Cross-condensation rearrangement

Because each monomer is potentially tetrafunctional, all sorts of partially hydrolyzed and partially condensed species are possible. For example, 10 distinct dimers may be formed. Many cyclic species can occur. Many of the species in solution are metastable with respect to anhydrous, amorphous Si02, and rearrangements occur with time (2). The complexity of this system means that the structure and growth of these polymers can be described only in a statistical and geometric fashion rather than by a more conventional description in terms of topologies (such as chains and cross-links) and molecular weights. The most useful and intuitive description of the structure of these polymers is in terms of fractal geometry. 

Other reactions must be mentioned beside the major reactions described above. These reactions may be responsible for the transformation of a toxic metabolite into the ultimate toxicant. Rearrangements and cyclizations are examples of reactions involved in these processes. In the case of the solvent hexane (Figure 33.19), the toxic metabolite, 2, 5-hexanedione, is formed by four successive oxidations of the molecule. The condensation of the -dicetone with the lysyl amino group of a neurofilament protein is followed by a Paal-Knorr cyclization reaction. This is the initial process that explains the hexane-induced neurotoxicity." A further auto-oxidation of the A-pyrrolyl derivatives leads to the cross-linking of the axonal intermediate filament proteins and the subsequent occurrence of peripheral neurotoxicity." ... 

From two conjugated carbonyl compounds the cross-benzoin condensation initiated by a chiral azolecarbene (199) sets up a sequence of oxy-Cope rearrangement, aldol reaction and decarboxylation/ ... 

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