Desymmetrization

Desymmetrization in stereochemistry is the modification of a molecule that results in the loss of one or more symmetry elements. A common application of this class of reactions involves the introduction of chirality.[1] Formally, such conversions required the loss of an improper axis of rotation (mirror plane, center of inversion, rotation-reflection axis). In other words, desymmetrisations convert prochiral precursors into chiral products.[2]

Examples

Typical substrates are epoxides, diols, dienes, and carboxylic acid anhydrides.[1]

One example is the conversion of cis-3,5-diacetoxycyclopentene to monoacetate. In this transformation, the plane of symmetry in the precursor is lost, and the product is asymmetric. The desymmetrisation itself is not usually considered useful. The enantioselective desymmetrisation however delivers a useful product. This particular conversion utilizes the enzyme cholinesterase.[3]

In another example, a symmetrical cyclic imide is subjected to asymmetric deprotonation resulting in a chiral product with high enantioselectivity.[4]

Transfer hydrogenation converts benzil (PhC(O)C(O)Ph) into one enantiomer of hydrobenzoin:[5]

PhC(O)C(O)Ph + 2 H2 → PhCH(OH)CH(OH)Ph

The precursor benzil has C2v symmetry, and the product is C2 symmetric.

Citric acid is also a symmetric molecule that can be desymmetrized by partial methylation.

References
  1. Willis, Michael C. "Enantioselective desymmetrization" Journal of the Chemical Society, Perkin Transactions 1: Organic and Bio-Organic Chemistry 1999, pp. 1765-1784. doi:10.1039/A906269B
  2. Basic Terminology of Stereochemistry, G.P. Moss Ed. Pure Appl. Chem., Vol. 68, No. 12, pp. 2193-2222, 1996.
  3. Donald R. Deardorff, Colin Q. Windham, and Chris L. Craney "Enantioselective Hydrolysis of cis-3,5-diacetoxycyclopentene: (1R,4S)-(+)-4-Hydroxy-2-cyclopentenyl Acetate" Organic Syntheses, Coll. Vol. 9, p.487 (1998); Vol. 73, p.25 (1996).
  4. Vincent Rodeschini, Nigel S. Simpkins, and Fengzhi Zhang "Chiral lithium amide base desymmetrization of a ring fused imide: formation of (3as,7as)-2-[2-(3,4-dimethoxyphenyl)-ethyl]-1,3-dioxo-octahydro-isoindole-3a-carboxylic acid methyl ester" Org. Synth. 2007, volume 84, 306. doi:10.15227/orgsyn.084.0306
  5. Takao Ikariya, Shohei Hashiguchi, Kunihiko Murata, and Ryōji Noyori (2005). "Preparation of Optically Active (R,R)-Hydrobenzoin from Benzoin or Benzil". Organic Syntheses: 10{{cite journal}}: CS1 maint: multiple names: authors list (link).


This article is issued from Wikipedia. The text is licensed under Creative Commons - Attribution - Sharealike. Additional terms may apply for the media files.