Diastereoisomers

The most widely used method for the separation of enantiomers, often called classical resolution, rests on the crystallization of diastereoisomers formed from a racemate (dl) and an enantiopure reagent (say, D), called a resolving agent:

It is convenient to designate with letters p (positive) and n (negative) the diastereoisomers resulting from reaction of the two constituents of like sign and opposite signs, respectively. In this convention, no account is taken of the sign of rotation of the dia-stereoisomers themselves, which, if needed, can be specified by adding a + or — subscript to the p and n descriptor. Accordingly, the above reaction yields a mixture of p (dD) and n (lD) diastereoisomers, which, for instance, can both be dextrorotatory, i.e. p+ and n + . The opposite enantiomer (L) of the resolving agent would then afford with the same racemate the diastereoisomer pair p_ (lL) and n_ (dL), mirror image of p+ and n# (Marckwald principle). Note that the reciprocal resolution of DL by, for instance, d, would yield a mixture of p + (dD) and n_ (dL).

Diastereoisomeric salts, formed from simple acid-base reactions, are central to such resolutions, although covalent diastereoisomers (esters, amides, etc.) are also occasionally resolvable by crystallization. To cite but a single example, the DSM company resolves dl-phenylglycine by crystallization of its dia-stereoisomeric salts with (+ )-10-camphorsulfonic acid, to produce the d-enantiomer required for the manufacture of the antibiotic ampicillin, at a scale of more than 1000 tonnes per year.

Diastereoisomeric inclusion complexes, in which the resolving agent is a chiral crystalline host lattice, represent an interesting alternative for the resolution of substances that cannot form salts, or do not possess functional groups suitable for formation of covalent diastereoisomers.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

Get My Free Ebook


Post a comment