Oxazoline iV-oxides

Camphor-derived oxazoline iV-oxides 1 have been employed as dipoles for [3+2] cyeloadditions with a,P-iinsaturated esters or nitroalkenes. The reactions proceed in good yields and are highly regio- and enob-stereoselective. Thus, regioisomers 2 are obtained preferentially to 3 (flie 2 3 ratios are about 95 5), with almost complete e [Pg.101]

The enhanced reactivity of oxazoline AAoxide 10 compared to nitrone 11 could be explained in terms of FMO theory. The data of HOMO energies indicate that 10 has a higher HOMO and a lower LUMO than 11. In reactions controlled by the HOMO dipole-LUMO dipolarophile interaction, as in Figure 15.3, as well as in processes controlled by the LUMO dipole-HOMO dipolarophile interaction (Fig. 15.4), oxazoline iV-oxide 10 will provide a smaller energy gap than nitrone 11 when reacting wifli a dipolarophile. 

The anodic behavior of A -substituted alkenes can be described as the oxidation of an electron-rich double bond. Tetraamino-substituted alkenes are extremely easily oxidized. Tetrakis(dimethylamino)ethylene exhibits two reversible one-electron processes at —0.75 and —0.61 V vs. SCE at a dropping mercury electrode in acetonitrile [140]. The anodic behavior of A, A -dimethylaminoalkenes has been studied intensively by cyclic voltammetry and electron spin resonance (ESR) spectroscopy [141]. The anodically E° = 0.48 V vs. SCE) generated cation radical of l,l-bis(iV,iV-dimethylamino)ethylene is shown to undergo C-C coupling, forming l,l,4,4-tetrakis(A, iV-dimethylamino)butadiene, which subsequently is further oxidized to its dication at —0.8 V [141,142]. With vicinal diamino ethylenes, usually two reversible one-electron oxidations are observed [143], while gem-inal diamino ethylenes exhibit an irreversible behavior [141]. Aryl-substituted vicinal diamino ethylenes (endiamines) can undergo a double cyclization to give an indolo-oxazoline when oxidized at 0.4 V vs. SCE in acetonitrile in the presence of 2,6-lutidine [144] ... 

The DFT study of the 3 + 2-cycloaddition between ketene and TV-silyl-, IV-germyl-, and TV-stannyl-imines shows that the TV-germylimine reaction is a two-step process the TV-stannylimine reaction is a competition between two- and three-step processes whereas the TV-silyl process follows a three-step process44 A new and convenient synthesis of functionalized furans and benzofurans based on 3 + 2-cycloaddition/oxidation has been reported. The cyclization of cyclic 1,3-bis-silyl enol ethers (48) with l-chloro-2,2-dimethoxyethane (49), via a dianion, produced 5,6-bicyclic 2-alkylidenetetrahydrofurans (50), which are readily oxidized with DDQ to 2,3-unsubstituted benzofurans (51) (Scheme 13)45 The Evans bis(oxazoline)-Cu(II) complex catalyses the asymmetric 1,3-dipolar cycloaddition of a -hydroxyenones with nitrones to produce isoxazolidines.46 The... 

Physical Data bp 70 °C/0.02 mmHg [a]g = +434° (c 1, CHCI3). Solubility sol all common nonprotic organic solvents. Preparative Methods the acetal from serine methyl ester and pivaladehyde is IV-methoxycarbonylated and saponified to give the cw-half ester (2) (61% yield). Oxidative decarboxylation by electrolysis in AcOH (1 3 by weight) in the presence of 5% EtaN affords (3) (87%), which upon heating with NH4Br in toluene yields the title oxazoline (1) (75% after distillation in vacuo) (eq 1). ... 

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