Synthesis of ω-Aminodithioesters

Abstract

w-Aminodithioester derivatives were obtained from thionolactams by reaction with an alkyl triflate followed by thiolysis with hydrogen sulfide. The presence of an electron-withdrawing group was required on the N1 position (p-nitrophenyl or benzoyl) to favor the ring opening of g-, d-and e-thionolactams. In the case of b-thionolactam, activation was provided by a CF 2 motif in C3 position Dithioesters have attracted considerable attention in recent years as versatile tools for organic synthesis. 1-5 Methods of dithioester preparation have been known for a long time. 6 Generally, organometallic compounds or carbanions are treated with carbon disulfide followed by addition of an alkyl halide, 7-9 or reacted with aryl trithiocarbonates, 10 or alkyl dithiocarbonyl chlorides. 11 Another general approach was based on a Pinner-like reaction: 8 thiolysis with hydrogen sulfide of in situ-generated thiolimidoesters produced dithioesters. [12][13][14][15][16][17] In the course of a program dedicated to the 18 F-radiolabeling of perfluorinated amine derivatives (to be incorporated in nitroimidazolic markers of hypoxia), we became interested in dithioester compounds 1a,b (Scheme 1) as precursors of [ 18 F]CF 3 and [ 18 F]C 2 F 5 motifs by an oxidative fluorodesulfurization reaction using 1,3-dibromo-5,5-dimethylhydantoin (DBH) and [ 18 F]HF·pyridine. 18,19 We had already prepared 1a (X = H; R = Et); 20 the key step for the dithioester formation was the nucleophilic substitution of a thioacyl-N-phthalimide intermediate 21 with ethanethiol. The same strategy could be applied for the synthesis of 1b, but with lower yields. 19 In fact, the preparation of a-perfluorinated dithioesters was scarcely described in the previous literature and required particular methods. [22][23][24][25] Just having in hand a practical synthesis of 3,3-difluoro-1-benzhydrylazetidin-2-one (2), 26,27 we decided to examine the possibility of using the corresponding azetidin-2-thione 3 as precursor of 1b. Our plan was to transform the b-thionolactam into 2-thioalkyl-azetidinium salt 4 by selective S-alkylation; the subsequent thiolysis with hydrogen sulfide should create the dithioester function by nucleophilic addition on the C2 position followed by C2-N1 ring opening.The precursor 3 was readily obtained by treatment of azetidinone 2 with Lawesson's reagent (Scheme 2). Reaction of 3 with methyl triflate quantitatively furnished the salt 4, within 30 minutes at room temperature, in dichloromethane solution (control by 1 H NMR). Then a stream of hydrogen sulfide was bubbled into the solution of 4; the dithioester 5a was formed in about 80% yield, the sideproduct being the thiolester 5b due to some competitive hydrolysis. After chromatographic purification, com-