Axially Chiral Compounds by Tan Bin;

Author:Tan, Bin; [Tan, Bin]
Language: eng
Format: epub
Publisher: John Wiley & Sons, Incorporated
Published: 2021-08-12T00:00:00+00:00

Scheme 6.22 Pd-catalyzed synthesis of optically active allenoates with a stoichiometric proton source.

6.3.1 Catalytic Enantioselective Synthesis from Achiral Substances

6.3.1.1 Enantioselective Proton Migration (Isomerization) of Alkyne

Alkynes bearing a relatively more acidic propargylic methylene are suitable substrates for enantioselective isomerization. In 2000, Arai and Shiori et al. reported the asymmetric isomerization of 1,3-diaryl alkyne to axially chiral allene under phase transfer catalysis (PTC) conditions, but only a single example was shown with low enantioselectivity [60].

In 2009, Huang and Tan et al. showed that a highly enantioselective route to chiral disubstituted 2,3-allenoates 76 could be realized with a chiral guanidine catalyst 75 from isomerization of 3-alkynoates, which bears more acidic propargylic protons (Scheme 6.23a) [61]. However, the incomplete conversion and inseparability of the starting materials with the products undermined its synthetic utility. Takemoto and coworkers reported in 2011 a benzothiadiazine-1,1-dioxide-type catalyst 78 promoted highly enantioselective isomerization of propargyl esters, ketones, and amides 77 to the corresponding allenes 79 (Scheme 6.23b) [62]. To overcome the issue of inseparability with an unreacted starting material, the allenes were in situ converted to Diels–Alder and [3 + 2] dipolar cycloaddition products.

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