A Versatile Selenium Transfer Reagent in Organic Synthesis

Chemistry in india Part i CHIMIA 2012, 66, No. 12 921

doi:10.2533/chimia.2012.921 Chimia 66 (2012) 921–929 © Schweizerische Chemische Gesellschaft Tetraethylammonium Tetraseleno- tungstate: A Versatile Selenium Transfer Reagent in Organic Synthesis

Devarajulu Sureshkumar§, Purushothaman Gopinath§, and Srinivasan Chandrasekaran*ab§

Abstract: Organoselenium compounds have attracted intense research owing to their unique biological properties as well as pharmaceutical significance. Progress has been made in developing reagents for incorporation of selenium in an efficient and controlled manner. Herein, we present a review on the recently developed selenium reagent, tetraethylammonium tetraselenotungstate, [Et N] WSe as a versatile selenium transfer reagent in organic 4 2 4 synthesis. Tetraselenotungstate has been successfully used for the synthesis of a number of functionalized diselenides, sugar- and nucleoside-derived diselenides, seleno-cystines, selenohomocystines, selenoamides, selenoureas and sugar- and nucleoside-based cyclic-selenide derivatives. Additionally, this reagent has been employed for the ring opening of aziridines to synthesize a variety of β-aminodiselenides. A new selena-aza- Payne type rearrangement of aziridinemethanoltosylates mediated by tetraselenotungstate for the synthesis of allyl amines is also discussed. Keywords: Aziridines · Diselenides · Selena-aza-Payne and cyclic-diselenides · Selenium · Selenium-transfer reagent · Seleno-cystines · Selenohomocystine · Sugar diselenides · Tetraselenotungstate

Srinivasan of Chemistry – An Asian Journal and zyme inhibitors, antitumor, anti-infective Chandrasekaran Regional Editor of Tetrahedron Letters. He agents and also for their potential phar- was born in Tamil was an elected member of the Bureau and maceutical significance (Fig. 1).[2] Much Nadu, India in Executive committee of the International of the biological activity of selenium is 1945. He received Union of Pure and Applied Chemistry. His associated with its incorporation into the his early education research interests include exploitation of seleno-cysteine residue of enzymes such as at the University of new methods and reagents in organic syn- glycine reductase, formatedehydrogenase, Madrasandobtained thesis and their utilization in the synthesis hydrogenase, glutathione peroxidase, tet- his PhD degree in of biologically active natural products. raiodothreonine 5-deiodinase and plasma 1972 working with proteins.[3] Recently, it has been reported Prof. S. Swaminathan at the University that by incorporation of selenium into pep- of Madras. He worked as a Research 1. Introduction tides, it is possible to study the conforma- Associate (1973–75, 1976–77) with Prof. tional folding preferences of peptides and E. J. Corey at Harvard University, USA Selenium was discovered by Berzelius proteins. Despite the importance of the se- and at Syntex Research, USA (1976). in 1817 and is considered as an essential lenium analogues of amino acids, there are He started independent research at the micro-mineral necessary for cellular func- very few synthetic methodologies avail- Department of Chemistry, Indian Institute tion in mammals but toxic at higher levels. able for the synthesis of selenium com- of Technology, Kanpur where he was The first organo-selenium compound, di- pounds due to difficulties in purification, promoted to Professor in 1985. In May ethyl selenide, was prepared by Lowig in and relative instability. For example, the 1989, he moved to the Indian Institute of 1836. In the past decades, organoselenium synthesis of amino acid, selenocysteine, is Science to take up his current position. He chemistry has made significant progress difficult because it readily oxidizes to form has received the Shanti Swarup Bhatnagar starting from the synthesis of simple sele- the diselenide, selenocystine, in the pres- Prize (Council of Scientific and Industrial nols, selenides and diselenides to the de- ence of air. Research, 1989). He is an elected fel- velopment of organoselenium compounds Diselenides are important motifs in se- low of the Indian Academy of Sciences, having therapeutic potential. lenocystine containing proteins, enzymes Indian National Science Academy and the Following the discovery of seleno-en- and are very good chiral ligands. The most Academy of Sciences of the Developing zymes, selenium-containing compounds frequently used method for the prepara- World (TWAS). He is currently a mem- have been studied extensively in recent tion of organic diselenides is the reaction ber of the International Advisory Board years because of their interesting reactiv- of alkali metal diselenides with various ity profile,[1] anti-oxidant properties, as en- electrophiles.[4] Generally the metal dis-

*Correspondence: Prof. S. Chandrasekaranab aDepartment of Organic Chemistry Fig. 1. Biologically Indian Institute of Science NH2 O active selenium-con- Bangalore-560012, India COOH HSe Se COOH b taining molecules. Jawaharlal Nehru Center for Advanced Scientific HOOC Se N Research NH2 NH2 Se Jakkur, Bangalore, India Selenocystiene Selenocystine E-mail: [email protected] Ebselen (anti-oxidant) §All three authors contributed equally to this article 922 CHIMIA 2012, 66, No. 12 Chemistry in india Part i

i. W(CO)6 ,DMF,90°C, 1h Se Se SeR RX Se SeR 2K +3Se K2Se3 [Et4N]2WSe4 Se RX W W ii. Et4NBr 1 W Se Se Se Se SeR 60 % Se X1 X2 1 Scheme 1. Synthesis of tetraethylammonium tetraselenotungstate (1).

WeSe + Se R + Se R 2 R Se WeSe3 R Se [Et N] WSe 1 3 4 2 4 Se R 3 RX R Se CH3CN, rt, 5-90 min 2 3 Scheme 3. Tentative mechanism of 1 with alkyl halides. 82 -95% R=Alkyl X=Halides

Simple benzyl bromides and its deriva- Se ) Y 2 tives were easily (5–15 min) converted to Se ) HO 2 the corresponding diselenides using 1 in ()5 Se ) 2 CH CN at ambient temperature (Scheme 3a;Y=H; 92% 3 [9] 3b;Y=p-CN; 95% 2). Generally, the reaction is clean and 4;89% 5;92%; dr =1:1 a simple filter column yields diselenides 3c;Y=p-CO2Et; 84% 3d;Y=p-OMe; 91% of high purity and chemical yields (81– 3f;Y=p-COPh; 81% 95%). Tetraselenotungstate 1, reacts with the alkyl halides to form the corresponding diselenides chemoselectively without Se ) EtO interfering with the easily reducible func- 2 Se ) 2 O tional groups such as cyano, ester and keto O N 6 84% 2 82% 8 functionalities. Thus, this reagent has a + (1:1) (1:1) + definite advantage over other procedures ) 2 EtO ) involving the use of metal diselenides and Se 2 Se reducing agents, which cannot be utilized O N O 2 9 in some of these cases. Simple long-chain 7 hydroxy alkyl bromides show low reactiv- Scheme 2. Reaction of tetraselenotungstate 1 with alkyl halides. ity with 1 as compared to benzyl bromides towards the formation of alkyl diselenides. Secondary benzylic bromides are easily elenides are prepared under strongly basic 2. Synthesis of Tetraethylammonium transformed into the corresponding disel- conditions or using reducing agents such Tetraselenotungstate, [Et N] WSe enides as a diastereomeric mixture. In the 4 2 4 as NaBH and LiEt BH, which are often in- case of highly reactive bromides like 4-ni- 4 3 compatible with many functional groups. Tetraselenotungstate 1 is prepared by trobenzyl bromide and ethyl bromoacetate, Another drawback is the formation of a slight modification of the original pro- reaction of 1 furnished a mixture of mono- monoselenides and triselenides as byprod- cedure reported by Kollis and O’Neal[8] and diselenides in 1 : 1 ratio in reasonable ucts, which are often very difficult to pu- by treatment of K Se with W(CO) in dry yields without affecting the nitro and ester 2 3 6 rify. Furthermore, such procedures require DMF at 90 ºC for 1 hr followed by the ad- functionalities (Scheme 2). refluxing conditions for the formation of dition of Et NBr (exchange of potassium Although reaction of 1 with alkyl ha- 4 diselenides. counter cation with tetraethylammonium lides seems to follow the bimolecular S 2 N Although several methods are avail- cation makes the reagent more soluble in substitution pathway, the mechanism of able for the synthesis of organo-selenium organic solvents). After filtration, THF is alkylation of selenometallates of this kind compounds,[5] there still exist challenges to slowly added and the reaction mixture is has not been completely studied and fully develop new versatile selenating reagents, stored at 4 ºC overnight. This furnishes understood. Since reactivity of tetraseleno- which can perform regio-, and stereo con- tetraethylammonium tetraselenotungstate, tungstate 1 is similar to tetrathiotungstate, trolled selenium transfer reactions effi- [Et N] WSe (1) as a red crystalline solid (WS )2–, a plausible mechanism based on 4 2 4 4 ciently in a single step. These challenges in 60% yield (Scheme 1).The reagent can the analogous reaction of tetrathiotungstate arise partly because of the relative insta- be stored under argon atmosphere for a few with alkyl halides is presented in Scheme bility of existing selenating reagents at months; however it decomposes to the cor- 3. In the first step, tetraselenotungstate 1 ambient conditions. Development of prac- responding oxometallic species upon pro- reacts with alkyl halides by alkylation of tically useful and efficient new selenating longed exposure to air. selenium to form monoalkylated interme- reagents is still necessary. Therefore, we diate X and subsequently the second sele- 1 have been involved in the development of nium alkylation forms dialkylated interme- new selenating reagents in organic synthe- 3. Synthesis of Simple Diselenides diate X . The intermediates X and X can 2 1 2 sis for the last few years. Müller and co- undergo induced internal redox reaction workers first reported tetraselenotungstate Tetraselenotungstate 1 is a highly with the oxidation of the selenium ligand in 1981,[6] and since then there have been reactive and soft nucleophile that can and concomitant reduction of the metal no comprehensive studies on the use of this transfer selenium to a variety of organic center to form diselenide 3 along with a selenating reagent. In this review we focus compounds. The study of its chemistry mixture of selenotungstates, which could on our work on tetraethylammonium tetra- began with the nucleophilic displacement not be purified and characterized.[9] selenotungstate, [Et N] WSe (1),[7] and its of alkyl halides in an S 2 fashion to offer 4 2 4 N applications in organic synthesis. alkyl diselenides in a highly pure form. Chemistry in india Part i CHIMIA 2012, 66, No. 12 923

X OAc Se ) OAc ()n () 2 n O [Et N] WSe 1 [Et4N]2WSe4 1 AcO 4 2 4 O CO R1 AcO AcO Se ) RHN 2 RHN CO R1 AcO CH3CN, rt, 45 min AcO 2 CH3CN, rt, 0.5-1.5 h 2 Br AcO 10 12a 91 % 79 -93% 11 13a n=1,2,3 OAc OAc OAc O OAc R=Boc, Cbz AcO O O [Et4N]2WSe4 1 O O 1 AcO AcO O R =Me, Bn AcO AcO Se ) AcO CH3CN, rt, 45 min AcO AcO 2 Br AcO AcO X=OTs,Br 12b 92 % 13b

Se ) Se ) 2 Se ) 2 Scheme 5. Synthesis of carbohydrate-derived diselenides from anomeric 2 bromides. CO R1 1 t RHN 2 RHN CO2R RHN CO2 Bu 11a11b 11c

R=Boc, R1=Me; 85% R=Boc, R1= tBu; 86% R=Boc, R1= tBu; 83% O O R=Cbz, R1=Me; 79% R=Cbz, R1= tBu; 85% R=Cbz, R1= tBu; 85% NH NH

R=Cbz, R1=Bn; 85% R=Cbz, R1=Me; 93% R=Cbz, R1=Me; 91% [Et N] WSe 1 ) Se TsO N O 4 2 4 2 N O Scheme 4. Synthesis of selenium-containing amino acid derivatives. O O CH3CN, rt, 20 h 74 % O O O O 4. Synthesis of Selenium- containing Amino Acid Derivatives 14 15

Selenium-containing amino acids such Scheme 6. Synthesis of uridine-derived diselenide 15. as selenocystine, selenohomocystine and selenocysteine play a vital role in biology. in chemical biology. Additionally, seleno- from the corresponding nucleoside tosyl- Forexample,selenocystinehasbeenusedin glycosides have been used as efficient gly- ates as starting materials. The uridine- protein ligation to incorporate selenocyste- cosyl donors in a variety of oligosaccha- derived tosylate 14 was subjected to sele- [10] ine in the active site of metalloproteins. ride synthesis in carbohydrate chemistry. nium transfer reaction with 1 (1.1 equiv., A few reports available in the literature for Tetraselenotungstate 1 was successfully CH CN, rt, 20 h) to furnish the correspond- 3 the direct synthesis of selenocystine and utilized for the synthesis of carbohydrate- ing uridine derived diselenide 15 in 74% selenohomocystine give poor to moder- derived diselenides from the correspond- yield (Scheme 6).[13] Interestingly, in the [11] ate yields. Using tetraselenotungstate ing anomeric bromides. The selenium case of acetyl-protected thymidine-derived 1 as a selenium transfer reagent, direct transfer reaction of 1 with glucose and lac- tosylate, an unusual intramolecular reac- synthesis of selenocystine 11a, selenoho- tose derived anomeric bromides 12a and tion with 1 was observed yielding a cyclic mocystine 11b and selenobishomocystine 12b was efficiently used for the synthesis diselenide as the only product. This reac- 11c derivatives have been achieved starting of β-anomeric diselenides 13a and 13b re- tion is discussed in detail later in the sec- from the corresponding amino acid deriva- spectively in high chemical yield as shown tion on cyclic diselenides. tives 10 (Scheme 4).[12] This methodology in Scheme 5.[9] is very useful for the synthesis of seleno- amino acid derivatives in small quantities 7. Direct One-pot Conversion of with good chemical yield. In addition, the 6. Synthesis of Diselenide Alcohol to Diselenides methodology is highly chemoselective Derivatives of Nucleosides and is compatible with different protecting Direct conversion of alcohols to disel- groups in the molecule. Utilizing 1 as the key reagent, seleno- enides is an interesting functional group nucleoside derivatives can be synthesized transformation and would lead to environ- 5. Synthesis of Glycosyl

Diselenides Ts NHTs N [Et4N]2WSe4 1 Se R R Se R CH Cl ,rt, 2-5 h NHTs Selenium-containing glycosides are 2 2 19 potent antiviral and antitumor compounds 18 74 -85%

NHTs NHTs

Se NHTs Se CH3 H3C Se TsHN Se H3C Se Se CH3 H NHTs NHTs DCC, CuCl 5mol% N N 85% 74% 78% R OH Se ) O 2 16 CH3CN, rt, 8-36 h R X NHTs CH3 CH3 NHTs R=Alkyl, aryl 71 -84% 17 H3C Se Se CH3 X=H;81% Se CH3 H3C Se X=Cl; 75% CH3 NHTs NHTsCH3 X=OMe; 71% 80% [Et4N]2WSe4 –DCU X=p-COPh; 81% 82%

()6 Se ) 2 NHTs NHTs DCC =Dicyclohexylcarbodiimide 84% Ph Se Se Se R Se Ph Se DCU =Dicyclohexyl Urea R Se NHTs NHTs 3 85% 80%

Scheme 7. Direct transformation of alcohols to diselenides using tetrase- Scheme 8. Regiospecific ring opening of mono-substituted aziridines by lenotungstate 1. 1. 924 CHIMIA 2012, 66, No. 12 Chemistry in india Part i

Ts NHTs O O 'Payne' rearrangement O R 3 N OH 2 [Et4N]2WSe4 1 R 3 2 O 1 Se ) 1 R 1 2 ref. [15] OH 1 2 R R R CH CN, rt, 10-12 h 3 R2 20a; R1 =R2=Et 72 -76% 1 2 O O NH 1 2 21a; R =R =Et 'Aza-Payne' rearrangement R 20b; R =Me; R = i-Pr NH2 1 2 R NH 21b; R =Me; R = i-Pr ref. [16] R OH

NHTs O S Ts 'Thia-Payne' rearrangement O R SH N [Et4N]2WSe4 1 S Se ) R R R 3 2 ref. [17] OH R 3 R 4 CH CN, rt, 7-8 h 3 R4 1 2 80% 1 2 X 22a; R =R =Et 23a; R =R =Et Y2– X X –Y XH OTs Y 1 2 1 2 Y 22b; R =Me; R = i-Pr 23b; R =Me; R = i-Pr R ref. [18 and 19] X=O,NTs Y=Se, Te Scheme 9. Regio and stereospecific ring opening of disubstituted aziri- Ts 'Thia-aza-Payne' dines with 1. N R type rearrangement NTs R S OTs R NHTs ref. [20] S mentally benign methodology in terms of minimal number of steps and waste gener- Scheme 10. Different types of Payne rearrangements. ated during the reaction. In order to perform the direct transformation, alcohols were activated first using dicyclohexylcar- in 76% yield. In the case of (±)-trans-N- type rearrangement of N-tosyl aziridine- [20] bodiimide and a catalytic amount of CuCl tosyl-2,3-diethylaziridine (22a), syn-β- methanoltosylates has been reported. to form activated isourea intermediate 17 aminodiselenide 23a was obtained exclu- During this rearrangement different in situ, which upon reaction with tetrasele- sively in 80% yield under the same reac- N-tosylaziridinemethanoltosylates were notungstate 1 in CH CN at room tempera- tion conditions (Scheme 9). transformed to the corresponding thiirane 3 ture furnished the required diselenides 3 in Additionally, regio- and stereospecific derivatives. good yields under mild and neutral reac- ring opening of (±)-cis-N-tosyl-1-isopro- In a similar fashion, an unusual rear- tion conditions (Scheme 7).[9] pyl-2-methylaziridine (20b) with 1 was rangement of N-tosylaziridinemethanol reported with the formation of anti-β- tosylates was observed with tetraseleno- aminodiselenide 21b exclusively in good tungstate 1. In this selena-aza-Payne type 8. Aziridine Ring-opening yield. Similarly, in the case of (±)-trans- rearrangement, N-tosylaziridinemethanol Reactions N-tosyl-1-isopropyl-2-methylaziridine tosylates were converted to allyl amine (22b), the syn-β-aminodiselenide 23b was derivatives with excellent regio- and ste- [21] Aziridines are very good electrophiles obtained under similar reaction conditions reo control as depicted in Scheme 11. and reactivity of aziridines can be tuned (Scheme 9). This method provides easy During the rearrangement, the selenirane by varying different substituents on the access to a number of β-aminodiselenides intermediate underwent spontaneous sele- nitrogen atom.[14] For example, simple in an efficient manner starting from substi- nium elimination to yield allyl amine de- [14b] N-alkyl-substituted aziridines are poor tuted aziridines. rivatives. electrophiles whereas incorporation of The overall outcome of this rearrange- electron-withdrawing groups like Ns, Ts, ment is nitrogen migration from C(2), and acyl make them very reactive electro- 9. Selena-Aza-Payne-type C(3) to C(1) carbon to form allyl amine philes. Aziridines can be used as substrates Rearrangement derivatives via the formation of seleni- for ring-opening reactions with 1. Simple rane intermediate. This rearrangement Intramolecular nucleophilic (S 2) dis- is totally different from the well-studied non-activated aziridines like N-alkyl sub- N stituted aziridines failed to undergo aziridi- placement of aziridines/epoxides with Payne, aza-Payne and thia-Payne and tel- [15] ne ring-opening reactions with 1. However, oxygen (Payne rearrangement), nitro- luride mediated rearrangements reported [16] [17] activated N-tosyl aziridines are good sub- gen (aza-Payne), sulfur (thia-Payne), in the literature so far. In all the previously [18] [19] strates and they underwent smooth ring selenium and tellurium nucleophiles reported cases, leaving groups such as to- opening in the presence of 1 without use of in the presence of a base is well studied in sylate or mesylate undergo reaction first any external Lewis acid catalyst. Utilizing the literature (Scheme 10). Recently, tet- followed by aziridine ring opening to give this methodology, a number of chirally rathiomolybdate mediated thia-aza-Payne- 2-substituted allyl amine or allyl alcohol pure β-amino diselenides were synthesized starting from the corresponding chirally pure mono-substituted N-tosyl aziridines TsN NTs Ts [Et N] WSe 1 R OTs R using 1 as a selenium transfer reagent. In N 4 2 4 –WSe3 R R NTs all cases, regiospecific ring opening took OTs 'Selena-Aza-Payne' Se Se 3 2 Se Se Se 1 type rearrangement place at the less substituted carbon to fur- W W selenirane Se nish chirally pure β-amino diselenides in Se Se Se intermediate good yields (Scheme 8).[14b]

Further, this methodology was ex- Selenium –Se tended to stereospecific ring-opening re- elimination actions of 2,3-disubstituted aziridines. When meso-N-tosyl-2,3-diethylaziridine 1 R 3 NHTs (20a) was subjected to ring opening with 2 1 (1.2 equiv.; CH CN, 28 ºC, 10 h) the 3 anti-β-aminodiselenide 21a was obtained Scheme 11. New selena-aza-Payne-type rearrangement. Chemistry in india Part i CHIMIA 2012, 66, No. 12 925

Ts O O N R [Et4N]2WSe4 OTs NHTs (COCl)2,[Et4N]2WSe4 BnO CH CN, rt, 7-8 h BnO 24 3 R N 92-95% 25 N CH2Cl2,–78 °Cto25°C R=H,Me 1.5 h, 79% 38a 36a O Se Ts Se Se Se R N [Et4N]2WSe4 O R NHTs + 3 1 3 2 OTs 2 R POCl3,[Et4N]2WSe4 1 CH3CN, rt, 10-12 h 3 1 2 N N 26 80-86% 27 NHTs CH2Cl2,–78 °Cto25°C 4:1 R=Me, n-Pr, n-Bu, n-Pen 28 O 2h,55% O 36b 38b

H Ts NHTs H POCl3,[Et4N]2WSe4 N N [Et N] WSe Se Se N 4 2 4 NHTs + + OTs R R R CH2Cl2,-78 °Cto25°C Se R CH3CN, rt, 13 h 30 O 1.5 h, 60% 29 70-78% 31 NHTs 36c 38c X=CH3,-CH2OBn 2:2:1 32 O POCl3,[Et4N]2WSe4 No reaction NH Ts Se Se 2 CH2Cl2,–78 °Cto25°C N NHTs [Et4N]2WSe4 1 (1.1 equiv) 12 h OTs + 36c CH3CN, rt, 8h 33 34 NHTs 88% 35 O 4:1 (COCl)2,[Et4N]2WSe4 No reaction N CH2Cl2,–78 °Cto25°C Scheme 12. Selena-aza-Payne-type rearrangement of different aziridine- 12 h methanoltosylates. 36d Se O (COCl)2,[Et4N]2WSe4 N N CH Cl ,–78 °Cto25°C O Se 2 2 1.5 h, 91% (COCl)2 or POCl3 Cl R1 R R1 36e 38e R N 1 N Cl R N R R R2 R2 2 Scheme 14. Synthesis of various selenoamides. 36 37 38 10 examples

POCl3,CH2Cl2 Scheme 13. Synthesis of selenoamides from the corresponding amides () () n O n Se via chloroiminium salt. N –78 °Cto25°C, 1 N n=1,39a n=1,40a, 62% n=3, 39b n=3,40b, 70% derivatives, whereas in the present case the amides are rather difficult to synthesize ring opening of aziridine takes place first owing to the difficulty in preparation and Scheme 15. Synthesis of N-methyl seleno- followed by reaction of tosylate to give their stability. In general, selenoamides lactams. 2,3-disubstituted allyl amine derivatives as are synthesized by the reaction of amides the major product (Scheme 11). with various selenating reagents such as Reaction of simple aziridinemetha- (iBu Al) Se,[5c] (Me Si) Se[23] selenium tor, the corresponding selenoamide 38c 2 2 3 2 noltosylates 24 with 1 furnished exclu- version of Lawesson’s reagent[24] etc. was obtained in good yield. On the other sively allyl amine derivatives 25 in quanti- Selenoamides are also synthesized from hand, benzamide (36d, N-unsubstituted tative yield with excellent regioselectivity. chloroiminium salts by reaction with sel- amide) gave no product under similar re- Similarly, the trans-aziridinemethanolto- enating reagents such as LiAlSeH [25] and action conditions. Steric bulk at the nitro- 2 sylates 26 gave trans-allyl amine deriva- NaSeH.[26] Most of these reactions suffer gen has a pronounced effect on the reaction tives 27 as the major product along with from lower reaction yields, longer reaction yield compared to the steric bulk at the car- the cyclic diselenides as minor product in time and harsh reaction conditions. bonyl carbon. For example, N-isopropyl- 4:1 ratio. In the reaction of cis-aziridine- While direct treatment of tetraseleno- N’-cyclohexylbenzamide (36d) did not methanoltosylates 29 with 1, a mixture of tungstate 1 with amides 36 failed to give the furnish the corresponding selenoamide regio isomers of allyl amine products 30, selenoamides, the reaction of amides with whereas N-pivaloylpyrrolidine (36e) gave 31 and cyclic diselenides 32 were formed oxalyl chloride or POCl in CH Cl (–78 the corresponding selenoamide 38e in 91% 3 2 2 in 2:2:1 ratio. Even 2,3,3-trisubstituted °C to 25 °C) generated the corresponding yield (Scheme 14). aziridinemethanoltosylate (33) underwent chloroiminium salts 37 in situ which on Using the same protocol N-methyl- rearrangement as expected with 1 and fur- treatment with reagent 1, afforded the cor- pyrrolidone (39a) and N-methyl capro- nished tertiary allyl amine derivative 34 as responding selenoamides 38 in moderate lactam (39b) were converted to the cor- the major product and the cyclic diselenide to excellent yields (Scheme 13).[27] responding selenolactams 40a and 40b in 35 (4:1) as the minor product in good yield Utilizing this methodology, a num- 62% and 70% yields respectively (Scheme (Scheme 12). ber of amides were transformed to the 15). The advantages of this methodology corresponding selenoamides in moder- are milder reaction conditions and high ate to good yields without any problem. chemoselectivity compared to other re- 10. Synthesis of Selenoamides and Additionally, chemoselective transfor- ported methods. Selenolactams mation could be performed without affecting ester (36a) and keto functionality Selenoamides are important synthetic (36b). N-Mono-substituted amides such as 11. Synthesis of Selenoureas precursors for the synthesis of various N-benzyl phenylacetamide (36c) gave very selenium-containing heterocycles such as poor yield of the resultant selenoamide Selenoureas are also one of the key selenazole and selenazine derivatives.[22] when oxalyl chloride was used as the pro- synthetic precursors for the synthesis of Unlike amides and thioamides, seleno- moter whereas with POCl as the promo- many pharmologically important selenium 3 926 CHIMIA 2012, 66, No. 12 Chemistry in india Part i

Se Me Cl Me Cl [Et N] WSe N 4 2 4 Me R1 Cl Me 1 R1NHR2 + N + K2CO3 N N Me NRR Me Cl CH CN, rt Me Cl N Cl 3 N + RR1NH 41 Me R2 Me NRR1 Se Cl Me Cl Se 42 43 41 A W Cl Se Se 5examples 42

Scheme 16. Synthesis of selenourea derivatives. 1 Me NRR Me Cl S N N –WSe3 1 Me Me Se Se Me NRR Se 1 Se N NRR W Se Me W Me Se Se NH Me Cl [Et4N]2WSe4,K2CO3 N N 43 Se Se N C + Me B Me Cl CH3CN, rt, 77% Cl 42 Scheme 18. Proposed reaction mechanism for the formation of sele- 41a 43a nourea derivatives. Se H N Me Cl [Et4N]2WSe4,K2CO3 Me + N N N OTs Me Cl CH3CN, rt, 64% Me O i. 33% HBr in AcOH Se O Cl O 43b AcO ii. [Et4N]2WSe4, 1 OAc 41b O AcO Se OAc CH3CN, rt, 94% NH2 OAc [Et N] WSe ,K CO Me 44a Me Cl 4 2 4 2 3 N NH OAc OAc + N 45a CH CN, rt, 63% Me Me Cl 3 OTs Cl i. 33% HBr in AcOH Se OMe O AcO ii. [Et4N]2WSe4, 1 OAc 41c 43c O OMe AcO CH3CN, rt, 94% 44b OAc NH2 Se OAc 45b

Me Cl [Et4N]2WSe4,K2CO3 Me + N N NH AcO OTs i. 33% HBr in AcOH Se Me Cl CH3CN, rt, 65% Me O Cl ii. [Et4N]2WSe4, 1 OAc O Br 43d Br AcO CH CN, rt, 94% AcO 41d OAc OAc 3 44a 45c OAc Se H2N Me Cl [Et4N]2WSe4,K2CO3 Me + N NH Scheme 19. Synthesis of 1,6-episeleno sugar derivatives. N Me Me Cl CH3CN, rt, 50% Cl

41e 43e B. This intermediate eliminates another chloride ion in a similar fashion to form an Me Cl [Et N] WSe ,K CO 4 2 4 2 3 Complex mixure iminium intermediate C, which then forms + N the corresponding selenourea derivatives N Me Cl CH3CN, rt H Cl 43 via selenium transfer reaction (Scheme 41f 18). Scheme 17. Reaction of various amines with Viehe’s iminium salt and reagent 1. 12. Synthesis of Cyclic Selenides heterocycles.[28] In general selenoureas N-Isopropyl cyclohexylamine (41a) 12.1 Cyclic-monoselenides are synthesized from isoselenocynates and morpholine (41b) (secondary amines) Tetraselenotungstate 1 reacts with a by reaction with secondary amines.[29] gave better yields of the resultant sele- variety of 1,6-diactivated carbohydrate Alternatively they are also synthesized nourea derivatives as compared to other derivatives to furnish the corresponding from phosgene iminium chloride, lithium primary amines such as p-methoxyaniline 1,6-episeleno hexoses via selenium trans- aluminium hydride and elemental sele- (41c), p-bromobenzylamine (41d) and fer at the anomeric carbon.[32] Treatment nium.[30] Our group developed a one-pot (S)-1-(naphthalen-1-yl)ethanamine (41e). of 6-O-tosyl-1,2,3,4-tetra-O-acetyl-α-d- protocol for the synthesis of selenourea On the other hand, reaction of indole with glucopyranoside (44a) with HBr in ace- derivatives in good yields under mild con- Viehe’s iminium salt gave a mixture of tic acid gave the corresponding glycosyl ditions by the reaction of primary and sec- products, which were difficult to purify bromide. The glycosyl bromide was di- ondary amines with Viehe’s iminium salt because the selenourea derivatives decom- rectly treated with tetraselenotungstate 1 (1.2 equiv., CH CN, 28 °C, 0.5 h) to (phosgene iminium chloride) and tetrase- posed during purification and were not 3 lenotungstate 1 (Scheme 16). The addi- isolable (Scheme 17). give 2,3,4-tri-O-acetylselenolevoglucosan tion of potassium carbonate increased the The reaction mechanism involves a (45a) in 94% yield. Similarly, 2-deoxy-3,4- chemical yield of the reaction by quench- simple nucleophilic addition of amine 41 di-O-acetyselenolevoglucosan (45b) and ing the hydrochloric acid generated during to Viehe’s iminium salt 42 to form the 1,6-episeleno-β-d-galactopyranose (45c) the reaction.[31] In the absence of potassi- corresponding intermediate, A which on were synthesized in excellent yields under um carbonate the yields are lower because displacement of one of the chloride ions similar reactions (Scheme 19). of partial decomposition of the reagent 1 by the amine followed by reaction with Using the same protocol a number of with hydrochloric acid. tetraselenotungstate 1 gave intermediate deoxy amino sugars were synthesized. Chemistry in india Part i CHIMIA 2012, 66, No. 12 927

OTs OH i. 33% HBr in AcOH Se Se i. NaOMe, MeOH, Ac2O R 6 O O ii. [Et N] WSe , 1 4 R1 ii. p-TsCl, Pyridine AcO 4 2 4 OAc 5 1 R4 HO OAc O Se OH AcO [Et4N]2WSe4 1 HO NHAc CH3CN, rt, 75% R NH .HCl iii. Ac2O O NTs 3 R 2 47a 1 46 50% 3steps OAc NHAc 48a 4 2 CH3CN :EtOH (1:1) HO R 3 R R2 3 2 6-9 h, rt NHTs OTs 58 OH i. 33% HBr in AcOH Se 59 i. p-TsCl, Pyridine O 85-92% O ii. Ac O AcO ii. [Et4N]2WSe4, 1 OAc HO 2 OAc O OH AcO HO NHTs CH3CN, rt, 85% NH .HCl 50% 2steps 2 47b 46 OAc NHTs 48b Se Se Se Se Se Se Se Se CH Scheme 20. Synthesis of deoxy amino sugars. 3 H3C HO H3C HO H3C HO H3C HO NHTs NHTs NHTs NHTs 59b,92% 59c,88% 59d,90% OTs OTs Se 59a, 90% O Br 33% HBr in AcOH O [Et N] WSe , 1 AcO AcO 4 2 4 O AcO Br Se Se Se CH3CN, rt, 82% 49 50 Br OAc Se CH3 Se TBDMSO Se 51

H3C HO H3C HO HO Scheme 21. Synthesis of 2,3,6-trideoxy sugars. NHTs NHTs NHTs 59e,89% 59f,90% 59g,85%

Se TsO O i. SOCl2,CH2Cl2 O Scheme 24. Synthesis of cyclic diselenides. OH ii. [Et4N]2WSe4, 1 O O CH3CN, rt, 90% O O TBDMSO 53a 52a Se Se OTBDMS [Et N] WSe 1 TBDMSO Se Se 4 2 4 + Se O NTs TsO i. HCl (dry), AcOH O CH3CN :EtOH (1:1) HO HO O NHTs NHTs OH 60 7h,rt 61a ii. [Et4N]2WSe4, 1 91% 4:1 61b CH CN, rt, 61% OTs 3 OTs 52b 53b Se OTBDMS O NTs TBDMSO Se [Et4N]2WSe4 1 Se + Se Scheme 22. Synthesis of 1,5-episeleno-pen- CH3CN :EtOH (1:1) 7h,rt HO HO toses. OTBDMS NHTs NHTs 83% 62 63a 1:1 63b

O O NH Scheme 25. Synthesis of sterically hindered diselenides. NH N O [Et N] WSe 1 TsO 4 2 4 N O O Se O CH3CN, rt, 22 h, 82% Se OAc 55 54 3,4-di-O-acetyl-d-glucal (49) was treated 12.2 Cyclic-diselenides with HBr/acetic acid to give the dibromide Tetraselenotungstate 1 reacts with thy- 50, which on treatment with tetraseleno- midine-derived tosylate 54 to give an un- [Et N] WSe 1 Br 4 2 4 Se tungstate 1 afforded 2,3-dideoxy-3-bromo- expected cyclic diselenide 55 in 82% yield Br CH CN, rt, 30 min Se 3 4-O-acetyl-1,6-episelenoglucopyranose [13] 89% with an inverted configuration at C(3). 56 57 (51) in very good yield (Scheme 21). Although similar diselenides are known, The reagent 1 has also been success- their synthesis involves longer routes and Scheme 23. Synthesis of cyclic diselenides. fully used for the synthesis of 1,5-episele- lower yields.[36] In a similar fashion di- no-pentoses. Treatment of 5-O-tosyl-2,3- bromide 56 on treatment with tetraseleno- At first 2-acetamido-1,3,4-tri-O-acetyl- isopropylidine-d-ribose (52a) with SOCl tungstate 1 gave the corresponding cyclic 2 2-deoxy-6-O-p-toluenesulfonyl-β-d- in dichloromethane gave the correspond- diselenide 57 in 89% yield (Scheme 23).[9] glucopyranose (47a) was synthesized from ing ribosyl chloride which was immedi- Tetraselenotungstate 1 on reaction with glucosamine hydrochloride (46) via acety- ately treated with 1 to give the correspond- 1,4-cis-aziridino epoxides 58,[37] furnished lation, tosylation followed by acetylation ing 1,5-episeleno-2,3-isopropylidine-d- the corresponding bicyclic diselenides 59 in moderate yield (50%). Treatment of ribose (53a) in very good yield. Similarly, in good to excellent yields.[38] In the case 47a with HBr/AcOH followed by treat- 2-deoxy-3,5-di-O-tosyl-d-erythropento- of tri- and tetra-substituted cis-aziridino- ment with tetraselenotungstate 1 furnished side (52b) on treatment with dry HCl in epoxides, aziridine ring opening always 2-acetamido-3,4-di-O-aetyl-2-deoxy- glacial acetic acid gave the correspond- took place at the more substituted carbon selenolevoglucosan (48a). In a similar ing 2-deoxy-3,5-di-O-tosyl-d-erythro- atom[39] whereas in case of the epoxide, fashion 2-deoxy-2-tosylamino-3,4-di-O- pentosyl chloride derivative, which upon the ring opening took place both at the less acetyl-1,6-episelenoglucose (48b) was treatment with 1 afforded 2-deoxy-1,5- substituted and more substituted carbon at- synthesized in 85% yield (Scheme 20). episeleno-3-O-tosyl-ribose (53b) in 61% om depending on the substrate. Diselenide Tetraselenotungstate 1 was also used yield (Scheme 22). These compounds are 59d was obtained by ring opening of aziri- for the preparation of synthetic precursors excellent precursors for the preparation of dine from the more hindered carbon and of 2,3,6-trideoxy carbohydrate derivatives, deoxynucleotides, 2,3-dideoxy-3-thiocyti- epoxide from the less substituted carbon as they are present in natural products like dine (3TC), which are potent anti-human whereas diselenide 59e was obtained by aclacinomycin as terminal sugars and are immunodeficiency virus (HIV) active[34] the ring opening of aziridine and epoxide also intermediates in the synthesis of antibi- and anti-human hepatitis B virus active.[35] from the more substituted carbon centers otic, amicetin.[33] Accordingly, 6-O-tosyl- (Scheme 24). 928 CHIMIA 2012, 66, No. 12 Chemistry in india Part i

Se Se OTs Se OR' Se Se Se OTs Se Se O [Et N] WSe , 1 OMe OR' 4 2 4 O O [Et4N]2WSe4, 1 OR' MeO CH CN, rt, 8h O 3 OR' MeO CH CN, rt, 8h,67% Br OR' 3 MeO OR' Br MeO R' =Ac, 64a 66 OMe R' =Pv, 64b R' =Ac, 65a,73% 67 R' =Pv, 65b,55%

Scheme 26. Synthesis of cyclic tetraselenides. Scheme 28. Synthesis of 1,5-cis-cyclic tetraselenide.

OTs OTs OAc OTs O O O O O O AcO O AcO AcO AcO AcO Berlin, 2000; (b) T. Wirth, Angew. Chem. Int. AcO Se Se 64a Br D E W Ed. 2000, 39, 3740 and references cited therein. Se Se Se Se W [2] (a) ‘Organic Selenium Compounds: Their Se Se Chemistry and Biology’, Eds. D. L. Klayman, [Et4N]2WSe4, 1 W. H. H. Gunther, Wiley, New York, 1973; (b) K. C. Nicolaou, N. A. Petasis, ‘Selenium in Natural Products Synthesis’, CIS, Philadelphia, Se Se Se Se Se Se 1984. Se Se W Se Se W W Se Se Se [3] (a) G. Mugesh, W. W. Du Mont, H. Sies, Chem. Se Se Se OAc O AcO O Rev. 2001, 101, 2125 ; (b) G. Mugesh, H. B. O AcO AcO AcO Singh, Chem. Soc. Rev. 2000, 29, 347. AcO AcO OAc OAc Se Se F W [4] (a) L. Syper, J. Miochowski, Tetrahedron 1988, 65a Se Se 44, 6119; (b) J. A. Gladysz, J. L. Hornby, J. E. Garbe, J. Org. Chem. 1978, 43, 1204; (c) D. P. Scheme 27. Tentative mechanism for the formation of tetraselenide 65a. Thompson, P. Boudjouk, J. Org. Chem. 1988, 53, 2109; (d) J. X. Wang, C. H. Wang, W. Cui, Y. Hu, J. Chem. Soc, Perkin Trans. 1 1994, 16, 2341; (e) R. I. Cordova, E. Vanden Hoven, A. Introduction of a bulky substituent at gave the corresponding tetraselenide 65a Mohammed, B. M. Pinto, Can. J. Chem. 1995, the aziridine carbon did not change the (Scheme 27). 73, 113. regioselectivity of aziridine ring opening; On the other hand, reaction of manno- [5] (a) D. L. Klayman, T. S. Griffin, J. Am. Chem. aziridino-epoxide 60 on treatment with tet- syl bromide 66 (lacking neighboring group Soc. 1973, 95, 197; (b) C. M. Copeland, J. Ghosh, D. P. McAdam, B. W. Skelton, R. V. raselenotungstate 1 gave 61a (obtained by participation) with tetraselenotungstate 1 Stick, A. H. White, Aust. J. Chem. 1988, 41, the ring opening of aziridine from the more gave exclusively 1,5-cis-substituted cyclic 549; (c) G. M. Li, R. A. Zingaro, J. Chem. Soc., substituted carbon) as the major product. tetraselenide 67 in 67% yield (Scheme 28). Perkin Trans. 1 1998, 647; (d) H. Ishihara, M. On the other hand introduction of a bulky Koketsu, Y. Fukuta, F. Nada, J. Am. Chem. Soc. substituent at the epoxide carbon showed 2001, 123, 8408. [6] A. Müller, E. Diemann, R. Jostes, H. Bogge, a pronounced effect on the regioselectivity 13. Conclusion and Outlook Angew.Chem. Int. Ed. Engl. 1981, 20, 934. of ring opening; aziridino-epoxide 62 gave [7] (a) P. Gopinath, S. Chandrasekaran, ‘e- a 1:1 mixture of products 63a and 63b, re- Organoselenium compounds play an EROS Encyclopedia of Reagents for Organic sulting from the ring opening of epoxide important role in biochemical processes Synthesis’, Eds. D. Crich, A. B. Charette, P. L. Fuchs, T. Rovis, John Wiley & Sons LTD, 2011, from both the possible sites (Scheme 25). and serve to function as antioxidants, an- DOI: 10.1002/047084289X.rn01353; (b) P. R. ticancer and antiviral agents. Hence, de- Sridhar, Synlett 2004, 744. 12.3 Cyclic-tetraselenides velopment of new and efficient selenating [8] S. C. O’Neal, J. W. Kollis, J. Am. Chem. Soc. Tetraselenotungstate 1 on treatment reagents is becoming more important. In 1988, 110, 1971. with mannose-derived anomeric bromides recent years, applicability and utility of [9] V. Saravanan, E. Porhiel, S. Chandrasekaran, Tetrahedron Lett. 2003, 44, 2257. 64a and 64b furnished novel 1,5-trans- tetraselenotungstate as an efficient sele- [10] S. M. Berry, M. D. Gieselman, M. J. Nilges, substituted cyclic tetraselenides 65a and nium transfer reagent in organic synthesis W. A. van der Donk, Y. Lu, J. Am. Chem. Soc. 65b with a carbohydrate backbone, instead has been demonstrated in different areas of 2002, 124, 2084. of the expected 1,6-episelenide (Scheme organic chemistry, which includes the syn- [11] (a) E. P. Painter, J. Am. Chem. Soc. 1947, 69, 229; (b) C. S. Pande, J. Rudock, R. Walter, J. [40] 26). The trans-stereochemistry of 65a thesis of simple diselenides, amino acid- Org. Chem. 1970, 35, 1440; (c) H. D. Jakubke, was confirmed by X-ray crystallography. derived diselenides, carbohydrate-based J. Fischer, K. Jost, J. Rudinger, Collect. Czech. In order to account for the trans-stereo- diselenides, selenoamides, selenoureas, cy- Chem. Commun. 1968, 33, 3910. chemistry in the formation of tetraselenide clic diselenides, mono-selenides and con- [12] R. G. Bhat, E. Porhiel, V. Saravanan, S. 65a the following tentative mechanism formationally locked diselenides. Despite Chandrasekaran, Tetrahedron Lett. 2003, 44, 5251. was proposed. these significant advancements, further [13] K. Sivapriya, P. Suguna, S. Shubashree, P. R. At first, neighboring group participa- developments are necessary to broaden the Sridhar, S. Chandrasekaran, Carbohydr. Res. tion of the axial acetate group in mannosyl application by making this reagent more 2007, 342, 1151. bromide 64a led to the corresponding oxo- versatile and explore the possibility of de- [14] (a) D. Tanner, Angew. Chem. Int. Ed. 1994, 33, 599; (b) J. B. Sweeney, Chem. Soc. Rev. 2002, nium ion intermediate D, which on reaction veloping chiral selenium transfer reagents 31, 247; (b) D. Sureshkumar, Ph.D Thesis, with tetraselenotungstate 1 from the axial in organic synthesis. Indian Institute of Science, Bangalore, 2007. side gave the corresponding organosele- [15] (a) G. J. Payne, J. Org. Chem. 1962, 27, 3819; nium intermediate E. Further reaction of Received: October 10, 2012 (b) K. B. Sharpless, C. H. Behrens, T. Katsuki, E with tetraselenotungstate 1 at the C(5) A. W. Lee, V. S. Martin, M. Takatani, S. M. Viti, [1] (a) ‘Topics in Current Chemistry: Organo- G. J. Walker, S. S. Woodard, Pure Appl. Chem. tosylate furnished intermediate F, which selenium Chemistry, Modern Developments 1983, 589; (c) ‘Organic Reactions’, Vol. 60, Ed. then through an internal redox process[41] in Organic Synthesis’, Ed. T. Wirth, Springer, L. E. Overman, John Wiley and Sons, Inc. 2002. Chemistry in india Part i CHIMIA 2012, 66, No. 12 929

[16] T. Ibuka, Chem. Soc. Rev. 1998, 27, 145 and re- [24] (a) J. Bethke, K. Karaghiosoff, L. A. Wessjohan, J. W. Beach, W. B. Choi, S. Yeola, D. C. Liotta, ferences therein. Tetrahedron Lett. 2003, 44, 6911; (b) P. Antimicrob. Agents Chemother. 1992, 36, 672; [17] J. K. Branalt, I. Kvarnstrom, J. Org. Chem. Bhattacharyya, J. D. Woolins, Tetrahedron Lett. (b) K. C. Chung, J. W. Beach, L. S. Jeong, B. G. 1996, 61, 3604. 2001, 42, 5949. Choi, F. I. Comer, A. J. Alves, R. F. Schinazi, J. [18] G. Polson, D. C. Dittmer, J. Org. Chem. 1988, [25] M. Koketsu, Y. Okayama, H. Aoki, H. Ishihara, Org. Chem. 1991, 56, 6503. 53, 791. Heteroat. Chem. 2002, 13, 195. [35] (a) S. L. Doong, C. H. Tsai, R. F. Schinazi, D. C. [19] (a) A. S. Pepito, D. C. Dittmer, J. Org. Chem. [26] (a) D. H. R. Barton, G. Fontana, Tetrahedron Liotta, Y. C. Chen, Proc. Natl. Acad. Sci. USA 1997, 62, 7920; (b) B. Chao, D. C. Dittmer, 1996, 52, 11163; (b) M. P. Cava, L. A. Saris, J. 1991, 56, 6503; (b) J. W. Beach, L. S. Jeong, A. Tetrahedron Lett. 2001, 42, 5789. Chem. Soc. Chem. Commun. 1975, 617. J. Alves, D. Pohl, H. O. Kim, C. N. Chang, S. L. [20] (a) D. Sureshkumar, S. Koutha, S. Chandra- [27] V. Saravanan, C. Mukherjee, S. Das, S. Doong, R. F. Schinazi, Y. C. Cheng, C. K. Chu, sekaran, J. Org. Chem. Soc. 2010, 75, 5533; (b) Chandrasekaran, Tetrahedron Lett. 2004, 45, J. Org. Chem. 1992, 57, 2217. D. Sureshkumar, S. Koutha, S. Chandrasekaran, 681. [36] G. Adiwidjaja, O. Schulze, J. Voss, J. Wirsching, J. Am. Chem. Soc. 2005, 127, 12760. [28] (a) R. Caujolle, H. Amarouch, M. Payard, P. M. Carbohydr. Res. 2000, 325, 107. [21] D. Sureshkumar, S. Koutha, S. Chandrasekaran, Loiseau, C. Bories, P. Gayral, M. D. Linas, J. P. [37] (a) O. P. Brien, C. D. Pilgram, Org. Biomol. Eur. J. Org. Chem. 2007, 4543. Seguela, Eur. J. Med. Chem. 1995, 30, 801; (b) Chem. 2003, 1, 523; (b) D. Sureshkumar, S. [22] (a) H. R. Zhao, Q. S. Yu, Chinese Chem. Lett. T. Phoung, T. Khac-Minh, N. T. Van Ha, H. T. Maity, S. Chandresekaran, J. Org. Chem. 2006, 2002, 13, 729; (b) M. Koketsu, Y. Takenaka, N. Phuong, Bioorg. Med. Chem. Lett. 2004, 14, 71, 1653. H. Ishihara, Synthesis 2001, 5, 731; (c) M. 653 and references therein. [38] D. Sureshkumar, V. Ganesh, S. Chandrasekaran, Koketsu, T. Senda, K. Yoshimura, H. Ishihara, [29] M. Koketsu, N. Suzuki, H. Ishihara, J. Org. J. Org. Chem. 2007, 72, 5313. J. Chem. Soc., Perkin Trans. 1 1999, 453; (d) P. Chem. 1999, 64, 6473. [39] a) P. Lin, K. Bellos, H. Stamm,A. Onistschenko, F. Zhang, Z. C. Chen, Synthesis 2000, 9, 1219; [30] M. Koketsu, Y. Fukuta, H. Ishihara, J. Org. Tetrahedron 1992, 48, 2359; (b) P. Li, E. M. (e) M. L. Petrov, M. A. Abramov, Phosphorus, Chem. 2002, 67, 1008. Forbeck, C. D. Evans, M. M. Joullié, Org. Lett. Sulfur, Silicon Related Elements 1998, 134, [31] K. Sivapriya, P. Suguna, A. Banerjee, V. 2006, 8, 5105; (c) E. M. Forbeck, C. D. Evans, 331; (f) H. Mizuno, M. Kita, J. Fujita, M. Saravanan, N. R. Desirazu, S. Chandrasekaran, J. A. Gilleran, P. Li, M. M. Joullié, J. Am. Chem. Nonoyama, Inorg. Chim. Acta 1992, 202, 183; Bioorg. Med. Chem. Lett. 2007, 17, 6387. Soc. 2007, 129, 14463. (g) M. Nonoyama, K. Nonoyama, Polyhedron [32] P. R. Sridhar, V. Saravanan, S. Chandrasekaran, [40] K. Sivapriya, P. Suguna, S. Chandrasekaran, 1991, 10, 2265; (h) M. Koketsu, S. Hiramatsu, Pure Appl. Chem. 2005, 77, 145. Tetrahedron Lett. 2007, 48, 2091. H. Ishihara, Chem. Lett. 1999, 485. [33] E. L. Albano, D. Horton, J. Org. Chem. 1969, [41] (a) W. H. Pan, M. A. Harmer, T. R. Halbert, E. [23] G. M. Li, R. A. Zingaro, M. Segi, J. H. 34, 3519. I. Stiefel, J. Am. Chem. Soc. 1984, 106, 459; Reibenspies, T. Nakajima, Organomettalics [34] (a) R. F. Schinazi, C. K. Chu, A. Peck, A. (b) C. L. Coyle, M. A. Harmer, G. N. George, 1997, 16, 756. Mcmillan, R. Mathis, D. Cannon, L. S. Jeong, M. Daage, E. I. Stiefel, Inorg. Chem. 1990, 29, 14; (c) P. M. Boorman, M. Wang, M. Parvez, J. Chem. Soc., Chem. Commun. 1995, 999; (d) N. L. Kruhkak, M. Wang, P. M. Boorman, M. Parvez, Inorg. Chem. 2001, 40, 3141.