Ketene acetals reactions with isocyanates

Condensations. Reaction of tertiary alcohols with cyclic anhydrides, Baylis-Hillman reaction, aldol reaction involving bis(trimethylsilyl) ketene acetals, reaction of R(XIH(CN)2 with activated imines, and the [2+2]cycloaddition between isocyanates and enol ethers are assisted by high pressure. 

In contrast, a-ketosulphoxides react with isocyanates to give the products of a monoaddition only550 (equation 290). Reaction of dimsyl anion with trithiocarbonates 481 followed by alkylation results in the formation of (methylsulphinyl)ketene dithio-acetals 482551 (equation 291). 

The methods for generating acyl ketenes (Scheme 7-V) and their subsequent in situ participation in [4 + 2] cycloadditions with a wide range of hetero- or olefinic and acetylenic dienophiles (Scheme 7-VI), including acyl ketenes,185 186,197 carbonyl compounds, 86-188 nitriles,1874,189,191 isocyanates and isothiocyanates,1864,190,191 ketenes,191 imines,1864,1874,191,192 carbo-diimides,l87c 190,191,193 ynamines,194 ketene acetals,1864,195 enol ethers,1864,191,196 and V-sulfinylamines197 have been extensively reviewed.5,9,12 Two reports have detailed the 4-n- participation of allenic ketones in [4 + 2] cycloaddition reactions [Eq. (51)].198,199... 

Aromatic and aliphatic acyl isocyanates participate in a similar range of [4 + 2] cycloadditions although [2 + 2] and simple addition reactions often are observed. The acyl isocyanate substituents may determine or alter the observed course of the reaction, and the substituent effects have been detailed in extensive reviews.7,71 Observed [4 + 2] cycloadditions of acyl isocyanates with selected olefins, p-quinones, allenes, the carbon-carbon double bond of ketenes, electron-rich acetylenes, imines, dianils, ethy-lenediimines, enamines, enol ethers, ketene acetals, carbodiimides, azirines, and vinyl sulfides have been described.7 0 The reaction of aromatic acyl isocyanates with carbodiimides is not a simple, direct [4 + 2] cycloaddition but proceeds by a kinetic [2 + 2] cycloaddition followed by a subsequent rearrangement to provide the observed [4 + 2] cycloadduct [Eq. (40)].97... 

The [2+2] cycloaddition reaction of isocyanates proceeds better with olefins having electron donating groups attached to the double bond system. Examples include vinyl ethers, enamines, ketene acetals, tetraalkoxy- or tetraalkylaminooleflns. The more reactive sulfonyl and carbonyl isocyanates undergo cycloaddition reactions with vinyl ethers especially well. For example, the reaction of vinyl ethers with p-toluenesulfonyl isocyanate affords the [2+2] cycloadducts in a stereospecific reaction... 

Ketene-0,0-, N,N-, N,0-, N,S- and 5,5-acetals react readily with alkyl, aryl and especially arenesulfonyl isocyanates to give 1 1 adducts. The structure of the adducts depends on the degree of substitution on the jS-carbon atom. The /3-disubstituted ketene acetals give [2+2] cycloadducts In the reaction of phenyl isocyanate with ketene diethylacetal, a six-membered ring 2 1 cycloadduct is obtained and mixtures of the [2+2] and the [2+2+2] are obtained from substituted keten 0,0-acetals Heterocyclic e-caprolactim ethers 92, which are in equilibrium with ketene OW-acetals, react with aryl isocyanates at room temperature to give [2+2] cycloadducts 93 and at 150 °C to produce [2+2+2] cycloadducts 94. ... 

Heterocyclic imines with a thiomethyl group adjacent to nitrogen are in equilibrium with ketene 5,Af-acetals. On reaction with aryl isocyanates, linear 1 1 adducts 95 are obtained. ... 

A 2 1 adduct is also obtained from 3,4-dihydroisoquinoline and phenyl isocyanate Caprolactim ethers, which are in equilibrium with ketene-0,A acetals also undergo [2+2+2] cycloaddition reactions with aryl isocyanates at 150 °C. At room temperature, [2+2] cycloadducts are obtained (see Section 3.3.1.4). 

The 1,3-dipolarophiles participating in the [3+2] cycloaddition reaction with azides include carbon-carbon multiple bonds, such as alkynes, olefins, vinyl ethers and ketene acetals, but addition across C=N multiple bonds, such as in nitriles and isocyanates, C=S and P=C bonds is also observed. The yields observed for many of these reactions approach quantitative. 

Several approaches to the 1,2,3-triazole core have been published in 2000. Iodobenzene diacetate-mediated oxidation of hydrazones 152 led to fused 1,2,3-triazoloheterocycles 153 <00SC417>. Treatment of oxazolone 154 with iso-pentyl nitrite in the presence of acetic acid gave 1,2,3-triazole 155, a precursor to 3-(W-l,2,3-triazolyl)-substituted a,P-unsaturated a amino acid derivatives <00SC2863>. Aroyl-substituted ketene aminals 156 reacted with aryl azides to provide polysubstituted 1,23-triazoles 157 <00HC387>. 2-Aryl-2T/,4/f-imidazo[43-d][l,2,3]triazoles 159 were prepared from the reaction of triethyl AM-ethyl-2-methyl-4-nitro-l//-imidazol-5-yl phosphoramidate (158) with aryl isocyanates <00TL9889>. 

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