A formal [3 + 3] cycloaddition reaction between alkyl 2-aroyl-1-chlorocyclopropanecarboxylates 1 and common hydrazones 2 was developed. This process readily proceeded in diastereo-and regioselective fashion and gave alkyl 1,4,5,6tetrahydropyridazine-3-carboxylates in high yields under mild basic conditions. Treatment of the annulation products with DDQ leads to the formation of functionalized pyridazine-3-carboxylates, analogues of nicotinic acid ester.
Keto–enol tautomerism is a
special example of structural
isomerism. In this experiment, 6-chlorofulvene is predominantly yielded
during the chlorination reaction of 6′-hydroxyfulvene with
POCl3 in the presence of DMF. The high regioselectivity
of this reaction is dominated by intramolecular hydrogen-bonding and
steric hindrance, providing an illustrative experiment for the students
to understand the affecting factors of keto–enol equilibria
clearly. Treatment of the 6-chlorofulvene with primary amines such
as n-butylamine in acetonitrile quickly gives 6-aminofulvene,
a regioisomer of the direct amination product of 6′-hydroxyfulvene.
The strong auxochromic effect of the amino group enables us to identify
6-chlorofulvene and 6-aminofulvene by UV spectra and to monitor the
reaction progress between 6-chlorofulvene and amines. The concepts
of keto–enol tautomerism and regioselective addition–elimination
are illustrated, and this helps students understand the factors that
influence the keto–enol tautomerism equilibria and the reaction
sites of conjugate addition in a large conjugated system.
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