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

The regioselective ring‐opening reactions of some epoxides with ammonium thiocyanate in the presence of a series of new 9 H ‐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 NH 4 SCN 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 NH 4 SCN, 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.