ARTICLE OBC www.rsc.org/obc Internally stabilized selenocysteine derivatives: syntheses, 77 Se NMR and biomimetic studies† Prasad P. Phadnis and G. Mugesh* Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore, 560 012, India. E-mail: mugesh@ipc.iisc.ernet.in; Fax: +91 80 2360 1552/2360 0683 Received 15th April 2005, Accepted 11th May 2005 First published as an Advance Article on the web 9th June 2005 Selenocystine ([Sec] 2 ) and aryl-substituted selenocysteine (Sec) derivatives are synthesized, starting from commercially available amino acid L-serine. These compounds are characterized by a number of analytical techniques such as NMR ( 1 H, 13 C and 77 Se) and TOF mass spectroscopy. This study reveals that the introduction of amino/imino substituents capable of interacting with selenium may stabilize the Sec derivatives. This study further suggests that the oxidation–elimination reactions in Sec derivatives could be used for the generation of biologically active selenols having internally stabilizing substituents. Introduction Selenium, an essential trace element, 1 exerts its biological effect through several selenoenzymes, which include glutathione peroxidase (GPx), 2,3 iodothyronine deiodinase (ID) 4 and thiore- doxin reductase (TrxR). 5 Although these enzymes have seleno- cysteine (Sec), the 21st amino acid, in their active site, 6–10 their substrate specificity and cofactor systems are strikingly different. GPx is an antioxidant selenoenzyme 11 that protects various organisms from oxidative stress by catalyzing the reduction of hydroperoxides at the expense of thiols. 12 ID, particularly the type I enzyme (ID-I), on the other hand, is responsible for the activation of thyroid hormones by deiodination reactions. The role of TrxR is to reduce thioredoxin (Trx) by NADPH, which is important for a variety of biological functions such as DNA synthesis. 5,13 However, the synthetic and biological studies on Sec are hampered by instability of Sec derivatives as compared with the cysteine (Cys) analogues. In general, Sec derivatives undergo fast and mild oxidative elimination to produce dehydroalanines. 14 Initial attempts to synthesize Sec derivatives in the laboratory met with limited success. Soda et al. synthesized selenocys- teine by treating b-L-chloroalanine with disodium diselenide 15 whereas Pete Silks et al. approached the synthesis from tosylated N-Boc-serine methylester. The conversion of this serine ester into an iodide or a bromide derivative followed by treatment with dilithium diselenide and then deprotection with TFA afforded the required Sec. 16,17 Walther and coworkers reported one of the first syntheses of optically pure Sec derivatives. 18 A key step in the synthesis involved the nucleophilic displacement of an O-tosylated L-serine derivative. An alternative approach has been reported by Shirahama and coworkers, in which diphenyl diselenide was first reduced with sodium metal and the resulting selenolate was treated with tert-butyloxy carbonyl (Boc) protected serine b-lactone. 19 Recently, this approach has been modified by performing an in situ reduction of diphenyl dis- elenide with sodium trimethoxyborohydride [NaBH(OMe) 3 ]. 14,20 More recently, a modified synthetic route to Sec derivatives using Fmoc strategy has also been reported. 21 Recent studies on aryl substituted Sec conjugates show that the oxidative elimination reactions could lead to the generation of catalytically active selenols. 22 Therefore, the fast and mild oxidative elimination of selenocysteine derivatives can be utilized for further catalytic reactions. However, the instability of such †Electronic supplementary information (ESI) available: 77 Se NMR data for selected compounds. See http://www.rsc.org/suppdata/ob/ b5/b505299h/ derivatives poses a major problem in synthetic and purification procedures. To overcome this difficulty we employed an aryl selenium moiety having internally chelating substituents, which would increase the stability by Se ··· N non-covalent interac- tions. These substituents may also modulate the reactivity of selenols that are generated in situ by elimination reactions. In this article, we report the first examples of Sec derivatives having basic amino/imino groups in the close proximity to selenium. Results and discussion The synthesis of selenocysteine derivatives was approached with protected L-serine. Our initial route to these derivatives focused on modification to literature preparations of selenocysteine and the corresponding phenyl derivative. Boc-protected serine ester (1) was converted into bromoalanine methyl ester (3), which was treated with Li 2 Se 2 to afford the protected selenocystine (4) (Scheme1). Scheme 1 Synthetic route to diselenide 4. Although this reaction afforded the expected compound in low yield, our initial attempts towards scale up did not work. The 77 Se NMR spectrum showed a number of signals which were probably due to different diastereomers. The activation of serine –OH in 1 with phenylmethanesulfonyl fluoride (PMSF) or tosyl chloride (TsCl) followed by reaction with NaHSe and subsequent oxidation also did not look promising. The major problem in these reactions lies in the tosylation step, which leads to the formation of dehydroalanine derivatives. In view of these results, we decided to find an alternative synthetic approach to 4 that does not involve the tosylation and would be amenable to scale up. Scheme 2 depicts the conversion of the protected serine into selenocystine, in which the –OH group can be converted directly to a bromide without involving the tosylation step. 23 Scheme 2 Modified route for the synthesis of 4. DOI: 10.1039/b505299h 2476 Org. Biomol. Chem. , 2005, 3 , 2476–2481 This journal is © The Royal Society of Chemistry 2005