9b endo shows two singlets at 3.53 ppm (protons a to the imido carbonyls) and at 5.05 ppm (bridgehead protons), while the corresponding signals for 6b appear as doublets of doublets at 4.27 ppm (2.0 and 2.7 Hz, protons a to the imido carbonyls) and at 5.06 ppm (2.0 and 2.7 Hz, bridgehead protons). The resonances for the methyl goups are found at 3.10 ppm for 6a and at 2.38 ppm for 6b, respectively. These data are also in very close agreement with ...
![1,3-dihydrothieno[3,4-b]quinoxaline Structure](https://image.chemsrc.com/caspic/142/3138-80-5.png)