Imino esters rearrangement

Imino esters with any or all of the three groups being alkyl also rearrange, but they require catalysis by H2SO4 or a trace of methyl iodide or methyl sulfate. The mechanism is different, involving an intermolecular process.This is also true for derivatives for formamide (Ar = H). 

The formation of tluorinated Q -hydroxy-jS-imino esters (180) by treatment of fluorinated imino ethers (179) with lithium 2,2,6,6-tetramethylpiperidide has been reported. A possible explanation for this interesting intramolecular rearrangement is proposed in Scheme 64. Acyclic imides derived from primary benzylic amines and amino acid esters have been found to undergo a novel nitrogen to carbon acyl migration via a base-generated carbanion to yield the corresponding a-amino... 

Imino esters with any or all of the three groups being alkyl also rearrange, but they... 

Under the influence of bases IV-acylaminosulfonium salts are rearranged to imino esters (257 equation 141). Photolysis of aromatic or heterocyclic azides in the presence of alkoxides affords imino esters, e.g. (258 equation 142). ... 

Organoaluminum compounds, reaction with imino carbocations, 66, 189 Orthoester Claisen rearrangement, 66, 22 Orthoformic acid, triethyl ester, 65, 146 Oxalic acid, diethyl ester, 65, 146... 

N-t-butyl derivatives (e.g., 58) can give satisfactory results but are prone to a number of side reactions, including that just mentioned.173-175 Diacyl-amides (63) may arise as by-products from the ketoketenimine via an imino-anhydride (as shown in Scheme 11) rather than from the enol ester.176 Tetrahydrobenzisoxazolium ions (59) and related compounds are promising as far as freedom from rearrangement and lack of promotion of racemiza-tion are concerned, but they do not appear to have been evaluated in actual peptide syntheses.171 Although beyond the scope of this review, benzisox-azolium salts have also been applied as reagents for peptide synthesis.177,178... 

Assorted anions. These are generated by deprotonation of allylic halides," chloromethylphosphonic esters, conjugated hydrazones, chiral carbamates,unsaturated a-aminonitriles, phosphonamides," and sulfonamides. The dianions derived from tu-haloalkanecarboxylic acids cyclize, and this reaction forms the basis of a synthesis of V-Boc cyclic imino acids. The conjugate bases of 2-(arylmethoxy)-methyl-2-oxazolines are unstable as [2,3]sigmatropic rearrangement takes place even at — 75°C. 

The synthesis of a-imino aldehydes through the thermal [l,3]-rearrangement of 0-alkenyl benzophenone oximes has been reported. A copper-mediated C-O bond coupling between benzophenone oxime and alkenyl boronic acids provides access to 0-alkenyl oximes and a Horner-Wadsworth-Emmons olefination applied to the a-imino aldehyde products gives y-imino-a, -unsaturated esters (Scheme 166). ... 

Shin was unable to observe enamino-imino tautomerism in the case of dehydrovaline compounds (253, 373). E. Ohler and U. Schmidt (287, 350) were able to convert a-iminocarboxylic acid esters into a-enaminocarboxylic acid esters (42) by preparing the hydrochlorides of the former, which slowly rearranged into the hydrochlorides of the latter. This reaction is the simplest means of preparing a-enaminocarboxyhc acid esters, as the a-iminocarboxylic acid esters obtained by N-chlorina-tion/dehydrochlorination of amino acid esters can be rearranged directly, without intermediate purification. 

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