Carbonyl isothiocyanates 4+2 cycloaddition reactions

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... 

Several reactive isothiocyanates undergo dimerization reactions in which one of the isothiocyanates functions as the diene and the other isothiocyanate reacts as the dienophile. These [4-1-2] cycloaddition reactions are very pronounced with heterocyclic isothiocyanates and isothiocyanates having carbonyl, thiocarbonyl and imidoyl substituents. For example, 2-pyridyl isothiocyanate undergoes dimerization to give the triazine derivative 2. ... 

The dimerization of carbonyl-, thiocarbonyl-, imidoyl- and thiocarbamoyl isothiocyanates proceeds in a [4-1-2] cycloaddition reaction (see Section 3.3.2.2). Carbonyl isothiocyanates also readily undergo [4-1-2] cycloaddition reactions with aliphatic and aromatic azomethines, the former being considerably more reactive. For example, phenylcar-bonyl isothiocyanate reacts with benzylidenemethylamine 184 to give a [4-1-2] cycloadduct 185... 

Cycloadditions The [4+2] cycloaddition reactions of carbodiimides with phenyl-carbonyl isocyanate, phenylcarbonyl isothiocyanate and thiocarbamoyl isothiocyanate have been discussed above. In the dimerization reactions the functional carbodiimides react as both the diene and the dienophile. Unsaturated carbodiimides, generated in situ, can be trapped with N=N bond- or C=N bond-containing substrates. 

Diazo transfer from p-toluenesulfonyl azide onto acylthioacetamide (256) takes place in EtOH to give 90% of 4-acyl-5-phenylamino-l,2,3-thiadiazole (258).1 The diazo intermediate (257) could not be isolated, since the cycloaddition is evidently faster than the diazo transfer reaction.1 Thiadiazoles are also prepared by 1,3-dipolar cycloaddition of diazo carbonyl compounds to isothiocyanates in moderate yields.1... 

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