Hydroxyl group, alkyl stabilization

The ease of selective protection can be explained in terms of maximum anomeric stabilization such that both anomeric methoxy groups of the product are oriented in the axial position with respect to the central dioxane ring. All four sterically demanding alkyl substituents are then placed in the favored equatorial positions. The poor selectivity for the protection of glucose results from the existence of two pairs of trans-hydroxyl groups in this sugar. 

The relative stability of the oxiranes to bases at room temperature allows the selective ammonolysis of carboxylic ester groups with ammonia in methanol,49 and, conversely, free hydroxyl groups may be esterified with acid chlorides or anhydrides in pyridine without affecting the oxirane ring. Hydroxyl groups may also be converted into alkyl ethers by using either the Haworth50 or the Purdie-Irvine procedure.5152... 

The strongest evidence in favor of propagation at the transition metal-alkyl bond is the existence of one-component, that is, metal-alkyl-free polymerization catalysts. Of these systems the Phillips catalyst was studied most thoroughly because of its commercial importance. Originally it was believed that Cr(VI) ions stabilized in the form of surface chromate and perhaps dichromate resulting from the interaction of Cr03 with surface hydroxyl groups above 400°C are the active species in polymerization 286,294... 

The treatment of 2-imino-4-alkyl-2//-pyrido[l,2-a]pyrimidine hydrochlorides 288 with sodium carbonate in 95% ethanol at room temperature for 10 hours gave 4-hydroxyl-4-alkyl derivatives 289 as hydrates [88JCS(P1)975]. If the reaction mixtures were refluxed for 24 hours, then ring-opened products 290 were isolated. The presence of a hydroxyl group at position 9 increased the stability of bicyclic 289. 

Alkylations with sodium hydride in dipolar aprotic solvents usually proceed very efficiently and with high yields [50]. Dibenzyl ether, the major byproduct when alkali hydroxide is used, is not formed even under forcing conditions necessary for benzylation of sterically hindered hydroxyl groups [51]. Under limiting conditions of base, the rate of formation, the stability, and the rate of further reaction of competing alkoxides determine the distribution of products. 

Heddle and Percival48 have demonstrated that the C2- and C6-hydroxyl groups of glucosides possess acidic properties, and the instability of aryl jS-D-glucosides as compared to alkyl /J-D-glucosides is undoubtedly related to the high stability of aryloxide ions as compared to that of alkoxide ions.33... 

The chemical transformations that lead to the conversion of cellulose to mixed polysaccharides differing from cellulose in the conformation of the pyranose ring and the number and configuration of the hydroxyl groups of the repeating unit of the macromolecule, may exert a considerable effect on the structure of the material as well as on its important chemical properties (rate of acetylation and O-alkylation of OH groups, stability of the acetal linkage) and physicochemical indices (solubility of modified preparations of cellulose and cellulose ethers and esters). 

A para alkyl substituent may function in the same manner as a hydroxyl group in promoting loss of water from the phenylhydroxylamine and stabilizing the quinonoid intermediate. 5-Nitroacenaphthene is thus in acid solution reduced to 5-aminoace-naphthene in a six-electron reduction [144]. 

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