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DOI: 10.1055/s-0029-1217712
Asymmetric Synthesis of γ-Hydroxy-α,β-acetylenic Esters Catalyzed by Oxazolidine-Titanium Complex
Publication History
Publication Date:
03 August 2009 (online)
Abstract
An efficient catalytic system has been developed for the enantioselective reaction of alkynoates with aromatic aldehydes for the synthesis of optically active γ-hydroxy-α,β-acetylenic esters (with up to 81% isolated yield and up to 84% enantioselectivity).
Key words
asymmetric synthesis - oxazolidine - aldehyde - addition - asymmetric alkynylation
- Supporting Information for this article is available online:
- Supporting Information
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References and Notes
General Procedure
for the Addition of Methyl Propiolate to Benzaldehyde
All
manipulations were carried out under an argon atmosphere. The ligand 2 (0.1 mmol), base (0.5 mmol), and DIMPEG
(0.05 mmol) were mixed in dry toluene (2.0 mL) at r.t. Then, a solution
of Me2Zn (1.2 M in toluene, 1.5 mmol) and methyl propiolate
(1.5 mmol) were added in turn. After the mixture was stirred at
r.t. for 7 h, Ti(Oi-Pr)4 (0.2 mmol,
60 µL) was added and the stirring continued for another
0.5 h. The yellow solution was cooled to 0 ˚C and treated
with benzaldehyde (0.5 mmol, 50 µL), then the resultant
mixture was allowed to warm up to r.t. naturally and stirred for
20 h. After the reaction was completed, it was cooled to 0 ˚C
again and quenched by 5% aq HCl (2 mL). The mixture was
extracted with EtOAc (2 × 10 mL). The organic
layer was dried over Na2SO4 and concentrated
under vacuum. The residue was purified by flash column chromatography
(silica gel H, 10% EtOAc in PE) to give the pure product.
Methyl 4-Hydroxy-4-phenylbut-2-ynoate
Yield
78%; 84% ee determined by HPLC analysis (Chiralcel OD-H
column, IPA-hexane = 20:80). t
R(minor) = 6.60
min, t
R(major) = 7.33
min. ¹H NMR (400 MHz, CDCl3): δ = 2.67 (d, J = 6.4 Hz,
1 H), 3.80 (s, 3 H), 5.58 (d, J = 6.4
Hz, 1 H), 7.35-7.43 (m, 3 H), 7.52 (d, J = 6.8
Hz, 2 H). ¹³C NMR (100 MHz, CDCl3): δ = 53.4,
64.4, 77.8, 87.4, 127.1, 29.22, 129.3, 138.9, 154.4.