insufficient to assign the relative stereochemistry of the
pyrrolizidine. Although 5 is new to natural products chem-
istry, two stereoisomers were reported in 2006 by Pfizer
of a differing ∆18 stereochemistry. This hypothesis was
confirmed by a diagnostic 2D ROESY NMR correlation
1
8
between 17-Me and H-20, confirming a Z ∆ stereochem-
istry for 3. All other stereochemical assignments, and the
inclusion of twin pyrrolizidine amides, were assigned on the
basis of comparable arguments as presented above for 2.
Bistellettazole A (4) returned a molecular/pseudomolecular
ion in the high-resolution mass spectrum consistent with the
formula C H N O (-0.5 ∆mmu). Unlike the bistellet-
1
researchers as synthetic intermediates in a medicinal
chemistry investigation of pyrrolizidine amides as 5-HT
4
receptor antagonists and agonists. Unfortunately the spec-
troscopic characterization reported at that time was inad-
equate for a definitive comparison, and the lack of optical
rotation data prevented a comparative assignment of
stereochemistry.
3
1
43
4
2
tazines, the +ve ESI mass spectrum of 4 did not feature
Bistellettazine B (2) displayed similar mass spectral
properties to 1, with a +ve ESI mass spectrum dominated
doubly charged ions, requiring that C H N O correspond
3
1
43
4
2
+
to either the singly charged molecular species (M ) or a
2
+
by a doubly charged molecular ion (M , m/z 341) and a
+
pseudomolecular M + H ion. The NMR (DMSO-d
6
) data
+
singly charged fragment ion (M - H , m/z 681). High-
for 4 (Table S4b, Supporting Information) revealed two
exchangeable amide protons (δ 7.9 (1-CONH, t, 5.8 Hz)
and 8.0 (18-CONH, t, 5.8 Hz)) with HMBC correlations to
two amide carbonyl carbons (δ 165.6 (C-1) and 166.4 (C-
8), respectively) and two amino methylene carbons (δ 36.1
C-1′) and 35.6 (C-10′), respectively). The NMR (MeOH-
) data for 4 (Table S4a, Supporting Information) provided
better dispersion of resonances compared to DMSO-d and
resolution analysis of the M - H+ fragment ion was
H
consistent with a molecular formula (C44
mmu) incorporating two +ve charged functional groups
and 15 DBE. Again, in an analysis similar to that presented
above for 1, the NMR (DMSO-d ) data for 2 (Table S2b,
Supporting Information) revealed two exchangeable amide
protons (δ 8.05/8.07 (1-CONH/22-CONH, t, 5.8 Hz)) with
HMBC correlations to two amide carbonyl carbons (δ
68.47/168.49 (C-1 and C-22)) and two equivalent amino
methylene carbons (δ 40.1 (1′/1′′-CH ). The NMR (MeOH-
) data for 2 (Table S2a, Supporting Information) provided
64 4 2
H N O , 2.0
∆
C
1
C
6
(
d
4
H
6
C
2
revealed a further 15 sp carbons attributed to a total of seven
double bonds and an “imine-like” moiety and suggestive that
1
C
2
4
be tricyclic. Following a similar approach as outlined above
for the bistellettazines, analysis of the 2D COSY and HMBC
NMR (MeOH-d ) data revealed correlations consistent with
a) a dienyl amide (C-1 to C-5) attached to (b) a cyclohexenyl
moiety (C-6 to C-11), in turn further substituted at C-11 by
c) a tertiary methyl (11-Me) and (d) a trienyl amide (C-12
to C-18)sthe (b) to (d) substructural units being in common
d
4
2
better dispersion of resonances and revealed 16 sp olefinic
carbons accounting for 10 DBE and defining 2 as pentacyclic.
These observations, together with significant similarities in
the NMR data between 1 and 2, suggested that 2 was a
biosynthetic homologue of 1, featuring a common C-1 to
C-13 terpenyl substructure but differing about the C-13
terpenyl side chain. That this side chain in 2 featured a
propenyl moiety inserted adjacent to the terminal amide (C-
4
(
(
2
4
with 1. An E stereochemistry was assigned to ∆ and ∆ on
the basis of J2,3 15.3 Hz and 2D ROESY NMR (MeOH-d
4
)
correlations between H-3 and H-5. The relative stereochem-
istry about the cyclohexenyl ring was established as indi-
cated, by ROESY correlations between H-5 and 11-Me, and
H-6 and H-12. An all-E stereochemistry was assigned about
2
2) was confirmed by 2D COSY and HMBC (MeOH-d
correlations (Table S2a, Supporting Information). Key
observations included HMBC correlations from H-20 (δ
.98) to the amide carbonyl (C-22, δ 172.6) and COSY
correlations between H-20 and H-19 (δ 6.53). The all-E
4
)
H
6
C
1
2
14
16
∆
1
, ∆ , and ∆ on the basis of J12,13 14.8 Hz and J16,17
5.3 Hz and the shielded chemical shift for 15-Me (δ 12.9),
which was in common with the comparable structure
fragment (17-Me, δ 12.7) in 1 that was independently
H
C
2
4
6
14
16
stereochemistry about ∆ , ∆ , ∆ , ∆ , and ∆ and the
relative stereochemistry about C-8 and C-13 in the cyclo-
hexenyl moiety in 2 were determined to be the same as those
for 1, based on the observation of comparable coupling
C
determined to have an E stereochemistry by diagnostic
ROESY correlations. Further examination of the 2D COSY
and HMBC NMR (MeOH-d ) data permitted extension of
4
4
constants and 2D ROESY NMR (MeOH-d ) correlations.
1
8
An E stereochemistry was also assigned to ∆ on the basis
the assigned structure from 1-CONH through a 1,2-ethanyl
moiety to C-2′, and 18-CONH through a 1,3-propanyl moiety
to C-8′, with the remaining structure fragment being nomi-
nally attributed to an N-methylimidazole (C-3′-N-7′). Deg-
radation of 4 as described for 1 returned the imidazole
hydrochloride salt 6. Spectroscopic analysis (Table S6,
Supporting Information) supported the identity of 6, which
was confirmed by total synthesis as outlined in Scheme 1,
2
0
of J18,19 15.0 Hz and to ∆ on the basis of a 2D ROESY
NMR (MeOH-d ) correlation between H-19 and 21-Me. The
4
presence of twin pyrrolizidine amides in 2 (incorporating
the amine 5) was inferred from comparisons of NMR
(
4
MeOH-d ) data with 1 was assigned on spectroscopic and
biogenetic grounds by comparison to the cometabolite 1.
Bistellettazine C (3) was determined to be isomeric with
2
, with the two isomers displaying a very high degree of
1
utilizing the method of Cohen. In this synthesis, histamine
similarity between their spectroscopic data (Tables S3a and
S3b, Supporting Information). The only significant spectro-
scopic difference between 2 and 3 was in the NMR (MeOH-
(7) is converted to the cyclic urea 8, alkylated to 9, ring
opened to 10, methylated to 11, and finally deprotected to
yield imidazole 6 identical in all respects to that obtained
from chemical degradation of 4. Thus, the total relative
stereostructure for bistellettazole A (4) can be assigned as
shown.
d
4
) resonances for H-18 and H-19, which in the latter
appeared as an overlapping multiplet (δ 6.23) suggestive
H
(
1) Jain, R.; Cohen, L. A. Tetrahedron 1996, 52, 5363.
Org. Lett., Vol. 10, No. 19, 2008
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