Hydrogen bonding cyclic amides

Witt, M. Kreft, D. Griitzmacher, H.-F. Effects of Internal Hydrogen Bonds Between Amide Groups Protonation of Ali-cyclic Diamides. Eur. J. Mass Spectrom. 2003,9, 81-95. 

The diastereoselective formation of a cyclic array of intramolecular hydrogen bonds between amide and carbonyl functions at the narrow rim was also observed for the tetraamides 121 and for similar 1,3-di- or monoamides.242... 

Other common supramolecular synthons for solid-state supramolecular construction are formed via hydrogen bonding of amide groups [64]. Primary and secondary amides [65], pyridones [66] and 2-aminopyrimidines [67] form cyclic hydrogen-bonded dimers, as shown in Figure 5. 

The amide group shows a prominent directivity in the hydrogenation of cyclic unsaturated amides by a cationic iridium catalyst, and much higher diastereo-selectivity is realized than in the corresponding ester substrates (Table 21.7). In the case of / ,y-unsaturated bicyclic amide (entry 3), the stereoselectivity surpasses 1000 1 [41]. An increase of the distance between the amide carbonyl and olefmic bond causes little decrease in the selectivity (d, -unsaturated amide, entry 6) compared with the case of the less-basic ester functionality (Table 21.6, entry 5). 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

From a structural point of view the OPLS results for liquids have also shown to be in accord with available experimental data, including vibrational spectroscopy and diffraction data on, for Instance, formamide, dimethylformamide, methanol, ethanol, 1-propanol, 2-methyl-2-propanol, methane, ethane and neopentane. The hydrogen bonding in alcohols, thiols and amides is well represented by the OPLS potential functions. The average root-mean-square deviation from the X-ray structures of the crystals for four cyclic hexapeptides and a cyclic pentapeptide optimized with the OPLS/AMBER model, was only 0.17 A for the atomic positions and 3% for the unit cell volumes. 

Oxidative attack on a carbon-hydrogen bond of an alkyl group a to a nitrogen atom is not restricted to saturated aliphatic amines. In fact X in an X-N-CH- structural subunit can be virtually any common atomic grouping that can be found in stable organic molecules. For example, w-carbon hydrogens of Aralkyl-substituted aromatic cyclic amines (119), aryl amines (120), amides (121), amidines (122), A-nitrosodialkylamines (123), etc. are all subject to oxidative attack, carbinolamine formation, and in most cases release of an aldehyde or ketone depending on the substitution pattern (1° or 2°)... 

They are of interest in studies on the thermodynamic behavior of nonionic compounds in aqueous media because they have the ability of forming hydrogen bonds with the solvent (through the two r/j-amide groups in the cyclic dipeptide ring), and give rise to hydrophobic interactions. ... 

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

---