O-Substitution-rearrangement

N-Substitution (s. a. N-Alkyla-tion, N-Arylation) 9, 539 o-Substitution rearrangement... 

A useful synthesis of allylstannanes from primary alcohols involves conversion of the alcohols into their O-substituted 5-methyl carbonodithioates, thermolysis to effect [3,3] rearrangement to the corresponding 5-substituted 5-methyl carbonodithioates, and treatment with a trialkyl-tin hydride under free-radical conditions to form the allylstannane21. This procedure has been applied to the synthesis of functionalized allylstannanes including (5)-( )-4-(benzyloxy)-2-pen-tenyl(tributyl)stannane22. 

The conversion of anomerically linked enol ethers 29 into either the cis- or trans-substituted pyranyl ketones with high diastereoselectivity and yield involves a Lewis acid-promoted O —> C rearrangement (Scheme 19) <00JCS(P1)2385>. Under similar conditions, homoallylic ethers 30 ring open and the oxonium ions then recyclise to new pyran derivatives 31. Whilst the product is a mixture of alkene isomers, catalytic hydrogenation occurs with excellent diastereoselectivity (Scheme 20) <00JCS(P1)1829>. 

The N-diazeniumdiolates are quite photosensitive. Studies of various 02-substi-tuted compounds, both alkyl and aryl, revealed a primary photochemical reaction involving cleavage of the N=N bond to yield a nitrosoamine and an O-substituted nitrene which rearranges to a C-nitroso compound (Scheme 3.31), the latter is often isolated as the oxime [224]. 

O-Substituted phenylhydroxylamines also undergo rearrangement to give the 2-isomers. For example O-(arenesulphonyl) phenylhydroxylamines 47 readily form the 2-sulphonyl derivatives 48. Experiments with 180-labelled compounds led to the suggestion54 of a mechanism involving an ion pair which has only a very short lifetime. 

Pd-catalyzed reactions proceed via 7i-allyl complexes, which at room temperature isomerize via a ti-o-ti rearrangement. As a consequence, branched as well as Z-and B-linear starting materials yield the same products, with memory effects being minimal at room temperature [16], The isomerization processes of (allyl)Ir complexes are usually slow, and accordingly any memory effects are pronounced. The configurational stability of (allyl) Ir intermediates of the Ir-catalyzed allylic substitution was studied by an investigation of substitutions at nonracemic allylic substrates (Scheme 9.5). 

Alternatively, oximes may be converted to O-substituted oximes (typically O-tosyl oximes) making the rearrangement much easier. Sometimes, these oxime derivatives rearrange spontaneously under the condition of their formation and cannot be isolated. Usually, O -tosyl ketoximes rearrange smoothly with exclusive anti migration. Relative to the acid-promoted Beckmann rearrangement, the rearrangement of O -tosyl oximes is much milder and specific. 

Anomalous Fischer cyclizations are observed with certain o-substituted aryl-hydrazones, especially 2-alkoxy derivatives[l]. The products which are formed can generally be accounted for by an intermediate which would be formed by ( p.s o-substitution during the sigmatropic rearrangement step. Nucleophiles from the reaction medium, e.g. Cl or the solvent, are introduced at the 5-and/or 6-position of the indole ring. Even carbon nucleophiles, e.g. ethyl acetoacetate, can be incorporated if added to the reaction solution[2]. The use of 2-tosyloxy or 2-trifluoromethanesulfonyloxy derivatives has been found to avoid this complication and has proved useful in the preparation of 7-oxygen-ated indoles[3]. 

A similar rearrangement of 2-O-substituted methyl 3,4-dideoxy-o -D-en/t/iro-hex-3-enopyranosides (184a,b) into 4-C-substituted a-D-en/tforo-hex-2-enopyranosides (185a,b) was used for the chiral synthesis... 

Because substituted anilines are widely available, they are ideal starting materials for the synthesis of indoles. Although anilines are often the precursors of the hydrazones used in the Fischer cyclization, more direct methods for conversion of anilines to indoles would be highly desirable. Such a process would, in general, have to involve some method for specific o -substitution of the aniline derivative. In the most successful method of this type which has been developed to date, a specific rearrangement effecting o-substitution is the key to a synthetic scheme which constructs indoles from anilines via anilinosulfonium ions. The procedure, which in appropriate circumstances can be carried out without isolation of the intermediates, involves oxidation of the aniline to an A-chloroaniline, reaction with a thiomethylmethyl ketone and a weak base, followed by cyclizative condensation (equation... 

The effect of ring substitution on the O-ncophyl rearrangement of 1,1-diarylalkoxyl dadicals has been reported.174... 

Esters Rather weak intensity Base peak3 at m/e equal to the mass of R-C=0 peaks at m/e equal to the mass of 0=C-0R, the mass of OR and R McLafferty rearrangement possible in case of (1) presence of a beta hydrogen in R (peak at m/e equal to the mass of R-C( OH)OH, and (2) presence of a gamma hydrogen in R peak at m/e equal to the mass of (CH2=C( OH)OR) loss of 42 (CH2=C=0) in case of benzyl esters loss of ROH via the ortho effect in case of o-substituted benzoates... 

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