Loncar-Tomaskovic, Linda and Pustet, Nikola and Mrvos-Sermek, Draginja and Nagl, Ante and Mintas, Mladen and Mannschreck, Albrecht (2001) Spirobipyridopyrans and indolinospiropyridopyrans: Synthesis, x-ray crystal structure, separation of enantiomers, and barriers to thermal cleavage of the C(sp(3))-O bond. CHIRALITY, 13 (2). pp. 81-88. ISSN 0899-0042, 1520-636X
Full text not available from this repository. (Request a copy)Abstract
The novel chiral spirobipyridopyrans 1 and 2 were synthesized by the acid catalyzed aldol type condensation of 5-deoxypyridoxal with the appropriate ketone and subsequent reaction of the resulting pyrylium salt with base. The indolinospiropyridopyrans 3-5, which contain the modified B-6 unit, were prepared by aldol reaction of 5-deoxypyridoxal with 1,3,3-trimethyl-2-methylenindolines. Analytical separation of enantiomers was accomplished by low-pressure liquid chromatography (LPLC) on triacetylcellulose. The barriers to thermal racemization were determined by on-line measurements of the enriched enantiomers after LPLC. Gibbs energies of activation DeltaG(not equal) for reversible cleavage of the C-spiro-O bond in 1, 3, and 4 were in the range 103-108 kJ/mol. The lower limits of the barriers in 2 and 5 were estimated to be greater than 102 and 109 kJ/mol by attempted thermal racemizations. The increase of the barriers from 3 to 4 and 5 was explained by the influence of electron withdrawing groups, which reduce the stability of the ring-opened transition states to C (sp3)-O bond cleavage. Geometrical data from X-ray structure analysis showed that the angle [C3-C2-C3'] around the spiro carbon atom increases with elongation of the chain in the C3-C3' bridge. This angle widening is explained by a ring-strain effect, which is greater in the five-membered ring in the skeleton of 7 than in the six- and seven-membered rings of 1 and 2, Chirality 13.81-88, 2001. (C) 2001 Wiley-Liss. Inc.
| Item Type: | Article |
|---|---|
| Uncontrolled Keywords: | CIRCULAR-DICHROISM; RACEMIZATION; chiral spirobinaphthopyrans; enantioselective liquid chromatography; tri-acetylcellulose; barriers to racemization; x-ray crystallography |
| Subjects: | 500 Science > 540 Chemistry & allied sciences |
| Divisions: | Chemistry and Pharmacy > Institut für Organische Chemie > Alumni or Retired Professors > Prof.Dr. Mannschreck |
| Depositing User: | Dr. Gernot Deinzer |
| Date Deposited: | 28 Feb 2022 07:15 |
| Last Modified: | 28 Feb 2022 07:15 |
| URI: | https://pred.uni-regensburg.de/id/eprint/41840 |
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