Question

Comparing enantioselective carbonyl reduction using CBS reagent, and
enantioselective biological reduction using an enzyme, what is an advantage
and disadvantage for each?

Answer 1

CBS

advantages:

The CBS reduction has proven to be an effective and powerful method to reduce a wide range of different types of ketones in both a stereoselective and chemoselective manner.

disadvantages:

The presence of water in the reaction mixture has been shown to have a significant effect on enantiomeric excesses, and thus the CBS reduction must be conducted under anhydrous conditions.^[14] Temperature also plays a critical role in the observed stereoselectivity. In general, at lower temperatures enantiomeric excesses (ee’s) are obtained. However, when the temperature is increased, the ee values reach a maximum value that is dependent on the catalyst structure and borane reducing agent used

enantioselective biological reduction using an enzyme ;

advantages:

Many of the building blocks of biological systems, such as sugars and amino acids, are produced exclusively as one enantiomer. As a result of this living systems possess a high degree of chemical chirality and will often react differently with the various enantiomers of a given compound. Examples of this selectivity include:

• Flavour: the artificial sweetener aspartame has two enantiomers. L-aspartame tastes sweet, yet D-aspartame is tasteless
• Odor: R-(–)-carvone smells like spearmint yet S-(+)-carvone, smells like caraway.
• Drug effectiveness: the antidepressant drug Citalopram is sold as a racemic mixture. However, studies have shown that only the (S)-(+) enantiomer is responsible for the drug's beneficial effects.^[5][6]
• Drug safety: D‑penicillamine is used in chelation therapy and for the treatment of rheumatoid arthritis. However L‑penicillamine is toxic as it inhibits the action of pyridoxine, an essential B vitamin
• Diadvantages:
• isolation and purification of enzymes needs careful efforts