Formation and Reaction of Carbonyl Ylides. Reactions of 1-Methoxy-2-benzopyrylium-4-olate with Isothiocyanates

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Abstract
The 1,3-dipolar cycloaddition of 1-methoxy-2-benzopyrylium-4-olate (2) generated by Cu(acac)2-catalyzed decomposition of methyl 2-diazoacetylbenzoate with various aryl isothiocyanates and methyl isothiocyanate gave 2 : 1-, 3 : 1-, 4 : 1-, and 5 : 1-adducts without affording 1 : 1-adduct. The first cycloaddition of 2 occurs on the C=N double bond of isothiocyanate. The cycloadditions of the following steps, such as second, third, fourth, and fifth cycloadditions, proceed on C=O group of the cycloadducts of each previous steps. The molecular structure of the 2 : 1-adduct of 2 with phenyl isothiocyanate was determined by means of X-ray crystallography. Colorless crystals of the adduct are monoclinic with the space group P21/a, and with the unit-cell dimensions of a = 12.619 (4), b = 16.432 (4), c = 11.238 (2) Å, β = 97.39 (1)°, and Z = 4. The structure was calculated by direct methods (MULTAN 80) and refined by block-diagonal least-squares methods. All hydrogen atoms were revealed by a difference-Fourier synthesis, and further least-squares refinements gave the final R value of 0.046.

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