Synlett 2019; 30(10): 1222-1227
DOI: 10.1055/s-0037-1611831
letter
© Georg Thieme Verlag Stuttgart · New York

Oxidative Cyclization of Naphtholic Sulfonamides Mediated by a Chiral Hypervalent Iodine Reagent: Asymmetric Synthesis versus Resolution

Nikita Jain
,
Jason E. Hein*
,
Marco A. Ciufolini*
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada   Email: ciufi@chem.ubc.ca   Email: jhein@chem.ubc.ca
› Author Affiliations
We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for support of this research.
Further Information

Publication History

Received: 24 April 2019

Accepted after revision: 26 April 2019

Publication Date:
15 May 2019 (online)


§ Author to whom correspondence regarding coupled preferential crystallization technology should be addressed.

Abstract

A chiral aryl iodide promotes the enantioselective oxidative cyclization of 1-naphtholic sulfonamides, albeit in moderate ee and low yield. The products tend to crystallize as conglomerates. Recrystallization thus increases their ee to > 99% ee. This highly enantioenriched material provides seed crystals for the resolution of the racemate (prepared in high yield by oxidative cyclization with (diacetoxyiodo)benzene in trifluoroacetic acid) by coupled preferential crystallization. This enables the production of significant quantities of highly enantioenriched products, despite the low efficiency of the enantioselective reaction.

Supporting Information

 
  • References

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  • 11 Experimental protocols: Most commercial reagents and solvents were used as received, except CH2Cl2, which was freshly distilled from CaH2 under Ar. All reactions were performed under Ar in flame- or oven-dried flasks equipped with Teflon™ stirring bars and fitted with rubber septa for the introduction of materials via syringe. Reactions were monitored by thin-layer chromatography (TLC) using silica gel 60 F254 pre-coated plates. Spots were visualized by UV or with KMnO4 or vanillin stains. Flash chromatography utilized Silicycle® 230–400 mesh silica gel. Unless otherwise stated, 1H (300 MHz) and 13C (75 MHz) NMR spectra were recorded from CDCl3 solutions at r.t. Chemical shifts are reported in parts per million (ppm) on the δ scale and coupling constants, J, in hertz (Hz). Multiplicities are reported as ‘s’ (singlet), ‘d’ (doublet), ‘t’ (triplet), ‘q’ (quartet), ‘dd’ (doublet of doublets), ‘ddd’ (doublet of doublet of doublets), ‘m’ (multiplet), and further qualified as ‘br’ (broad), ‘app’ (apparent). Infrared (IR) spectra (cm–1) were recorded from films (Perkin Elmer® Universal ATR Sampling Accessories). Low- and high-resolution mass spectra (m/z) were obtained in the electron impact (EI) or electrospray (ESI; MeOH solutions) mode, as specified. Optical rotations were measured at the Na D line (589 nm) with a Perkin Elmer® 241 polarimeter. Enantiomeric excesses reported in this experimental section were determined with a Thar SFC station (Model 840) equipped with chiral columns OD-H (0.46 cm × 25 cm × 5 μm), AD-H (0.46 cm × 25 cm × 5 μm), AS-H (0.46 cm × 25 cm × 5 μm), OJ-H (0.46 cm × 25 cm × 5 μm) and Lux 3u Cellulose 2 (50 × 4.60 mm). The operating pressure was 120 bar and the column temperature was 33–34 °C. Enantiomeric excesses of some of the compounds were determined by high-performance liquid chromatography (HPLC), which was performed with an Agilent 1100 HPLC, equipped with a variable wavelength UV/Vis detector and Chiralcel OD-H chiral column (0.46 cm × 25 cm × 5 μm), Chiralcel AD-H chiral column (0.46 cm × 25 cm × 5 μm), Chiralcel OJ-RH (0.46 cm × 15 cm × 5 μm). The intensities of the signals of each enantiomer in enantioenriched products were corrected on the basis the ratios of the signal intensities observed for the racemates. More detailed experimental protocols are provided in the Supporting Information. Enantioselective oxidative cyclization of sulfonamides 1: Typical procedure for (R)-(+)-2a: A solution of MCPBA (150 mg of commercial, 70% pure reagent, corresponding to 105 mg of MCPBA, 470 μmol, 1.3 equiv) in CH2Cl2 (8 mL) was added over 5 min (syringe) to a cold (–20 °C) solution of 1a (100 mg, 360 μmol, 1.0 equiv) and iodide 3 (50 mg, 72 μmol, 0.2 equiv) in CH2Cl2 (12 mL). The mixture was stirred at –20 °C and monitored for completion by TLC. After 13 h, the reaction was quenched with aq. sat. NaHCO3 (10.0 mL) and aq. sat. Na2S2O3 (5.0 mL), and extracted with CH2Cl2 (5 × 15 mL). The combined extracts were dried (Na2SO4) and evaporated. The residue was purified by flash column chromatography (EtOAc/hexanes, 2:3) to afford 2a (20 mg, 20%); [α] d 24 = +36.5° (c 0.59, CHCl3). SFC analysis [Column: OD-H; 5:95:0.1 iPrOH/liq CO2/diethylamine, flow rate = 1.0 mL/min; tR (1) = 44.53 min (minor), tR (2) = 52.79 (major)] indicated an ee of 67%. Spectral properties of the racemic compound are provided in ref. 5b. Compound (S)-(–)-2a: The same procedure described above was used, except that iodide 4 was employed instead of 3, to give (S)-(–)-2a (20 mg, 20%); [α] d 24 = –24.2° (c 0.58, CHCl3). SFC analysis under the same conditions detailed above indicated an ee of 46% [tR (1) = 44.50 min (major), tR (2) = 53.15 (minor)]. (+)-2-(3-(Methylsulfonamido)propyl)-1-oxo-1,2-dihydronaphthal-en-2-yl 3-Chlorobenzoate [(+)-18]: Yield: 38 mg (25%) from 100 mg of 1a and catalyst 3; yellow oil; eluent: 2:3 Et2O/pentane; [α] d 23 = +16.8° (c 0.06, CHCl3). SFC analysis [Column: OD-H; 10:90:0.1 (iPrOH/liq CO2/diethylamine), flow rate = 1.0 mL/min; tR (1) = 44.10 min (minor), tR (2) = 48.87 (major)] indicated an ee of 80%. IR (film): 3390–3156 (broad), 1721, 1690 cm–1. 1H NMR (CDCl3, 300 MHz): δ = 8.09 (d, J = 7.8 Hz, 1 H), 8.00 (s, 1 H), 7.91 (d, J = 7.7 Hz, 1 H), 7.64 (t, J = 7.5 Hz, 1 H), 7.56 (d, J = 8.1 Hz, 1 H), 7.45–7.37 (m, 2 H), 7.32 (d, J = 7.7 Hz, 1 H), 6.78 (d, J = 10.0 Hz, 1 H), 6.13 (d, J = 9.9 Hz, 1 H), 4.33 (t, J = 5.9 Hz, 1 H), 3.15 (app q, J = 6.7 Hz, 2 H), 2.93 (s, 3 H), 2.20–2.10 (m, 1 H), 2.06–1.96 (m, 1 H), 1.91–1.75 (m, 1 H), 1.68–1.57 (m, 1 H). 13C NMR (CDCl3, 75 MHz): δ = 195.7, 164.0, 136.8, 135.0, 134.6, 133.5, 132.6, 130.9, 129.9, 129.8, 129.4, 128.8, 128.2, 128.1, 127.7, 127.3, 80.6, 43.1, 40.4, 35.0, 23.5. HRMS: m/z [M – H] calcd for C21H19NO5S35Cl: 432.0672; found: 432.0670.