Investigation of Four Sets of NMR Spectra for One Racemic Compound of Pseudoresonance Structures

A racemic compound, (S*)-3-[(1R*,3R*)-3-hydroxy-1,3-dihydroisobenzofuran-1-yl]-2-(naphthalen-1-yl)-isoindolin-1-one (3), was synthesized and found to exhibit four distinct sets of 1H and 13C NMR signals in CDCl3 solution. The four sets of 1H NMR signals displayed relative integration ratios of approximately 1:1:0.8:0.8. Subsequent PCC oxidation of 3 yielded racemic (S*)-2-(naphthalen-1-yl)-3-[(R*)-3-oxo-1,3-dihydroisobenzofuran-1-yl]-isoindolin-1-one (5), which showed simplified NMR behavior with only two sets of 1H and 13C NMR signals (1:1 ratio) in CDCl3. Interestingly, compound 5 exhibited two sets of 1H NMR signals in DMSO-d 6 that gradually coalesced into a single set upon variable-temperature NMR analysis (298-353 K). To further investigate the structural dynamics, racemate 3 was converted to its corresponding acetate derivative (8) in CHCl3. Notably, the acetate 8 displayed only two sets of NMR signals (near 1:0.9 ratio) in CDCl3, contrasting with the original four sets (1:1:0.8:0.8) observed for compound 3. This critical observation eliminated the possibility that hemiacetal isomeric structures were responsible for the NMR multiplicity in CDCl3. These findings suggest that the two sets of 1H NMR signals with 0.8:0.8 integration ratios originated from two atropisomers with near 1:1 ratios.