Synthesis of Some Novel Thioxanthenone‐Fused Azacrown Ethers, and Their Use as New Catalysts in the Efficient, Mild, and Regioselective Conversion of Epoxides to β‐Hydroxy Thiocyanates with Ammonium Thiocyanate

Abstract

AbstractThe regioselective ring‐opening reactions of some epoxides with ammonium thiocyanate in the presence of a series of new 9H‐thioxanthen‐9‐one‐fused azacrown ethers, i.e., 7–11 (Scheme 1), and also of dibenzo[18]crown‐6 (12), Kryptofix® 22 (13), and benzo[15]crown‐5 (14) were studied (Tables 1 and 2). The epoxides were subjected to cleavage by NH4SCN in the presence of these catalysts under mild conditions in various aprotic solvents. Reagents and conditions were identified for the synthesis of individual β‐hydroxy thiocyanates in high yield and with more than 90% regioselectivity. The results can be discussed in terms of a four‐step mechanism (Scheme 2): 1) formation of a complex between catalyst and NH4SCN, 2) release of SCN− from the complex, 3) reaction of the released SCN− at the sterically less hindered site of the epoxide, and 4) regeneration of the catalyst. The major advantages of this method are the high regioselectivity, the simple regeneration of the catalyst, the reuse of it through several cycles without a decrease of activity, and the ease of workup of the reaction mixtures.