Amide natural occurrence

The presence of isoretulinal (111) in the root bark of Strychnos variabilis de Wild, has been confirmed,79 and it has also been shown that retulinal (112), with which it is very easily equilibrated, occurs in smaller amounts in the same extracts both alkaloids exist in two forms, which are amide rotamers, according to their n.m.r. spectra. The 12-hydroxy-derivatives, (113) and (114), form a similar equilibrating pair of alkaloids, and were also isolated from this source. The occurrence of these easily interconvertible C-16 epimers explains the simultaneous occurrence of retuline and isoretuline derivatives in Nature, in both the monomeric and dimeric alkaloid series. 

This section will have a detailed look at the occurrence of the different cyclic amides in nature and their physiological role in various metabolic pathways. Allantoin is widely distributed in nature and is an important metabolite in the degradation of purine nucleotides (see Fig. 12.4-4). Allantoin occurs in all organisms that do not have uric acid as the final product of their purine degradation pathways, and is the substrate for the enzyme allantoinase or 5-ureidohydantoinase (E.C. 

The carboxyl group of the lactic acid side chain of A -acetylmuramic acid forms an amide bond with the N-terminal end of a tetrapeptide that has the sequence L-Ala-D-Gln-L-Lys-D-Ala. Recall that bacterial cell walls are one of the few places where D-amino acids occur in nature. The occurrence of D-amino acids and A acetylmu-ramic acid in bacterial cell walls but not in plant cell walls shows a biochemical as well as structural difference between prokaryotes and eukaryotes. 

CARBOXYLIC ACIDS ARE another class of organic compounds containing the carbonyl group. Their occurrence in nature is widespread, and they are important components of foodstuffs such as vinegar, butter, and vegetable oils. The most important chemical property of carboxylic acids is their acidity. Furthermore, carboxylic acids form numerous important derivatives, including esters, amides, anhydrides, and acid halides. In this chapter, we study carboxylic acids themselves in Chapters 14 and 15, we study their derivatives. 

For organisms which express both pathways for nitrile hydrolysis, the stereochemical pathways can be very complex. The latter is illustrated by the microbial resolution of cx-aryl-substituted propionitriles using a Rhodococcus butanica strain (Scheme 2.109) [697]. Formation of the natural L-acid and the o-amide indicates the presence of an L-specific amidase and a nonspecific nitrile hydratase. However, the occurrence of the (5)-nitrile in case of Ibuprofen (R = i-Bu, e.e. 73%) proves the enantioselectivity of the nitrile hydratase [694]. In a related approach, Brevibacter-ium imperiale was used for the resolution of structurally related a-aryloxypropionic nitriles [698]. 

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