Microwave-promoted synthesis of β-hydroxy sulfides and β-hydroxy sulfoxides in water

Abstract

A simple, efficient and environmentally friendly one-pot synthesis of β-hydroxy sulfoxides in water under microwave irradiation is reported.

---

Introduction

β-Hydroxy sulfides and β-hydroxy sulfoxides are important intermediates in organic synthesis.¹

The former are versatile building blocks for synthesizing cyclic sulfides,² allylic alcohols,¹ benzoxathiepines,³ benzothiazepines⁴ and thioketones.⁵ On the other hand the readily accessible β-hydroxy sulfoxides have an extended use in asymmetric synthesis.⁶ They find also a useful application in the preparation of naturally occurring compounds such as leukotrienes.⁷

The most important synthetic route to β-hydroxy sulfide is the ring opening of epoxides with thiolates.⁷ It can be promoted by acids, Lewis catalyst or metal salts.⁸ Thiolysis in water was investigated for the first time by Fringuelli and coworkers,⁹ where the formation of β-hydroxy sulfides was promoted by Lewis acids such as InCl₃⁹ and ZnCl₂.¹⁰

β-Hydroxy sulfoxides are commonly prepared by oxidation of β-hydroxy sulfides with conventional oxidizing agents.

We herein report the optimization of a one-pot protocol for the preparation of β-hydroxy sulfoxides in water, promoted by microwave irradiation. The opening of the epoxide and the subsequent sulfoxidation were accelerated by microwave and the desired product was obtained in only 15 minutes with very good yield. (Scheme 1) (see Table 1). Microwave activation as a non-conventional energy source has become an important method that can be used to carry out a wide range of reactions with short reaction times and in high yield and regioselectivity.¹¹ Indeed microwave dielectric heating in a pressurized system rapidly increases temperature far above the boiling point of the solvent and leads to a uniform energy transfer to the reactants of the chemical reaction.

Scheme 1 Example of one-pot synthesis.

Table 1 Thiolysis by thiophenol of epoxides (1a–6a) in water to give β-hydroxy sulfides

Entry	Epoxide	T/°C	Time/min	Power/Watt	Yield (%)	a/b^a
a Refers to the ratio of a-carbon and b-carbon, determined by ^1H-NMR and HPLC analyses.
1	1a	25	240	—	80
2	1a	150	5	400	97
3	1a	110	5	—	30
4	2a	25	300	—	72
5	2a	150	5	400	95
6	3a	25	1440	—	43
7	3a	150	10	400	91
8	4a	25	300	—	77	5/95
9	4a	150	5	400	85	3/97
10	5a	25	270	—	72	80/20
11	5a	150	5	400	88	65/35
12	6a	25	180	—	85	5/95
13	6a	150	5	400	98	3/97

---

Water is cheap, readily available and nontoxic resulting in an economic process and has clear advantages as an environmentally friendly solvent alternative in organic synthesis. The heating effect utilized in microwave-assisted organic transformations is due to the dielectric constant of the solvent. For this reason, water is therefore a very useful solvent for microwave-assisted organic synthesis.¹¹

Results and discussion

The reaction was performed with a laboratory microwave oven in a sealed tube controlled at 150 °C with an optical fiber in the presence of a catalytic amount of sodium hydroxide¹² (1.5%). The epoxide and the sulfide were used in a stoichiometric ratio. The β-hydroxy sulfide was obtained in quantitative yields (for reaction conditions see Table 1). In the one-pot procedure tert-butyl hydroperoxide (2.0 equiv.) was added to the same reaction media at 100 °C leading to β-hydroxy sulfoxides 1c and 1d in very good yields. No appreciable amounts of the corresponding sulfone were found.

This reaction was optimized using epoxide 1a as starting material and thiophenol. Different parameters such as temperature, equivalent of oxidant and reaction time were studied (see Table 2). The β-hydroxy sulfoxides were isolated in 89% yield as a 25 ∶ 75 mixture of diastereoisomers 1c and 1d of known configuration^1,9 (see Scheme 1).

Table 2 Optimization of one pot protocol using 1a and thiophenol

Entry	tert-ButOOH/mol equiv.^−1	T/°C	Time/min	Power/Watt	1c ∶ 1d	Yield (%)
1	3.0	25	720	—	35 ∶ 65	60
2	3.0	70	330	—	30 ∶ 70	65
3	1.0	70	10	400	29 ∶ 71	34
4	1.0	70	15	400	31 ∶ 69	43
6	2.0	100	15	400	25 ∶ 75	89

---

In order to extend our protocol we have also investigated the thiolysis of a variety of epoxides (1a–6a) with thiophenol.

When the thiolysis was performed with microwave irradiation the reaction was very rapid and the products isolated from the reaction media were pure and did not need further purification.

At room temperature and in the absence of microwave irradiation the reaction times were in the range of 3–24 hours and the yields were lower (Table 1).

The rate acceleration observed can be justified as a consequence of the higher temperature and of the microwave irradiation. The opening of 1a performed with conventional heating at 110 °C provided the expected product 1b with only 30% yield (see Table 1 entry 3).

In all cases the ring opening was completely anti-stereoselective and the only products obtained were the trans-β-hydroxy sulfides.¹³ The reaction proceeded via an S_N2 mechanism with attack of the thiol at the less substituted b-carbon, except for compound 5a where the attack was driven predominantly at the benzylic a-carbon by an electronic effect.

Conclusion

In conclusion we have demonstrated that the thiolysis of several epoxides in water, under controlled microwave irradiation, can be performed in a short time and with quantitative yield without the use of any metal catalyst. By coupling this procedure with the in situ oxidation of sulfide mediated by tert-butyl hydroperoxide, we have optimized a one-pot procedure for the preparation of β-hydroxy sulfoxides.

Experimental

Typical procedure of opening of an epoxide 1a–6a in the presence of thiophenol

Thiophenol (1.5 mmol) and epoxide (1.5 mmol) were suspended in water (3 ml), then sodium hydroxide (0.022 mmol) was added . The reaction tube (25 ml) was sealed with a Teflon^® cap and irradiated in the cavity of a multimode Milestone MicroSYNTH labstation for the appropriate time at 150 °C, using an irradiation power of 400 Watt. The tube was cooled to 50 °C by gas-jet cooling. The reaction mixture was extracted with diethyl ether (3 × 5 ml). The combined organic layers were dried on magnesium sulfate and removed under reduced pressure to give pure product.

Optimized procedure for one-pot synthesis of β-hydroxy sulfoxide

Thiophenol (1.5 mmol.) and epoxycyclohexane (1.5 mmol) were suspended in water then sodium hydroxide was added (0.022 mmol). The reaction tube was sealed with a Teflon^® cap and irradiated in the cavity of a microwave apparatus at 150 °C for 5 minutes. The reaction tube was cooled to room temperature by gas-jet cooling, tert-ButOOH (3.0 mmol) was added and then irradiated in the cavity of microwave apparatus at 100 °C using an irradiation power of 400 Watt. After the appropriate reaction time the tube was cooled to 50 °C by gas-jet cooling. The reaction mixture was extracted with diethyl ether (3 × 5 ml). The combined organic layers were dried on magnesium sulfate and the solvent was removed under reduced pressure to give the β-hydroxy sulfoxides as a mixture of diastereoisomers.

Acknowledgements

This work was supported by a PRIN project of the Italian MIUR. We want to thank Dr. Giorgio Abbiati and Dr.ssa Alessandra Bellinazzi for their help with the microwave apparatus.

References

1. V. Kesavan, D. Bonnet-Delpon and J. P. Bégué, Tetrahedron Lett., 2000, 41, 2895 and references therein.
2. (a) E. Ozaki, H. Matsui, H. Yoshinaga and S. Kitagawa, Tetrahedron Lett., 2000, 41, 2621; (b) B. M. Adger, J. V. Barkley, S. Bergeron, M. W. Cappi, B. E. Flowerdew, M. P. Jackson, R. McCague, T. C. Nugent and S. M. Roberts, J. Chem. Soc., Perkin Trans. 1, 1997, 3501.
3. (a) H. Sugihara, H. Mabuchi, M. Hirata, T. Iamamoto and Y. Kawamatsu, Chem. Pharm. Bull., 1987, 35, 1930; (b) C. G. Aifheli and P. T. Kaye, Synth. Commun., 1996, 26, 4459; (c) R. Krishnamutri, S. Nagy and T. F. Smolka, US Patent NO 5621153, 1997.
4. A. Scwartz, P. B. Madan, E. Mohacsi, J. P. O'Brien, E. Todaro and D. L. Coffen, J. Org. Chem., 1992, 57, 851.
5. J. P. Bégué, D. Bonnet-Delpon and A. Kornilov, Synthesis, 1996, 529.
6. (a) M. C. Carreňo, Chem. Rev., 1995, 95, 1717; (b) M. C. Carreňo, M. Ribagorda and G. H. Posner, Angew. Chem., Int. Ed., 2002, 41, 2753 and references therein.
7. E. J. Corey, D. A. Clark, G. Goto, A. Marfat, C. Mioskowski, B. Samuelsson and S. Hammarstrom, J. Am. Chem. Soc., 1980, 102, 1436.
8. (a) A. E. Vougiokas and H. B. Kagan, Tetrahedron Lett., 1987, 28, 6065; (b) J. Iqbal, A. Pandey, A. Shukla, R. R. Srivastava and S. Tripathi, Tetrahedron, 1990, 18, 6423; (c) K. S. Ravikumar, F. Barbier, J. P. Bégué and D. Bonnet-Delpon, J. Fluorine Chem., 1999, 95, 123.
9. F. Fringuelli, F. Pizzo, S. Tortoioli and L. Vaccaro, Adv. Synth. Catal., 2002, 344, 379.
10. (a) D. Amantini, F. Fringuelli, F. Pizzo, S. Tortoioli and L. Vaccaro, Synlett, 2003, 15, 2292; (b) F. Fringuelli, F. Pizzo, S. Tortoioli and L. Vaccaro, J. Org. Chem., 2003, 68, 8248.
11. Microwaves in Organic Synthesis, ed. A. Loupy, Wiley-VCH, Weinheim, 2002.
12. In the absence of a catalytic amount of sodium hydroxide, compound 1b was obtained in 13% yield.
13. The trans configuration was deduced from the J_H–H coupling constants: for more information see ref. 1 .

---