Experiment 3, page 1

EXPERIMENT 3:
THE PENICILLIN–CEPHALOSPORIN CONVERSION

Aims of the experiment
To use chromatographic techniques to follow a reaction and to purify the products.

Techniques used/learned:
Flash chromatography for small scale purifications; analysis of complex nmr spectra.

Introduction
The routine purification of organic compounds, especially in large quantities, was originally
carried out by tedious long column chromatography. Good separations often requiresprolonged
elution with solvents of low polarity. Nowadays, the technique of flash chromatography1has
become almost universal for bench-top separations. Flash chromatography involves the
purification of an organic (or inorganic) compound by partition between a finely divided stationary
phase, usually a specially manufactured grade of silica gel, and a rapidly moving organic solvent.
The technique is highly attractive in that separations are rapid (10-20 min is possible), resolution of
similar compounds is often excellent, and the technique reasonably inexpensive. In many cases
reasonably unstable compounds, such as diazoketones, can be purified easily by this technique.
The choice of eluant is easily found by prior testing by thin layer chromatography (t.l.c.).

In this experiment, the rearrangement2of the penicillin 1to the cephalosporin 2is conveniently
followed by t.l.c. and the product isolated by flash chromatography.

Department of Chemistry, Imperial College

Third Year Advanced Practical Organic Chemistry

N

S+

H
N

PhO

O–

CO2CHPh
2

O

H

H

O

N

H
N

PhO

S

CO2CHPh2

O

H

H

O

2

1