Acylative and Silylative Dynamic Kinetic Resolutions of 6-Hydroxy-2H-pyran-3(6H)-one Catalyzed by Chiral Guanidines

Abstract

Chiral guanidine-catalyzed acylative and silylative dynamic kinetic resolutions of 6-hydroxy-2H-pyran-3(6H)-one were achieved. We succeeded in developing a non-enzymatic acylative dynamic kinetic resolution of 6-hydroxy-2H-pyran-3(6H)-one with high enantioselectivity, which could be improved by recrystallization. In contrast, the non-enzymatic silylative dynamic kinetic resolution of 6-hydroxy-2H-pyran-3(6H)-one was accomplished with a moderate selectivity for the first time.

Key words

acylation - guanidines - dynamic kinetic resolution - organocatalysis - silylation - hydroxypyranone

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• 17 When using Ph₂MeSiCl, the yield of 1 was 7% with 71% ee; when using PhMe₂SiCl, the yield of 1 was 25% with 41% ee; and when using TBSCl, no reaction occurred.
• 18 Acylative Dynamic Kinetic Resolution of Lactol 1; Typical Procedure (Table [2], Entry 5) Catalyst C (3.1 mg, 9.9 μmol) and cyclohexanecarboxylic anhydride (54.5 μL, 0.24 mmol) were added successively to a solution of lactol 1 (22.8 mg, 0.20 mmol) in Et₂O (1.0 mL) at r.t., and the mixture was stirred for 24 h. The reaction was then quenched with sat. aq NaHCO₃ at 0 °C. The organic layer was separated and the aqueous layer was extracted with EtOAc. The combined organic layers were dried (Na₂SO₄), filtered, and concentrated, and the resulting residue was purified by column chromatography [silica gel, hexane–EtOAc (9:1)] to give (R)-7 as a colorless oil; yield: 38.1 mg (85%, 94% ee). 1 mmol Scale Acylative Dynamic Kinetic Resolution of Lactol 1 (Table [2], Entry 6) The above procedure was followed using lactol 1 (114.2 mg, 1.00 mmol) in Et₂O (5.0 mL) with catalyst C (15.7 mg, 0.05 mmol) and cyclohexanecarboxylic anhydride (272.4 μL, 1.20 mmol) to give (R)-7 as a pale-yellow oil; yield: 211.2 mg (94% yield, 94% ee). Recrystallization of Acylate (R)-7 (Table [2], entry 6) The oily acylate product (R)-7 (127.2 mg, 97:3 er) was stored in a refrigerator (4 °C) overnight to give crystals that were recrystallized from hexane with a small amount of CH₂Cl₂ to afford a white solid; yield: 66.2 mg (52% recovered yield, >98.5:1.5 er); mp 58–60 °C. Silylative Dynamic Kinetic Resolution of Lactol 1; Typical Procedure (Table [3], Entry 7) Catalyst B (3.9 mg, 10.0 μmol) and i-Pr₂NEt (51.3 μL, 0.30 mmol) were successively added to a solution of lactol 1 (22.8 mg, 0.20 mmol) in toluene (4.0 mL) at r.t., and the mixture was cooled to 0 °C. A 0.20 mM solution of Ph₃SiCl in toluene (1.50 mL, 0.30 mmol) was slowly added to the mixture over 15 h by using a syringe pump. The reaction was additionally stirred for 7 h at 0 °C, and then the reaction was quenched with sat. aq NaHCO₃ at 0 °C. The organic layer was separated and the aqueous layer was extracted with EtOAc. The combined organic layers were dried (Na₂SO₄), filtered, and concentrated to give a residue that was purified by column chromatography [silica gel, toluene–EtOAc (100:1)] to give (S)-8 as a colorless oil; 88% NMR yield (59% ee). 1 mmol Scale Silylative Dynamic Kinetic Resolution of Lactol 1 (Table [3], entry 8) The above procedure was followed using lactol 1 (114.1 mg, 1.00 mmol) in toluene (20 mL), catalyst B (19.4 mg, 0.05 mmol), and i-Pr₂NEt (256.4 μL, 1.50 mmol), with slow addition (15 h) of a 0.20 mM solution of Ph₃SiCl in toluene (7.50 mL, 1.50 mmol). The product was purified by flash column chromatography [silica gel, toluene to toluene–EtOAc (99:1)] to give (S)-8 as a white solid: 348.0 mg (93%, 47% ee).