Asymmetric Synthesis Research Paper

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According to R.A Aitken, “An asymmetric synthesis may be defined as a synthesis in which an achiral unit is an ensemble of substrate molecules is converted to a chiral unit such that the possible stereoisomers are formed in unequal amounts.” It relates to any synthetic process that incorporates one or more new elements of chirality during a functional group transformation.

Asymmetric synthesis involves the formation of chiral molecules. An object is said to be chiral, if it cannot be superimposed upon its mirror image meaning it is not identical and has no plane of symmetry. Any structure that has no plane of symmetry can exist as two mirror image forms. Many compounds may be obtained in two different forms in which the molecular structures
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DOPA must be administered as the pure (-) – enantiomer, this is necessary to prevent a build-up of (+) – DOPA in the body which could not be metabolized by the enzymes present. The asymmetric synthesis of DOPA is carried out on an industrial scale. The two enantiomers in many chiral compounds have distinct biological activities. Propranolol was introduced in the 1960’s and it belongs to a group of medicines known as beta-blockers. It is used to treat many different medical conditions such as heart disease, Propranolol works on the heart and blood vessels. For clinical use, enantiomeric purity is essential but the (+) – enantiomer acts as a contraceptive. The asymmetric synthesis of the beta-adrenergic blocker (S) – (-) – propranolol is described by the famous Sharpless expoxidation. The alkaloid (-) – levorphanol is an opiate medication used to treat moderate to severe pain. First prescribed in Germany in 1948 as an orally-active morphine-like analgesic. It has been in clinical use in the U.S. since 1953. Its enantiomer, (+) –dextrorphan is entirely lacking of this activity, but is useful as a cough suppressant. “For more complex chiral molecules there are not only two possible stereoisomeric forms but anywhere up to 2n where n is the number of stereogenic units. While a conventional synthesis followed by separation of the two enantiomers is feasible when there is only …show more content…
The use of the second generation method has a number of advantages, the diasteriomer generated from the use of the chiral auxiliary can be separated by the use of conventional methods ie. Chromatography and crystallization. Because it can be recycled, this reduces the costs greatly. This method does have its disadvantages also as it needs to be synthesized. The chiral auxiliary is introduced and then removed once its proposed is fulfilled.

In the case of the third generation or reagent controlled methods, the two extra steps needed in the second generation methods to attach and remove the auxiliary can be avoided completely by using this method. This method converts an achiral substrate to the chiral product by the use of a chiral reagent.
S ---R*--- P*
The control is now intermolecular whereby the first and second generations were intramolecular. An example is provided by the hydroboration of 1 – methylcyclohexene using isopinocampheyl – borane derived from (+) – alpha – pinene to give alcohol with two adjacent stereogenic

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