Photochemical Reaction of 1,2-Naphthoquinone toward Enantioselective Synthesis of γ-Rubromycin: Viability Dependence on Chromophore

Abstract

Aiming at the enantioselective total synthesis of γ-rubromycin, we reported the photochemical reaction of 1,2-naphthoquinone as a promising solution to otherwise-difficult enantiocontrol of the single spiroacetal stereogenic center. The present study examined the applicability of this approach to more functionalized substrates, which revealed viability dependence on the chromophore structure differing in the position and number of methoxy substituents.

Key words

photochemical reactions - 1,2-naphthoquinone - spiroacetals - redox - rubromycin

• References and Notes

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Related works in stereospecific photochemical reaction:
• 7a Ando Y, Hanaki A, Sasaki R, Ohmori K, Suzuki K. Angew. Chem. Int. Ed. 2017; 56: 11460
• 7b Ando Y, Tanaka D, Sasaki R, Ohmori K, Suzuki K. Angew. Chem. Int. Ed. 2019; 58: 12507
• 7c Ando Y, Matsumoto T, Suzuki K. Helv. Chim. Acta 2021; 104: e2100008
• 7d Ando Y, Ogawa D, Ohmori K, Suzuki K. Angew. Chem. Int. Ed. 2023; 62: e202213682

For review and account:
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• 12 The experimental procedures and characterization data of spiroacetal 12c are shown as follows: A solution of 1,2-naphthoquinone 4c (12.8 mg, 0.0351 mmol) in CH₃CN (3.5 mL) was placed in a Pyrex^® two-necked round-bottom flask, and degassed three times by purging with argon under sonication. An LED lamp (Techno Sigma PER-AMP, PAR-448, 680 mW) was inserted into the solution, and the flask was covered with aluminum foil. The solution was irradiated at room temperature for 40 min. Then, to the solution was added cerium(IV) ammonium nitrate (43.0 mg, 0.0784 mmol) in H₂O (1.8 mL) at 0 °C. After stirring for 10 min at the same temperature, the reaction was quenched by adding saturated aqueous NaHCO₃ at 0 °C. The mixture was extracted with CH₂Cl₂ (×3), and the combined organic extracts were washed with brine, dried (Na₂SO₄), and concentrated in vacuo. The residue was purified by PTLC (silica gel, CH₂Cl₂) to afford spiroacetal 12c (8.3 mg, 68%) as a yellow solid. 12c: R_f 0.54 (hexane/EtOAc = 1:1). ¹H NMR (600 MHz, CDCl₃): δ = 2.23 (td, J = 13.4, 6.0 Hz, 1 H), 2.39 (ddd, J = 13.4, 5.8, 2.0 Hz, 1 H), 2.82 (ddd, J = 16.5, 6.0, 2.0 Hz, 1 H), 3.25 (d, J = 17.9 Hz, 1 H), 3.30 (ddd, J = 16.5, 13.4, 5.8 Hz, 1 H), 3.46 (d, J = 17.9 Hz, 1 H), 3.95 (s, 3 H), 6.82 (d, J = 8.0 Hz, 1 H), 6.95 (t, J = 8.0 Hz, 1 H), 7.11 (dd, J = 8.6, 2.7 Hz, 1 H), 7.12 (d, J = 8.0 Hz, 1 H), 7.13 (t, J = 8.0 Hz, 1 H), 7.55 (d, J = 2.7 Hz, 1 H), 8.02 (d, J = 8.6 Hz, 1 H). ¹³C NMR (150 MHz, CDCl₃): δ = 21.2, 30.4, 39.5, 56.0, 110.6, 112.2, 116.9, 118.6, 121.0, 122.0, 122.6, 124.9, 127.7, 129.0, 129.2, 135.3, 151.4, 158.5, 164.5, 176.5, 181.9. IR (neat): 2940, 2843, 1674, 1651, 1627, 1592, 1489, 1456, 1362, 1288, 1247, 1226, 1180, 1143, 1095, 1081, 1030, 965, 939, 926, 906, 731 cm^–1. HRMS (ESI-TOF): m/z [M + H]⁺ calcd for C₂₁H₁₇O₅: 349.1071; found: 349.1071.