Arylpyran Pseudoacid Racemization: Rate Estimation and Structural Influences

Arylpyran pseudoacids showed slow ring-opening to the oxocarboxylic acids, and fast ring-closure to the lactols (pseudoacids) in a process that amounts to racemization. Compounds studied were patterned on “Cooper’s Pseudoacid” [3-hydroxy-4,4-dimethylisobenzopyran-1-one, 1 ], to which modifications w...

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Bibliographic Details
Published in:Journal of chemical crystallography 2021-03, Vol.51 (1), p.14-41
Main Authors: Fung, Jonathan, Duong, Truc-Vi, Braceros, Keean C. A., Brooks, Marilyn L., Schloesser-Lingscheit, Katie, Tagawa, Tristen K. S., Wilson, Johanna M., Jones, Kevin K., Valente, Edward J.
Format: Article
Language:English
Subjects:
Aromatic compounds
Chemical equilibrium
Chemistry
Chemistry and Materials Science
Closures
Coalescing
Crystal structure
Crystallography and Scattering Methods
Enantiomers
Inorganic Chemistry
Isomers
NMR
Nuclear magnetic resonance
Organometallic Chemistry
Original Paper
Physical Chemistry
Racemization
Resonance lines
Ring opening
Room temperature
Temperature
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Summary:Arylpyran pseudoacids showed slow ring-opening to the oxocarboxylic acids, and fast ring-closure to the lactols (pseudoacids) in a process that amounts to racemization. Compounds studied were patterned on “Cooper’s Pseudoacid” [3-hydroxy-4,4-dimethylisobenzopyran-1-one, 1 ], to which modifications were introduced to influence racemization rates by steric compression. Pseudoacid 1 in d 6 -DMSO had a barrier to racemization (ΔH ‡ ) of +77.8(3) kJ/mol determined by dynamic nuclear magnetic resonance line-broadening measured from 25 °C to 100 °C, and coalescence of its diastereotopic gem -dimethyl signals at 78.5 °C. The solution half-life of 1 at room temperature is a few seconds. Introduction of various aryl 5-substituents lead to similar racemization barriers for the 5-fluoro-8-methyl derivative, and increasingly higher barriers for 5-methyl-8-methyl, 5-chloro-8-methyl, and 5-bromo-7-methyl derivatives. In 3-hydroxy-4,4,5,8-tetramethylisobenzopyran-1-one, the barrier was + 104(6) kJ/mol, and the solution enantiomer t 1/2 at room temperature was of the order of hours. Chemical shift differences (Δν Me ) between diastereotopic methyls of 8-methyl-5-substituted enantiomeric pseudoacids were observed in the order F 
ISSN:1074-1542
1572-8854
DOI:10.1007/s10870-020-00829-2