Asymmetric Aldol Reaction of Glyoxal Catalyzed by Diarylprolinol

Diarylprolinol as an Effective Organocatalyst in Asymmetric Cross-aldol Reactions of Two Different Aldehydes

The aldol reaction is one of the most important carbon-carbon bond-forming reactions in organic chemistry. Asymmetric direct cross-aldol reaction of two different aldehydes has been regarded as a difficult reaction because of the side reactions such as self-aldol reaction and over reaction. We found that trifluoromethyl-substituted diarylprolinol, α,α-bis[3,5-bis(trifluoromethyl)phenyl]-2-pyrrolidinemethanol (1), is an effective organocatalyst that promotes several cross-aldol reactions of aldehydes with excellent diastereo- and enantioselectivities. Acetaldehyde can be employed as a suitable nucleophilic aldehyde. Successful electrophilic aldehydes are ethyl glyoxylate, chloroacetaldehyde, dichloroacetaldehyde, chloral, α-alkyl-α-oxo aldehyde, trifluoroacetaldehyde, glyoxal, alkenyl aldehyde, alkynyl aldehyde, and formaldehyde. Some of the aldehydes are commercially available as a polymer solution, an aqueous solution, or in the hydrated form. They can be used directly in the asymmetric aldol reaction as a commercially available form, which is a synthetic advantage. Given that the obtained aldol products possess several functional groups along with a formyl moiety, they are synthetically useful chiral building blocks.

Three-Pot Synthesis of Chiral Anti-1,3-diols through Asymmetric Organocatalytic Aldol and Wittig Reactions Followed by Epoxidation and Reductive Opening of the Epoxide

A three-pot asymmetric synthesis of the anti-1,3-diol unit was developed. In the first pot, enantioselective aldol reaction of aldehydes proceeds, catalyzed by organocatalyst, followed by either Wittig or Horner-Wadsworth-Emmons reactions to afford δ-hydroxy α,β-unsaturated carbonyls with excellent enantioselectivity. Diastereoselective hydroxy-directed anti-epoxidation proceeds in the next pot by the use of tert-BuOOH and LiHMDS. Reductive opening of the epoxide proceeds in a third pot to afford anti-β,δ-hydroxy carbonyl compounds with excellent diastereo- and enantioselectivity.

Organocatalytic Asymmetric Aldol Reaction of Arylglyoxals and Hydroxyacetone: Enantioselective Synthesis of 2,3-Dihydroxy-1,4-diones

A highly efficient quinine-derived primary-amine-catalyzed asymmetric aldol addition of hydroxyacetone to arylglyoxals is described. Structurally diverse anti-2,3-dihydroxy-1,4-diones were generated in high yields, with good diastereoselectivities and enantioselectivities.

Diarylprolinol-Mediated Asymmetric Direct Cross-Aldol Reaction of α,β-Unsaturated Aldehyde as an Electrophilic Aldehyde

The diarylprolinol-mediated asymmetric direct cross-aldol reaction of α,β-unsaturated aldehyde as an electrophilic aldehyde was developed. The reaction becomes accelerated by an acid when a carbonyl group is introduced at the γ-position of the α,β-unsaturated aldehyde. Synthetically useful γ,δ-unsaturated β-hydroxy aldehydes were obtained with high anti-selectivity and excellent enantioselectivity.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Regioselectivity switch in chiral amine-catalysed asymmetric addition of aldehydes to reactive enals

A regioselectivity switch was realized in the chiral amine-catalysed asymmetric addition of aldehydes to reactive enals, and either aldol adducts or conjugate adducts were obtained in a stereoselective fashion.