N-O turn

Interestingly, 8-aminoxy acids which are homologs of y-amino acids have also been found to promote the formation of turns and helices. In apolar solvent and in the solid state, model diamides consisting of /9 -aminoxy add residues adopt a novel N-O turn stabilized by both a nine-membered H-bonded ring between C=0 and NHj+2, and a six-membered ring formed between N-O and NH +1. The X-ray crystal structure of a corresponding triamide revealed two consecutive C9 N-O turns suggesting a novel 1.79-helical fold [279]. 

Equation (3-178) suggests that a plot of A obs/[ROH] vs. [N] will be linear. Because the conversion to the intermediate is quantitative, [N] = [N]o — [AXJq. Plots according to Eq. (3-178) were linear, permitting ky and A in to be estimated. Turning to the acetic anhydride—alcohol system, it is inferred that (in the absence of water) itj/lLi is close to zero (Scheme XXIV). Although the intermediate could not be detected spectrally, its possible presence is admitted in the rate equation for the loss of anhydride ... 

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°...

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...

An intramolecular nitrone 1,3-dipolar cycloaddition reaction to give 46 from 45 followed by reductive N-O bond cleavage afforded a stereoselective synthesis of the tetrahydro 177-1-benzazepines 47 the nitrone precursors 44 were prepared in turn by a Claisen rearrangement from an IV-allylamine <06SL2275>. 

The trans isomer turned out to be favored over the cis isomer by 41.8 kJmol at B3LYP/aug-cc-pvTZ level of theory. In both isomers the C—N—O moiety is strongly polarized. The charges are 4-0.24 e for both nitrogens and —0.50 and 0.51 e for both... 

A novel route to the azepinone system in 93 and 94, based on an intramolecular nitrone-eneallene cycloaddition (in 91 and 92, which were accessed in turn from 88 via 89 and 90) and subsequent rearrangement via N-O bond homolysis and an electrocyclic recyclization step) has been described (Scheme 11) <2005EJ02715>. On heating 93 in toluene, equilibrium with the isomeric azepinone 95 was established, although comprising less than 3% of 95. The general synthetic approach was applied to the synthesis of an analogue of the alkaloid astrocasine. 

The synthesis of conformationally constrained bicyclic and tricyclic compounds, obtained via 5-Exo-Trig iodocyclisation of polybenzylated sugars with an allylic substituent at the anomeric position, is described. The conformation of these molecules has been studied by n.O.e. experiments and Molecular Dynamics calculations. The introduction of an amino and a carboxylic group resulted in the formation of conformationally constrained bicyclic glyco-aminoacids that mimic protein turn conformation. 

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