Stereochemical aspects of the "tert-amino effect". 2. Enantio- and diastereoselectivity in the synthesis of quinolines, pyrrolo[1,2-a]quinolines, and [1,4]oxazino[4,3-a]quinolines

Abstract

Thermal isomerization of the optically pure 2-vinyl-N,N-dialkylanilines, with a methyl or an ethyl substituent (R2) at the α-position of the N,N-dialkyl moiety, 4a (R1= H), 4b (R1= CH3), 5a (R1= H), and 6a (Rl= H), affords enantioselectively the optically pure pyrrolo[l,2-α] quinolines 7a and 7b and the [1,4]oxazino[4,3-a]quinoline 11, with the methyl or ethyl substituent (R2) at the bridgehead carbon atom, and the quinoline 13, respectively. The optical purity of these quinoline derivatives was determined by 1H NMR spectroscopy in the presence of chiral shift reagents. Heating of the optically pure analogues in which R2is a methoxymethyl group (4c and 4d) in refluxing 1-butanol yields, besides the compounds 7c,d with the methoxymethyl group at the bridgehead carbon atom, also the regioisomers 8c,d and 9c,d that are enantiomerically pure. Mixtures of the diastereomers 12a,b and 14a,b were obtained by cyclization of compound 5b, with a 3-ethylmorpholinyl group, and of the acyclic amine 6b, respectively, in refluxing 1-butanol. The compounds 12a and 14a,b were proven enantiomerically pure. The configuration of the compounds 8, 9, 12, and 14 was determined by X-ray analysis [(±)-12a] and NMR 1H and 1H NOE difference spectroscopy. These results provide conclusive evidence for the mechanism of these cyclization reactions, which are further examples of the “tert-amino effect”. The effect of substituents on the enantio- and diastereoselectivity of the cyclization is discussed.