Nikola Pustet

1′,3′,3′-Trimethyl-6-nitrospiro[2H-1-benzopyran-2,2′-indoline]: its thermal enantiomerization and the equilibration with its merocyanine†

Abstract The enantiomers of the title compound, the important photochromic material (RS)-1b, have been enriched semipreparatively by liquid chromatography. As a consequence, we were able to determine the barrier of the thermal interconversion (R)-1b⇌(S)-1b via time-dependent polarimetry, amounting to ΔG≠=85.9 kJ/mol at 22.0°C in d6-DMSO ( Table 2 ). The thermal equilibration of the corresponding merocyanine 2b was monitored in d6-DMSO by time-dependent 1H NMR, resulting in ΔG1≠=102.8 and ΔG2≠=92.0 kJ/mol at 22°C ( Table 1 Table 1 . Equilibration between spiro compounds (RS)-1 and merocyanines 2 at 22°C, measured by time-dependent UV absorptions 3 for (RS)-1a⥄2a and by time-dependent 1H NMR intensities for the other compounds ). This means that, starting from (RS)-1b, the opened isomer 2b is attained by a slow reaction (ΔG1≠=102.8 kJ/mol, Fig. 4 ). Therefore, the merocyanine 2b cannot be identified with the intermediate required for the fast process of C(sp3)–O bond cleavage (ΔG≠=85.9 kJ/mol) upon the above enantiomerization of (RS)-1b. Apparently, these two thermal isomerizations ( Fig. 4 ) are independent of each other. The structure of the unknown intermediate of the interconversion (R)-1b⇌(S)-1b must therefore differ from the known one of merocyanine 2b.

Chiroptical detection during liquid chromatography 7: The rotation angle/absorbance ratio of chiral molecules. Its possible use for on-line analysis during preparative separations of enantiomers☆

The rotation angle/absorbance ratios C+ = alpha+/A+ and C- = a-/A-, determined via detection by a polarimeter and a photometer, were checked for the first time with reference to their use for on-line analysis during preparative separations. For this purpose, (+)-, (-)- and (+/-)-carvones were investigated by liquid chromatography (LC) on microcrystalline tribenzoylcellulose. It turned out that the ratios C+ and C- depend only slightly upon concentration (Table 1). Overlapped peaks of enantiomers were successfully submitted to computer deconvolution (e.g. Fig. 2, bottom). A procedure for on-line analysis during preparative LC is proposed.

Syntheses, separation of enantiomers and barriers to racemization of some sterically hindered N-aryl-1,2,3,4-tetrahydro-3,3-dimethyl-2,4-quinolinediones and their thio analogues

SummaryThe novel N-aryl-1,2,3,4-tetrahydro-3,3-dimethyl-2,4-quinolinediones1,4 and8 were thiated with Lawesson reagent and P4S10 to yield the monothio derivatives2 and5 and the dithio compounds3, 6 and9. Their enantiomers were separated by liquid chromatography on triacetyl- or tribenzoylcellulose. Rotation barriers for1–4 and8 were determined by thermal racemization and discussed in terms of steric effects.ZusammenfassungDie neuen N-Aryl-1,2,3,4-tetrahydro-3,3-dimethyl-2,4-chinolindione1, 4 und8 wurden mit Lawesson-Reagens und P4S10 zu den Monothioderivate2 und5 und den Dithioverbindungen3, 6 und9 thioniert. Ihre Enantiomeren wurden mittels Flüssigchromatographie an Triacetyloder Tribenzoylcellulose getrennt. Die Rotationsbarrieren für1–4 und8 wurden mittels thermischer Racemisierung bestimmt und aufgrund der sterischen Gegebenheiten diskutiert.

The Enantiomers of Spiro(cyclohexadiene-indolines) and their Thermal Racemization

Summary. The separation of the enantiomers of substituted spiro(cyclohexadiene-indolines) was performed by enantioselective liquid chromatography with the sorbent/solvent systems triacetylcellulose/methanol, triacetylcellulose/ethanol/water, and (+)-poly(trityl methacrylate)/SiO2/n-hexane/isopropanol. The interconversion barriers of the enantiomers were determined for a series of derivatives by thermal racemization. A mechanism for the thermal ring opening/ring closure is discussed in terms of the Woodward-Hoffmann rules.Zusammenfassung. Die Enantiomeren von substituierten Spiro(cyclohexadien-indolinen) wurden durch enantioselektive Flüssigkeitschromatographie mit den Solvent/Sorbens-Systemen Triacetylcellulose/Methanol, Triacetylcellulose/Ethanol/Wasser und (+)-Poly(tritylmethacrylat)/SiO2/n-Hexan/Isopropanol getrennt. Aus thermischen Racemisierungen wurden die Barrieren der Enantiomerisierung für eine Reihe von Derivaten bestimmt. Ein Mechanismus der thermischen Ringöffnung und des Ringschlusses wird auf der Grundlage der Woodward-Hoffmann-Regeln diskutiert.

The enantiomers of N-aryl-2-thioxo-4-oxazolidinones and N-arylrhodanines. Investigation by liquid chromatography, circular dichroism and thermal racemization

The enantiomers of the sterically hindered title heterobiaryls 1 and 2 have been investigated for the first time analytically and enriched semi-preparatively by liquid chromatography on triacetyl- and tribenzoylcellulose. The circular dichroism spectra of the enriched enantiomers provide relative configurations which are discussed with respect to the order of chromatographic elution. Barriers to rotation about the C–N bond have been determined (104–121 kJ mol–1) and are rationalized qualitatively by repulsive interactions in a planar transition state traversed upon partial rotation.