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The labile unrearranged hydroxymethylbilane (5) is synthesised unambiguously, is proved to be identical with the product from deaminase acting on porphobilinogen, and is shown to be the substrate for deaminase-free cosynthetase which quantitatively ring-closes (5) with rearrangement to uro'gen-III (7).
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Battersby et al. (1979) studied this question.
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