Asymmetric Desymmetrization of 2-Substituted 1,3-Propanediols via Catalytic Enantioselective Ring-Cleavage Reaction of Cyclic Acetal Derivatives

Abstract

Non-enzymatic desymmetrization of 2-substituted 1,3-propanediols leading to the enantiomerically enriched 3-benzyloxy-1-propanols was achieved by using oxazaborolidinone-catalyzed enantioselective ring-cleavage reaction of the cyclic acetal derivatives with dimethylsilyl ketene S,O-acetal as a key reaction.

asymmetric - catalysis - cleavage - diols - Lewis acids

References

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10

Acetal 4a was prepared from 2-phenyl-1,3-propanediol and 1-naphthaldehyde (1.1 equiv) under the conventional conditions (p-TsOH, toluene reflux). A pure trans-isomer isolated by recrystallization from EtOAc and hexane (74% yield) was used in ring-cleavage reaction.

11

For the use of diethyl ether as an additive, see ref. 8 and 9d.

12 Harada T. Nakamura T. Kinugasa M. Oku A. J. Org. Chem.  1999,  64:  7594 

13

The overall enantioselectivity of 6, for example, is calculated by [(syn-6 + anti-6) - (ent-syn-6 + ent-anti-6)}/{(syn-6 + anti-6) + (ent-syn-6 + ent-anti-6)].

14

Representative Procedure for Catalytic Ring-Cleavage Reaction (Table 1, Entry 6). To a solution of N-tosyl-3-(2-naphthyl)-l-alanine (57.5 mg, 0.15 mmol) in CH₂Cl₂ (1.6 mL) under argon atmosphere at r.t. was added dibromo-4-chlorophenylborane (23 µL, 0.15 mmol). After being stirred for 30 min, the mixture was concentrated in vacuo. To a solution of the resulting OXB 3 in CH₂Cl₂ (1.5 mL) at -50 °C were added dimethylsilyl ketene acetal 5d (428 mg, 2.25 mmol), Et₂O (117 µL), and a toluene (1.5 mL) solution of acetal 4a (218 mg, 0.75 mmol). After being stirred for 18 h at -50 °C, the mixture was quenched by the addition of aq NaHCO₃ and filtered. The filtrate was extracted three times with Et₂O. The organic layers were dried (MgSO₄) and concentrated in vacuo. The residue was treated with aq 70% HOAc (0.5 mL) in THF (0.5 mL) at r.t. for 1 h. The mixture was diluted with H₂O, extracted three times with Et₂O, and washed with aq NaHCO₃. The organic layers were dried (MgSO₄) and concentrated in vacuo. Purification of the residue by flash chromatography (SiO₂, 5-20% EtOAc in hexane) gave 262 mg (83%) of 7a ⁸ (syn:anti = 15:1, 93% ee and 17% ee for syn- and anti-isomers, respectively). ¹H NMR (500 MHz, CDCl₃): δ = 1.53 (9 H, s), 2.41 (1 H, br), 2.90 (1 H, dd, J = 2.7, 15.4 Hz), 3.12 (1 H, dd, J = 8.3, 15.4 Hz) 3.19 (1 H, m), 3.68-3.80 (2 H, m), 3.88 (1 H, dd, J = 5.7, 11.0 Hz), 4.09 (1 H, dd, J = 7.6, 11.0 Hz), 5.61 (1 H, dd, J = 2.7, 8.3 Hz), 7.09-7.30 (5 H, m), 7.42-7.59 (4 H, m), 7.81 (1 H, d, J = 8.1 Hz), 7.89 (1 H, br d, J = 8.0 Hz), 8.15 (1 H, br d, J = 8.2 Hz) [a minor anti-isomer resonated at δ = 1.55 (9 H, s), 3.95 (1 H, dd, J = 5.0, 11.2 Hz)]. ¹³C NMR (125.8 MHz, CDCl₃): δ = 29.8, 47.9, 48.5, 52.0, 65.3, 66.2, 71.7, 122.7, 123.9, 125.4, 125.7, 126.3, 126.8, 128.2, 128.38, 128.39, 129.0, 130.4, 133.8, 135.7, 139.9, 198.4. HPLC: ee determination (HPLC, Chiralpak AD-H, 1.0 mL/min, 3% 2-PrOH in hexane, (anti- 7a) τ₁ = 31.3 min, (ent-syn-7a) τ₂ = 34.3 min, (syn-7a) τ₃ = 38.8 min, (ent-anti-7a) τ₄ = 46.8 min).

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19

[α]_D ²⁵ -22.9 (c 1.06, CHCl₃) {lit.^6d [α]_D +24.8 (c 1.00, CHCl₃)}.