2- pyridine, conformational

Pyridine, 2-(dimethylamino)-5-nitro-nitration, 2, 189 Pyridine, 3,5-dinitro-conformation, 2, 110 Pyridine, 3,5-diphenyl-synthesis, 2, 0... 

NQR, 2, 125 Pyridines, acetylalkyl-alkyl deacetylation, 2, 301 Pyridines, acetyltrimethyl-synthesis, 2, 470 Pyridines, acyl-conformation, 2, 162 reactions, 2, 337 Pyridines, alkenyl-ozonolysis, 2, 334 reactions, 2, 334 Pyridines, alkenyldihydro-disproportiation, 2, 62 Pyridines, alkyl-... 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

Fig. 8-1. Schematic representations of the interaction of the (R)NapEtNH enantiomer guest with a chiral pyridine-18-crown-6 host (S,S)-1 and possible conformations of the (R)NapEt com-...

O-isopropylidene derivative (57) must exist in pyridine solution in a conformation which favors anhydro-ring formation rather than elimination. Considerable degradation occurred when the 5-iodo derivative (63) was treated with silver fluoride in pyridine (36). The products, which were isolated in small yield, were identified as thymine and l-[2-(5-methylfuryl)]-thymine (65). This same compound (65) was formed in high yield when the 5 -mesylate 64 was treated with potassium tert-hx Xy -ate in dimethyl sulfoxide (16). The formation of 65 from 63 or 64 clearly involves the rearrangement of an intermediate 2, 4 -diene. In a different approach to the problem of introducing terminal unsaturation into pento-furanoid nucleosides, Robins and co-workers (32,37) have employed mild base catalyzed E2 elimination reactions. Thus, treatment of the 5 -tosylate (59) with potassium tert-butylate in tert-butyl alcohol afforded a high yield of the 4 -ene (60) (37). This reaction may proceed via the 2,5 ... 

High-valency metal fluoride fluorination of pyridine [82JFC(21)171], quinoline [82JFC(21)413], and 2-methylfurans [91 JFC(51)179] has been reported. With 2-methylfuran a complex mixture of stereoisomers of partially fluorinated oxolans was obtained. These can be dehydrofluorinated to fluorooxolens and no furans have been observed. Conformation and structural group were found to influence the direction and readiness toward dehydrofluorination [91 JFC(52) 165]. 

Pentadienyl radical, 240 Perturbation theory, 11, 46 Propane, 16, 165 n-Propyi anion conformation, 34 n-Propyl cation, 48, 163 rotational barrier, 34 Propylene, 16, 139 Protonated methane, 72 Pyrazine, 266 orbital ordering, 30 through-bond interactions, 27 Pyridine, 263 Pyrrole, 231... 

The recognition that short chain / -peptides can form regular secondary structures initially came from detailed conformational analysis of y9 -peptides 1 and 66 (which incorporates a central (2S,3S)-3-amino-2-methylbutanoic acid residue) by NMR in pyridine-d5 and CD3OH [10, 103, 164] and homooUgomers (as short as four residues) of trons-2-amino-cyclohexanecarboxyhc acid (trans-ACHC) (e.g. hex-amer 2 for the (S,S) series) by NMR and X-ray diffraction [6, 126, 159]. 

Detailed NMR conformational analysis of y -peptides 139-141 (Fig. 2.35) in pyri-dine-d5 revealed that y-peptides as short as four residues adopt a 2.6-hehcal fold stabilized by H-bonds between C=0 and NH +3 which close 14-membered pseudocycles [200, 201]. The 2.614-helical structure of a low energy conformer of y-hex-apeptide 141 as determined from NMR measurements in pyridine-d5 [200], is shown in Fig. 2.36A and B). Determination of the structure of y" -peptides in CD3OH was hampered by the much lower dispersion of the diasterotopic H-C(a) protons compared to their dispersion in pyridine-d5. However, the characteristic and properly resolved i/ir-2 NOE crosspeacks between H-C(y) and NH +2 in the NH/H-C(y) region of the ROESY spectrum were an indication that the 2.6-helical structure is at least partially populated in CD3OH. 

Stmctural characterization of y and y peptides 148 and 149 by NMR in both pyridine-d5 and CD3OH was hampered by the low dispersion and strong signal overlap, and so far the conformational preferences of these peptides, if any, remain undetermined. 

Optimal pre-organization of the y-peptide backbone towards the formation of open-chain turn-like motifs is promoted by unlike-y " -amino acid residues. This design principle can be rationalized by examination of the two conformers free of syn-pentane interaction (f and II", Fig. 2.34). Tetrapeptide 150 built from homo-chiral unlike-y -amino acid building blocks 128e has been shown by NMR experiments in pyridine to adopt a reverse turn-like structure stabilized by a 14-mem-bered H-bond pseudocycle [202] (Fig. 2.37 A). 

Fig. 2.47 The (P)-2.5-helical structure of N.N -linked oligoureas as determined by NMR meaurements in pyridine-c/5. (A) Stereo-view along the helix axis of a low energy conformer of nonamer 178 generated by restrained molecular dynamics calculations. (Adapted from [274]). The helix is characterized by (i) a rigid +)-SYnclinal arrangement around the C(a)-...

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