Biosynthesis, carbocation reactivity

In the preceding sections we have noted the biosynthesis and/or catabolism of cyclopropyl moieties by carbocation, by carbanion and by radical processes in specific contexts. At least two naturally occurring cyclopropyl metabolites and several synthetic ones have been observed to induce inactivation of specific target enzymes. The inactivation is presumptively related to chemical reactivity of loci at or adjacent to the cyclopropane equivalent. In the case of hypoglycine A (7) the methylene cyclopropane group is the problematic moiety, while in coprine (29) the hemiaminal is a latent cyclopropanone equivalent we shall analyze the proposed enzyme killing routes for each. Then we will turn to cyclopropyl... 

There is no final consensus on whether procyanidin biosynthesis is controlled thermodynamically or enzymatically. In either case proanthocyanidins are synthesized through sequential addition of flavan-3,4-diol units (in their reactive forms as carbocations or quinone methides) to a flavan-3-ol monomer [218]. Based on the latest findings there is some evidence that different condensation enzymes might exist which are specific for each type of flavan-3,4-diol [64] and that polymer synthesis would be subject to a very complex regulatory mechanism [63]. But so far, no enzyme synthetase systems have been isolated and enzymatic conversion of flavanols to proanthocyanidins could not be demonstrated in vitro [219]. If biosynthesis was thermodynamically controlled, the variation in proanthocyanidin composition could be explained by synthesis at different times or in different compartments [64], The hypothesis of a thermodynamically controlled biosynthesis is based on the fact that naturally and chemically synthesized procyanidin dimers occur as a mixture of 4—>8 and 4—>6 linked isomers in approximate ratios of 3-4 1 [220]. Porter [164] found analogous ratios of 4—>8 and 4—>6 linkages in proanthocyanidin polymers. 

Each of the carbocations discussed to this point has been a species in which all of the electrons were spin-paired. Another type of positively charged reactive intermediate is the radical cation—a species that has both an impaired electron and a positive charge. Radical cations play important roles in many radiochemical and photochemical reactions, and they may also be important in biological processes, including photosynthesis and the biosynthesis of natural products. ... 

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