Enamides tautomerization

Methyl-3-oxidopyrazinium salt (421) undergoes 1,3-dipolar cycloaddition with a variety of dipolarophiles to give the bicyclic compounds (422) existing in enamide tautomeric form (87TL2187). 

The A-acylimine-enamide tautomerism of methyl 2-acetamidoacrylate has been studied641 by means of ab initio calculations. A 13C NMR investigation has been undertaken to study the tautomerism between the hydrazone imine and diazenylen-amine forms of 3-(arylhydrazono)methyl-2-oxo-l,2-dihydroquinoxalines,642 and the effects of temperature and side-chain on the imine-enamine tautomerism in quinoxalinone and pyridopyrazinone systems have been studied.643 A detailed... 

A possible side reaction in A-acyliminium chemistry is caused by deprotonation, giving rise to the formation of an enamide. Though this tautomerization is in principle reversible in acid media, this is not always the case. The enamide may react as a nucleophile with the /V-acyliminium ion still present, to produce dimers14. 

Condensation of an amino acid-derived anilide and a /3-ketoamide afforded l,4-benzodiazepin-2-ones in which the initially formed imine tautomerizes to an exocyclic enamide (Scheme SO) <2001TL3227>. Only the (Z)-isomer of the enamide was isolated, assigned based on NOE data, and presumably reflecting stabilization by an intramolecular H-bond between the ring NH and exocyclic amide carbonyl. 

Iminopropadienones are highly reactive species that have been used as three-carbon fragments in the synthesis of 1,4-diazepines <2002JOC2619>. Aryl- and neopentyl-substituted iminopropadienone derivatives are stable at 25 °C and the latter, 99, reacted with iV,iV -dimethyl-l,2-diaminoethane to afford a 55% yield of the l,4-diazepin-5-one 100, a compound that partially tautomerized to the enamide 101 upon Kugelrohr distillation. 

A two-electron oxidation of N-acetyltyrosine ethyl ester with mushroom tyrosinase, or with periodate, afforded the N-acetyIdopa ester 142, together with the (Z)-enamide 145 and the 6-acetoxydopa amide 146 (Fig. 40) (284). It is assumed that 145 originates from dopaquinone 143 via 144 by tautomerization. Michael addition of acetate to quinone 143 is believed to be the origin of 146. The formation of quinone methide 144 from dopa ester 142 by tyrosinase is reminiscent of the formation of iminochromes and quinone methides catalyzed by this enzyme in their formation from a-methyl dopa ester (285), and such reactions may well occur in mammalian systems. 

N-Sulfonyl imines derived from enolizable aldehydes and ketones are, in principle, capable of tautomerization to the corresponding ene sulfonamides. There has been no systematic study of this process, probably due in large part to the fact that only relatively few sulfonyl imines of this type have to date been prepared and characterized. Trost and Marrs [17], however, have found that aldehyde 35 on conversion to imine 36 using tellurium reagent 3 led to enamide 37 upon workup (Scheme 9). Similarly, aldehyde 38 was converted to imine 39 which tautomerized to 40 upon isolation. 

The oxygen-substituted 1,3-azoles exist in their carbonyl tautomeric forms. The bromination of thiazol-2-one, at C-5, is a nice demonstration of relative reactivity here the double bond carries both sulfur and nitrogen, and it is the latter, i.e. the enamide rather than the enethiol ester character, that dictates the site of electrophilic attack. ... 

In the hypothesized catalytic cycle, in the presence of catalyst and the co-catalyst acetic acid, the enamide 1 is tautomerized to the corresponding imine, which is activated by the acid via an iminium intermediate. In the following step, only chiral phosphoric acid is active enough to catalyze the hydrogenation of the imine, while the acetic acid role is probably only to help keep a sufficient concentration of iminium intermediate present since it was used in such small quantities (Figure 15.7). 

A [3+3]-type condensation of O-acetyl ketoximes and a, -unsaturated aldehydes yields pyridines " for example, Ph-(Me)C=N-OAc and trans-cinnamaldehyde (trans-Ph-CH=CH-CHO) give 2,4-diphenylpyridine (54) using copper(I) iodide as catalyst and a salt of a secondary amine only a trace of the 2,6-product is observed. A synergistic copper/iminium catalysis is proposed the oxime reacts with the copper iodide to give an iminyl copper species, Ph-(Me)C=N-Cu-X (i.e., N-O reduction), which tautomerizes to a copper(II) enamide, Ph-C(=CH2)-NH-CuX, which then acts as a nucleophile towards the iminium ion (formed from the aldehyde and 2° amine). 

Ruthenium/Br0nsted Acid System The tandem isomerization/Friedel-Crafts reaction of aUylamides reported by Sorimachi and Terada was another early example of a cascade catalysis procedure using Brpnsted acid and metal salts as catalysts (Scheme 2.93) [127]. With the promotion of the ruthenium catalyst 342, the aUylamides isomerized to enamide X32, which was then tautomerized by a Brpnsted acid (343 or 344) to form imine X33 then the electron-enriched aromatic substrates 341 attacked the Brpnsted acid-activated imine intermediates to afford Friedel-Crafts products 345 in up to 91% yield. 

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