The 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 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 b-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 À from the complex, 3) reaction of the released SCN À 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.Introduction. -Epoxides are one of the most versatile intermediates in organic synthesis, and a large variety of reagents are known for the ring opening of these compounds [1]. Their electrophilic reaction with different nucleophiles has been a permanent subject in organic synthesis [2 -7]. Among these nucleophiles, the reaction of the thiocyanate ion with epoxides, in the absence or in the presence of a catalyst, is a widely studied and suitable method for the preparation of thiiranes [8 -15]. The formation of thiiranes from the reaction of epoxides and thiocyanate ions has been proposed to occur through the intermediacy of the corresponding b-hydroxy thiocyanate, but this intermediate has not been isolated due to its rapid conversion to the corresponding thiirane [13 -17]. There are two methods reported in the literature for the synthesis of b-hydroxy thiocyanates. In one method, thiocyanohydrins are prepared by opening of a cyclic sulfate with NH 4 SCN to form the corresponding bsulfate, which is then hydrolyzed to the thiocyanohydrines. A second method employs the addition of thiocyanic acid, generated in situ at low temperature, to the epoxide [18 -21]. It has been reported for these syntheses that the presence of some hydroquinone (¼ benzene-1,4-diol) or DDQ (¼ 4,5-dichloro-3,6-dioxocyclohexa-1,4-diene-1,2-dicarbonitrile) is required to stabilize the produced b-hydroxy thiocyanate and to inhibit its conversion to thiirane [16] [22]. Although the reagents such as Ti(O
