Catalytic Asymmetric Direct-Type 1,4-Addition Reactions of Alkanesulfonamides

Abstract

Catalytic asymmetric 1,4-addition reactions of alkanesulfonamides were developed on the ‘product base’ strategy. Alkanesulfonamides reacted with α,β-unsaturated amides in good to high yields with good to high diastereo- and enantioselectivities using a chiral alkaline metal amide. To our knowledge, this is the first example of a catalytic asymmetric C–C bond-forming reaction using an alkanesulfonamide without any activating group at its α-position.

Key words

alkanesulfonamide - 1,4-addition - asymmetric - catalyst - strong base - product base - enantioselective reaction - crown ether

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• 10 Experimental procedure for the reaction of 1c with 2b to afford 1,4-adduct 3cb (Table 2, entry 11) is described as a general method. Under an argon atmosphere, KHMDS (8.0 mg, 0.040 mmol), (R,R)-34-crown-10 (L2, 19.6 mg, 0.0220 mmol), and 1c (81.3 mg, 0.400 mmol) were placed in a well-dried 10 mL one-neck flask with a three-way stopcock, and ethylbenzene (1.0 mL) was added at –78 °C. After stirring for 1 h at the same temperature, 2b (109.8 mg, 0.800 mmol) in ethylbenzene (1.0 mL) was added, and the whole was stirred for 18 h. After addition of water (1 mL) to stop the reaction, the mixture was extracted with CH₂Cl₂ (30 mL) three times. The organic layers were combined, dried over Na₂SO₄, and concentrated under reduced pressure after filtration. The crude product obtained was purified by column chromatography (silica gel, hexanes–EtOAc = 1:1) to afford the desired product 3cb (125.4 mg, 92%). Analytical Data of 3cb Colorless solid; mp 91–94 °C. [α]_D +27.7 (c 0.102, CHCl₃, 82% ee). HPLC analysis using Daicel Chiralpak AS-3 column (Hex–2-PrOH = 9:1, 1.0 mL/min, 210 nm): t _R = 20.2 min (minor), 23.5 min (major). ¹H NMR (600 MHz, CDCl₃): δ = 7.24 (2 H, t, J = 7.6 Hz), 7.20 (2 H, d, J = 6.9 Hz), 7.15 (1 H, t, J = 7.2 Hz), 3.89–3.86 (1 H, m), 3.36 (1 H, dq, J = 14.0, 3.3 Hz), 3.32–3.20 (3 H, m), 3.18–3.13 (2 H, m), 2.83 (6 H, s), 2.83 (3 H, s), 2.75 (1 H, dd, J = 16.0, 10.0 Hz), 1.27 (3 H, d, J = 6.9 Hz), 1.10 (3 H, t, J = 7.2 Hz), 0.91 (3 H, t, J = 6.9 Hz). ¹³C NMR (100 MHz, CDCl₃): δ = 169.6, 141.0, 128.4, 128.0, 126.9, 60.0, 41.8, 40.1, 37.5, 32.0, 14.2, 12.8, 11.0. IR (KBr disc): 3435, 3255, 3060, 2980, 2939, 2814, 2448, 2269, 1994, 1832, 1638, 1459, 1378, 11319, 1270, 1245, 1220, 1199, 1140, 1076, 1041, 965, 927, 878, 804, 767, 741, 711, 687, 622, 601, 559, 527, 504, 452 cm^–1. ESI-HRMS: calcd for C₁₇H₂₆N₂NaO₃S [M + Na]⁺: 363.1718; found: 363.1733.