Page 57 - Visions of the Future Chemistry and Life Science

46 J. M. GOODMAN

there are several alcohols, and several ketones. A synthesis could not finish
by oxidising just some of the alcohols to ketones, because the reagent
would not know which alcohols should be oxidised and which should not.
This is a major problem for synthesis. How is it possible to selectively
make ketones in the presence of alcohols? More generally, how can a trans-
formation be made to act on only a part of a molecule?
Two general approaches can be used to find solutions to this problem.
Selective reagents could be developed, so that it is possible to change some
alcohols into ketones without affecting others in the same molecule. For
example, the lower left hand ketone in PM-toxin is close to the end of the
carbon chain. Might it be possible to develop a reagent that only oxidises
alcohols which are close to the end of carbon chains? An approach of this
sort would require a very good knowledge of the properties of reagents.
Alternatively, a strategic approach could be tried. The molecule could be
joined together in such a way that the question does not arise, because the
alcohols and ketones are already in the right places as a result of the choice
of joining processes. In practice, a combination of these methods may be
required in order to make complex molecules.
As a result, organic synthesis is an extremely demanding discipline,
requiring both a wide knowledge of chemistry and also the ability to
develop complete strategies for the construction of molecules. If the last
step of a synthesis does not work, then it may be necessary to begin again
by altering the first step. The science fiction character, Dr Who, has
machines which can synthesise molecules, just given the target structure.
Might it be possible to build such a machine? The physical manipulations
of mixing and purifying compounds can be automated to a large extent, and
it is possible to imagine building a machine which could do the mechani-
cal tasks of a highly trained synthetic chemist, although it would be far
more expensive and probably less effective than a skilled individual. The
main difficulty in the construction of such a machine would be to provide
the machine with suitable instructions for the synthesis.
Organic synthesis is sometimes compared with a game of chess, where
the effects of the opening moves are felt right through to the end game, and
where the total number of possible situations is greater than can be com-
prehensively analysed by any computer. Chess games require an opponent,
whose responses to the strategy chosen by the opening moves determine
the course of the game. Organic synthesis may be regarded as a similar
challenge. A good chess player may reasonably be expected not to make a