b-Orientation of the C-21 side-chain was inferred from NOESY
cross-peaks (Fig. 4) ofH-17 with H-22 and H3-29 and that of21-OH
with H3-30 and H3-28. In case of an a-isopropyl side chain, H-22
would show NOESY interaction with H3-28, which was not
observed in 1 indirectly supporting the b-orientation of C-21 side
chain. A slight downfield shift of H3-28 signal ( dH 0.66 to dH 0.89 in
1) due to the proximity with the a-OH at C-21, as compared to nonsubstituted isohopanes, further supported this assignment (Ageta
et al., 1993; Kildahl-Andersen et al., 2010). It is important to note
that C-21 resonated farther downfield because of the presence of
an OH, which also deshield its b carbons C-17, C-20 and C-22. On
the basis of these data, 1 was assigned the structure, 3 b,21 adihydroxyisohopane.