Alkoxyl migration

Azide addition to open-chain vinyl ethers results in different thermolysis products, depending on the azide and the enol ether employed. In Scheme 180, when R = benzoyl or ethoxycarbonyl, R2 = OR1 and R3 = H, imino ethers are obtained by alkoxyl migration along with minor amounts of azir-idine.269 525 However, when R = Ph and the carbons are fully substituted, the imine (108) is obtained apparently from the reversibility of the imine-diazo compound addition (Section IV,B,2) (Scheme 181).270... 

These rearrangement processes are characterised by a transition state in which the alkoxyl group migrates from the amide nitrogen to the carbonyl carbon (Fig. 17a) and therefore involves HEteroatom Rearrangements On Nitrogen the HERON... 

The formation of anhydrides 101 in the thermal decomposition reactions of TV-acyloxy-TV-alkoxyamides in non-polar mesitylene (Scheme 21, pathway (ii)) can thus be attributed to HERON migration of acyloxyl groups. However, the similarity of Eas for the two isomeric transition states raises the possibility that in these reactions some, or all of the ester might be generated by HERON migration of the alkoxyl substituent (Scheme 21, pathway (iii)). 

Clearly the ESI-MS-MS data reflect the expected migration tendencies of acyloxyl versus alkoxyl moieties in these substrates. 

The characteristics of the HERON transition states for both steps in the thermal decomposition of Ai,Ai -diacyl-Ai,A -diaIkoxyhydrazines facilitate the synthesis of esters. Notably, the alkoxyl group migrations in the internal 8 2 displacement of the 1,1-diazene in the first step, and nitrogen in the second step, do not involve a tetrahedral alkoxide intermediate and both Barton and coworkers and Glover and have utilized... 

A. Hasegawa, M. Goto, and M. Kiso, An unusual behavior Of methyl or benzyl 3-azido-5-0-benzoyl-3,6-dideoxy-a-L-talofuranoside with (dimethylamino) sulfur trifiuoride migration of the alkoxyl group from the C-l to the C-2 position, J. Carbohydr. Chem. 4 627 (1983). 

In the reaction of benzyl 3-azido-5-0-benzoyl-3,6-dideoxy-a-L-talofuranoside (4) with (diethylamino)sulfur trifluoride (DAST) the anomeric alkoxyl group migrated to the C-2 position and a fluorine atom entered into C-l from the P-side to give the 2-benzyl ether 5 having P-l-galacto configuration [101]. Methyl 2-0-benzoyl-5-0-... 

A functionalization that converts C—H bonds to C— NO bonds occurs when nitrite esters are photo-lyzed (the Barton reaction Scheme 4). Again an alkoxyl radical abstracts a 8-hydrogen, and the resulting carbon radical picks up NO. The product nitroso compounds convert easily to oximes. Particularly valuable examples have bMn studied in the steroid field. If the photolysis is performed in the presence of copperfll) acetate the intermediate carbon radical can be oxidized to an alkene, rather than capture NO. If the alcohol whose nitrite ester is photolyzed is part of a cyanohydrin, then the Heusler-Kalvoda reaction occurs, and the product is a ketone with a migrated cyano group (Scheme 5) ... 

Several benzo-fused tetrahydropyrans have been prepared either from aryl-propanhydroperoxides in the presence of iron(II) and copper(II) salts, from the corresponding arylpropanol either by oxidation with FeS20g, or in a photochemical reaction with iodine and mercury(II) oxide (type III radical precursors). These reactions are considered to proceed via alkoxyl radical intermediates which add to the aromatic part of the molecule. Subsequent C- or O-migration in spirocyclized intermediates affords substituted chromanes. However, ionic cyclizations may significantly interfere with the key step of the C-O bond formation [57-59]. 

TABLE 14. lUite Coefficients for Isomerization of Alkoxyl Radicals by H[Pg.236]

This reaction involves the following steps the addition of an alkoxyl group to the carbonyl group, the formation of epoxide, and the migration of the alkyl group, as illustrated below. 

There was no evidence from cell-free assay systems that carvedilol is a scavenger for 02 or NO (Asbrink et al. 2000). Carvedilol did not affect other reactions dependent on NO, e.g. spontaneous of formyl-methionyl-leucyl-phenylalanine-stimulated polymorphonuclear leucocyte migration or hpoxin A4-, fMLP-, or A23187-induced neutrophil cytotoxicity for human umbiUcal vein endothehal cells. Thus, these effects point to the possibility that carvedilol modulates the NADPH oxidase of polymorphonuclear leucocytes but leaves the nitric oxide synthase of phagocytes intact. Carvedilol exerted poor reactivity toward phenoxyl, alkoxyl, and per-oxyl radicals in acetonitrile solution nor did it show an appreciable antioxidant effect against either the peroxyl radical-induced oxidation of methyl Unole-... 

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