924647-48-3Relevant articles and documents
Substituted 3-hydroxypyrroles from 1-azapenta-1,4-dien-3-ones: The aza-Nazarov reaction - Synthesis and quantum chemical calculations
Dieker, Juergen,Froehlich, Roland,Wuerthwein, Ernst-Ulrich
, p. 5339 - 5356 (2006)
On the basis of quantum chemical calculations, 1-azapenta-1,4-dien-3-ones were chosen as candidates for a thorough theoretical and experimental investigation of their electrocyclization reactivity upon protonation. According to G3 theory, the O-protonated 1-azapenta-1,4-dien-3-one cyclizes with high exothermicity and modest activation barrier to give the corresponding 3-hydroxydihydropyrrolium ion ("aza-Nazarov reaction"), whereas the corresponding O-protonated 2-azapenta-1,4-dien-3-one shows endothermicity and a huge barrier to the electrocyclization reaction. The stereochemistry of the cyclization reaction (torquoselectivity) was studied in detail theoretically as well as the cyclization properties of vinylogous system 4, which may give either five- or seven-membered heterocyclic cations (5 versus 6). 1-Amino-and 1-alkoxy-1-azapenta-1,4-dien-3-ones 9 and 10 were easily prepared from corresponding α-imino-carbonyl compounds 7 and 8 by aldol condensation. Experimentally, as evidenced by in situ NMR experiments, 1-azapenta-1,4-dien-3- ones gave dihydropyrrole cations upon protonation at a low temperature. For preparative purposes, trapping of the 3-hydroxypyrrole intermediates with the use of anhydrides proved to be advantageous. Thus, a large variety of new, fully substituted 3-hydroxypyrrole derivatives 13-17 have become accessible in moderate-to-good yields, including two bis-pyrrole compounds 17a,b. All new compounds were thoroughly characterized, including a number of X-ray diffraction studies. Wiley-VCH Verlag GmbH & Co. KGaA, 2006.
