C-13-NMR study of 4H-1,3,4-thiadiazino [5,6-b] quinoxalines. A proof for the N-5-deuteration in deuteriotrifluoroacetic acid

The carbon signals of the 2-acylamino-4H-1,3,4-thiadiazino[5,6-b]quinoxalines la,b, 2-acylamino-4H-1,3,4-thiadiazino[5,6-b]quinoxaline 1,l-dioxides 2a,b, and 2-amino-4H-1,3,4-thiadiazino[5,6-b]quinoxaline 3 in deuteriodimethyl sulfoxide and in deuteriotrifluoroacetic acid were assigned by the nmr (HMBC, HMQC) spectroscopy. The comparison of the carbon chemical shifts in deuteriodimethyl sulfoxide with those in deuteriotrifluoroacetic acid clarified that compounds la, Ib, and 3 were deuterized at the N-5-position in deuteriotrifluoroacetic acid, while the 1,l-dioxides 2a,b did not undergo the N-5-deuteration in deuteriotrifluoroacetic acid.