Prototropic ene

Scheme 8C.24. Asymmetric aldol (prototropic ene-type) reaction.

One report has indicated the potential of this mild reagent for a-oxidation of silyl ketoie acetals. Once again the usefulness is restricted by competition between the required silatropic ene process and the prototropic ene reaction giving rise to the enoate. For substrates lacking -protons the a-hydroperoxy ester is readily obtained in good yield. In one case where p-protons were present (methyl grotq)) an 80% yield of the silatropic product was obtained by carefully optimizing the reaction conditions. This may not however always be possible. 

It seems likely that the reaction proceeds through a prototropic ene reaction pathway, a pathway that has not been previously recognized as a possible mechanism in the Mukaiyama aldol condensation. Usually an acyclic antiperiplanar transition-state model has been used to explain the formation of the syn diastereomer from either ( )- or (Z)-silyl enol ethers [91]. The cyclic ene mechanism, however, now provides another rationale for the syn diastereoselectivity irrespective of enol silyl ether geometry (Sch. 32). 

Mikami reported that BINOL derived titanium complex efficiently catalyzed the aldol reaction of silyl enol ether with excellent control of both absolute and relative stereochemistry [106] (Scheme 14.37). The reaction was proposed to proceed via a prototropic ene reaction pathway that is different from that of Mukaiyama aldol condensation. A cyclic antiperiplanar transition-state model was proposed to explain the pref erential formation of the syn diastereomer from either (E)- or (Z)-silyl enol ethers [106]. Further modifications of the catalyst system include the use of perfluorophenols and other activating additives [107], or performing the reaction in supercritical fluids [108]. Furthermore, the nucleophile could be extended to enoxysilacyclobutane derivatives [109]. 

The cyclic enediynyl sulfide 93 is also prone to undergo prototropic rearrangement (Scheme 20.21) [57]. When the l,8-diazabicydo[5.4.0]undec-7-ene (DBU)-induced isomerization was conducted in carbon tetrachloride, three cycloaromatized products, 96 to 98, were isolated, indicating the formation of the biradical 95a as a transient intermediate. In a polar solvent, such as methanol or ethanol, the formation of 99 can best be accounted for by regarding the biradical 95a as the zwitterion ion 95b. A related process involving the oxidation of 93 with selenium dioxide has also been reported [58],... 

The double sequence of dehydrohalogenation of a 7,7-dihalobicyclo[4.1.0]heptane derivative to give a bicyclo[4.1.0]hept-l(7)-ene, followed by base-induced prototropic shifts to move the double bond into the six-membered ring132-141, finds continuing applications in routes to cycloproparenes100,142 ... 

While the reaction of singlet oxygen with silyl enol ethers was governed by competing prototropic and silatropic ene processes (see Section 2.3.2.1.3.ii), the interaction with dienol ethers displays a different mode of reactivity. Singlet oxygen generated from triphenyl phosphite ozonide at low temperature... 

The Grigg group also studied the tautomerization of oximes to N-H nitrones followed by a dipolar cycloaddition reaction. The well-known H-bonding dimeric association of oximes, in both solution and the solid state, allows for a concerted proton transfer to occur and provides nitrone 56 (Scheme 11) (91TL4007). Another possible pathway involves tautomerization of the oxime to an ene-hydroxylamine (i.e. 57) followed by a 1,4-hydride shift to give nitrone 58. To probe the ene-hydro-xylamine mechanism, deuterated oxime 59 was prepared and heated at 140 °C in xylene. The physical characteristics of the isolated product, however, were consistent with compound 60, suggesting that the 1,2-prototropic reactions does not proceed... 

Diazabicyclo[4.3.0]non-5-ene Prototropic isomerization of carbon-nitrogen double bonds... 

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