Tautomerism enamine-imine—

The enamine-imine tautomerism of the indolenine system gives rise to rearrangement reactions of interest in indole alkaloid chemistry. Thus the synthesis of dihydroburnamicine (625) utilized the rearrangement of an acetoxyindolenine to an a-hydroxyalkyl indole, presumably through an intermediate enamine. Similarly 2,3-dialkyl indoles undergo oxidations to 2-acyl indoles (626-631). 

Meanwhile, Nikolay had started his new research on the preparation, reactions, and synthetic applications of p-chloro vinyl ketones and related compounds (p-aminovinyl ketones, p-kctoacetals). Efficient synthetic methods were developed and improved, including C-ketovinylation (introduction of the RCOCH = CH group), and a number of useful heterocyclic compounds (pyrazoles, triazoles, pyridines, and so on) were synthesized. The discovery of enamine-imine tautomerism in p-aminovinyl ketones was another remarkable achievement by Nikolay at that time. For these studies he received in 1953 the degree of Doctor of Science in chemistry. He continued as a lecturer and supervisor of postgraduates at the University, becoming docent (associate professor) in 1951 and full professor in 1955. 

The tertiary enamines, in contrast to the secondary derivatives, cannot exhibit enamine-imine tautomerism. As the free bases, they exist only in the vinylamino form. Their physico-chemical properties are in agreement with this structure, especially the spectral properties. The bands due to the stretching frequency of the carbon-carbon double bond in their infrared spectra1-25-27 (situated at 1630-1660 cm-1 according to the nature of the substituents) occur at somewhat lower frequencies, but their intensities are greatly increased in comparison to those of simple olefins because of conjugation with the free electron pair on the nitrogen atom. Indications of cis-trans isomerism... 

The steric influence on the enamine-imine tautomerism has also been observed in the cyclic ketone derivatives. Cyclohexanone imines of w-propylamine, cyclohexylamine and 2-bornylamine show no signals ascribed to the enamine tautomer in their NMR spectra, but the /-butylamine38 does display signals of the enamine. In DMSO-d6 it comprises 38% of enamine 56 at equilibrium. The proportion of the 3,3,5,5-tetra-methylcyclohexanone and cyclopentanone enamines 57 and 58 is even higher, rising to 52% and 58%, respectively, in DMSO-d663. 

Very recently, an enamine-imine tautomerism of ketene aminals has been reported by Huang and coworkers64-67. Due to competition by increased aromaticity, very stable enaminone isomers 63-65 equilibrate to their imine isomers 63-65 (equation 10). It is... 

Although this methodology is widely applicable for the preparation of many 1,1-enediamines, 2-(aroylmethylene)benzimidazolines (29) cannot be obtained from the reaction of benzoyl-substituted ketene dithioacetals 30 and o-phenylenediamine. Instead of 29, 3H- 1,5-benzodiazepines 31 are isolated from the reaction64. Similar results have been reported for the reaction of 30 and other primary aromatic diamines70,71. A recent study64 has revealed that an enamine-imine tautomeric equilibrium is involved in the reaction process (Scheme 2). This equilibrium between 33A and 33B is affected by the... 

Extensive studies have shown that the enamine-imine tautomeric equilibrium of equation 1 shifts almost completely to the enamine side when an unsaturated electron-withdrawing group including nitrile nitro °, carbonyl, thiocarbonyP etc. ° is attached to the -carbon. Reviews related to these enamines which include enaminoni-triles 2, nitroenamines 3, enaminones 4 and the enaminothioketones 5, have appeared in the literature, ... 

Diethoxy-2,5-dihydropyrimidine (27) has a flat ring in the crystalline solid phase, and the molecule is not involved in enamine-imine tautomerism in solution <86JOC4623>. The 4,6-diphenyl analogue (28), however, is in equilibrium with its 1,2-dihydro tautomer (29) in solution (Equation... 

When enamine-imine tautomerization is possible 370, 437, the compounds existed as an equilibrium mixture, usually in favor of the penem form [140,169, 202]. Unfortunately, deprotection of the C3 ester (P = allyl or pNB) was accompanied in these products by extensive decomposition. By contrast, the azolyl penems 436 proved stable, and the corresponding free acid or sodium salts displayed excellent antimicrobial properties [140, 141]. 

The ultimate step in the total synthesis of ( )-dehydrotubifoline (157), a Strychnos alkaloid, is an intramolecular Mizoroki-Heck reaction of H-like (Figure 6.4) substrate 156 (Scheme 6.45) [115]. This ring closure preserved the integrity of the double-bond stereochemistry and delivered, after the excepted enamine-imine tautomerization, the desired product 157 in excellent yield. 

Recently, A(-sulfinyl L-proline amides have been used for the enantioselective reduction of a range of A(-aIkyl (i-enamino esters (Scheme 15.8) [32]. In this case, the use of water an additive is crucial for high reactivity and enantioselectivity, accelerating enamine-imine tautomerization and increasing the eletrophilicity of the imine thought protonation of the nitrogen atom. 

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