Abstract
Spiro 3-Oxazolines from the 1,3-Dipolar Cycloaddition of Benzonitrilio-2-propanide and 1,4-Quinones. On irradiation with light of wavelength 290-350 nm, 2,2-dimethyl-3-phenyl-2H-azirine (1b) reacts with 1,4-naphthoquinone to give the 1H-benzo[f]isoindol-4,9-dione (11) (Scheme 3) via cycloaddition of the benzonitrilio-2-propanide (2b) onto the quinone C,C-double bond. With 2-methyl- and 2,3-dimethyI-1,4-naphthoquinone, the nitrile ylide 2b undergoes cycloaddition preferentially onto the C,O-double bond of the quinone, leading to spiro-oxazolines 12 and 14 (Scheme 4). Steric as well as electronic effects can be discussed to explain the observed site selectivity of the cycloaddition.
With the 1,4-benzoquinones 15a, 15b, 15d and 15f, nitrile ylide 2b undergoes the 1,3-dipoIar cycloaddition exclusively onto the C,O-double bond. The corresponding spiro-oxazolines have been isolated in 17-32% yield. This contrasts with the previously reported results with benzonitrilio-phenylmethanide (2a), which undergoes cycloaddition to the C,C-double bond of 1,4-benzoquinones (cf. [11). This difference in the site selectivity of the 1,3-dipolar cycloaddition can be explained with Houk’s concept of LUMO-polarization, that is, the stronger nucleophilic dipole 2b polarizes the LUMO of a alpha,beta-unsaturated carbonyl compound more efficient than the less nucleophilic 2a. This leads to a preference of the cycloaddition to the C,O-double bond in the case of 2b. With 2,3-dimethyI- (15c) and 2,3,5,6-tetramethyl-1,4-benzoquinone (15e), nitrile ylide 2b undergoes C,O- as well as C,C-cycloaddition (Schemes 7 and 8 ) .
Stegmann, Werner