Acid-catalyzed rearrangement of 1-acyl-2-azabuta-1,3-dienes to 4-pyrrolin-2-ones
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Acid-catalyzed rearrangement of 1-acyl-2-azabuta-1,3-dienes to 4-pyrrolin-2-ones Nikolai V. Rostovskii1, Ilia A. Smetanin1, Alexander N. Koronatov1, Anastasiya V. Agafonova1, Vasilii V. Potapenkov1, Alexander F. Khlebnikov1, Mikhail S. Novikov1* 1
Institute of Chemistry, Saint Petersburg State University, 7/9 University Embankment, Saint Petersburg, 199034, Russia; e-mail: [email protected] Submitted June 16, 2020 Accepted June 29, 2020
Translated from Khimiya Geterotsiklicheskikh Soedinenii, 2020, 56(7), 881–887
1-Acyl-2-azabuta-1,3-dienes underwent rearrangement to 4-pyrrolin-2-ones in the presence of acidic catalysts (silica gel or ZnCl2) and water. The reaction proceeded via the formation of 2,3-dihydroxy-3,4-dihydropyrrole and 2-hydroxy-2Н-pyrrole intermediates, the latter of which was transformed by [1,5]-sigmatropic shift of carbonyl substituent and prototropic isomerization. 4-Chloro-substituted 1-acetyl2-azabuta-1,3-dienes were stable toward rearrangement, while brominated analogs rearranged into 4-pyrrolin-2-ones, probably as a result of the initial radical hydrodebromination. According to calculations performed at the DFT level, two steps of the reaction were catalyzed by acid: the cyclization of 2-azabutadiene and the sigmatropic shift in the 2-hydroxy-2Н-pyrrole intermediate. Keywords: azabuta-1,3-dienes, pyrrolinones, acid catalysis, sigmatropic shift.
electrocyclization reactions, which have been used in our earlier work for preparing some derivatives of 2,3-dihydroazete,5,6 2Н-1,4-oxazine,7 and 2Н-1,3-oxazine.8 While investigating the applicability of this approach to the synthesis of 2Н-1,3-oxazine derivatives in a reaction of azirine A with ethyl diazoacetoacetate in the presence of Rh2(OAc)4 catalyst, pyrrolinone D was obtained instead of the expected oxazine C (Scheme 1).8 It was proposed that 1-acetyl-4-formyl-substituted 2-azabutadiene B, which existed in a solution-phase equilibrium with oxazine C, was transformed into compound D during the chromatographic purification of the reaction mixture on silica gel. The cyclization of azabutadiene B into pyrrolinone D,
Azabuta-1,3-dienes exhibit strong reactivity of various types leading to a wide range of synthetic applications as building blocks in the assembly of nitrogen heterocycles.1 In particular, 2-azabuta-1,3-dienes can participate in (2+1), (2+2), (2+3), and (2+4) cycloaddition reactions, which provide convenient access to alkylideneamino-substituted cyclopropanes,2 β-lactams,3 derivatives of pyrazole, isoxazole, pyridine, and different polycyclic systems.1,4 Certain methods for the synthesis of pyrrole and oxazole derivatives have been also developed on the basis of 1,5-exo-trig cyclization of 2-azabuta-1,3-dienes.5 Electrondeficient 2-azabuta-1,3-dienes, generated by a catalytic reaction of azirines with diazo esters, easily undergo Scheme 1
O
MeO
N A
O
CO2Et O
N2 Me Rh2(OAc)4
N
1,2-dichloroethane, 84 °C
O
Me CO2Et
B
N MeO
MeO 0009-3122/20/56(7)-0881©2020 Springer Science+Business Media, LLC
O
881
C
COMe CO2Et
Silica g
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