Unexpected 5,6,7,8,9,10-Hexahydro-6,6-pentamethylenephenanthridines
and 2,3,4,5-Tetrahydro-4,4-tetramethylene-1<i>H</i>-cyclopenta[<i>c</i>]quinolines from Skraup–Doebner–Von Miller
Quinoline Synthesis and Their Implications for the Mechanism of That
Reaction
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Abstract
The real mechanism of the Skraup–Doebner–Von
Miller
quinoline synthesis remains controversial and not well understood
despite several mechanistic studies reported on the matter. A series
of unexpected and unusual 5,6,7,8,9,10-hexahydro-6,6-pentamethylenephenanthridines
and 2,3,4,5-tetrahydro-4,4-tetramethylene-1<i>H</i>-cyclopenta[<i>c</i>]quinolines have been obtained through the Skraup–Doebner–Von
Miller quinoline synthesis. On the basis of these unexpected results
and in agreement with some of the previously reported quinoline syntheses,
an alternative mechanistic pathway is proposed for this variant of
the reaction. It involves the formation of a Schiff base through a
reaction between the ketone and the aniline derivative in the first
step, followed by a cycloalkenylation at the <i>ortho</i>-position to the amine functional group of the aniline derivative,
and an annulation in the final step to close the quinoline ring, leading
to a dihydroquinoline derivative. To the best of our knowledge, this
is the first report of such a mechanistic pathway being proposed for
any variant of the Skraup–Doebner–Von Miller quinoline
synthesis