Nicholas Dennis

1,3-Dipolar character of six-membered aromatic rings. Part XXI. Thermal cycloadditions of 1-(5-nitro-2-pyridyl)- and 1-(4,6-dimethylpyrimidin-2-yl)-3-oxidopyridinium with 2, 4, and 6 π-electron components

The regio- and stereo-selectivities of reactions of the title compounds at the 2- and 6-positions with mono- and disubstituted ethylenes are elucidated and rationalised by FMO theory including secondary orbital overlap and taking into account steric and dipolar effects. For monosubstituted ethylenes, FMO theory correctly predicts that electron donor, electron acceptor, and conjugated olefins all add in the same regioselective sense. Stereoselectivity is shown to depend rationally on the structures of the betaine and the addend.Buta-1,3-diene, its 2,3-dimethyl and 1-monomethyl derivatives, and cyclopentadiene add as 4π-electron addends at the 2- and 4-positions of the betaines. Such reactions are favoured kinetically, but under thermodynamic control reaction also occurs with the diene acting as a 2π-addend at the 2- and 6-positions of the betaine. The peri-selectivities and stereoselectivities are discussed in terms of FMO theory: the reversal of electron demand leading to easier reaction with electron-rich dienes is explained by PNIFLS.Fulvenes add at the betaine 2- and 6-positions. Peri-, regio-, and stereo-selectivity are successfully rationalised in terms of FMO theory.

1,3-Dipolar character of six-membered aromatic rings. Part 52. 2π+ 8π Cycloaddition reactions of 1-substituted 3-oxidopyridinium betaines

Dichloroketen and a series of aryl(bromo)ketens react with various 1-substituted 3-oxidopyridiniums to give novel bicyclic compounds by addition across the C(4)–O and the C(2)–O positions. Frontier-MO theory is used to rationalise the orientation of these cycloadditions. Acid-catalysed hydrolysis of the C(2)–O adducts (9) yielded 3-hydroxy-2-benzylpyridines.

1,3-Dipolar character of six-membered aromatic rings. Part 45. Photochemically-induced dimerisation of 1-vinyl- and 1-heteroaryl-3-oxidopyridiniums and related compounds

Irradiations of 1-aryl-3-oxidopyridiniums and 2-methyl-4-oxidoisoquinolinium yield photochemically allowed symmetrical dimers, further chemical transformations of which are described. 1-H, 1-methyl, and 1-benzyl-3-oxidopyridiniums are photostable. 3-Oxido-1 -styrylpyridinium yields cycloadducts with a variety of 2π- and 4π-electron dipolarophiles as well as a photodimer.

1,3-Dipolar character of six-membered aromatic rings. Part 44. Further examples of troponoid synthesis

Adducts formed from 3-hydroxypyridine and two moles of the dipolarophiles, acrylonitrile and methyl acrylate, are converted by quaternisation and Hofmann elimination into 4-cyanotropolone and 4-methoxycarbonyltropolone. Overall yields of ca. 25% are achieved in one-pot reactions from 3-hydroxypyridine 2-Chloro-, 2-bromo-, and 2-cyano-1-methyl-3-oxidopyridinium form adducts with 2π addends, some of which were converted into tropone derivatives.

1,3-Dipolar character of six-membered aromatic rings. Part XVI. Reactions of benzyne with 3-hydroxypyridine, 1-methyl-3-oxidopyridinium, and related compounds

3-Hydroxypyridine, its 6-methyl analogue, and 4-hydroxyisoquinoline all react with 2 mol. equiv. of benzyne cycloadduct formation occurs across the 2- and 6-positions together with N-phenylation. 1-Methyl-3-oxidopyridinium and its 6-methyl analogue both react with 2 mol. equiv. of benzyne in two successive cycloadditions.

1,3-Dipolar character of six-membered aromatic rings. Part XXVII. Photochemically induced valence bond tautomerism and dimerisation of 3-oxido-1-phenylpyridinium

Irradiation of the title compound (1) yields one of the photochemically allowed symmetrical dimers (16) together with a bicyclic valence bond isomer (7) and two stereoisomers (18) and (20) of thermal addition products of (7) to a second molecule of the starting phenyl betaine. Analogues (19) and (21) of the dimers (18) and (20) were prepared by reaction of isolated (7) with the pyrimidine betaine (2). Irradiation of (1) in allyl alcohol affords an addition product (24) of (7).

Novel conversion of a 3-oxidopyridinium into a 4-oxidoisoquinolinium

Diels-Alder annelation of a 3-oxidopyridinium dimer followed by oxidation leads in high yield to a mixed 3-oxidopyridinium–4-oxidoisoquinolinium dimer which undergoes reversible dissociation to form a regioisomeric product; the 4-oxidoisoquinolinium is trapped as several adducts with dipolarophiles.

1,3-Dipolar character of six-membered aromatic rings. Part XXV. 5-Aryl-1-methyl-3-oxidopyridiniums

The title betaines react at the 2- and 6-positions with 2π addends and at the 2- and 4-positions with 4π addends. Adducts from the 2π addends are transformed into aryl-substituted tropones via methylation and Hofmann elimination.

1,3-Dipolar character of six-membered aromatic rings. Part X. Pyridazine and benzopyridazine betaines

The betaines 3-methyl-1-oxidophthalazinium and 6-chloro-2-methyl-4-oxidocinnolinium react as 1,3-dipoles across the 2- and 4-positions with acetylenic dipolarophiles. Phthalazin-1(2H)-one and 6-chloro-4-cinnolone also possess 1,3-dipolar reactivity and react with benzyne. The preparations of 1-methyl-3-oxidopyridazinium and 1,6-dimethyl-3-oxidopyridazinium are described, and the tautomerism of the betaine monoprotonated cations is discussed.

1,3-Dipolar character of six-membered aromatic rings. Part III. 2-Methyl-3-oxidoisoquinolinium. A novel route to benzotropones

The title compound reacts as a 1,3-dipole across the 1- and 3-positions with acrylonitrile and methyl acrylate. The stereochemistry of the adducts is deduced from n.m.r. spectra. The methiodides of the adducts undergo base-catalysed elimination to form 2-aminobenzotropones.