Pyridine TV-oxide

Various other chromene derivatives 176a-d could be epoxidized with Katsuki s Mn-salen catalyst 173d using either H2O2 or TMS2O2 as oxidant. With this catalytic system several axial ligands (none, 7V-methylimidazole, pyridine TV-oxide) and additives (none. 

Aminopyridine can be prepared by the reaction of 4-chloro-l,3-benzoxazines (47) with pyridine TV-oxides. Here it is proposed that an ion or radical pair is formed initially, which through displacement of hydrogen chloride and rearrangement leads to an TV-substituted benzoxazine (48). Finally, acid hydrolysis gives 2-aminopyridine and salicylic acid (49 Scheme 14) (80CPB465). 

A Oxides and their analogues (e.g., AMmides) can be considered as a special type of dipolar structure. They are usually dealt with by use of simple additive nomenclature (e.g., pyridine TV-oxide), but alternatives are possible. For example, other methods of indicating unusual valency could be applied, e.g., lA5-pyridin-l-one (see Section II, B, 2, d). 

Figure 7.41 Structure of the tubular lanthanide p-sulfonatocalix[4] arene assembly (a) partial spacefilling view perpendicular to the tube showing six turns of the helix. p-Sulfonatocalix[4] arene anions shown in stick mode mark two helix turns. Helix turns involve TT-stacking between calixarenes and pyridine-TV-oxide guests (also shown as sticks), (b) cutaway view showing hydrated Na+ and La3+ ions within the cylindrical channel. (Reprinted with permission from AAAS from [52]).

It was also demonstrated (1997HC183) that the reactions can be performed under analogous conditions not only with pyridine 130 but also with 2-chloro-formylpyri-dine Ar-oxide or 2-bromo-3-(l,3-dioxolan-2-yl)pyridine TV-oxide. The latter reactions produce the corresponding thieno[Z>][2,5]naphthyridine IV-oxides, which can also be prepared by oxidation of the parent systems with m-chloroperoxybenzoic acid. 

Treatment of furo[3,2-c]pyridine TV-oxide (41) with phosphorus oxychloride accomplished deoxygenation along with chlorination of the ring to afford 4-chlorofuro[3,2-c]pyridine (42) (Equation (3)) in 45% yield <75JHC705>. 

Azine approach. Pyridine TV-oxides undergo 1,3-dipolar cycloaddition reactions with aryl isocyanates. The first formed 1,2-dihydropyridines are unstable and rearrange at once to the 2,3-dihydropyridines (287). From 3-picoline 1-oxide both the 6-methyl and 7a-methyl isomers are formed. The adduct (288) from 3,5-dibromopyridine 1-oxide reacts further to eliminate HBr, thereby giving the fully aromatic system (79CPB2261). 

Palladium-catalyzed Arylation of Pyridine N-Oxides and N-Iminopyridinium Ylides. Tri-tert-butylphosphine has been used in the palladium-catalyzed C-arylation of pyridine 7/-oxides and derivatives. Using palladium acetate as the catalyst, tri-tert-butylphosphine as the ligand, and 3-bromopyridine as the electrophile, the C-H arylation of pyridine tV-oxide was accomplished, yielding the 2-arylated compound as the major product and the 2,6-diarylated pyridine as a minor product (eq 24). ... 

Considering that calixarenes have rather low molar absorption coefficients, more efficient chromophores were attached to calixarene moieties in order to obtain strongly absorbing ligands for and Tb + complexation. Complexes of ligand 54 containing four pyridine-TV -oxide units as chromophores showed metal luminescence upon ligand excitation in methanol (Pappalardo et al. 1991), The luminescence was completely... 

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