Reaction of aromatic N-oxides with dipolarophiles. VII. Effect of aromaticity on 1,3-dipolar cycloaddition reactivity of substituted pyridine N-oxides and preparation of oxazolo(4,5-b)pyridine derivatives.

Pericyclic Reaction of Ketenes with Cascade Reaction Products of Pyridine N-Oxides and Dipolarophiles. X-Ray Structures of the Adducts and Formation Mechanism

“…The similar reactions occurred with the dihydropyridine derivatives (1a-e) derived from the cascade reactions of pyridine N-oxides with phenyl isocyanates 3) and N-arylmaleimides. 4) For the azetidine formation reaction, we have proposed a sequential pericyclic reaction pathway which consists of the [4ϩ2] cycloaddition followed by the [3,3]-sigmatropic rearrangement of the [4ϩ2] cycloadduct (Path A in Chart 2).…”

“…3) The reaction conditions and products are summarized in Chart 3 and Table 1. The structures of the adducts are determined on the basis of the spectral data of a pair of the adducts (7aa, 8aa) of methoxyketene (6a) and 1a, whose structures are established by X-ray crystallography.…”

“…On the other hand, Machiguchi et al revealed that the socalled "[2ϩ2] adducts" of ketenes and cyclic dienes are derived from the [3,3]-sigmatropic rearrangements of the initially formed [4ϩ2] cycloadducts (4) of the dienes and the ketene carbonyl groups. 5) However, this reaction mechanism may not be applicable to the reaction of 1-azadienes.…”

Cascade Reaction of Imines with Phenylsulfinylallene. X-Ray Structure of the Product and Its Formation Mechanism.

“…Material Phenylsulfinylallene (2b) 9) and 6,7a-dimethyl-3-phenyl-3a,7a-dihydro-3H-oxazolo [4,5-b]pyridin-2-one (1a) 1) were prepared according to the reported methods.…”

“…IR (KBr) cm Ϫ1 : 1712 (imide ϾCϭO), 1668 (dihydropyridone ϾCϭO), 1632 (ϾCϭCH 2 ). 1 Single Crystal X-Ray Analysis of 3ab The reflection data were measured on a RIGAKU AFC7R four-circle autodiffractometer with a graphite monochromated MoKa radiation (lϭ0.7107 Å) and a rotating anode generator. The structures were solved by direct method.…”