SHORT COMMUNICATION

DOI: 10.1002/ejoc.201100628

[2 + 2] Annulation of Aldimines with Sulfonic Acids: A Novel One-Pot cis-Selective Route to β-Sultams

Ankita Rai,[a] Vijai K. Rai,[b] Atul K. Singh,[a] and Lal Dhar S. Yadav*[a]

Keywords: Sulfonamides / Annulation / Asymmetric synthesis / Diastereoselectivity

A novel DMF/dimethyl sulfate promoted [2 + 2] annulationof aldimines with sulfonic acids afforded β-sultams in goodto excellent yields (35–93%) under mild conditions. The pro-

Introduction

β-Sultams, the highly reactive sulfonyl analogues of β-lactams, are interesting from both chemical and pharmaco-logical viewpoints.[1a] They have revealed a wide spectrumof biological activities[1b–3] and are interesting buildingblocks for new synthetic drugs corresponding to β-lac-tams.[4] Bacterial resistance to β-lactam antibiotics by pro-ducing β-lactamase and elastase is a serious concern in thefight against bacterial infections by β-lactam antibiotics,which has motivated intensive research to overcome thisproblem. Recently, β-sultams have been reported to serve asβ-lactamase- and elastase inhibitors by irreversible sulf-onylation of the active site serine.[5a,5b] The β-sultam ring ismore distorted than the β-lactam ring, which makes β-sul-tams more suitable for the design of new inhibitors. Chanet-Ray and Vessiere have published an excellent review cover-ing the preparation and uses of β-sultams.[5c]

From a chemical viewpoint, β-sultams can act as pepti-domimetics in sulfonyl transfer reactions and as potent sul-fonating agents for a variety of nucleophiles[6–8] to afford awide range of important chemicals. Page et al. have exhaus-tively studied the reactivity of β-sultams with nucleophilesand found that it depends on the N-substituent.[9] Despitethese promising synthetic and pharmacological applica-tions, β-sultams have received less attention from syntheticchemists. Generally, construction of the β-sultam ring in-volves [2 + 2] cycloaddition or intramolecular cyclizationreactions. The [2 + 2] cycloaddition reported for β-sultamsynthesis utilizes mainly sulfene intermediates with im-ines,[10] mesyl chloride with chiral imines,[11] or alkenes with

[a] Green Synthesis Laboratory, Department of Chemistry, Univer-sity of Allahabad,Allahabad 211002, IndiaFax: +91-5322460533E-mail: ldsyadav@hotmail.com

[b] School of Biology & Chemistry, Shri Mata Vaishno Devi Uni-versity,Katra, Jammu & Kashmir 182320, India

© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Eur. J. Org. Chem. 2011, 4302–43064302

tocol offers a convenient alternative to highly corrosive andhygroscopic sulfonyl chlorides to provide β-sultams withcomplete diastereoselectivity in favour of the cis isomers.

N-sulfonylamines.[12] The intramolecular cyclization strat-egy involves use of amino acids followed by introduction ofthe sulfur moiety,[13] β-amino sulfonic acids,[6] heterocyclicsulfonates[14] or β-aminosulfonyl chlorides,[15] for diastereo-and enantioselective synthesis of β-sultams.[6,8,16]

Most of the methods available for the synthesis of β-sul-tams utilize sulfonyl chlorides A as C–SO2 transfer reagent(Figure 1). As they are highly corrosive and hygroscopic innature, their use is cumbersome. This prompted us to searchfor a new and efficient alternative to classical sulfonyl chlo-rides A for β-sultam synthesis, and we envisioned the useof the DMF/Me2SO4 system for the purpose, which workedwell. In fact, our choice for the DMF/Me2SO4 system wasguided by its application in β-lactam synthesis through theactivation of the COOH group,[17a,17b] Beckmann re-arrangement of cyclohexanone oxime to caprolactam,[17c]

formation of 1,3-dioxolanes,[17d] and synthesis of 3-acylatedindolizines,[17e] pyrimidines and pyridinones.[17f,17g] Herein,we report the first application of the DMF/Me2SO4 adductas a convenient and efficient reagent for the diastereoselec-tive synthesis of β-sultams, which is a novel entry into β-sultam chemistry. This is in continuation of our efforts todevelop new routes to chemically and pharmacologicallyrelevant, strained ring compounds,[18,19] especially β-lac-tams.[19] The protocol involves [2 + 2] annulation of ald-imines with sulfonic acids to afford β-sultams in good toexcellent yields (35–93%) with complete diastereoselectivityin favour of the cis isomer (Scheme 1).

Figure 1. C–SO2 transfer reagents in β-sultam synthesis.

[2 + 2] Annulation of Aldimines with Sulfonic Acids

Scheme 1. Synthesis of β-sultams 3.

Results and Discussion

To begin with, ethoxy- and methoxy-methylene-N,N-di-methyliminium salts (4a and 4b) were prepared by reactingN,N-dimethylformamide (DMF) with Et2SO4 and Me2SO4,respectively (Scheme 2).[17] The general procedure was firstoptimized with respect to reagent 4 and solvents. For thispurpose, aldimine 1a and sulfonic acid 2a were chosen as aset of model substrates for the synthesis of a representativeβ-sultam 3a (Table 1). First, we performed the reaction inTHF using reagent 4a, which afforded the target compound3a in 74 % yield (Table 1, Entry 4). We then examined an-other reagent 4b, which was found to be better than 4a(Table 1, Entries 4 and 5). However, the reaction did notoccur when using DMF or Me2SO4 alone (Table 1, Entries9 and 10). Next, optimization of the solvents for the synthe-sis of 3a by employing 4b was undertaken, and it was foundthat among THF, 1,4-dioxane, CHCl3 and CH2Cl2 (Table 1,Entries 5–8), the best solvent in terms of yield was THF(Table 1, Entry 5). Use of a base, for example Et3N, wasnecessary to bring about the reaction. It was noted that onperforming the reaction at 0 °C, instead of at room tem-perature, considerably lower yields was obtained in longerreaction times. In order to investigate the substrate scopefor generality of the present investigation, a variety of sub-stituted aldimines 1 and sulfonic acids 2 were used withthe optimized reaction conditions, and different β-sultams3 were synthesized. In most of the cases, the yields wereconsistently good (Table 2) and the highest yield was 93 %(Table 2, Entry 10). However, in the case of imines derivedfrom enolizable aldehydes (Table 2, Entries 17 and 18), theyields were considerably lower probably because of theireasy α-deprotonation that leads to several side reactions.

Scheme 2. Preparation of DMF/R2SO4 adducts.

The formation of sultam 3 may be tentatively rational-ized by initial formation of intermediate sulfene 5, as de-picted in Scheme 3. A formal [2 + 2] cycloaddition of sulf-ene 5 to imine 1 then affords the target β-sultams 3. The

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Table 1. Optimization of reaction conditions[a] for the formation of3a.

Entry Catalyst 4 Solvent Yield [%][b]

1 4a 1,4-dioxane 662 4a CHCl3 543 4a CH2Cl2 594 4a THF 745 4b THF 916 4b CHCl3 807 4b CH2Cl2 778 4b 1,4-dioxane 859 DMF THF –10 Me2SO4 THF –

[a] Reaction conditions: Aldimine 1a (1.0 mmol) and sulfonic acid2a (1.5 mmol) were used in dry THF (4 mL) in the presence ofalkoxy-methylene-N,N-dimethyliminium salt 4 and dry Et3N(5.0 mmol). [b] Yield of isolated and purified product.

Table 2. Synthesis of β-sultams 3.

Entry 3 R1 R2 R3 Time [h][a] Yield [%][b,c,d]

1 3a Ts 4-ClPh H 4 912 3b Ts 4-MeOPh H 4.5 853 3c Ts 4-ClPh OPh 5 874 3d Ts 4-MeOPh OPh 5 845 3e Ts Ph OPh 5 856 3f Ts CCl3 Et 4.5 887 3g Ts CCl3 nPr 4.5 868 3h Ts CCl3 (CH2)2Cl 5 829 3i Ts CCl3 CH2Ph 4.5 8910 3j Ts CCl3 Me 4 9311 3k c-C6H11 Ph Ph 4.5 8412 3l c-C6H11 Ph Me 4.5 8213 3m c-C6H11 Ph Et 5 8114 3n c-C6H11 tBu Ph 5 8515 3o c-C6H11 4-MeOPh Ph 4.5 8416 3p nBu Ph Ph 4.5 8317 3q Ts nBu H 5 3518 3r CH3 PhCH2 H 5 42

[a] See Experimental Section for general procedure. [b] Time re-quired for the one-pot procedure. [c] Yield of isolated and purifiedproducts. [d] All compounds gave C, H and N analyses of �0.39%and satisfactory spectral (IR, 1H NMR, 13NMR and EI-MS) data.

cycloaddition step involves Et3N-mediated attack of the α-carbon atom of 5 at the azomethine carbon atom of 1, fol-lowed by an intramolecular SN2 reaction (Scheme 3). Theformation of β-sultams 3 was entirely diastereoselective infavour of the cis isomer, which conforms with earlier obser-vations of similar reactions.[16b,21] The cis configuration ofthe substituted β-sultams was proved by nuclear overhausereffect measurements of 3 (Figure 2) along with the J valuesbetween the protons at C-3 and C-4 (J = 8.5–8.8 Hz), whichfavourably compare with those reported by Enders andMoll (8.4 Hz)[6] and Zajac and Peters (7.3–9.1 Hz).[16b]

However, cis-configured β-lactams typically exhibit a lower

A. Rai, V. K. Rai, A. K. Singh, L. D. S. YadavSHORT COMMUNICATION

Scheme 3. Tentative mechanism for the formation of β-sultams 3.

value for the coupling constants (around 5 Hz). The β-sul-tam ring is more distorted than the β-lactam ring,[11] whichpresumably causes a decrease in its outer H–C–C bondangles and results in the higher J value of the cis-configuredβ-sultams than the that for the corresponding β-lactams.

Figure 2. NOE experiment of 3e.

A strong NOE is observed between the proton at C-3and that at C-4. The proton at C-3 does not show any NOEwith the protons of substituent at C-4, and the proton atC-4 shows no NOE with that of the substituent at C-3. Thisconclusively demonstrates the cis stereochemistry of thesynthesized β-sultams 3, which is also finally confirmed bycomparison with authentic samples obtained by literaturemethods.[6,16,20,22]

A comparison of the results in the case of known com-pounds (cis-3f–3p) reveals that the present method is gen-erally superior to the literature methods[16b,20a] in terms ofyield and diastereoselectivity. Furthermore, it requiresshorter times, i.e. 4–5 h compared to 3 d,[20a] and is per-formed at room temperature instead of at –80 °C.[16b]

Conclusions

In summary, we have developed a novel, efficient asym-metric synthesis of cis-2,3,4-trisubstituted β-sultams fromaldimines and sulfonic acids by employing a readily access-ible DMF/dimethylsulfate adduct under mild conditions ina one-pot procedure. The protocol offers a convenient alter-native to highly corrosive and hygroscopic sulfonyl chlo-rides and would be a practical method for the synthesis ofβ-sultams.

Experimental SectionMelting points were determined by open glass capillary methodand are uncorrected. IR spectra in KBr were recorded on a Perkin–Elmer 993 IR spectrophotometer, 1H NMR spectra were recorded

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on a Bruker WM-40 C (400 MHz) FT spectrometer in CDCl3 byusing TMS as internal reference. 13C NMR spectra were recordedon the same instrument at 100 MHz in CDCl3, and TMS was usedas internal reference. Mass spectra were recorded on a JEOL D-300 mass spectrometer. Elemental analyses were carried out in aColeman automatic carbon, hydrogen and nitrogen analyzer. Allchemicals used were reagent grade and were used as received with-out further purification. Silica gel-G was used for TLC.

General Procedure for the Synthesis of β-Sultams: A mixture ofN,N-dimethylformamide (1.6 mmol) and dimethyl sulfate(1.5 mmol) was stirred at 60–80 °C for 2 h. After cooling to roomtemperature, the resulting solution was added to a mixture of aldi-mine 1 (1.0 mmol), sulfonic acid 2 (1.5 mmol) and dry Et3N(5.0 mmol) in dry THF (4 mL), and the reaction mixture wasstirred for 2–3 h at room temperature. After completion of the reac-tion, as indicated by TLC, HCl (2 mL of a 10% solution) wasadded, and the mixture was stirred well. It was then extracted withCH2Cl2 (3�5 mL). The combined organic layers were washed withsaturated NaHCO3 (10 mL) followed by brine (10 mL). The or-ganic phase was dried with anhydrous magnesium sulfate and con-centrated under reduced pressure to yield the crude product, whichwas purified by silica gel column chromatography (EtOAc/hexane)to give the corresponding β-sultams 3. The structure of the prod-ucts was confirmed by their elemental and spectral analyses. In caseof known compounds, the data compared favourably with thosereported in the literature.[6,16,20,22]

Characterization Data of Unknown Compounds 3

3-(4-Chlorophenyl)-2-tosyl-1,2-thiazetidine 1,1-Dioxide (cis-3a):White solid (0.34 g). Yield: 91%. M.p. 184–186 °C. IR (KBr) ν̃max

= 3034, 2934, 1491, 1459, 1442, 1370, 1359, 1350, 1207, 1190, 1179,1163, 1081, 1020, 905, 820, 815, 794 cm–1. 1H NMR (400 MHz.CDCl3): δ = 2.40 (s, 3 H, CH3-Ts), 3.97 (dd, J = 8.8 Hz, 2 H, CH2),5.65 (t, J = 8.8 Hz, 1 H, CH-4-ClC6H4), 7.30 (d, J = 8.1 Hz, 2 H,Ts), 7.65–7.70 (m, 2 H, 4-Cl-C6H4), 7.72 (d, J = 8.3 Hz, 2 H, Ts),8.10–8.17 (m, 2 H, 4-Cl-C6H4) ppm. 13C NMR (100 MHz, CDCl3):δ = 24.2, 50.4, 59.5, 127.8, 128.6, 129.9, 131.2, 133.7, 136.4, 139.8,142.2 ppm. EIMS: m/z = 371 [M+]. C15H14ClNO4S2 (371.85):calcd. C 48.45, H 3.79, N 3.77; found C 48.61, H 3.49, N 4.04.

3-(4-Methoxyphenyl)-2-tosyl-1,2-thiazetidine 1,1-Dioxide (cis-3b):White solid (0.31 g). Yield: 85%. M.p. 181–184 °C. IR (KBr) ν̃max

= 2950, 2935, 1507, 1374, 1369, 1230, 1191, 1165, 1053, 1030, 934,814 cm–1. 1H NMR (400 MHz. CDCl3): δ = 2.44 (s, 3 H, CH3-Ts),3.85 (s, 3 H, CH3O), 3.95 (dd, J = 8.5 Hz, 2 H, CH2), 5.63 (t, J =8.5 Hz, 1 H, CH-4-CH3OC6H4), 7.31 (d, J = 8.1 Hz, 2 H, Ts), 7.67–7.69 (m, 2 H, 4-CH3O-C6H4), 7.74 (d, J = 8.2 Hz, 2 H, Ts), 8.01–8.03 (m, 2 H, 4-CH3O-C6H4) ppm. 13C NMR (100 MHz, CDCl3):δ = 24.4, 46.5, 55.1, 59.6, 114.1, 127.7, 128.4, 129.9, 135.8, 137.2,

[2 + 2] Annulation of Aldimines with Sulfonic Acids

141.9, 158.2 ppm. EIMS: m/z = 367 [M+]. C16H17NO5S2 (367.43):calcd. C 52.30, H 4.66, N 3.81; found C 52.07, H 4.82, N 4.06.

3-(4-Chlorophenyl)-4-phenoxy-2-tosyl-1,2-thiazetidine 1,1-Dioxide(cis-3c): White solid (0.40 g). Yield: 87%. M.p. 176–178 °C. IR(KBr) ν̃max = 3031, 2936, 1494, 1459, 1441, 1378, 1366, 1341, 1209,1195, 1171, 1152, 1082, 1021, 914, 816, 809, 798 cm–1. 1H NMR(400 MHz. CDCl3): δ = 2.46 (s, 3 H, CH3-Ts), 4.93 (d, J = 8.9 Hz,1 H, CH- C6H5O), 5.65 (d, J = 8.9 Hz, 1 H, CH-4-ClC6H4), 7.32(d, J = 8.2 Hz, 2 H, Ts), 7.64–7.70 (m, 5 H, C6H5), 7.64–7.69 (m,2 H, 4-Cl-C6H4), 7.74 (d, J = 8.3 Hz, 2 H, Ts), 8.12–8.16 (m, 2 H,4-Cl-C6H4) ppm. 13C NMR (100 MHz, CDCl3): δ = 24.5, 51.9,106.9, 114.8, 120.2, 127.6, 128.7, 129.9, 130.0, 130.8, 131.7, 133.4,135.3, 141.7, 144.5, 157.8 ppm. EIMS: m/z = 463 [M+].C21H18ClNO5S (431.89): calcd. C 54.36, H 3.91, N 3.02; found C54.75, H 3.70, N 2.83.

3-(4-Methoxyphenyl)-4-phenoxy-2-tosyl-1,2-thiazetidine 1,1-Dioxide(cis-3d): White solid (0.39 g). Yield: 84%. M.p. 179–181 °C. IR(KBr) ν̃max = 2954, 2931, 1508, 1374, 1367, 1235, 1199, 1164, 1051,1036, 936, 811 cm–1. 1H NMR (400 MHz. CDCl3): δ = 2.40 (s, 3H, CH3-Ts), 3.89 (s, 3 H, CH3O), 4.95 (d, J = 8.5 Hz, 1 H, CH-C6H5O), 5.62 (d, J = 8.5 Hz, 1 H, CH-4-CH3OC6H4), 7.34 (d, J =8.1 Hz, 2 H, Ts), 7.61–7.72 (m, 5 H, C6H5), 7.63–7.70 (m, 2 H, 4-CH3OC6H4), 7.71 (d, J = 8.3 Hz, 2 H, Ts), 8.09–8.15 (m, 2 H, 4-CH3OC6H4) ppm. 13C NMR (100 MHz, CDCl3): δ = 24.1, 51.5,54.2, 106.3, 114.0, 114.9, 121.1, 126.4, 127.2, 128.8, 130.4, 134.9,136.1, 141.8, 157.2, 158.7 ppm. EIMS: m/z = 459 [M+].C22H21NO6S2 (459.53): calcd. C 57.50, H 4.61, N 3.05; found C57.20, H 5.00, N 2.87.

4-Phenoxy-3-phenyl-2-tosyl-1,2-thiazetidine 1,1-Dioxide (cis-3e):White solid (0.36 g). Yield: 85%. M.p. 188–189 °C. IR (KBr) ν̃max

= 3035, 2935, 1492, 1460, 1371, 1369, 1347, 1208, 1194, 1176, 1154,1084, 1025, 910, 821, 814, 792 cm–1. 1H NMR (400 MHz. CDCl3):δ = 2.41 (s, 3 H, CH3-Ts), 4.91 (d, J = 8.7 Hz, 1 H, CH- C6H5O),5.62 (d, J = 8.7 Hz, 1 H, CHC6H5), 7.33 (d, J = 8.2 Hz, 2 H, Ts),7.63–7.72 (m, 10 H, C6H5), 7.74 (d, J = 8.2 Hz, 2 H, Ts) ppm. 13CNMR (100 MHz, CDCl3): δ = 24.3, 51.7, 106.4, 114.1, 120.6, 126.4,127.1, 128.0, 128.8, 129.7, 130.5, 135.7, 140.8, 143.7, 157.1 ppm.EIMS: m/z = 429 [M+]. C21H19NO5S2 (429.50): calcd. C 58.72, H4.46, N 3.26; found C 58.89, H 4.15, N 3.51.

3-(n-Butyl)-2-tosyl-1,2-thiazetidine 1,1-Dioxide (cis-3q): White solid(0.11 g). Yield: 35%. M.p. 172–174 °C IR (KBr) ν̃max = 3032, 2924,1497, 1461, 1445, 1369, 1357, 1351, 1210, 1190, 1174, 1163, 1085,1024, 905, 820, 816, 792 cm–1. 1H NMR (400 MHz. CDCl3): δ =0.91 (t, J = 7.2 Hz, 3 H), 1.37 (m, 2 H), 1.62 (m, 2 H), 2.40 (s, 3H, CH3-Ts), 2.53 (m, 2 H), 3.97 (dd, J = 8.8 Hz, 2 H, CH2), 5.65(dt, J = 8.8, 4.8 Hz, 1 H, CH-nBut), 7.30 (d, J = 8.1 Hz, 2 H, Ts),7.72 (d, J = 8.3 Hz, 2 H, Ts) ppm. 13C NMR (100 MHz, CDCl3):δ = 14.2, 22.6, 24.9, 26.5, 32.7, 42.1, 55.9, 127.5, 129.6, 136.4,141.7 ppm. EIMS: m/z = 317 [M+]. C13H19NO4S2 (317.42): calcd.C 49.19, H 6.03, N 4.41; found C 48.90, H 6.23, N 4.16.

3-(Benzyl)-2-methyl-1,2-thiazetidine 1,1-Dioxide (cis-3r): White so-lid (0.08 g). Yield: 42%. M.p. 169–171 °C. IR (KBr) ν̃max = 3039,2937, 1490, 1455, 1448, 1372, 1354, 1348, 1202, 1195, 1176, 1162,1084, 1029, 902, 824, 815, 791 cm–1. 1H NMR (400 MHz. CDCl3):δ = 2.61 (s, 3 H, CH), 2.81 (m, 2 H, CH2C6H5), 3.94 (dd, J =8.6 Hz, 2 H, CH2), 5.52 (m, 1 H, CH-CH2C6H5), 7.59–7.71 (m, 5H, C6H5) ppm. 13C NMR (100 MHz, CDCl3): δ = 35.9. 41.9, 54.2,55.6, 126.4, 128.7, 129.8, 138.1 ppm. EIMS: m/z = 211 [M+).C10H13NO2S (211.28): calcd. C 56.85, H 6.20, N 6.63; found C56.54, H 6.44, N 6.83.

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Compounds cis-3f–3p are known compounds; their melting points,and IR, 1H NMR, 13C NMR, and EIMS data compare favourablywith those reported in the literature.[6,16,20,22]

Acknowledgments

We sincerely thank SAIF, Punjab University, Chandigarh, for pro-viding microanalyses and spectra. A. R. is grateful to the Councilof Scientific and Industrial Research, New Delhi, for the award ofa Research Associateship [CSIR File No. 09/001/(0327)/2010/EMR-I].

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Received: May 4, 2011Published Online: June 7, 2011