For fatty acid biosynthesis, reduction after each condensation step affords a growing hydrocarbon chain. In the absence of this reduction process, the growing poly-^-keto chain needs to be stabilized on the enzyme surface until the chain length is complete, at which point cyclization or other reactions can occur. The poly-^-keto ester is very reactive, and there are various possibilities for undergoing intramolecular Claisen or aldol reactions, dictated of course by the nature of the enzyme and how the substrate is folded. Methylenes flanked by two carbonyls are activated, allowing formation of carbanions/enolates and subsequent reaction with ketone or ester carbonyl groups, with a natural tendency to form strain-free six-membered rings.
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