Keto tautomer acyclic

The keto tautomer has been isolated in crystalline form for a number of D-fruc-tose-A -alkylanilines and the structure was unequivocally confirmed by IR and solid-state NMR spectroscopic data, " as well as by X-ray diffraction studies. " The ability of D-fructose-A -alkylanilines to crystallize spontaneously in the acyclic keto form is unprecedented among six-carbon reducing carbohydrates, and is matehed only by ribulos- and xylulos-amines, " which themselves are the Amadori rearrangement products. This phenomenon may be explained on the basis of an interplay of the hydrophobic effect and crystal-packing forces, given that many D-fruetose-A -alkylanilines still crystallize in the p-pyranose form, and for the majority of those which crystalhze in the keto form, the acyclic tautomer constitotes a minor eonstituent in equilibrated solutions. 

Fig. 1. A correlation between the rate constants h y for carbonyl hydration and the equilibrium molar liaction N of the acyclic keto tautomers of fructosamines in pyridine/Hj O at 25...

Tan also found that guanidine 21, acting as a base to activate the o [3], X [3] tautomers of diaryl phosphine oxides, catalyzes the asymmetric phospha-Michael reachon of aryl nitroalkenes (Scheme 5.42) [76]. He later employed 21 to realize highly enantioselective Michael additions of dithiomalonate and 3-keto thioesters with a range of acceptors, including maleimides, cyclic enones, furanone, and acyclic 1,4-dicarbonylbutenes [77]. 

V. V. Mossine, C. L. Barnes, M. S. Feadier, and T. P. Mawhiimey, Acyclic tautomers in crystalline carbohydrates The keto forms of 1-deoxy-l-carboxy-methylamino-D-2-pentuloses (pentulose-glycines), J. Am. Chem. Soc., 124 (2002) 15178-15179. 

The photochemical reactivity of P-ketoesters is different form that of P-diketones. Irradiation of a P-ketoester in the presence of an alkene produces oxetane via the ketone carbonyl instead of the desired cyclobutane ring system. Therefore, it is necessary to covalently lock the ketoesters as the enol tautomers. To this end, silyl enol ethers, 129 and 132a, and enol acetates, 130 and 132b, were prepared, but these substrates still fail to undergo the desired intramolecular [2 + 2] photocycloaddition with olefins. The only new products observed in these reactions result from the photo-Fries rearrangement of the cyclic enol acetate (130 to 131) and cis-trans isomerization of both acyclic substrates 132a/b. However, tetronates are appropriate substrates for both intermolecular and intramolecular photocycloadditions with olefins. In addition, enol acetates and silyl enol ethers of p-keto esters are known to undergo [2 + 2] photoaddition with cyclic enones (vide infra). 

Five-membered cyclic p-ketoesters (p-ketolactones 1) and cyclic p-ketothioesters (p-ketothiolactones 2) differ from their acyclic counterparts in at least two regards (1) by being mostly enolized and (2) by being so acidic that they are called tetronic acids (1) and thiotetronic acids (2), respectively (Fig. 1). Even 5-membered cyclic p-ketoamides, i.e., p-ketolactams 3, are sufficiently acidic to be known as tetramic acids. The parent compound 3 (Fig. 1 R t = H), for instance, exhibits a pKa of 6.4." Standard tetramic acids prefer the keto (keto-3) over the enol tautomer enol-3). 

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