Effective method of β-keto sulfones synthesis

Abstract

β-Keto sulfones are widely used in organic synthesis as reagents in Michael and Knoevenagel reactions [1–3], for preparation of vinyl sulfones [4], polyfunctional 4Н-pyrans [3], chalcones, alkynes, allenes, and compounds of other classes [1,5–9]. In this connection various approaches are developed to the synthesis of β-keto sulfones [10–18], but many among them require severe conditions, preparation of initial compounds, and the yields of products in these reactions are often low. Looking for an effi cient and cheap method of β-keto sulfones synthesis aiming at their application as methylene-active reagents in the preparation of 1,2,3-triazoles [19] we developed a procedure for the synthesis of β-keto sulfones with the use of available thiols I and β-haloketones II. The reaction of thiophenol (Ia) and 4-methylthiophenol (Ib) with β-haloketones IIа–IIh afforded β-keto sulfi des IIIа–IIIh. The oxidation of these compounds with hydrogen peroxide in acetic acid [20] did not result in selective formation of sulfones V, but gave a mixture of reaction products. This result may be ascribed to the concurrent reaction of the keto group oxidation analogously to Baeyer-Villiger reaction. For instance, the oxidation of compound IIIa with hydrogen peroxide in acetic acid at room temperature led to the formation of a mixture of β-keto sulfone Va and methylsulfonylbenzene in a ratio 2 : 1 [21]. Keto sulfi des may be oxidized with potassium permanganate [22], but at the oxidation β-keto sulfi des III in this way sulfones V formed in low yields. Considering these results we protected the keto group in compounds IIIа–IIIh by converting them in 1,3-dioxolanes IVа–IVh. At the oxidation of compounds IVа–IVh with hydrogen peroxide in acetic acid followed by treating the reaction mixture with water the target β-keto sulfones Vа–Vh formed in high yields (see the table).