Tetrahydroquinoxalines, chiral

Circular Dichroism Measurements. The absolute configurations of the C6 chiral center in tetrahydrobiopterin cofactor and related compounds were determined by comparison of their circular dichroism (CD) spectra with those of authentic (25)-2-methyl-l,2,3,4-tetrahydroquinoxaline.968... 

In a closely related study, Tung and Sun discussed the microwave-assisted liquid-phase synthesis of chiral quinoxalines [80], Various L-a-amino acid methyl ester hydrochlorides were coupled to MeOPEG-bound ortho-fluoronitrobenzene by the aforementioned ipso-fluoro displacement method. Reduction under microwave irradiation resulted in spontaneous synchronous intramolecular cyclization to the corresponding l,2,3,4-tetrahydroquinoxalin-2-ones (Scheme 7.71). Retention of the chiral moiety could not be monitored during the reaction, but after release of the desired products it was found that about 10% of the product had undergone racemization. 

Abstract Anhydro sugars have proved to be versatile chiral synthons for the synthesis of chiral heterocycles including pyradazines, oxazines, triazines, chiral morpholines, tetrahydroquinoxalines, benzodioxins, oxazolidinones, episulfides, thiazolines, and cyclic trithiocarbonates. Selected examples are described in this chapter. 

Chiral NHC ligands allowed the challenging asymmetric hydrogenation of the carbo-cyclic ring of quinoxalines, yielding enantioenriched 5,6,7,8-tetrahydroquinoxalines with an enantiomeric ratio of up to 94 6 in the presence of n-hexane and toluene in the presence of KOBu ... 

Ghorai MK, Sahoo AK, Kumar S. Synthetic route of chiral tetrahydroquinoxalines via ring-opening of activated aziridines. Org. Lett. 2011 13(22) 5972-5975. 

Fan et al. have successfully developed an efficient metal/ Bronsted acid (261) relay catalysis system for highly enantioselective hydrogenation of quinoxalines (374) through a convergent asymmetric disproportionation of dihydroquinoxalines (375) to chiral tetrahydroquinoxalines (377) with up to 94% ee (Scheme 99). ... 

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