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- W2144708969 abstract "The molecular structure of the vinylogous 4H-pyrone 2,6-dimethyl-4-(2-oxopropylidene)-4H-pyran (1) was determined by X-ray diffraction. The molecule is practically planar and has the s-cis conformation. The experimental dipole moment (3.71 D) of 1 agrees with the CNDO/2 calculated value (3.54 D) for the s-cis conformation 1 and rules out the s-trans conformation 2. The doubly vinylogous 4H-pyrone 4, Ar = p-C6H4Cl, has an experimental dipole moment of 5.65 D. Corrected 13C chemical shifts of the ring β- and especially α-carbons decrease in the series 2,6-dimethyl-4H-pyrone (10) to 1 to 4, indicating that the higher vinylogs have decreased contributions of the dipolar canonical structures. Dynamic 1H NMR data, obtained using the two-dimensional accordion technique, indicate that 1 undergoes rotation around the exocyclic C = C bond with a free energy of activation ΔG± = 22.4 kcal/mol in [D6]DMSO. Dipolmoment, dynamische NMR-Spektroskopie und Molekülstruktur von vinylogen 4H-Pyronen Die Molekülstruktur des vinylogen 4H-Pyrons 2,6-Dimethyl-4-(2-oxopropyliden)-4H-pyran (1) wurde durch Röntgenstrukturanalyse bestimmt. Das Molekül ist nahezu eben und besitzt s-cis-Konformation. Das experimentell bestimmte Dipolmoment (3.71 D) von 1 stimmt mit dem berechneten Wert von 3.54 D (CNDO/2) für die s-cis-Konformation 1 überein und schließt somit die s-trans-Konformation 2 aus. Für das zweifach vinyloge 4H-Pyran 4 (Ar = p-C6H4Cl) wurde ein Dipolmoment von 5.65 D gemessen. Die korrigierten 13C-Verschiebungen der Ring-β- und besonders der -α-Kohlenstoffe nehmen in der Reihenfolge 10 → 1 → 4 ab, woraus folgt, daß bei den höheren Vinylogen dipolare Resonanzformeln weniger am elektronischen Grundzustand beteiligt sind. Dynamische 1H-NMR-Spektren (2D-Technik) belegen, daß in 1 Rotation um die exocyclische C = C-Bindung mit einer Freien Aktivierungsenthalpie von ΔG± = 22.4 kcal/mol stattfindet (in [D6]DMSO)." @default.
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- W2144708969 date "1985-08-12" @default.
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- W2144708969 title "Dipole Moment, Dynamic NMR, and Molecular Structure of Vinylogous 4H-Pyrones" @default.
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- W2144708969 doi "https://doi.org/10.1002/jlac.198519850807" @default.
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