Planarity of amides

Full control of the formation of new bonds and associated torsions (e.g., avoidance of peroxo groups and the like, enforced planarity of amides, etc.). 

In view of the success that has so far been obtained by this method of attack, it seems justified to assume that proposed configurations of polypeptide chains in proteins that deviate largely from the structural principles that have now been formulated (planarity of amide group, correct interatomic distances and bond angles, formation of hydrogen bonds) may be ruled out of consideration. 

The higher frequencies of the /3-lactam carbonyl absorption in fused systems has been attributed to increased inhibition of amide resonance as the /3-lactam ring becomes less planar (b-72mI50900 p. 303). For the 3-cephems (61) there is also the possibility of enamine resonance which could further reduce the ability of the /3-lactam nitrogen to contribute to amide resonance. 

Check of the planarity of functional groups which are by definition planar, e.g., aniline nitrogen, double bonds or amides. 

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

Methodical details of crystal calculations have been published by several authors (8, 95-98). As applications we only mention the calculation of the crystal structure of diphenyl (98, 99) which has a planar structure in the solid state (100) whereas gaseous diphenyl is twisted around the CC-bond, which links both benzene rings, by about 40° (101). The calculations reproduce satisfactorily the experimental findings. A further application is the calculation of the crystal structures of a series of amides (102), with the object of deriving suitable functions for the description of hydrogen bonds (cf. previous Section). 

It would usually be assumed that abstraction/recombination reactions such as those illustrated in this section would proceed with racemization at the reacting centers. It has been reported22, however, that photocyclization of amide 1 proceeds with complete retention of absolute configuration. Racemization at the site of abstraction requires orbital rehybridization, passing through a planar intermediate. In this case rehybridization appears to be markedly slowed. This may be an electronic effect due to the heteroatom substituent on the intermediate radical, or simply a steric effect. The structure of product 2 was established by X-ray crystallographic analysis. 

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