Rhazinoline (1) was obtained as a c

Rhazinoline (1) was obtained as a colorless oil, [R]D +136 (c
0.07, CHCl3). The UV spectrum showed absorption maxima at 211
and 260 nm, characteristic of an indolenine chromophore, which
was confirmed by the observed carbon resonance at ä 187.3 due
to the imine carbon. The IR spectrum indicated the presence of an
aldehyde function (1718 cm-1). The EIMS spectrum showed a
molecular ion at m/z 292, which was also the base peak, with other
significant fragment ions observed at m/z 277 (M - CH3) and 263
(M - CHO). HREIMS measurements gave the molecular formula
C19H20N2O. The 13C NMR spectrum (Table 1) showed a total of
19 carbon signals (one methyl, four methylenes, eight methines,
and six quaternary carbons). The methine signal observed at ä 200.6
was readily attributed to an aldehyde carbonyl carbon. The 1H NMR
spectrum (Table 2) of 1 showed the presence of an unsubstituted
aromatic moiety and an ethylidene side chain. In addition, signals
observed at ä 4.82 (d, J ) 5.1 Hz), 3.31 (br s), and 4.13 (br d, J
) 16.4 Hz), corresponding to H-3, H-15, and H-21â, respectively,
are characteristic of an akuammiline-type alkaloid.17 The COSY
and HMQC spectral data also disclosed partial structures that are
reminiscent of an akuammiline skeleton, except that the NCHCH2-
CH partial structure, corresponding to C(3)-C(14)-C(15), has been
extended to include a CHCHO fragment, corresponding to C(16)-
C(17). These observations revealed the similarity of rhazinoline to
the akuammiline-type alkaloid strictalamine, previously reported
from Rhazya stricta.18 However, the W-coupling observed between
H-14R and H-16 requires the stereochemistry at C-16 to be assigned
as S, thus indicating that compound 1 is the C-16-epimer of
strictalamine.18 This was further supported by the NOEs observed
between H-6â/H-16, H-16/H-18, and H-14â/H-17. The latter NOE