Table 1.

Diagnostic ¹H- and ¹³C chemical shifts measured for 19a,b and calculated for the corresponding oxo- and enol forms by GIAO method ^a.

	19a			19b
	Experimental 1H- and 13C NMR Chemical Shift	oxo Calculated 1H- and 13C NMR Chemical Shift	enol Calculated 1H- and 13C NMR Chemical Shift	Experimental 1H- and 13C NMR Chemical Shift	oxo Calculated 1H- and 13C NMR Chemical Shift	enol Calculated 1H- and 13C NMR Chemical Shift
XH b	11.70	9.60	15.05	11.70	9.64	16.51
10-NH	11.31	12.78	8.88	11.32	12.72	8.22
H-5	8.05	8.61	8.73	8.03	8.62	8.65
H-6	7.26	7.72	7.98	7.28	7.72	7.96
H-7	7.58	8.07	8.16	7.60	8.07	8.09
H-8	7.81	7.83	8.37	7.84	7.82	8.30
C-2	143.5	148.3	153.7	143.9	148.2	156.2
C-3	115.1	124.3	119.5	114.2	124.2	118.8
C-4	177.3	186.3	173.5	177.8	186.2	170.0
C-4a	119.6	131.7	127.8	119.8	131.4	127.7
C-5	125.6	130.2	128.0	125.6	130.2	128.5
C-6	123.9	129.7	132.2	124.1	129.8	132.5
C-7	132.5	138.8	136.9	132.6	138.8	136.5
C-8	119.6	123.5	135.0	119.7	123.5	135.5
C-8a	139.6	144.2	152.6	139.7	144.2	153.7
C-10	163.3	171.2	173.4	163.5	171.4	174.4
C-18	47.5	49.6	58.2	50.8	53.5	62.8

^a Calculations were performed using B3LYP functional and 6-311++G(2d,p) basis set. ^b NH in the oxo tautomers, OH in the enol tautomers.