Thiazol-2-ylidene Catalysis in Intramolecular Crossed Aldehyde-Ketone Benzoin Reactions

Abstract

Intramolecular crossed aldehyde-ketone benzoin-type reactions catalyzed by nucleophilic carbenes, easily generated from commercially available thiazolium salts as precatalysts, are described. Five- and six-membered cyclic acyloins are obtained in moderate to good yields. Depending on the structure of the aldehyde-ketone substrate, an interchange of the alcohol and ketone function of the resulting acyloin is possible. Simple aldehyde-ketones are not good substrates due to the competing intermolecular reaction. Starting from biphenyl-2,2′-dicarbaldehyde, the intermediate acyloin is converted to 9,10-phenanthrenequinone by mild air oxidation.

Key words

organocatalysis - acyloins - carbenes - thiazolium salts - keto-aldehydes

References

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9

General Procedure for the Preparation of the Aldehyde-Ketones: The corresponding cycloalkene (in some cases, synthesized by Grignard reaction performed on the corresponding ketone and subsequent dehydration of the resulting alcohol )¹⁰ was dissolved in MeOH (5 mL/mmol), placed in an ozonolysis apparatus and flushed with argon. The solution was cooled to -78 °C and the ozonolysis was carried out until the color turned blue. After flushing with argon the solution was transferred to a round-bottomed flask and DMS (4.0 equiv) was added. The mixture was gradually warmed to r.t. and stirred for 24 h. The solution was concentrated, diluted with CH₂Cl₂, washed with H₂O, dried over MgSO₄ and concentrated under reduced pressure. Purification by flash column chromatography (silica gel, Et₂O-pentane) afforded the aldehyde-ketone.

12

General Procedure for the Intramolecular Aldehyde-Ketone Condensation: Catalyst, solvent and the appropiate aldehyde-ketone were placed under argon atmosphere in a round-bottomed flask with reflux condenser and heated at 60 °C. Upon completion of the addition of the base the solution was stirred at this temperature and the reaction progress was monitored by TLC. The reaction mixture was then poured into H₂O, extracted twice with CH₂Cl₂, dried over MgSO₄ and concentrated under reduced pressure. The residue was purified by flash chromatography (silica gel, Et₂O-pentane).

13

2-Hydroxy-2-methyl-3,4-dihydro-2H-naphthalin-1-one(9): ¹H NMR (300 MHz, CDCl₃): δ = 1.41 (s, 3 H, CH ₃), 2.26 (m, 2 H, CH ₂), 3.07 (m, 2 H, CH ₂), 3.91 (s, br, 1 H, OH), 7.25-8.07 (m, 4 H, ArH). ¹³C NMR (75 MHz, CDCl₃): δ = 23.89, 26.81, 35.88, 73.58, 126.88, 128.00, 128.99, 134.05, 129.93, 143.40, 201.75. The spectroscopic data were in accordance with those reported in the literature.¹⁴

15

1-Hydroxy-1-phenyl-3,4-dihydro-1H-naphthalin-2-one(13): Mp 89 °C. IR: 3469, 3062, 3025, 2948, 2913, 2857, 1962, 1917, 1716, 1601, 1487, 1449, 1402, 1353, 1282, 1215, 1174, 1143, 1055, 980, 931, 914, 873, 836, 758, 701, 630, 593, 554, 505 cm^-1. ¹H NMR (300 MHz, CDCl₃): δ = 2.57 (m, 1 H, CHH), 2.74 (m, 1 H, CHH), 2.85 (m, 2 H, CH ₂), 4.76 (s, 1 H, OH), 7.01-7.05 (m, 2 H, ArH), 7.18-7.43 (m, 6 H, ArH), 7.79 (m, 1 H, ArH). ¹³C NMR (75 MHz, CDCl₃): δ = 27.40, 33.38, 81.13, 127.00, 127.39, 127.50, 128.33, 128.48, 128.80, 135,94, 138.11, 139.77, 210.15. MS (EI): m/z (%) = 238 (17), 220 (13), 219 (7), 133 (13), 119 (6), 118 (60), 106 (8), 105 (100), 91 (5), 90 (18), 89 (7), 77 (16). Anal. Calcd for C₁₆H₁₄O₂: C, 80.65; H, 5.92. Found: C, 80.35; H, 6.13.

19

When our work was finished (O. Niemeier diploma work, RWTH Aachen, March 2004) we became aware of a parallel and independent investigation by K. Suzuki et al. (Adv. Synth. Cat. 2004, 346, in press).