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Synlett 2008(17): 2684-2688  
DOI: 10.1055/s-0028-1083517
LETTER
© Georg Thieme Verlag Stuttgart ˙ New York

A Novel Method for the Synthesis of Thioacetates Using Benzyltriethyl­ammonium Tetrathiomolybdate and Acetic Anhydride

Nasir Baig. R.B.a, Sai Sudhir. V.a, Srinivasan Chandrasekaran*a,b
a Department of Organic Chemistry, Indian Institute of Science, Bangalore 560012, Karnataka, India
Fax: +91(80)23602423; e-Mail: scn@orgchem.iisc.ernet.in;
b Honorary Professor, Jawaharlal Nehru Centre for Advanced Scientific Research, Bangalore 560 064, India
Further Information

Publication History

Received 1 July 2008
Publication Date:
01 October 2008 (online)

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Abstract

Herein we report a simple and efficient methodology for the synthesis of thioacetates using benzyltriethylammonium tetrathiomolybdate and acetic anhydride as the key reagents, starting from alkyl halides in a multistep, tandem reaction process. Its application in the synthesis of orthogonally protected cysteine and anomeric β-thioglycosides has also been demonstrated.

Key words

benzyltriethylammonium tetrathiomolybdate - acetic anhydride - thioacetates

    References and Notes

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General Procedure for the Synthesis of Thioacetates - Synthesis of 4a
To the solution of alkyl halide 5a (0.169 g, 1 mmol,) in MeCN (5 mL), tetrathiomolybdate 1 (0.37 g, 2.2 equiv) was added and the reaction mixture was stirred for 1 h. To this solution, Ac2O (0.408 g, 4.0 equiv) was added and the reaction mixture was stirred further for 1.5 h. The solvent was removed under vacuum, the solid residue was extracted with CH2Cl2-Et2O (3:7) 5 × 10 mL, filtered through Celite and concentrated. The crude product was purified by silica gel (100-200 mesh) column chromatography; EtOAc-hexane (19:1); gummy liquid, yield 94%. IR (neat): 3030 (w), 2924 (m), 1692 (s), 1353 (w), 1133 (m), 627 (m) cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 7.27 (s, 5 H), 4.09 (s, 2 H), 2.31 (s, 3 H). ¹³C (75 MHz, CDCl3): δ = 194.8, 137.4, 128.6, 128.4, 127.1, 33.2, 30.1. HRMS: m/z calcd for C9H10OS: 189.030 [M + Na]; found: 189.0358.
Compound 4d: oily liquid, yield 96%. IR: 2988 (w), 1676 (s), 1416 (m), 1176 (s), 625 (m) cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 7.88 (d, J = 8.1 Hz, 2 H), 7.30 (d, J = 8.1 Hz,
2 H), 4.13 (s, 2 H), 2.57 (s, 3 H), 2.35 (s, 3 H). ¹³C NMR (75 MHz, CDCl3): δ = 197.4, 194.5.
Compound 4l: oily liquid, yield 80%. IR (neat): 2903 (m), 2863 (w), 1732 (s), 1462 (m), 1110 (s) cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 2.71 (m, 1 H), 2.37 (s, 3 H), 1.70-1.30 (m, 8 H), 1.00 (t, J = 7.2 Hz, 3 H), 0.91 (t, J = 7.2 Hz, 3 H). ¹³C NMR (75 MHz, CDCl3): δ = 195.4, 54.4, 33.0, 26.7, 22.5, 13.8, 11.0. HRMS: m/z calcd for C9H18OS: 197.0976 [M + Na]; found: 197.0970.
Compound 8: white solid, mp 77 ˚C, yield 80%; [α]D ²8 -48 (c 1, CHCl3). IR (neat): 3351 (br), 2951 (w), 1698 (s), 1517 (m), 1212 (m) cm-¹. ¹H NMR (400 MHz, CDCl3): δ = 7.33 (m, 5 H), 5.55 (d, J = 7.2 Hz, 1 H), 5.11 (s, 2 H), 4.50-4.60 (m, 1 H), 3.75 (s, 3 H), 3.42 (dd, J = 14.0, 4.8 Hz, 1 H), 3.40 (dd, J = 14.0, 4.8 Hz, 1 H), 2.32 (s, 3 H). ¹³C NMR (100 MHz, CDCl3): δ = 194.8, 170.5, 155.6, 136.0, 128.4, 128.1, 128.0, 67.0, 53.4, 52.7, 31.1, 30.3. HRMS: m/z calcd for C14H17O5S: 334.0725 [M + Na]; found 334.0710.
Compound 11: gummy solid, yield 78%; [α]D ²8 +8.7 (c 1, CHCl3). IR (neat): 3318 (br), 2965 (m), 2930 (m), 1736 (s), 1695 (s), 1682 (s), 1635 (m), 1262 (m), 1213 (m), 1139 (m) cm-¹. ¹H NMR (400 MHz, CDCl3): δ = 7.34 (s, 5 H), 6.92 (d, J = 8.0 Hz, 1 H), 5.67 (d, J = 8.0 Hz, 1 H), 5.12 (s, 2 H), 4.52 (dd, J = 8.6, 4.8 Hz 1 H), 4.40 (m, 1 H), 3.35 (dd, J = 16.0, 4.0 Hz, 1 H), 3.12 (dd, J = 16.0, 8.0 Hz, 1 H), 2.30 (s, 3 H), 1.90-1.86 (m, 1 H), 1.44-1.36 (m, 1 H), 1.18-1.12 (m, 1 H), 0.90-0.86 (m, 6 H). ¹³C NMR (100 MHz, CDCl3): δ = 196.6, 171.8,169.6, 156.3, 136.0, 128.4, 128.1, 128.0, 67.7, 56.6, 55.0, 52.1, 37.6, 31.4, 30.4, 25.0, 15.3, 11.2. HRMS: m/z calcd for C20H28N2O8S: 447.1506 [M + Na]; found: 447.1563.

24

To a solution of dipeptide 9 (1.0 mmol, 0.366 g) and pyridine (5 mmol, 0.395 g) in CH2Cl2 (10 mL) at -10 ˚C MsCl (1.2 mmol, 0.136 g) was added and the reaction mixture was stirred for 6 h. The reaction was quenched with H2O and extracted with CH2Cl2 (3 × 15 mL). The organic extract was washed with brine, dried over Na2SO4, and concentrated. The crude product was purified by column chromatography on silica gel (100-200 mesh), white solid, mp 119 ˚C, yield 95%; [α]D ²8 +15.7 (c 1, CHCl3). IR (neat): 3334 (br), 2964 (m), 1731 (s), 1674 (s), 1529 (m), 1361 (m), 1170 (m) cm-¹. ¹H NMR (300 MHz, CDCl3): δ = 7.31 (s, 5 H), 6.95 (d, J = 7.5 Hz, 1 H), 5.68 (d, J = 7.2 Hz, 1 H), 5.14 (s, 2 H), 4.61-4.53 (m, 3 H), 4.35 (dd, J = 9.0, 7.2 Hz, 1 H), 3.80 (s, 3 H), 3.00 (s, 3 H), 1.93-1.84 (m, 1 H), 1.45-1.33 (m, 1 H), 1.25-1.00 (m, 1 H), 0.92-0.86 (m, 6 H). ¹³C NMR (100 MHz, CDCl3): δ = 171.7, 167.7, 155.9, 135.6, 128.54, 128.3, 128.1, 68.3, 67.5, 56.7, 53.6, 52.2, 37.6, 37.2, 24.9, 15.3, 11.4. HRMS: m/z calcd for C19H28N2O8S: 467.1467 [M + Na]; found: 467.1451.

25

Compound 11 was obtained in optically pure form and there was no racemization or formation of diastereomer.