?-? Molecular complexes with nitrones, II. Complexation between heterocyclic nitrones and tetracyanoethylene as well as 1,4-benzoquinones

Asymmetric Induction in the Cycloaddition of a Masked p-Benzoquinone to Cyclic Nitrones

“…For comparison, we first carried out the reaction between the more reactive nitrone 2,3,4,5-tetrahydropyridine 1-oxide ( 3 ) and an excess of unprotected p -benzoquinone in CHCl 3 at room temperature. The starting nitrone was completely consumed after 4 days, according to TLC analysis, but the 1 H-NMR spectrum of the crude product did not show any identifiable compound, as expected on the basis of the literature precedents …”

“…On the other hand, depending on the substitution of the starting nitrone and quinone, the resulting primary isoxazolidine may contain up to three new stereogenic centers from which various diastereoisomers originate. The reactivity of several p -benzoquinones toward nitrones was reported earlier and the expected cycloadducts were obtained only in very low yield, the suggested reason being the occurrence of complicated redox processes between reactants and products . Moreover, depending on the substitution of the starting quinone, the aromatization of the primary adducts can easily take place, obscuring the stereochemical information of the process.…”

Reaction of azomethine N-oxides. 6. Spectroscopic study of Lewis acid catalyzed reactions of nitrones with N-phenylmaleimide

“…The NMR spectrum of the residue showed two cycloaddition adducts in different ratios (Table I). The Isomers were separated by 4.1 (dd, 1 , </43 = 8 Hz, J4b = 9 Hz, H-4), 4.92 (d, Jb4 = 9 Hz, H-5), 5.4 (d, 1 H, J34 = 8 Hz, H-3), 6.0 (s, 2 H, CH2), 6.S-7.6 (m, 13 2.35 (s, 3 H, CH3), 4.0 (dd, 1 H, Jib = 9 Hz, J43 = 7 Hz, H-4), 4.91 (d, 1 H, Jb4 = 9 Hz, H-5), 5.29 (d, 1 H, J34 = 7 Hz, H-3), 6.8 (m, 14 3.95 (dd, 1 H, J43 = 1 Hz, J4b = 7 Hz, H-4), 5.1 (d, 1 H, Jb4 = 7 Hz, H-5), 5.62 (d, 1 H, J34 = 1 Hz, H-3), 6.0 (s, 2 H, CH2), 6.45-7.5 (m, 3 H, CeHs) 4b 127-128 59 (54) d 1780, 1710…”

“…3.82 (s, 3 H, OCH3), 3.95 (dd, 1 H, Ji3 = 1 Hz, J4b = 7.5 Hz, H-4), 5.1 (d, 1 H, Jb4 = 7.5 Hz, H-5), 5.7 (d, 1 H, J34 = 1 Hz, H-3), 6.55-7.55 (m, 14 3.77 (s, 3 H, CH3), 3.92 (s, 3 H, OCHg), 4.15 (t, 1 H, J = 8 Hz, H-4), 5.2 (d, 1 H, Jb4 = 7 Hz, H-5), 6.5 (d, 1 H, J3i = 2 Hz, H-3), 7.0-7.5 (m, 2.62 (s, 3 H, CH3), 3.85 (dd, 1 H, J4b = 7 Hz, Ji3 = 2 Hz, H-4), 5.1 (d, 1 H, Jb4 = 7 Hz, H-5), 5.83 (d, 1 H, J43 = 2 Hz, H-3), 6.5-7.82 (m, 14 H, C6H5) 4e 149-150 69 (53) g 1789, 1725…”

“…The two examples undergo rapid 1,3-dipolar cycloadditions in the presence of the Lewis acids AICI3 and BF3, respectively. In conjunction with our interest in the reactions of azomethine /-oxides with electron-deficient compounds, (12)(13)(14)(15)(16)(17) we now wish to report the spectroscopic study of the effect of the Lewis acid AICI3 on the reactivity and stereoselectivity of the 1,3-dipolar cycloaddition reactions of the azomethine N-oxides (nitrones) (1a-e) with the symmetrical electron-poor dipolarophile 2 in benzene as a solvent. The aldonitrones (1a-e) react with the A/-phenylmaleimide (2) in dry benzene at room temperature to give two stereoisomers, the cis -isoxazolidines (3a-e) and the trans -isoxazolidines (4a-e) (Figure 1).…”