Naslov
The Enantiomers of Chromenes and Their Barriers to Reversible Ring Opening

Autori
Leiminer, Andreas ; Lončar, Linda ; Otočan, Klaudio ; Stephan, Barbara ; Trötsch, Thomas ; Kastner, Fritz ; Mintas, Mladen ; Mannschreck, Albrecht

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Skup
3rd International Syimposium on Chiral Discrimination

Mjesto i datum
Tübingen, Njemačka, 05.10.1992. - 08.10.1992

Vrsta sudjelovanja
Poster

Vrsta recenzije
Nije recenziran

Ključne riječi
enantiomeri kromena; barijera za reverzibilno otvaranje prstena
(enantiomers of chromenes; barriers to reversible ring opening)

Sažetak
Racemic spiro-2H-chromenes l and their aza analoques 2 have been the subjects of extensive studies because of their photochromism which involves the rupture of the C(sp3)– 0 bond. Some derivatives of 2 received attention for protection against light (e.g. in sunglasses) and for the record of information. Much less is known about the corresponding thermal ring – opening reaction which can be investigated via racemization of enantiomers. Separations of enantiomers of 2H-chromenes were unknown but have now been accomplished for several examples of l-4 by semipreparative liquid chromatography, mainly on triacetyl- or tribenzoylcellulose (l, 2, 3). The barriers to enantiomerization omount to ∆ G = 85 - 105 kJ/mol for spiro– 2H– chromenes l, 2, and 3 and to ∆ G = 105– 123 kJ/mol for non-spiro 2H-chromenes 4. The latter show higher barriers, probably because their ring-opened dienone intermediates (and the corresponding transition states) are less stbilized than in the case of the spiro compounds. In several cases, barriers were obtained, in addition, via enantiomerization during HPLC of racemates on optically active sorbents (l, 3, 4). Circular dichroism was mainly used for deriving relative confifurations. When the barriers were lower than about l00 kJ/mol, non-racemic substrates could not be prepared for such measurements. In these cases, circular dichroism spectra were obtained by stop of flow during HPLC of racemates on optically active sorbents (4).

Izvorni jezik
Engleski

Znanstvena područja
Kemija

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