Reaction Generators, an Introduction

For explanation, the example 8 of Fig. 5 is again used (Fig. 7). The essential features of an aldol condensation in its retro-form are the breaking of a CC- and of an OH-bond, and the making of a CO- and of a CH-bond. Alternatively, if this process is considered in an even more general manner, two bonds between atoms I, J and K, L are broken and two new ones between the four atoms involved are made. 

For each set of two bonds broken there are two alternatives for making two new ones as indicated with reactions 9.1 and 9.2 as well as 9.4 and 9.5. The bonds made can be contained in two different molecules (reactions 9.3 and 9.6), or the bonds broken 

Observe that to generate the reactions of Fig. 9 no information was necessary on whether such a reaction is known no database of reactions is necessary. The problems in building, updating and maintaining a reaction library are thus avoided. The formal treatment of reactions as bond and electron-shifting processes allows the generation, in principle, of all conceivable reactions, and can be seen as a method to deal freely with molecular architecture. The program s result could be a known reaction, but equally a new, as yet undiscovered reaction which could be realised in the laboratory. 

But a price has to be paid for this potential advantage, because the number of reactions that could be obtained by applying a formal reaction generator scheme could be very high. Furthermore, most of the suggestions could be chemically mean- 

A system for synthesis design working with a database of reactions will initially contain only a few reactions and thus produce only a few alternatives. Thus, evaluation and selection are not very important. However, as the size of the database is expanded, more and more alternatives will be obtained for a given target structure. Here again, the development of a general evaluation and selection package will be required to handle the various transforms of the database. 

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