Spirocyclische 3‐Oxazoline durch 1,3‐dipolare Cycloaddition von Benzonitrilio‐2‐propanid mit 1,4‐Chinonen

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‐dimethyl‐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‐dipolar 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 benzonitrilipo‐phenylmethanide (2a), which undergoes cycloaddition to the C, C‐double bond of 1,4‐benzoquinones (cf. [1]). 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 dipol 2b polarizes the LUMO of a α,β‐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‐dimethyl‐ (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).