366
E. Sobarzo-S a´ nchez et al.
0
of 300.13 MHz and a 13C frequency of 75.47 MHz. All mea-
surements were performed at a probe temperature of 300 K,
using solutions of 4, 5 and 6 in DMSO-d (20–23 mg ml )
6
containing tetramethylsilane (TMS) as an internal standard.
All two-dimensional spectra were acquired with a Bruker
inverse 5 mm Z-gradient probe. The one-dimensional carbon
peaks were found between H-7/H-7 and H-6a (spin sys-
tem AMX).
ꢀ
1
With the C-7 methylene protons clearly differentiated
from those at C-4 by the connectivity of the former with
the methine proton, and by the concerted use of HMQC
and HMBC, it was possible to establish the long-range
connectivity between C-4 and the aromatic ring proton
resonating at υ 6.61. These results allowed the unambiguous
assignment of this resonance to the proton at C-3, correlated
with C-4, thus proving by default that the oxazole ring is
located at C-8/C-9 on the aporphine skeleton. Although the
chemical relationship between 4, 5 and 6, all derivatives of 8-
aminoboldine (3), leaves no doubt as to the position at which
the oxazole or oxazine ring is fused to the aporphine system,
analogous correlations between the C-4 methylene and the
upfield aromatic proton singlet confirmed the structures of
spectrum was obtained with a spectral width of 18 000 Hz
with 3 s between transients and the 90° pulse was 10 µs.
1
1
The homonuclear H– H shift-correlated 2D spectra were
obtained using standard Bruker software (cosygs). The spec-
tral widths were 3000 Hz. The spectra were collected as
512 ð 512 blocks of data and were processed by sinusoidal
multiplication in each dimension. Other parameters were as
follows: number of increments in t , 256; number of scans, 4;
1
and relaxation delay, 1 s. The HMQC spectra were acquired
using standard Bruker software (inv4gstp). These spectra
were collected with 512 ð 512 data points, a data acquisition
of four scans ðF and 256 increments in t . Spectral widths of
5
and 6. Similarly, the unambiguous determination of the
structures of these substances proves that the nitrosation of
boldine (1) affords 8-nitrosoboldine (2) as the only isolated
product.
The complete spectral assignment of 1, 4, 5 and 6 is
shown in Tables 2–5. The large vicinal coupling constant
2
1
1
2500 Hz and 18 000 Hz were employed in the F ( H) and F
2
1
1
3
( C) domains, respectively. Data were processed using Qsine
functions for weighting in both dimensions. The HMBC spec-
tra were obtained using the inv4gslplrnd pulse sequence in
the Bruker software and collected with 512 ð 512 data points,
(
10–14 Hz) observed for these compounds (H-6a,7˛ protons)
are in good agreement with those previously described for
a data acquisition of 10 scans ð F and 256 increments in t .
1
1
2
other aporphine alkaloids.
The spectral widths were 2500 Hz ꢀF ꢁ and 18 000 Hz ꢀF ꢁ
2
1
and the delays
tively. Data were processed using an exponential window in
with lb D 5 Hz and a Qsine window in F
1 2
and were set to 3.45 and 65 ms, respec-
EXPERIMENTAL
F
2
1
.
Preparation of aminoboldine (3)
Boldine (1) was dissolved in acetic acid and treated at room
Acknowledgements
E. S.-S. is the recipient of a Fundaci o´ n Andes fellowship. CEPEDEQ,
Facultad de Ciencias Qu ´ı micas y Farmac e´ uticas, Universidad de
Chile, is thanked for the free use of NMR facilities. This work was
funded in part by the Presidential Chair in Science (B.K.C.).
2
temperature with an equimolar amount of NaNO with
stirring for 1 h. After neutralization with ammonia solution
and extraction with ethyl acetate, 8-nitrosoboldine (2) was
isolated as the only product. Compound 2, dissolved in
EtOH, was hydrogenated catalytically over Pd/C at room
temperature at 50 psi for 2.5 h, to afford 8-aminoboldine (3)
in quantitative yield as gray needles.
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Copyright 2001 John Wiley & Sons, Ltd.
Magn. Reson. Chem. 2001; 39: 361–366
Sobarzo-Sanchez, Eduardo
