Intramolecular H-bonding

Intramolecular H bond Polyvalent alcohols Chelation 3600-3500 (s) 3200-2500 Sharper than dimeric band above Broad and occasionally weak the lower the frequency, the stronger the intramolecular bond... 

Ketones C=0 continued) o-Amino-aryl or o-hydroxy-aryl ketones 1655-1635 Low because of intramolecular H bonding. 

This reaction pathway is explained (71AKZ743) by intermolecular and intramolecular H-bonding with predominant participation of the more basic secondary amino group. 

Fig. 2.26 The crystal structure of / -tripeptide 113 with 1-aminomethylcyclohexanecarboxylic acid residues reveals a 10-membered H-bonded turn segment involving NH of residue 2 and C=0 of residue 3 [145]. The turn segment shows structural similarity to the central 10-membered turn of the 12/10/12-helix (see Section 2.2.3.2 and Fig. 2.19). The intramolecular H-bond is characterized by a N -O distance of 3.10 A and a (N-H -O) angle of 169.4°...

N-H--0) are 149.7° (inner H-bond) and 144.7° (outer H-bond). (B) Hybride -peptide 120 with a D-Pro-Gly type IT -turn segment (gray color) X-ray crystal structure [192]. The intramolecular H-bond N---0 distances are shown. The angles (N-H---0) are 147° (inner H-bond) and 155° (outer H-bond). The inter-molecular NH-0=C H-bonds (with N-H -O angles of 160 and 133 °) connect the hairpin into an infinitely extended -sheet... 

The formation of an intramolecular H-bond is supported by the slower rate of amide proton exchange in pyridine/10% CD3OD. The influence of the stereochemistry on turn formation and turn geometry has been investigated. Seebach and coworkers have demonstrated that dipeptide sequences consisting... 

Fig. 2.44 The N-0 turn formed from N-acety-lated a-aminooxy carboxamides of D-config-uration 169. Schematic representation of the N-0 turn with representative NOEs found in ROESY of 169c in CDCI3 and the X-ray crystal structure of 169c. The intramolecular H-bond is characterized by a short H- O distance of...

C(jS) bond very similar to that which is observed for helical fi- and y-peptides and (ii) two types of intramolecular H-bonds involving a given C=0, namely C=0,--. - -N, +2 snd C=0,---H-N +3 which close the 12- and 14-membered H-bonded rings, respectively. 

An interesting way to control the stereoselectivity of metathesis-reactions is by intramolecular H-bonding between the chlorine ligands at the Ru-centre and an OH-moiety in the substrate [167]. With this concept and enantiomerically enriched allylic alcohols as substrates, the use of an achiral Ru-NHC complex can result in high diastereoselectivities like in the ROCM of 111-112 (Scheme 3.18). If non-H-bonding substrates are used, the selectivity not only decreases but proceeds in the opposite sense (product 113 and 114). 

Similar studies have been performed for a series of nitrophenols [299], showing the ability of the A log P parameter to identify the presence of intramolecular H-bonds in a series of eongeners. This is of pharmaeeutical relevance, since, as mentioned above, it is often reeognized that the A log P parameter is a good predietor of biodistribution and passive diffusion [215,300]. This approaeh is as yet restrieted to neutral speeies, because of the diflieulty in establishing LSER for ions. It would yet be of great interest to enlarge the speetrum of LSERs to this field. 

These differences have been identified for two main classes of interactions, namely intramolecular H-bonding and charge delocalization. It has, for instance, been shown for VV-(p-methylbenzyl)hexylamine in water-DCE that its diff (log is 4.51, which is... 

The former structure contains an intramolecular H-bond within the host, which stabilizes its planar conformation, and an N-H. ..N link to the guest species. The other structure with acetic acid contains hydrogen-bond stabilized clusters of two hosts and two guests around the crystallographic inversion centers. A distortion of... 

Instead, the conformational characteristics are explained by the stringent requirement of the ionic interaction between the carboxylate/imidazolium ion pair coupled with the attempt of the former group to maintain as many H-bonds as possible (e.g. four H-bonds, cf. Ref. 75c). Such an attempt is obviously supported by the intramolecular H-bond in 1 Im 2 H20. The geometry of the corresponding moieties indicates the presence of strongly interacting ionic species (Fig. 42). 

Examples for the presence and absence in intramolecular H-bonds reflected in the hydration energies are shown below for cis- and frans-dicarboxyethylene.82 A similar presence and absence of H-bonding occurs for the phthalic acid isomers, 1,2-C02HC6H4C02, -AG°, =4.7 kcal/mol, and 1,4-C02HC6H4C02, -AG°0 l = 7.4 kcal/mol. 

AT1 cm ). and therefore, these dyes are excitable not only with red (635 or 670-nm) but also with blue (380, 405, and 470 nm) diode lasers or LEDs (Fig. 1). Carbonyl containing substituents such as 1,3-indanedione, cyanoacetic ester, barbituric, and thiobarbituric acid form intramolecular H-bond with the polymethine hydrogens of the squaraine bridge. As a result, the molar absorptivities and quantum yields of these dyes are substantially decreased. 

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