However, the key difference between the two
compounds was that compound 1 possessed one more ring. In the
HMBC spectrum, the cross-peak of H-17 with C-21 indicated the
linkage between C-17 and C-21 through an oxygen atom (Figure
1), which established the additional ring. In addition, the HMBC
correlation between the O-methyl hydrogens at δH 3.40 (3H, s) and
C-21 indicated that the methoxy group should be connected to C-21.
The relative configuration of 1 was assigned on the basis of a
ROESY experiment (Figure 1)