Hydroxylations with pyridine oxide

The acid 350 was demethylated with pyridine hydrochloride, then realkylated with benzyl bromide in aqueous potassium hydroxide to give 351. The latter was converted to the diazoketone 352 by the sequential treatment of 351 with oxalyl chloride and etheral diazomethane. Reaction of 352 with concentrated hydrobromic acid gave the bromoketone 353. The latter was reduced with sodium borohydride at pH 8 -9 to yield a mixture of diastere-omeric bromohydrins 354. Protection of the free hydroxyl as a tetrahydro-pyranyl ether and hydrogenolysis of the benzyl residue afforded 355. The phenol 355 was heated under reflux with potassium m/V-butoxide in tert-butyl alcohol for 5 hr to give a 3 1 epimeric mixture of dienone ethers 356 and 357 in about 50% yield. Treatment of this mixture with dilute acid gave the epimeric alcohols 358 and 359. This mixture was oxidized with Jones reagent to afford the diketone 349. 

Conversely, nucleophilic molecules (Nu) [Lewis bases e.g., catechols, hy-droquinones, phenols, alcohols, and thiols (and their anions) aromatic hydrocarbons and amines (benzene, toluene, pyridine, bipyridine)] can be oxidized by (1) direct electron-transfer oxidation [Eq. (12.3)] or (2) by coupling with the oxidation product of H20 (or HO-), hydroxyl radical (HO-) [Eq. (12-4)] ... 

Work on the pyridine-modified ozonization of tetramethylethylene showed that pyridine oxide is not-a product of ozonization (8). Most of the pyridine (— 90% ) remains unchanged during double bond cleavage. Only one mole of acetone, rather than two, is formed for each mole of olefin oxidized. Other work with a disubstituted olefin, trans-4-octene, showed that ozonides are formed in the reaction so that the reaction of pyridine with ozonide to form acid and aldehyde cannot occur (9). An NMR study of trans-4-octene ozonolysis in the presence of pyridine using 1,2-dichloroethane as the solvent shows that aldehyde and hydroxyl-containing material (carboxylic acid, peracid, and other OH species) are formed directly during double bond cleavage. 

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. 

Pyridine is frequently oxidized to pyridine oxide (equation 503) [729, 210, 263], Pyridine oxide is an oxidant capable of hydroxylating aromatic rings [994. But more important, the presence of oxygen on the nitrogen of the pyridine ring reverses the direction of electrophilic substitutions in the pyridine ring. Whereas electrophilic attacks on pyridine occur in P positions, attacks on pyridine oxide occur in a and -y positions. After the introduction of the electrophiles, the pyridine oxide is converted into pyridine by mild reductions, such as treatment with salts of iron or titanium. 

Avoidance by choice of oxygenated starting materials Oxidation through Lithiation and Ort/ro-Lithiation Hydroxylation of Pyridines by ortho-Lithiation Synthesis of Atpenin B Introducing OH by Nucleophilic Substitution Part II - Oxidation of Enols and Enolates Direct Oxidation without Formation of a Specific Enol Selenium dioxide Nitrosation with nitrites Nitrosation with stable nitroso compounds Indirect Oxidation with Formation of a Specific Enol Enone Formation Pd(II) oxidation ofsilyl enol ethers Bromination of enols in enone formation Sulfur and selenium compounds in enone formation Asymmetric Synthesis of Cannabispirenones... 

Dimethyl sulfoxide-Sulfur trioxide [1, 309, before references]. The combination of DMSO and sulfur trioxide, in the form of the pyridine complex, in the presence of trimethylamine oxidizes primary and secondary alcohols in good yield to aldehydes and ketones, respectively.55 The reaction usually is complete within minutes and the products are isolated by acidification and precipitation with water. The reagent also oxidizes allylic alcohols to the corresponding a,fi-unsaturated carbonyl compounds. One advantage over the DMSO-DCC method is that the elaborate purification required when dicyclohexylurea is a product can be dispensed with. Testosterone, with a 17/3-hydroxyl group, was oxidized toA -androstene-3,17-dione very rapidly the 17-epimer required a period of 35 min. 

Diphenylanthracene (DPA) has also been an important substrate in pyridination reactions. Anodic reaction leads to a di-pyridinium ion [32]. Marcoux (1971) found from the use of working curves that although the pyridination data could be described by both ECE and disproportionation processes, the data fitted disproportionation better. In contrast, Blount (1973), with transparent electrodes, and Svanholm and Parker (1973), with rotating disk electrodes, find that pyridination of DPA is an ECE reaction, following the pattern of Sioda s (1968) hydroxylation reaction. Anodic oxidation of DPA in the presence of 2,5-, 2,6-, and 3,5-... 

Pyridine methochloride is converted by aqueous alkali and a mild oxidizing agent (ferricyanide) to a solid compound of formula C6H7NO, which shows no hydroxyl absorption in the infrared. What is a likely structure for this substance and how is it formed Would you expect a similar reaction with pyridine ... 

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