DOI:
10.1039/C5RA06147D
(Paper)
RSC Adv., 2015,
5, 51807-51811
Synthesis of chiral Nβ-protected amino diselenides from the corresponding amino alkyl iodides using NaBH2Se3 as a selenating reagent and their conversion to seleninic acids†
Received
7th April 2015
, Accepted 30th May 2015
First published on 1st June 2015
Abstract
A convenient approach has been presented for the synthesis of Nβ-protected amino diselenides from the corresponding amino alkyl iodides using in situ generated NaBH2Se3 as an efficient selenating reagent. All the diselenides are obtained in good yields under very mild conditions, short duration times and the protocol is free from racemization. The methodology has been effectively extended to the synthesis of N-protected L-selenocystine methyl ester. Clean oxidation of Nβ-protected amino diselenides to Nβ-protected amino seleninic acids using 35% aqueous H2O2 has also been accomplished. The present protocol is compatible with all the common urethane protecting groups.
Introduction
Selenium is an essential trace element present in many different compounds with unequivocal functions.1 Organoselenium compounds find applications in organic synthesis,2 ligand chemistry3 and in biological processes.4 Selenium supplementation plays an important role in disease prevention like cancer, cardiovascular, neurodegenerative and Alzheimer's.5–8 The common dietary selenium compounds are selenite, selenomethionine, methylselenocysteine and selenocysteine (SeCys). In contrast to oxygen and sulphur, the selenium based methodologies offer several unique features in organic chemistry.9 Remarkably, diselenides have attracted much attention as intermediates in organic synthesis due to their higher stability and easier handling relative to parent selenols. Most importantly chiral diselenides have been employed as useful ligands and catalysts in various asymmetric transformations such as diethyl zinc addition to aldehydes,10 asymmetric hydrosilylation,11 1,4-addition of Grignard reagents to enones,12 stereo selective ring opening of epoxides,13 palladium-catalyzed asymmetric allylic substitution14 and electrophilic selenenylation of alkenes.15 SeCys, recognized as the 21st proteinogenic amino acid, often found in enzymatic active sites, where its known function is either acting as a nucleophile, a metal ligand or a redox element. In mammals, selenium is an integral part of selenoproteins as selenocysteine.16 The derivatives of SeCys are the convenient precursors for the synthesis of dehydroamino acids which are useful in the preparation of peptide conjugates.17
There are few methods reported for the synthesis of β-amino acid derived diselenides, which are employed in the synthetic organic chemistry. Braga et al., reported the synthesis of β-amino diselenides from N-Boc-aziridines18 as well as N-Boc-2-oxazolidinones19 employing Li2Se2 as a selenating reagent. But these protocols are limited to the synthesis of Nβ-Boc amino diselenides, take more time and give moderate yields. Chandrasekaran et al., reported the synthesis of N-Ts-β-amino diselenides from aziridines using tetraethylammonium tetraselenotungstate [(Et4N)2WSe4] as a selenating reagent.20 Later a variety of Nβ-protected amino diselenides and selenocystine derivatives from sulfamidates using potassium selenocyanate (KSeCN) and benzyltriethylammonium tetrathiomolybdate [(BnEt3N)2MoS4] have also been reported.21 However, longer reaction duration and use of expensive reagents which decompose on prolonged exposure to air are some of the disadvantages of these protocols. Cu(II) catalyzed synthesis of dialkyl, aryl or heteroaryl diselenides has also been known but the alkaline conditions employed in these protocols are not compatible with Fmoc chemistry.22,23 A few other approaches have also been reported for the synthesis of dialkyl, aryl or heteroaryl diselenides from the corresponding halides.24–27 Thus a method circumventing the use of aqueous alkali, compatible with common urethane protecting groups (Boc/Cbz/Fmoc), using easily prepared starting material is desirable for the synthesis of Nβ-protected amino diselenides.
Seleninic acids are of synthetic importance due to their utility in the construction of seleninates, selenonates28 and other reaction intermediates.29,30 They have also found widespread application as catalysts in oxidation reactions.31,32 Seleninic acid group is the most stable, among the four organic acids of selenium (selenol, selenenic, seleninic, and selenonic acids).33 Kehler et al., reported the synthesis of 3-aminopropaneseleninic acid dihydrotosylate and piperidine-4-seleninic acid dihydrotosylate from the corresponding diselenides using p-toluene sulfonic acid monohydrate and 35% aqueous H2O2.34 Knapp et al., reported the clean oxidation of selenoesters to the seleninic acids using dimethyldioxirane (DMDO) in stoichiometric amounts.28 Oxidative cleavage of organodiselenides by aqueous Br2,35 aqueous H2O2
36 or ozone37 has also been reported for the synthesis of seleninic acids. In continuation of our efforts in organoselenium chemistry38,39 and in order to overcome the drawbacks associated with the existing protocols, we envisaged to devise a simple, mild and efficient protocol for the synthesis of Nβ-protected amino diselenides from the corresponding alkyl iodides using in situ generated NaBH2Se3 as a selenating reagent. Further, the oxidation of Nβ-protected amino diselenides to the seleninic acids using 35% aqueous H2O2 is delineated.
Results and discussion
The selenating reagent for the present study was prepared by the method of Lalancette et al., which involved the treatment of one mole of NaBH4 with three moles of selenium in THF at 0 °C under nitrogen atmosphere (Scheme 1).40 Hydrogen gas gets evolved immediately and within 10 minutes the conversion of black selenium powder into reddish suspension indicates the formation of sodium selenoborate (NaBH2Se3).41,42 So far NaBH2Se3 has been employed as a reducing agent however its selenating ability has not been explored.43
![image file: c5ra06147d-s1.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-s1.gif) |
| Scheme 1 Preparation of sodium selenoborate (NaBH2Se3). | |
Our initial efforts were focused on the synthesis of Boc–Phe–CH2–Se–)2 (2c). In a typical reaction, to the freshly prepared NaBH2Se3 suspension in THF at 0 °C in a two-necked round-bottom flask under nitrogen atmosphere, a solution of Boc–Phe–CH2–I (1c) in THF was added drop wise using dropping funnel at 0 °C. As monitored by TLC, the desired Boc–Phe–CH2–Se–)2 (2c) was formed within 20 min. After completion of the reaction, the reaction mixture was filtered through celite and washed with THF. After simple workup, Boc–Phe–CH2–Se–)2 (2c) was isolated by column chromatography in 95% yield (Scheme 1). The structure of Boc–Phe–CH2–Se–)2 (2c) was confirmed by mass spectral and NMR spectroscopic analysis [1H NMR δ: 3.16–3.26 (m, 2H, –CH2–Se–); 13C NMR δ: 34.92 ppm for –CH2–Se– carbon; 77Se NMR δ 280.00 ppm]. Other selenating reagents such as Li2Se2,14 (Et4N)2WSe4
21 and Na2Se2
44 were found to be inefficient in affording the Boc–Phe–CH2–Se–)2 (2c) in good yield (Table 1). Moreover Na2Se2 is not compatible with Fmoc chemistry and large scale preparation of Li2Se2 using super hydride (Liet3BH) is inconvenient due to the formation of pyrophoric byproducts. Preparation of some of these selenating reagents (Table 1, entry 2, 3) also take longer reaction duration.
Table 1 Screening of other selenating reagents for the synthesis of 2c
![image file: c5ra06147d-u1.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u1.gif)
|
Entry |
Selenating reagent |
Time |
Yield (%) |
1 |
Li2Se2 |
2.5 h |
62 |
2 |
(Et4N)2WSe4 |
70 min |
68 |
3 |
Na2Se2 |
3 h |
55 |
The milder condition, shorter reaction duration (20 min) and the excellent yield obtained in case of 2c prompted us to examine the scope and generality of the present protocol for the conversion of a series of N-protected amino alkyl iodides 1 to the corresponding diselenides 2 (Table 2). The present methodology is an efficient one due to the following attributes: compatibility with the common urethane protecting groups like Boc/Cbz/Fmoc, starting material can be prepared easily, monoselenides and triselenides are not formed as byproducts, simple workup and purification procedure (Scheme 2).
Table 2 List of Nβ-protected amino diselenides 2
Entry |
Amino alkyl iodide 1 |
β-Amino diselenide 2 |
Yield (%) |
a |
![image file: c5ra06147d-u2.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u2.gif) |
![image file: c5ra06147d-u3.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u3.gif) |
94 |
b |
![image file: c5ra06147d-u4.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u4.gif) |
![image file: c5ra06147d-u5.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u5.gif) |
91 |
c |
![image file: c5ra06147d-u6.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u6.gif) |
![image file: c5ra06147d-u7.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u7.gif) |
95 |
d |
![image file: c5ra06147d-u8.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u8.gif) |
![image file: c5ra06147d-u9.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u9.gif) |
93 |
e |
![image file: c5ra06147d-u10.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u10.gif) |
![image file: c5ra06147d-u11.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u11.gif) |
91 |
f |
![image file: c5ra06147d-u12.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u12.gif) |
![image file: c5ra06147d-u13.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u13.gif) |
92 |
g |
![image file: c5ra06147d-u14.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u14.gif) |
![image file: c5ra06147d-u15.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u15.gif) |
94 |
h |
![image file: c5ra06147d-u16.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u16.gif) |
![image file: c5ra06147d-u17.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u17.gif) |
92 |
i |
![image file: c5ra06147d-u18.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u18.gif) |
![image file: c5ra06147d-u19.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u19.gif) |
94 |
j |
![image file: c5ra06147d-u20.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u20.gif) |
![image file: c5ra06147d-u21.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-u21.gif) |
93 |
![image file: c5ra06147d-s2.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-s2.gif) |
| Scheme 2 Synthesis of Nβ-protected amino diselenides 2. | |
Using chiral HPLC, the racemization study of the prepared compounds Fmoc–L–Phe–CH2–Se)2– 2h and Fmoc–D–Phe–CH2–Se)2– 2h* was analyzed. The retention times were found to be at Rt = 15.45 min for 2h and Rt = 17.54 min for 2h* respectively. Intentionally prepared equimolar mixture of 2h and 2h* showed distinct peaks at Rt = 15.80 min and Rt = 17.38 min. This confirms that the present protocol is free from racemization.
L-Serine is the most widely used starting material for the synthesis of L-selenocysteine among the amino acids (serine, asparatic acid, glycine, cysteine) employed. The conversion of L-serine to the L-selenocysteine can be achieved directly using Woollin's reagent but the protocol is restricted to N-acetyl protection45 and the activation through tosylates,46 halides,44,47,48 β-lactones,49 or sulfamidates21 using conventional selenating reagents is also known. But these protocols often suffer from several limitations, which include harsh reaction conditions, longer reaction duration, use of expensive reagents, inseparable byproducts formation, difficulty in workup, incompatibility with base sensitive groups, etc. A protocol which circumvents some of the drawbacks associated with reported protocols is desirable for the synthesis of L-selenocysteine from L-serine.
In order to demonstrate the wider scope of this reagent, we investigated NaBH2Se3 for the synthesis of N-protected L-selenocystine methyl ester 5 from N-protected L-serine methyl ester 3 through the activation to iodide 4 (Scheme 3). In brief, commercially available N-protected L-serine methyl ester 3 was converted to the N-protected L-serine iodide methyl ester 4 in excellent yield using imidazole, PPh3 and I2.50 The solution of 4 in THF was then treated with in situ generated NaBH2Se3 to afford the N-protected L-selenocystine methyl ester 5 in good yield. Due to the instability and high reactivity of selenol (SeH) functional group, Secys is usually prepared as diselenide, which is reduced51,52 in situ to the selenol when needed. The present methodology is more convenient than existing procedures for the synthesis of N-protected L-selenocystine methyl ester 5 from N-protected L-serine methyl ester 3.
![image file: c5ra06147d-s3.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-s3.gif) |
| Scheme 3 Synthesis of N-protected L-selenocystine methyl ester 5 from N-protected L-serine methyl ester 3. | |
Since there are no reports on the synthesis of Nβ-protected amino seleninic acids, in the next part of the work, we concentrated on the synthesis of amino acid derived seleninic acids 6 through the oxidation of the diselenides 2 using mild H2O2 (35% aqueous). In a typical study, to a solution of Boc–Phe–CH2–Se–)2 (2c) in DCM at 0° C, 35% aqueous H2O2 was added drop wise under vigorous stirring till the completion of reaction (monitored by TLC, reaction mixture became yellow to colorless). After 30 min, Boc–Phe–CH2–SeO2H (6c) was formed as white precipitate, which was filtered off. A simple recrystallization from ethanol yielded Boc–Phe–CH2–SeO2H (6c) as white stable solid in 91% (Scheme 4). Other solvents such as MeCN, THF, and 1,4-dioxane were found to be inefficient in affording the desired product 6c in good yield. The standardized reaction conditions provided all Nβ-protected amino seleninic acids 6 in good yields (Table 3). The present protocol is more convenient for the synthesis of seleninic acids than reported synthesis from selenoesters and moreover handling of 35% aqueous H2O2 is easier than DMDO.28
![image file: c5ra06147d-s4.tif](/image/article/2015/RA/c5ra06147d/c5ra06147d-s4.gif) |
| Scheme 4 Synthesis of Nβ-protected amino seleninic acid 6. | |
Table 3 List of Nβ-protected amino seleninic acids 6
Entry |
Pg |
R |
Yield (%) |
a |
Boc |
CH(CH3)2 |
90 |
b |
Boc |
CH2CH(CH3)2 |
90 |
c |
Boc |
CH2C6H5 |
91 |
d |
Boc |
C6H5 |
89 |
e |
Cbz |
CH3 |
92 |
f |
Cbz |
CH(CH3)2 |
90 |
g |
Cbz |
CH2C6H5 |
92 |
h |
Fmoc |
CH2C6H5 |
90 |
i |
Fmoc |
CHCH3CH2CH3 |
91 |
Conclusion
In conclusion, an efficient protocol for the synthesis of N-protected L-selenocystine methyl ester, Nβ-protected amino diselenides employing in situ generated NaBH2Se3 as a selenating reagent from the corresponding amino alkyl iodides is developed. The preparation of NaBH2Se3 is simple and fast, acts as a good selenating reagent under mild conditions without any undesirable side reaction. The Nβ-protected amino seleninic acids were also prepared by the oxidation of diselenides with 35% aqueous H2O2 at 0 °C. All the diselenides and seleninic acids prepared during the course of these investigations were found to be stable and were characterized by mass spectrometry, and NMR analyses.
Experimental section
General procedure for the synthesis of Nβ-protected amino diselenides 2, 5
To a solution of sodium borohydride (NaBH4, 10 mmol) in dry THF, black selenium powder (30 mmol) was added at 0 °C under N2 atmosphere. The consumption of selenium powder in less than 10 min lead to heterogeneous reddish suspension which indicated the formation of NaBH2Se3. To the resulting NaBH2Se3 suspension, a THF solution containing Nα-protected amino alkyl iodide 1 or 4 (10 mmol) was added drop wise at 0 °C and the stirring was continued for another 20 min. After completion of the reaction (as monitored by TLC), the reaction mixture was filtered through celite, washed with THF and the solvent was removed under reduced pressure. The crude mass was diluted with EtOAc, washed with water (2 × 20 mL), brine (2 × 20 mL), dried over Na2SO4 and the solvent was removed under reduced pressure. The crude residue 2 or 5 was purified by column chromatography on silica gel with EtOAc/hexane (2
:
8).
General procedure for the synthesis of Nβ-protected amino seleninic acids 6
To a solution of Nβ-protected amino diselenide 2 (10 mmol) in DCM at 0 °C, 35% aqueous H2O2 (64 mmol) was added drop wise under vigorous stirring till the completion of reaction (monitored by TLC). The change in color from yellow to colorless indicated the complete consumption of diselenide. After 30 min, the desired product seleninic acid 6 was formed as white precipitate, which was filtered off and recrystallized using ethanol as a solvent. All seleninic acids were isolated as stable solids.
Acknowledgements
We thank the Council of Scientific and Industrial Research (CSIR), Government of India (no. 02(0149)/13/EMR-II) for the financial assistance. Nageswara Rao Panguluri and Veladi Panduranga are thankful to CSIR for the SRF fellowship.
Notes and references
- C. M. Weekley and H. H. Harris, Chem. Soc. Rev., 2013, 42, 8870–8894 RSC.
- J. Mlochowski, M. Brzaszcz, M. Giurg, J. Palus and H. Wojtowicz, Eur. J. Org. Chem., 2003, 4329–4339 CrossRef CAS PubMed.
- A. Kumar, G. K. Rao, F. Saleem and A. K. Singh, Dalton Trans., 2012, 41, 11949–11977 RSC.
- C. Narajji, M. D. Karvekar and A. K. Das, Indian J. Pharm. Sci., 2007, 69, 344–351 CrossRef CAS PubMed.
- L. Wang, M. J. L. Bonorden, G.-x. Li, H.-J. Lee, H. Hu, Y. Zhang, J. D. Liao, M. P. Cleary and J. Lu, Cancer Prev. Res., 2009, 2, 484–495 CrossRef CAS PubMed.
- S. Tanguy, S. Morel, C. Berthonneche, M.-C. Toufektsian, M. de Lorgeril, V. Ducros, A. Tosaki, J. de Leiris and F. Boucher, Antioxid. Redox Signaling, 2004, 6, 792–796 CAS.
- J. van Eersel, Y. D. Ke, X. Liu, F. Delerue, J. J. Kril, J. Gotz and L. M. Ittner, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 13888–13893 CrossRef CAS PubMed.
- M. A. Lovell, S. Xiong, G. Lyubartseva and W. R. Markesbery, Free Radical Biol. Med., 2009, 46, 1527–1533 CrossRef CAS PubMed.
- D. M. Freudendahl, S. A. Shahzad and T. Wirth, Eur. J. Org. Chem., 2009, 1649–1664 CrossRef CAS PubMed.
- C. Santi and T. Wirth, Tetrahedron: Asymmetry, 1999, 10, 1019–1023 CrossRef CAS.
- Y. Nishibayashi, K. Segawa, J. D. Singh, S.-i. Fukuzawa, K. Ohe and S. Uemura, Organometallics, 1996, 15, 370–379 CrossRef CAS.
- A. L. Braga, S. J. N. Silva, D. S. Ludtke, R. L. Drekener, C. C. Silveira, J. B. T. Rocha and L. A. Wessjohann, Tetrahedron Lett., 2002, 43, 7329–7331 CrossRef CAS.
- S.-i. Fukuzawa, K. Takahashi, H. Kato and H. Yamazaki, J. Org. Chem., 1997, 62, 7711–7716 CrossRef CAS.
- M. Zielinska-Blajet, R. Siedlecka and J. Skarzewski, Tetrahedron: Asymmetry, 2007, 18, 131–136 CrossRef CAS PubMed.
- T. G. Back, Z. Moussa and M. Parvez, J. Org. Chem., 2002, 67, 499–509 CrossRef CAS PubMed.
- M. Muttenthaler and P. F. Alewood, J. Pept. Sci., 2008, 14, 1223–1239 CrossRef CAS PubMed.
- M. Iwaoka, R. Ooka, T. Nakazato, S. Yoshida and S. Oishi, Chem. Biodiversity, 2008, 5, 359–374 CAS.
- A. L. Braga, M. W. Paixao, D. S. Ludtke, C. C. Silveira and O. E. D. Rodrigues, Org. Lett., 2003, 5, 2635–2638 CrossRef CAS PubMed.
- J. A. Sehnem, F. Vargas, P. Milani, V. Nascimento and A. L. Braga, Synthesis, 2008, 1262–1268 CAS.
- D. Sureshkumar, T. Gunasundari, V. Saravanan and S. Chandrasekaran, Tetrahedron Lett., 2007, 48, 623–626 CrossRef CAS PubMed.
- N. B. R. Baig, R. N. Chandrakala, V. S. Sudhir and S. Chandrasekaran, J. Org. Chem., 2010, 75, 2910–2921 CrossRef PubMed.
- D. Singh, A. M. Deobald, L. R. S. Camargo, G. Tabarelli, O. E. D. Rodrigues and A. L. Braga, Org. Lett., 2010, 12, 3288–3291 CrossRef CAS PubMed.
- Z. Li, F. Ke, H. Deng, H. Xu, H. Xiang and X. Zhou, Org. Biomol. Chem., 2013, 11, 2943–2946 CAS.
- R. Ming-De, Z. Hua-Rong, F. Wei-Qiang and Z. Xun-Jun, J. Organomet. Chem., 1995, 485, 19–24 CrossRef.
- V. Saravanan, E. Porhiel and S. Chandrasekaran, Tetrahedron Lett., 2003, 44, 2257–2260 CrossRef CAS.
- K. R. Prabhu and S. Chandrasekaran, Chem. Commun., 1997, 1021–1022 RSC.
- M. Iwaoka, C. Haraki, R. Ooka, M. Miyamoto, A. Sugiyama, Y. Kohara and N. Isozumi, Tetrahedron Lett., 2006, 47, 3861–3863 CrossRef CAS PubMed.
- M. Abdo and S. Knapp, J. Am. Chem. Soc., 2008, 130, 9234–9235 CrossRef CAS PubMed.
- M. Abdo and S. Knapp, J. Org. Chem., 2012, 77, 3433–3438 CrossRef CAS PubMed.
- M. Abdo, Y. Zhang, V. L. Schramm and S. Knapp, Org. Lett., 2010, 12, 2982–2985 CrossRef CAS PubMed.
- E. A. Mercier, C. D. Smith, M. Parvez and T. G. Back, J. Org. Chem., 2012, 77, 3508–3517 CrossRef CAS PubMed.
- D. Crich and Y. Zou, Org. Lett., 2004, 6, 775–777 CrossRef CAS PubMed.
- L. Syper and J. Mlochowski, Tetrahedron, 1987, 43, 207–213 CrossRef CAS.
- N. Stuhr-Hansen, B. Ebert, P. Krogsgaard-Larsen and J. Kehler, Org. Lett., 2000, 2, 7–9 CrossRef CAS PubMed.
- L. Pichat, M. Herbert and M. Thiers, Tetrahedron, 1961, 12, 1–6 CrossRef CAS.
- J. D. McCullough and E. S. Gould, J. Am. Chem. Soc., 1949, 71, 674–676 CrossRef CAS.
- H. J. Reich, C. A. Hoeger and W.W. Willis Jr, Tetrahedron, 1985, 41, 4771–4779 CrossRef CAS.
- T. M. Vishwanatha, N. Narendra, B. Chattopadhyay, M. Mukherjee and V. V. Sureshbabu, J. Org. Chem., 2012, 77, 2689–2702 CrossRef CAS PubMed.
- G. Chennakrishnareddy, G. Nagendra, H. P. Hemantha, U. Das, T. N. G. Row and V. V. Sureshbabu, Tetrahedron, 2010, 66, 6718–6724 CrossRef CAS PubMed.
- J. M. Lalancette and M. Arnac, Can. J. Chem., 1969, 47, 3695–3697 CrossRef CAS.
- J. M. Lalancette, A. Freche, J. R. Brindle and M. Laliberte, Synthesis, 1972, 526–532 CrossRef CAS.
- D. L. Klayman and T. S. Griffin, J. Am. Chem. Soc., 1973, 95, 197–199 CrossRef CAS.
- Z. Hua and Y. Y. Bi, J. Yangzhou Univ., Nat. Sci. Ed., 1999, 2, 19–21 Search PubMed.
- A. H. G. Siebum, W. S. Woo, J. Raap and J. Lugtenburg, Eur. J. Org. Chem., 2004, 2905–2913 CrossRef CAS PubMed.
- A. Makiyama, I. Komatsu, M. Iwaoka and M. Yatagai, Phosphorus, Sulfur Silicon Relat. Elem., 2011, 186, 125–133 CrossRef CAS PubMed.
- R. G. Bhat, E. Porhiel, V. Saravanan and S. Chandrasekaran, Tetrahedron Lett., 2003, 44, 5251–5253 CrossRef CAS.
- E. M. Stocking, J. N. Schwarz, H. Senn, M. Salzmann and L. A. Silks, J. Chem. Soc., Perkin Trans. 1, 1997, 2443–2448 RSC.
- P. P. Phadnis and G. Mugesh, Org. Biomol. Chem., 2005, 3, 2476–2481 CAS.
- A. Schneider, O. E. D. Rodrigues, M. W. Paixao, H. R. Appelt, A. L. Braga and L. A. Wessjohann, Tetrahedron Lett., 2006, 47, 1019–1021 CrossRef CAS PubMed.
- B. M. Trost and M. T. Rudd, Org. Lett., 2003, 5, 4599–4602 CrossRef CAS PubMed.
- N. Metanis, E. Keinan and P. E. Dawson, J. Am. Chem. Soc., 2006, 128, 16684–16691 CrossRef CAS PubMed.
- M. D. Gieselman, L. Xie and W. A. van der Donk, Org. Lett., 2001, 3, 1331–1334 CrossRef CAS PubMed.
Footnote |
† Electronic supplementary information (ESI) available: Details of mass spectrometry, 1H, 13C and 77Se NMR spectra of synthesized compounds and chiral HPLC chromatograms. See DOI: 10.1039/c5ra06147d |
|
This journal is © The Royal Society of Chemistry 2015 |