only one L-rhamnose residue in the hexasaccharide repeating-unit, later demonstrated to have the structure 60, was oxidized and, consequently, /3-L-linked.156 Replacement of 2,3,6-tri-O-methyl-D-glucose in the methylation analysis of the original polysaccharide by 2,3,4,6-tetra-O-methyl-D-glucose in that of the oxidized polysaccharide established that this L-rhamnose residue is linked to 0-4 of a D-glucose residue. The analysis also showed that it was an L-rhamnose residue in the chain (and not the branching L-rhamnose residue) that was /3-linked. 

Trisaccharide 93, isolated from the urine of patients having the 

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We shall describe a specific synthetic example for each protective group given above. Regiosdective proteaion is generally only possible if there are hydroxyl groups of different sterical hindrance (prim < sec < tert equatorial < axial). Acetylation has usually been effected with acetic anhydride. The acetylation of less reactive hydroxyl groups is catalyzed by DMAP (see p.l44f.). Acetates are stable toward oxidation with chromium trioxide in pyridine and have been used, for example, for protection of steroids (H.J.E. Loewenthal, 1959), carbohydrates (M.L. Wolfrom, 1963 J.M. Williams, 1967), and nucleosides (A.M. Micbelson, 1963). The most common deacetylation procedures are ammonolysis with NH in CH OH and methanolysis with KjCO, or sodium methoxide. 

If homolytic reaction conditions (heat and nonpolar solvents) can be avoided and if the reaction is conducted in the presence of a weak base, lead tetraacetate is an efficient oxidant for the conversion of primary and secondary alcohols to aldehydes and ketones. The yield of product is in many cases better than that obtained by oxidation with chromium trioxide. The reaction in pyridine is moderately slow the intial red pyridine complex turns to a yellow solution as the reaction progresses, the color change thus serving as an indicator. The method is surprisingly mild and free of side reactions. Thus 17a-ethinyl-17jS-hydroxy steroids are not attacked and 5a-hydroxy-3-ket-ones are not dehydrated. 

The intermediacy of dipolar species such as 186 has been demonstrated by reaction of enamines with 2-hydroxy-1-aldehydes of the aromatic series (129). The enamine (113) reacts in benzene solution at room temperature with 2-hydroxy-1-naphthaldehyde to give the crystalline adduct (188) in 91 % yield. Oxidation with chromium trioxide-pyridine of 188 gave 189 with p elimination of the morpholine moiety. Palladium on charcoal dehydrogenation of 189 gave the known 1,2-benzoxanthone (129). 

D. Oxidation with Chromium Trioxide-Pyridine Complex General Procedure... 

Oxidation with chromium trioxide in acetic acid cleaved the indole ring to produce intermediate 23 which cyclodehydrated to give prazepam (24). 

ALDEHYDES FROM PRIMARY ALCOHOLS BY OXIDATION WITH CHROMIUM TRIOXIDE 1-HEPTANAL, 52, 5 ALDEHYDES FROM sym-TRITHIANE n-PENTADECANAL, 51, 39 Aldehydes, acetylenic, 54, 45 Aldehydes, aromatic, 54, 45 Aldehydes, benzyl, 54, 45 Aldehydes, olefinic, 54, 45... 

ALDEHYDES FROM PRIMARY ALCOHOLS BY OXIDATION WITH CHROMIUM TRIOXIDE 1-HEPTANAL... 

Collin s oxidation with chromium trioxide-dipyridine complex, or by use of... 

Thiochromenes (e.g., 15) are converted to thiocoumarins by oxidation with chromium trioxide in pyridine. In a similar way thiochromylium salts (e.g., 16) are converted to thiocoumarins by oxidation with either active... 

Oxidations with chromium trioxide.6 Secondary alcohols can be oxidized to ketones in good yields by Cr03 in the presence of catalytic amounts of tetraalkyl-ammonium halides. Yields from oxidation of primary alcohols are moderate. 

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