Unusual Diels—Alder reaction of 7,12-ethenoquinolino[6,7-f]quinoline derivatives

Abstract

Earlier1,2 during synthesis of 2 spiroannulated 1,2,3,4 tetrahydroquinolines, original spiro structures 1 with the framework of 7,12 ethenoquinolino[6,7 f]quinoline (Scheme 1) have been isolated and characterized. The study of chemical transformations of polycycles 1 revealed unusual reluctance of endocyclic C=N bonds. For instance, they were not involved into the reduction (NaBH4 in boiling methanol or ethanol, LiAlH4 in diethyl ether or THF), the Grignard (MeMgI in diethyl ether or THF), or oxidation (Na2WO4/H2O2) reactions charac teristic of azomethines. These facts, probably, can be explained by steric hindrance, which is created by the rigid bicyclic framework, as well as the cycloalkane substituents at α position to the nitrogen atoms of the molecule. Maleic anhydride does not react with the imine frag ments either. Instead of usual acylation products at the nitrogen atom, after heating of 7,12 ethenoquinolino [6,7 f]quinolines 1 with an excess of maleic anhydride the earlier unknown spiroheterocycles, 5,8 ethenoquinolines 2 (see Scheme 1), were isolated in high yields. We suggest that the reaction proceeds through the 2,3,4,4а tetrahydroquinoline intermediates 3, which re sult from the retro diene cleavage of the starting polycy cles 1 on heating. Such unstable dienes 3 are trapped by the dienophile, which leads to the endo adducts of [4+2] cycloaddition 2. Configurations of new asymmetric centers (at the C(9) and C(10) atoms) were established from the data of corre lation NMR spectroscopy. In particular, the two dimen sional NOESY spectrum exhibits an intensive cross peak between the proton H(4) and H(10) signals, which indi cates their closeness in space. Note that the intermediate 2,3,4,4а tetrahydroquino lines 3 are virtually unknown in the literature, there are only several reports on their formation in low yields as the intermediate products in the synthesis of 1,2,3,4 tetra hydroquinolines.3—5