New Thiochromans via Reductive Cyclization of Thiophenol Derivatives

Abstract

Reductive cyclization of sulfur-containing substrates 1 and 5 with samarium diiodide afforded the corresponding thiochroman derivatives with excellent diastereoselectivities. Cyclization of 1 is facilitated by geminal dimethyl substitution, which accelerates the reductive coupling and prevents samarium diiodide induced dehalogenation. Bromo-substituted dihydrothiochroman derivative 8 was further functionalized in subsequent reactions. Analogously, bromo-substituted hexahydroquinoline derivative 10 was diastereo­selectively prepared in satisfying yield.

Key words

samarium diiodide - radical - cyclization - thiochroman - geminal disubstitution - γ-aryl ketone

• References and Notes


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• 10 General Procedure for Samarium Diiodide Induced Cyclizations of Aryl Ketones: HMPA (10 equiv) was added to a previously prepared stock solution of SmI₂ in THF (0.1 M, 2.05 equiv) under argon and the solution was stirred for 20 min. During this time the solution turned from dark blue to dark violet. In a separate flask, the substrate (1 equiv) and t-BuOH (2 equiv) were dissolved in THF (10 mL/mmol cyclization precursor) under argon. Argon was bubbled through the solution for 20 min. The substrate solution was then transferred with a syringe to the samarium diiodide solution and the resulting mixture was stirred at r.t. or –20 °C until the color changed from violet to grey. Saturated aq Na-K-tartrate solution was added, the organic layer was separated and the aqueous layer was extracted three times with Et₂O. The combined organic layers were washed with H₂O and brine, dried with MgSO₄ and the solvents were removed under reduced pressure to give the crude product, which still contained small amounts of HMPA. Flash chromatography with aluminum oxide (activity grade III) yielded the cyclization products.
• 11 Niermann A. Dissertation . Freie Universität Berlin: 2012
• 12 Cyclization of Compound 1: According to the general procedure, the samarium diiodide solution in THF (27.2 mL, 2.59 mmol), HMPA (2.21 mL, 12.6 mmol), 1 (0.361 g, 1.26 mmol), and t-BuOH (0.187 g, 2.52 mmol) afforded after purification by flash chromatography (hexane–EtOAc, 9:1) compound 8 in 73% yield (265 mg) as a colorless oil. Analytical data of (4R*,4aS*)-6-Bromo-2,2,4-trimethyl-3,4,4a,7-tetrahydro-2H-thiochromen-4-ol (8): ¹H NMR (700 MHz, CDCl₃): δ = 1.21, 1.30, 1.37 (3 × s, 3 H each, CH₃), 1.57 (br s, 1 H, OH), 1.99, 2.02 (AB system, J _AB = 13.4 Hz, 1 H each, 3-H), 3.02 (m_c, 1 H, 4a-H), 3.07 (dddd, J = 1.2, 3.8, 7.5, 22.4 Hz, 1 H, 7-H), 3.17 (dddd, J = 1.9, 3.4, 7.4, 22.4 Hz, 1 H, 7-H), 5.96 (ddd, J = 1.3, 3.4, 3.8 Hz, 1 H, 8-H), 6.30 (ddd, J = 1.2, 1.9, 3.5 Hz, 1 H, 5-H). ¹³C NMR (176 MHz, CDCl₃): δ = 24.6, 30.6, 32.7 (3 × q, CH₃), 37.2 (t, C-7), 43.6 (s, C-2), 53.7 (d, C-4a), 56.8 (t, C-3), 73.6 (s, C-4), 119.6 (s, C-6), 126.2 (d, C-5), 126.9 (d, C-8), 129.3 (s, C-8a). IR (film): 3400 (O–H), 3010–2850 (=C–H, C–H), 1665 (C=C) cm^–1. Anal. Calcd for C₁₂H₁₇BrOS (289.2): C, 49.83; H, 5.92. Found: C, 49.78; H, 5.77.
• 13 Large amounts of dehalogenated compounds were isolated indicating that the increase in rate due to geminal dimethyl substitution cannot compensate the very fast samarium diiodide mediated deiodination.
• 14 Oxidation of Compound 8 with m-CPBA: m-CPBA (0.313 g, 1.82 mmol) was added to a solution of alkenyl bromide 8 (0.150 g, 0.52 mmol) in CH₂Cl₂ (2 mL) at 0 °C. After stirring for 3 h at 0 °C, the solvent was removed under reduced pressure and the crude material was subjected to column chromatography on silica gel (hexane–EtOAc, 7:3). Sulfone 14 was isolated as a colorless solid (70 mg, 42%). Analytical data of (4R*,4aS*)-6-Bromo-2,2,4-trimethyl-1,1-dioxo-3,4,4a,7-tetrahydro-2H-benzo-thiopyran-4-ol (14): mp 160–164 °C. ¹H NMR (500 MHz, CDCl₃): δ = 1.24, 1.34, 1.41 (3 × s, 3 H each, CH₃), 1.83 (br s, 1 H, OH), 1.88, 2.38 (2 × d, J = 14.4 Hz, 1 H each, 3-H), 3.26 (dddd, J = 1.3, 3.9, 7.8, 23.6 Hz, 1 H, 7-H), 3.36 (dddd, J = 2.0, 3.1, 7.5, 23.6 Hz, 1 H, 7-H), 3.63 (m_c, 1 H, 4a-H), 6.29 (m_c, 1 H, 5-H), 6.78 (m_c, 1 H, 8-H). ¹³C NMR (126 MHz, CDCl₃): δ = 22.0, 22.8, 24.5 (3 × q, Me), 36.0 (t, C-7), 49.7 (d, C-4a), 51.9 (t, C-3), 57.2 (s, C-2), 72.9 (s, C-4), 118.8 (s, C-6), 125.5 (d, C-5), 132.8 (s, C-8a), 135.2 (d, C-8). IR (film): 3480 (O–H), 2990–2855 (=C–H, C–H), 1130 (S=O) cm^–1. HRMS (ESI–TOF–MS): m/z [M + Na]⁺ calcd for C₁₂H₁₇BrO₃SNa: 342.9979; found: 342.9977.