Pyridine-1-oxides rearrangement

Prostaglandins 624, 725, 960 Prostanoids 620 Protonation 565-567, 1049 photochemical 882 Pseudopotential methods 15, 16 Pummerer rearrangement 240, 243, 470, 843 Pyramidal inversion 602, 604 Pyrazolenines 749 Pyridazine oxides 640 Pyridine aldehydes, synthesis of 310 Pyridine oxides 640 Pyrolysis 102-105 of sulphones 110, 679-682, 962 of sulphoxides 739, 740 Pyrroles 265, 744... 

Intramolecular Lewis base complexation of the germanium atom also did not appear to be a valuable alternative to overcome the lability of these elusive intermediates, since the germanones expected from oxidation of the base-stabilized germylenes 140131 (with Me3NO) and 141132 (with 02, DMSO, or pyridine oxide) either rearrange as already mentioned [Section VI,A,4, Eq. (33)]131 or dimerize to digermadioxetane 160132 [Eq. (35)]. 

In the H-NMR spectra of pyridine oxides (e.g., 443) a significant low-field shift of H-9 is observed (Table XII). Pyridine oxide 446 shows a transformation, similar to the acetic anhydride rearrangement [84CHEC(2), pp. 224 and 228], to give pyridone 447 (87USP4639457). 

Allyloxy derivatives from pyridine-1-oxides posed still a further element of uncertainty. This arose from the facile isomerization of 2-alkoxypyridine-l-oxides into the corresponding l-aIkoxy-2-pyri-dones even under the conditions of synthesis. Thus, treatment of 2-chloropyridine-l-oxide with sodium allylalcoholate afforded the 2-allyloxypyridine-l-oxide whereas treatment of the same starting compound with sodium benzyloxide gave l-benzyloxy-2-pyridone. A closer investigation revealed that 2-benzyloxypyridine-l-oxide rearranged readily under the experimental conditions of synthesis.6... 

Allyl ethers situated on the imidazole moiety in this system show interesting behavior. 2-Allyloxybenzimidazole affords a quantitative yield of l-allyl-2-(3.flr)-benzimidazolinone on heating at 180° [Eq. 7]22b. On the other hand, 1-allyloxybenzimidazole (12) does not rearrange on heating even at its decomposition point. This observation is in line with the findings in the pyridine series where 2-allyloxypyridine-l-oxide rearranges irreversibly to the l-allyloxy-2-pyridone. 

MEISENHEIMER N-Oxide Rearrangement Rearrangement of tertiary amine oxides to trisubstituted hydroxylamines via a [2,3] sigmatropic shift. Also chlorination of pyridines via N-oxides (see 1st edition). 

MEISENHEIMER N-OxideRearrangement Chlorination of pyridines via rearrangement o< N-oxides. 

Phosphorus oxide chloride pyridine Beckmann rearrangement Aeylamines from oximes... 

Recently, alkynyl oximes have attracted attention as an intriguing substrate in catalytic skeletal rearrangements. Nakamura et al. reported that 0-propargylic oximes 88 derived from a,(3-unsaturated aldehydes in the presence of eopper catalysts afforded 2,3,4,5-tetrasubstituted pyridine oxides 89 (Scheme 27.32) [41]. The reaction proceeds via tandem copper-catalyzed 2,3-rearrangement and b-ir-electrocyclization of the resulting N-allenylnitrone intermediate 90. 

The gold(I)-catalysed oxidative rearrangement of propargyl alcohols (238), facilitated by pyridine-Ai-oxides, has been developed as a new method for the preparation of P-diketones (239). ... 

The N-oxide function has proved useful for the activation of the pyridine ring, directed toward both nucleophilic and electrophilic attack (see Amine oxides). However, pyridine N-oxides have not been used widely ia iadustrial practice, because reactions involving them almost iavariably produce at least some isomeric by-products, a dding to the cost of purification of the desired isomer. Frequently, attack takes place first at the O-substituent, with subsequent rearrangement iato the ring. For example, 3-picoline N-oxide [1003-73-2] (40) reacts with acetic anhydride to give a mixture of pyridone products ia equal amounts, 5-methyl-2-pyridone [1003-68-5] and 3-methyl-2-pyridone [1003-56-1] (11). 

Synthesis From Other Ring Systems. These syntheses are further classified based on the number of atoms in the starting ring. Ring expansion of dichlorocyclopropane carbaldimine (53), where R = H and R = ryl, on pyrolysis gives 2-arylpyridines. Thermal rearrangement to substituted pyridines occurs in the presence of tungsten(VI) oxide. In most instances the nonchlorinated product is the primary product obtained (63). 

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