Naslov
Sirobipyridopyrans and Indolinospiropyridopyrans : Synthesis, X-ray Crystal Structure, Separation of Enantiomers and Barriers to Thermal Cleavage of the C(sp^3)-O Bond

Autori
Lončar-Tomašković, Linda ; Pustet, Nikola ; Mrvoš-Sermek, Draginja ; Nagl, Ante ; Mintas, Mladen ; Mannschreck, Albrecht

Izvornik
Chirality (0899-0042) 13 (2001), 2; 81-88

Vrsta, podvrsta i kategorija rada
Radovi u časopisima, članak, znanstveni

Ključne riječi
chiral spirobinaphthopyrans; enantioselective liquid chromatography; triacetylcellulose; barriers to racemization; X-ray crystallography

Sažetak
The novel chiral spirobipyridopyrans 1 and 2 were synthesized by the acid catalysed 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^# for reversible cleavage of the C_spiro-O bond in 1, 3 and 4 were found to be 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

Izvorni jezik
Engleski

Znanstvena područja
Kemija

POVEZANOST RADA