Written By: Mark Redlich, Ph.D.

I just came across an ASAP article on Organic Letters which describes a clean and fast aziridination of diazoacetamides with imines.  Typically, yields and chemoselectivities of Lewis or Brønsted acid catalyzed asymmetric aziridinations of N-Boc-imines have been modest. Zhong and coworkers achieved excellent results when a chiral phosphoric acid derived from anthryl-substituted (R)-BINOL was used as the catalyst.  In these cases, the reactions were all completed in 10 minutes at room temperature, providing high yields and displaying excellent chemo-, diastereo-, and enantioselectivities.

 
Zeng, X. et al. Org. Lett. 2009, ASAP.

The conversion of alcohols to their corresponding carbonyl compounds is a critical functional group transformation and green processes, which can effect these transformations, are of significant interest.  While nitroxyl-based TEMPO has had a long-standing reputation as an environmentally friendly oxidant, the bulky nature of TEMPO’s piperidine skeleton precludes its use in sterically demanding environments. Iwabuchi and coworkers have shown that the related azaadamantane organocatalyst, AZADO (2-Azaadamantane N-Oxyl) exhibits enhanced reactivity over TEMPO in mild catalytic oxidations of secondary alcohols such as menthol.

Shibuya, M.; Tomizawa, M.; Suzuki, I.; Iwabuchi, Y. J. Am. Chem. Soc. 2006, 128, 8412.

Written By: Mark Redlich, Ph.D.

Here is a recent report I came across which highlighted the NHC-catalyzed ring expansion of heterocyclic carboxaldehydes to provide lactones.  The best results were achieved when the 2,6-diisopropylphenyl substituted imidazolinium catalyst was employed.  Thiazolium and triazolium catalysts were also screened in the transformation but both provided either complex reaction mixtures or trace amounts of lactone.  Five-, six-, and seven-membered as well as bicyclic lactones are all accessible from their corresponding aldehydes, and the reaction is able to be performed under air without a significant decrease in yield.  Interestingly, the corresponding imidazolium NHC catalyst is ineffective in this transformation.

Wang, L. et al. Org. Lett. 2009, 11, ASAP.

Written by: Dr. Sharbil J. Firsan

The first catalytic, highly enantioselective synthesis of chiral N-benzoyl hemiaminals has recently been reported by Antilla and co-workers.  Their approach utilizes the direct addition of alcohols to imines derived from aromatic aldehydes, which is facilitated by a BINOL-derived, chiral phosphoric acid organocatalyst.  Some of the attractive features of this method include:

• Good-to-high yields and enantioselectivities
• Mild reaction conditions
• Wide alcohol scope: primary, secondary, tertiary, and cyclic secondary
• A range of substituents in the aryl portion of the imine are tolerated
• Double and triple bonds in the alcohol substrate are compatible with the reaction conditions

(1) Li, G.; Fronczek, F. R.; Antilla, J. C. J. Am. Chem. Soc. 2008, 130, 12216.  (2) For a very recent review on the use of chiral phosphoric acid organocatalysts in asymmetric synthesis, see Adair, G.; Mukherjee, S.; List, B. Aldrichimica Acta 2008, 41, 31