Table 1: 1H NMR (500 MHz) data of compounds 2a2f in .

EntryC-6 methylC-5 methineC-2 methineCO-NH

2a1.55 (d,  Hz)4.13 (qd,  Hz, 0.97 Hz)
4.23 (qd, , 1.47 Hz)
5.92 (s)10.94 (s)

2b1.54 (d, )
1.55 (d,  Hz)
4.12 (q,  Hz)
4.22 (q,  Hz)
5.90 (s)10.94 (s)
10.95 (s)

2c1.55 (d,  Hz)
1.57 (d,  Hz)
4.15 (q,  Hz)
4.25 (qd, , 1.47 Hz)
6.01 (d,  Hz)
6.02 (s)
10.99 (s)
11.01 (s)

2d1.53 (d,  Hz)
1.55 (d,  Hz)
4.10 (q,  Hz)
4.16 (qd, , 1.47 Hz)
5.87 (s)
5.88 (d,  Hz)
10.90 (s)

2e1.54 (d, )
1.55 (d,  Hz)
4.11 (q,  Hz)
4.18 (qd, , 1.46 Hz)
5.89 (s)
5.90 (s)
10.97 (s)

2f1.48 (d, )
1.52 (d,  Hz)
4.10 (q,  Hz)
4.21 (qd, , 1.96 Hz)
6.22 (d,  Hz)
6.26 (s)
11.04 (s)
11.05 (s)

For 1H NMR data of the other protons, see Section 2.
The signals corresponding to major diastereomer.
Coupling with C-2 methine was observed as a shoulder.