Erythrina Alkaloid Analogues as nAChR Antagonists-A Flexible Platform for Leads in Drug Discovery

Article abstract of DOI:10.1021/acs.joc.1c00707

Erythrina alkaloids and their central nervous system effects have been studied for over a century, mainly due to their potent antagonistic actions at β2-containing nicotinic acetylcholine receptors (nAChRs). In the present work, we report a synthetic approach giving access to a diverse set of Erythrina natural product analogues and present the enantioselective total synthesis of (+)-Cocculine and (+)-Cocculidine, both found to be potent antagonists of the β2-containing nAChRs.

Full text of DOI:10.1021/acs.joc.1c00707

pubs.acs.org/joc
Article
Erythrina Alkaloid Analogues as nAChR Antagonists−A Flexible
Platform for Leads in Drug Discovery
Sebastian Clementson, Sergio Armentia Matheu, Emil Märcher Rørsted, Henrik Pedersen,
Anders A. Jensen, Rasmus P. Clausen, Paulo Vital, Emil Glibstrup, Mikkel Jessing,
and Jesper L. Kristensen*
Cite This: J. Org. Chem. 2021, 86, 8248−8262
Read Online
sı
*
Metrics & More
Article Recommendations
Supporting Information
ACCESS
ABSTRACT: Erythrina alkaloids and their central nervous system effects have
been studied for over a century, mainly due to their potent antagonistic actions at
β2-containing nicotinic acetylcholine receptors (nAChRs). In the present work, we
report a synthetic approach giving access to a diverse set of Erythrina natural
product analogues and present the enantioselective total synthesis of (+)-Cocculine
and (+)-Cocculidine, both found to be potent antagonists of the β2-containing
nAChRs.
The structural complexity and pharmacological profile of
Erythrina alkaloids have made them popular targets for total
synthesis, resulting in several notable approaches.10a−m,11
However, only a few of these efforts have produced optically
pure natural products.7,10a,11
Moreover, the oxidative diversity found in the D-ring of
Erythrinanes has also made a unifying synthetic approach to
various subcategories very challenging. In the present
work,12,14,16 we expand upon our previously developed
synthetic route to the Erythrina alkaloids and demonstrate
that our divergent synthetic strategy allows access to the
lactonic and aromatic Erythrina natural products for the first
time.10k,l
INTRODUCTION
■
Extracts from the seeds of Erythrina plants have been utilized
as hypnotics, sedatives, and arrow poison by indigenous South
Americans for the past centuries. Many alkaloids of this natural
product family possess a curare-like neuromuscular blocking
effect, and comprehensive studies in the early 1940s concluded
that a majority of the isolated Erythrinanes exert their principal
pharmacological activity via the central nervous system by
acting as competitive antagonists of nicotinic acetylcholine
receptors (nAChRs), a heterogeneous class of pentameric
ligand-gated ion channels mediating fast cholinergic trans-
mission.1−3
(+)-Dihydro-β-erythroidine [DHβE (1), Figure 1B] was
previously used in the treatment of Parkinson’s disease to
relieve tetanus and spastic disorders. As one of the most potent
members of the Erythrina alkaloid family, DHβE (1) has
become a key pharmacological tool in the nAChR field as it
displays pronounced subtype-selectivity for β2-subunit-con-
taining nAChRs (e.g., α4β2) over β4-containing (e.g., α3β4)
and other nAChR subtypes (e.g., α7).4,5,6a−c 1 and the other
Erythrina alkaloids act as competitive antagonists at the
nAChR, and their binding modes to the receptors have been
elucidated by a co-crystal structure of 1 and the ACh-binding
protein (AChBP), a mollusk protein homologous to the
extracellular domain of the nAChR.6c
The Erythrina alkaloids all possess a structurally rigid
spiroamine scaffold, where the A-, B-, and C-rings are generally
decorated in an oxidatively similar manner−Erythrivarine B
(2) represents an exception to this. Most subcategories of
Erythrina alkaloids distinguish themselves from each other in
the nature of their D-ring, where lactones, aryls, and
heteroaryls comprise the majority of the reported moieties
(Figure 1B).7−9
We also characterize the binding and functional properties of
this series of Erythrina alkaloids at nAChRs. In a previous
investigation, we found that the AB-ring system of DHβE (1)
is bound deep in the nAChR-binding pocket with the lactonic
D-ring protruding from it (Figure 2A).6c In the present study,
we take advantage of our recently disclosed total synthesis of 1
to access a library of ligands.10k,l These ligands maintain the
structural features of 1 embedded in the binding pocket, while
allowing for easy manipulation of the part protruding from the
binding pocket (Figure 2B). The MeO-appendage on the A-
ring established an interaction with a water molecule in the
binding pocket, and we also wanted to probe the importance of
this interaction by varying the stereochemical orientation at
Received: March 25, 2021
Published: June 1, 2021
https://doi.org/10.1021/acs.joc.1c00707
© 2021 American Chemical Society
J. Org. Chem. 2021, 86, 8248−8262
8248

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
Figure 1. (A) General overview of a divergent approach to various Erythrina alkaloid subcategories and (B) representative members of the alkaloid
family.
resulted in faster reaction time and increased scalability and
yield. This is a notable improvement to the previously reported
total synthesis.
Subsequent allylation of the C3 with (+)-allylboronic acid
pinanediol ester 1119 generated the desired diastereomer 12 in
total 60% yield. Methylation and ring closing metathesis20a,b
furnished the bicyclic ester 15, and this compound could be
further enantioenriched through the means of chiral
chromatography. The final carbons of the C-ring were
introduced through a one pot deprotection−reductive
amination.20,28−31 The tricyclic ring system could then be
assembled by refluxing bis-ester 17 with potassium tert-
butoxide, furnishing 18 in 98% yield. Further functional
group manipulation produced the silyl-protected triflate 19 in
73% (see Scheme 2). Installation of the final two carbons of 1
proved problematic as the triflate 19 rapidly decomposed
under basic coupling conditions.21a−d Eventually, we found
that a base-free decarboxylative α-Cyanation formed the
desired carbon−carbon bond, and the process was ultimately
telescoped to 1, using HCl.22
Total Synthesis of Aromatic Erythrinanes (+)-Coccu-
line and (+)-Cocculidine. We then turned our attention to
the aromatic Erythrinanes looking to utilize intermediate 18 to
access representative members from this structural class. The
required carbons of the D-ring could be introduced via a
Michael addition with methyl-vinyl-ketone 22 in quantitative
yield (see Scheme 2).
Decarboxylation−condensation of 23 with potassium
hydroxide in two iterations delivered enone 24/24′ as a
1.8:1 mixture of C-12 epimers. Enone 24 could later be
Figure 2. (A) DHβE (1) bound in the nAChR model.6b (B) Outline
of the contributions of different parts of DHβE (1) in its nAChR
binding and the targeted library, wherein R and the stereochemistry at
C3 varied.
this carbon−given that our synthetic route provides access to
both diastereisomers of the DHβE-scaffold.
RESULTS AND DISCUSSION
■
Our synthesis commenced with revisiting our total synthesis of
1 (see Scheme 1). Asymmetric allylic allylation using Trost
ligand 7 delivered allyl prolinone 8.12a,b The terminal double
bond was then oxidatively cleaved, and the product isolated as
the corresponding dimethyl acetal 9.13,14a,b Methenylation of
the ketone moiety proved problematic, and all attempts at
using titanium carbenoids15a−g or addition of organometallic
sources of methylene16a,b failed. Wittig procedures using
potassium tert-butoxide furnished the desired olefin 10;
however, the reaction suffered from solubility issues and the
yield was consistently around 50%.17 Both aspects were
resolved by forming the ylide in a separate flask and then
transferring the said ylide-solution to the neat ketone,18 which
8249
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
a
Scheme 1. Previous Synthesis of Intermediate 1810k,l
,
a
Reagents and conditions: (a) 6, 7, [PdCl(allyl)]2, K2CO3, DMF, −40−0 °C. (b) OsO4, NMO, THF:H2O, rt, 12 h, then NaIO4, DCM, rt, 4 h. (c)
CH(OMe)3, CSA, toluene, rt, 2 h. (d) MePPh3Br, NaH, toluene, 90 °C, 10 h, then 9. (e) 11, HCl, DCM: H2O, rt, 8 h, d.r. 1.8:1. (f) Me3OBF4,
proton-sponge, DCM, rt, 3 h. (g) 14, toluene, 80 °C. (h) TFA:DCM, rt, 30 min, then 16, AcOH, NaBH3CN, THF:MeOH, rt. (i) KOtBu, toluene,
95 °C, 1 h.
a
Scheme 2. Total Synthesis of (+)-DHβE (1), (+)-Cocculine (25), and (+)-Cocculidine (3)
a
Reagents and conditions: (a) 20 (3 equiv), [PdCl(allyl)]2 (0.2 equiv), S-Phos (0.4 equiv), mesitylene, 110 °C, 30 min, then HCl (aq), MeOH, 85
°C, 30 min, 54%. (b) Et3N (5 equiv), 22 (5 equiv), MeOH, 70 °C, 14 h, >99%. (c) KOH (5 equiv), water, 105 °C, 2 h, then KOH (15 equiv), 105
°C, 2h, 78%. (d) HCl (6.3 equiv), CHCl3, room temperature, 24 h, 74%. (e) CuBr2 (4 equiv), MeCN, 40 °C, 5 h, 60%. (f) CuBr2 (2 equiv),
CH(OMe)3 (15 equiv), methanol, 80 °C, 14 h, then BF3·OEt2 (2 equiv), DCM, 0 °C-rt, 2 h, 62%. S-Phos = 2-dicyclohexylphosphino-2′,6′-
dimethoxybiphenyl. DCM = dichloromethane.
obtained as a single epimer in 56% yield following treatment
with HCl. The final oxidation of the D-ring proved difficult
(see the Supporting Information) but was ultimately
accomplished using stoichiometric amounts of CuBr2,23
delivering the phenolic natural product (+)-Cocculine 25 in
60% yield. Standard O-methylation procedures (methyl iodide
or diazomethane) did not produce (+) Cocculidine (3), but
upon treatment of enone 24 with CuBr2 in the presence of
trimethyl orthoformate, the bromo-ketal 26 (tentatively
assigned) was formed as the major component. Subsequent
Lewis acid-mediated elimination delivered (+)-Cocculidine
(3) in good 62% yield from 24 (see the Supporting
Information for a complete list of attempted oxidations).
This approach represents the first synthesis of (+)-Cocculine
(25) as well as the shortest and first asymmetric synthesis of
(+)-Cocculidine (3), both in 12 steps, respectively.
Synthesis of Erythrina Library. Using key intermediate
18 as the starting point, we set out to access a library of
Erythrina alkaloid analogues (see Scheme 3). To be able to
investigate the significance of the stereochemistry of the C3
methyl ether, both epimers 18 and epi-18 were decarboxylated
to ketones 29 and epi-29 using 4 and 3 molar aqueous HCl,
respectively. Interestingly, when exposed to 4−6N HCl, the
C3-epimer underwent O-demethylation and subsequent
lactonization to form the tetracyclic lactone 31. Triflation of
29 and epi-29 with KHMDS and bis-triflimide24 generated the
8250
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
a
Scheme 3. Synthesis of Erythrina Analogues
a
Reagents and conditions: (a) Hydrazine−hydrate (10 equiv), 1,4-dioxane, 100 °C, 72 h, 30%. (b) Thiourea (2.6 equiv), KOtBu (2.6 equiv),
MeOH, 90 °C, 12 h, 5%. (c) HCl (4N), 110 °C, 3 h, 68%. (d) HCl (3N), 110 °C, 3 h, 56%. (e) HCl (6M), 115 °C, 16 h, 30%. (f) KHMDS (2
equiv), PhN(OTf)2 (1.8 equiv), THF, −78−0 °C, 3.5 h, 68%, resp. 56%. (g) Pd(dppf)Cl2 (0.1 equiv), R−B(OH)2 (1.7 equiv), NaOH (1.8 equiv),
DME, 85 °C, 75 min. DME = dimethoxyethane. KHMDS = potassium bis(trimethylsilyl)amide. THF = tetrahydrofuran.
Scheme 4. Structures of Ligands in the Erythrina Library Accessed in Scheme 3
corresponding triflates 30 and epi-30, which set the stage for
the assembly of the final carbon−carbon bondconsequently
introducing the D-ring as a non-fused, appended ring system.
Following a standard Suzuki coupling24 of the triflates, a library
containing over 22 unnatural Erythrina alkaloids bearing
various functionalities in their D-ring was synthesized.
With the exception of 33 and 38, both epimers were
obtained from the analogues 32−43. Furthermore, 18 was
condensed with hydrazine and thiourea to give heterocyclic
compounds 27 and 28, respectively. See Scheme 4 for the
structure of all the ligands in the Erythrina library.
Erythrina alkaloids as nAChR ligands were determined at
two β2-containing (α4β2 and α6/α3β2β3V9′S, a surrogate for
the α6β2β3 receptor) and two β4-containing (α3β4 and α4β4)
subtypes in a [3H]epibatidine competition binding assay, as
previously described (see Table 1).25a,b The functional
properties of the compounds as nAChR antagonists were
investigated at the α4β2 and α3β4 receptors in a Ca2+ imaging
assay using the Ca2+ fluorophore Fluo-4.25b Full details on all
of the synthesized compounds are given in Tables 1 and 2.
In agreement with the literature, 1 was found to exhibit
pronounced β2-over-β4 selectivity at the nAChRs, both in
terms of its binding affinities and functional antagonist
potencies (see Table 1). In striking contrast and highly
Pharmacological Characterization of the Erythrina
Alkaloids at nAChRs. The binding properties of the
8251
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
Table 1. Binding Affinities of the Erythrina Alkaloid Analogues at hα4β2-, hα6/α3β2β3V9’S-, rα3β4-, and rα3β4-HEK293 Cell
a
Membranes in the [3H]epibatidine Competition Binding Assay
hα4β2
hα6/α3β2β3V9’S
Ki [pKi S.E.M.](n)
0.60 [6.22 0.10](4)
rα3β4
Ki [pKi S.E.M.](n)
∼50 [∼4.3](3),
IC50 > 100
∼20 [∼4.7](3),
IC50 > 100
IC50 > 100
IC50 > 100
∼10 [∼5.0](3),
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
rα4β4
Ki [pKi S.E.M.](n)
∼20 [∼4.7](3),
Ki [pKi S.E.M.](n)
0.27 [6.57 0.03](4)
c
c
(+)-DHβE (1)
(−)-DHβE (1′)
3
18
epi-18
24
25
27
28
29
epi-29
31
32
epi-32
33
34
epi-34
35
epi-35
36
epi-36
37
epi-37
38
c
b
b
b
(3),
∼30 [∼4.5](3),
IC50 > 100
IC50 > 100
(3),
(3),
b
0.0049 [8.31 0.08](3)
0.60 [6.22 0.05](4)
0.075 [7.13 0.01](3)
1.6 [5.80 0.12](3)
3.4 [5.47 0.08](3)
b
b
b
b
(3),
(3),
IC50 > 100
IC50 > 100
b
b
b
(3),
(3),
(3),
(3),
IC50 > 100
IC50 > 100
c
0.59 [6.23 0.03](3)
4.0 [5.39 0.02](3)
0.042 [7.37 0.11](3)
∼20 [∼4.7](3),
(3),
c
0.0018 [8.75 0.13](3)
1.2 [5.91 0.05](3)
c
b
b
b
b
b
b
b
b
b
b
b
b
b
b
b
∼10 [∼5.0](3),
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
IC50 > 100
∼50 [∼4.3](3),
(3),
(3),
(3),
(3),
(3),
(3),
(3),
(3),
(3),
(3),
(3),
c
∼20 [∼4.8](3),
(3),
4.3 [5.37 0.09](4)
(3),
b
(3),
(3),
IC50 > 100
IC50 > 100
b
(3),
(3),
c
c
c
c
c
4.8 [5.32 0.06](4)
4.6 [5.34 0.10](3)
7.0 [5.16 0.09](3)
3.4 [5.47 0.10](3)
2.9 [5.54 0.10](3)
∼50 [∼4.3](3),
∼50 [∼4.3](3),
∼50 [∼4.3](3),
∼50 [∼4.3](3),
c
b
∼10 [∼5.0](3),
IC50 > 100
(3),
c
c
2.0 [5.70 0.04](3)
3.4 [5.46 0.07](4)
∼20 [∼4.7](3),
∼20 [∼4.7](3),
c
c
b
b
(3),
∼10 [∼5.0](3),
∼10 [∼5.0](3),
IC50 > 100
IC50 > 100
(3),
c
c
c
0.39 [6.40 0.04](3)
4.8 [5.32 0.10](3)
1.2 [5.92 0.02](3)
1.9 [5.71 0.13](3)
7.1 [5.15 0.08](3)
1.8 [5.75 0.09](4)
∼50 [∼4.3](3),
∼50 [∼4.3](3),
∼50 [∼4.3](3),
4.9 [5.31 0.07](3)
b
(3),
IC50 > 100
c
∼30 [∼4.5](3),
b
(3),
c
b
b
∼30 [∼4.5](3),
IC50 > 100
IC50 > 100
∼30 [∼4.5](3),
IC50 > 100
IC50 > 100
(3),
(3),
c
0.22 [6.66 0.03](4)
5.5 [5.26 0.05](3)
1.3 [5.90 0.05](3)
0.87 [6.06 0.06](4)
0.63 [6.20 0.01](3)
1.2 [5.94 0.08](4)
2.8 [5.55 0.08](3)
0.81 [6.09 0.06](3)
6.5 [5.19 0.01](3)
3.0 [5.52 0.02](4)
0.88 [6.06 0.06](4)
4.2 [5.38 0.07](3)
0.96 [6.01 0.11](3)
0.70 [6.16 0.11](3)
0.33 [6.48 0.08](3)
1.4 [5.86 0.09](3)
5.2 [5.29 0.06](3)
1.8 [5.75 0.08](3)
4.7 [5.33 0.05](3)
1.4 [5.87 0.12](2)
6.0 [5.22 0.01](3)
1.1 [5.97 0.06](3)
0.23 [6.63 0.10](3)
2.9 [5.54 0.09](n)
b
b
(3),
(3),
IC50 > 100
c
c
c
c
c
c
c
c
c
c
c
∼50 [∼4.3](3),
∼20 [∼4.7](3),
∼10 [∼5.0](3),
∼50 [∼4.3](3),
∼30 [∼4.5](3),
∼20 [∼4.7](3),
∼30 [∼4.5](3),
∼20 [∼4.7](3),
∼50 [∼4.3](3),
∼20 [∼4.7](3),
∼10 [∼5.0](3),
39
epi-39
40
epi-40
41
epi-41
42
b
(3),
IC50 > 100
c
∼10 [∼5.0](3),
b
(3),
IC50 > 100
b
b
c
(3),
IC50 > 100
∼50 [∼4.3](3),
c
b
(3),
(3),
epi-42
43
∼10 [∼5.0](3),
IC50 > 100
IC50 > 100
c
c
c
c
0.83 [6.08 0.04](3)
1.0 [5.98 0.03](3)
∼20 [∼4.7](3),
∼20 [∼4.7](3),
epi-43
∼10 [∼5.0](3),
∼50 [∼4.3](3),
a
The binding affinities of the compounds are given as Ki values in μM [with pKi standard error of mean (S.E.M.) in brackets] with the number of
b
experiments (n) indicated in superscript. At 100 μM, the compound mediated less than 50% inhibition of specific [3H]epibatidine binding (i.e.,
c
IC50 > 100 μM). The concentration−inhibition relationship for the compound was not complete at 100 μM. Thus, the Ki value for the compound
was calculated based on an IC50 value estimated by visual inspection from the data.
interestingly, however, an overall difference was observed for
these properties exhibited by most of the 33 other Erythrina
alkaloids. However, all active analogues analogously to 1
displayed significantly higher binding affinities to α4β2 and
α6/α3β2β3V9′S than to α3β4 and α4β4, they displayed
substantially lower degrees of α4β2-over-α3β4 selectivity in the
Ca2+/Fluo-4 assay, so much so that many of the analogues
were equipotent antagonists at these two receptors. Since the
binding affinities and antagonist potencies exhibited by the
analogues at the α4β2 nAChR correlated fairly well (Figure
S9), the observed loss of functional β2-over-β4 selectivity
arises from the analogues possessing higher α3β4 activity in
this Ca2+/Fluo-4 assay than that in the binding assay. Since
data from radioligand-binding competition binding experi-
ments predominantly are believed to reflect the binding affinity
of the ligand to a desensitized nAChR conformation, the
antagonist potency of the competitive antagonist in a
functional assay mainly reflects its binding to the resting
receptor conformation. The apparent discrepancy between the
β2-over-β4 selectivity exhibited by the Erythrina alkaloids in
the two assays could thus be speculated to be a reflection of
different binding properties of the compounds to different
receptor conformations, in particular in the case of α3β4 and
other β4-containing nAChRs. Overall, the accessed Erythrina
ligands retained the high affinity/antagonist potency of 1 at the
two β2-containing nAChRs. Among the exceptions was the
significant loss of activity in the heterocyclic derivatives 27, 28,
36, and 42. Strikingly, with low nanomolar Ki values at α4β2
and α6/α3β2β3V9′S, natural products (+)-Cocculine (25)
and (+)-Cocculidine (3) displayed 10−100-fold higher
binding affinities at the two β2-containing nAChRs than 1
itself, and these two natural products exhibited pronounced
β2-over-β4 selectivity in both binding and functional assays.
Finally, comparison of the binding affinities exhibited by the 10
epimeric pairs at the two β2-containing nAChRs reveals very
similar Ki values for the 32/epi-32, 40/epi-40, and 43/epi-43
epimers, whereas for the seven other epimeric pairs, it is the
natural (S)-epimer that is a somewhat better binder than the
8252
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
group to the water molecule; and (3) the varying substituents
protruding toward the β2 subunit. In the proposed binding
mode from the docking of 3 and 25, the aromatic ring forms π-
stacking interactions to W57 of the β2 (t-shaped) and possibly
also to Y197 of the α4 subunit (parallel) (see Figure 3b). This
interaction is stronger compared to the van der Waals
interactions of 1 and could explain the very high binding
affinity of these two compounds compared to 1. In contrast,
the two heterocyclic derivatives 27 and 28 have a hydroxyl
group protruding into this area, and the unfavorable
interactions arising from this could be the root of the
significantly reduced binding affinity exhibited by these
analogues (see Figure 3c).
A total of 10 epimeric ligand pairs were accessed. In general,
the inversion of the C3-stereocenter does not have a significant
impact to the affinity of the ligands for α4β2−with the notable
exception of 36/epi-36, where the epimer displayed a
substantially lower binding affinity. When docked into the
α4β2 model, most epimers exhibited the highest scoring
binding pose, where the fused ring system is overlapping with 1
and the methoxy group is adopting a pseudo-axial position
with no H-bond to the water molecule (Figure 3e). This is
independent of including the water molecule found in the X-
ray structure in the docking. Thus, the importance of the
interaction with the water molecule found in the X-ray
structure for binding of Erythrina alkaloids may be over-
estimated. As the size of the substituent in the Erythrina
alkaloid is increased (39 and 43), binding modes wherein the
AB-ring systems rotate in the binding pocket become
predominant for the epimeric derivatives, as illustrated with
epi-43 in Figure 3f. This suggests that the styryl derivatives
epi-39 and epi-43 may have a different binding mode than the
rest of the epimer analogues.
Table 2. Functional Properties Displayed by the Erythrina
Alkaloid Analogues at hα4β2-HEK293 and rα3β4-HEK293
a
Cell Lines in the Ca2+/Fluo-4 Assay
hα4β2
rα3β4
IC50 [pIC50 S.E.M.](n)
0.35 [6.45 0.06](6)
IC50 [pIC50 S.E.M.](n)
b
(+)-DHβE (1)
(−)-DHβE (1′)
3
18
epi-18
24
25
27
28
29
>100 [<4.0](6),
c
b
∼30 [∼4.5](3),
∼100 [∼4.0](3),
0.26 [6.59 0.10](4)
4.8 [5.32 0.08](3)
4.2 [5.37 0.12](4)
b
>100 [<4.0](3),
>100 [<4.0](3),
b
b
>100 [<4.0](3),
c
2.0 [5.69 0.08](3)
0.12 [6.92 0.05](4)
∼100 [∼4.0](3),
c
∼20 [∼4.7](3),
c
b
∼100 [∼4.0](3),
>100 [<4.0](3),
c
b
b
b
b
∼20 [∼4.7](3),
>100 [<4.0](3),
>100 [<4.0](3),
>100 [<4.0](3),
>100 [<4.0](3),
b
b
b
>100 [<4.0](3),
>100 [<4.0](3),
>100 [<4.0](3),
epi-29
31
32
epi-32
33
34
epi-34
35
epi-35
36
epi-36
37
epi-37
38
7.4 [5.13 0.07](3)
2.1 [5.68 0.07](3)
0.49 [6.31 0.08](4)
1.5 [5.81 0.10](3)
0.99 [6.00 0.07](3)
0.33 [6.49 0.02](3)
0.33 [6.49 0.12](3)
2.6 [5.59 0.11](3)
5.0 [5.30 0.07](3)
1.6 [5.79 0.14](4)
2.6 [5.89 0.08](3)
4.2 [5.38 0.07](3)
0.79 [6.10 0.05](3)
1.2 [5.91 0.06](3)
c
c
∼20 [∼4.7](3),
∼30 [∼4.5](3),
c
c
∼20 [∼4.7](3),
∼50 [∼4.3](3),
1.8 [5.75 0.11](3)
5.6 [5.25 0.13](3)
11 [4.96 0.06](3)
b
∼30 [∼4.5](3),
c
c
∼20 [∼4.7](3),
∼100 [∼4.0](3),
39
epi-39
40
epi-40
41
epi-41
42
epi-42
43
epi-43
1.3 [5.89 0.11](3)
1.1 [5.94 0.11](4)
0.52 [6.29 0.10](3)
0.14 [6.84 0.12](3)
0.24 [6.62 0.07](3)
0.44 [6.36 0.08](3)
1.2 [5.91 0.09](4)
1.3 [5.89 0.08](4)
0.29 [6.54 0.07](3)
0.54 [6.27 0.13](3)
5.7 [5.25 0.09](3)
5.6 [5.25 0.06](3)
2.9 [5.54 0.03](3)
1.6 [5.80 0.11](4)
0.78 [6.11 0.07](3)
1.5 [5.82 0.04](3)
2.6 [5.59 0.09](3)
2.5 [5.61 0.10](3)
1.5 [5.82 0.06](3)
1.1 [5.96 0.10](3)
CONCLUSIONS
■
A library of Erythrina alkaloid derivatives were accessed in a
concise and convergent manner from a common intermediate,
giving access to ligands with a complex molecular architecture.
The enantioselective total synthesis of two Erythrina alkaloids
was completed. Subsequently, the pharmacological properties
of the synthesized analogues at nAChRs were determined in
binding and functional assays. The natural products
(+)-Cocculidine (3) and (+)-Cocculine (25) exhibited
remarkably higher affinities at α4β2 and hα6/α3β2β3V9′S
and higher β2-over-β4 selectivity than (+)-DHβE (1) in the
binding assay and thus constitute promising leads for future
nAChR ligand development. Also, notably was the loss of the
inherent α4β2-over-α3β4 selectivity observed for most of the
synthetic analogues in the functional assay as this indicates that
the nAChR selectivity profile of the Erythrina alkaloids can be
tweaked by structural modifications to its scaffold. Finally, a set
of epimeric ligands were used to investigate a specific
interaction with a water molecule in the receptor, which
showed that the binding pocket can accommodate both
epimers with comparable affinity. In conclusion, the Erythrina
alkaloids constitute a promising scaffold for ligand develop-
ment in the nAChR and potentially other fields, and the
divergent synthetic strategy presented in this work will enable
the exploration of this potential by allowing access to diverse
series of derivatives.
a
The antagonist properties were determined using (S)-nicotine
∼EC80 (EC70−EC90) as an agonist. Antagonist potencies are given
as IC50 values in μM [with pIC50 standard error of mean (S.E.M.)
in brackets] with the number of experiments (n) indicated in
b
c
superscript. No significant inhibition observed at 100 μM. The
concentration−inhibition relationship was not complete at 100 μM.
The IC50 value was estimated from the fitted curve by visual
inspection.
unnatural (R)-epimer. The most prominent example of the
latter is the 30−100-fold higher binding affinities exhibited by
36 compared to epi-36 at α4β2 and α6/α3β2β3V9′S.
Elucidation of nAChR SAR Properties Using α4β2
Homology Model. In order to aid the interpretation of the
pharmacological properties exhibited by the alkaloids at the
nAChRs, all analogues were docked in the homology model of
the α4β2 nAChR reported by Yu et al6b (see the Supporting
Information for details). Docking of 1 into this homology
model replicated the binding mode exhibited by the compound
in the AChBP/DHβE co-structure.6c All synthesized ligands
could be accommodated in the binding pocket, where they
adopted a similar conformation to 1 (see Figure 3a): (1) a H-
bond from the protonated amine to the backbone carbonyl of
W156 in the α4-subunit; (2) a H-bond from the methoxy
8253
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
Figure 3. Docking poses exhibited by (a) 1, (b) 3, (c) 27, (d) 37, (e) epi-37, and (f) epi-43 in the α4β2 nAChR homology model.6b
doublet of triplets, ddd = doublet of doublet of doublets, ddt =
doublet of doublet of triplets, ddq = doublet of doublet of quartets,
dddd = doublet of doublet of doublet of doublets, m = multiplet, and
EXPERIMENTAL SECTION
■
General Information. Unless otherwise stated, all reactions were
performed in an oven-dried or flame-dried glassware under an
atmosphere of dry nitrogen or argon. Dry tetrahydrofuran (THF),
N,N-dimethylformamide (DMF), dichloromethane (DCM), toluene,
hexane, acetonitrile, and diethyl ether were used. Reagents were
purchased at the highest commercial quality and used without further
purification, unless otherwise stated. Reactions were monitored by
thin-layer chromatography (TLC) on Merck TLC-Silica gel 60 F254
Aluminum sheets (5 × 7.5 cm) and visualized by UV irradiation and
staining with the potassium permanganate developing agent. LC−MS
data were acquired using a waters acquity UPLC−MS consisting of a
waters acquity system including column manager, binary solvent
manager, sample organizer, Photodiode-Array Detection (PAD)
detector (operating at 254 nm), Evaporative light scattering (ELS)
detector, and Triple Quad Mass Spectrometry (TQ−MS) equipped
with atmospheric pressure photoionization-source operating in the
positive ion mode. LC conditions were as follows: the column was
Acquity UPLC BEH C18 1.7 μm; 2.1 × 50 mm operating at 60 °C
with 1.2 mL/min binary gradient consisting of H2O + 0.05%
trifluoroacetic acid (TFA) (A) and MeCN + 5% H2O + 0.05% TFA
(B). Gradient: 0.00 min: 10% B; 1.00 min: 100% B; 1.01 min: 10% B;
and 1.15 min: 10% B. The retention times provided in the
Experimental Section shall be compared to the total run time of
1.20 min. Yields refer to chromatographically and spectroscopically
(1H NMR and 13C NMR) homogeneous material, unless otherwise
stated. Structural assignments were made with additional information
from gradient correlation spectroscopy (gCOSY), gradient hetero-
nuclear single quantum coherence (gHSQC), and gradient hetero-
nuclear multiple bond correlation (gHMBC) experiments. Volatile
solvents were removed under reduced pressure using a rotary
evaporator. Flash column chromatography was performed using
RediSep silica gel (60 Å, 230−400 mesh, 35−70 μm). Ethyl acetate
and heptane were purchased from Fisher Chemical and used for
chromatography without further purification. NMR data were
collected with a Bruker 600-AVANCE-III spectrometer equipped
app = apparent. Melting points were determined using a Buchi
̈
Melting Point B-540 apparatus. IR spectra were recorded on a Bruker
Platinum ATR, Tensor 27 spectrometer. HRMS data were acquired
with a Bruker Daltonic MicroTOF with internal calibration using
electrospray ionization (ESI) in the positive mode. Optical rotations
were measured on an Anton Paar MCP 300 polarimeter. Preparative
supercritical fluid chromatography (SFC) was performed on a Berger
Multigram II operating at 50 mL/min at 35 °C and 100 bar
backpressure using stacked injections. The column was a Diacel IA
CHIRALPAK (25 cm, 5μ) (250 × 20 mm). The eluent was CO2
(95%) and ethanol + 0.1% diethylamine in ethanol (5%). UV
detection was performed at 230 nM. Enantiomeric excess (ee) was
determined on an Aurora Fusion A5/Agilent SFC system operating at
4 mL/min at 40 °C and 150 bar backpressure. The column was an
AD (3× AD 3 columns in serial for compound 6) 3μ, 15cm (150 ×
4.6 mm). The eluent was CO2 (95%) and 2-propanol (for compound
7) or ethanol (for compounds 6 and 1) + 0.1% diethylamine in
ethanol (10%). For the synthesis and characterization of compounds
5−21, and 1, see ref 10k.
(2S,13bS)-2,12-Dimethoxy-2,3,5,6,8,9-hexahydro-1H-indolo-
[7a,1-a]isoquinoline (+)-Cocculidine (3). An oven-dried 5 mL
microwave vial equipped with a stir bar was charged with enone
24′ + 24 (20.0 mg, 0.073 mmol) and methanol (1.5 mL). Copper (II)
bromide (33.0 mg, 0.15 mmol) and trimethylorthoformate (0.12 mL,
1.10 mmol) were added at room temperature. The vial was capped,
and the reaction mixture was heated to 80 °C (using an oil bath) and
stirred for 14 h. At this point, the starting material was completely
consumed and the intermediate bromo-ketal is primarily formed, as
indicated by TLC and LC−MS (see [M + 2]+ in Figure S1). The
reaction mixture was concentrated in vacuo and subsequently diluted
with DCM (1.5 mL). Boron trifluoride diethyl etherate (18.0 μL, 0.15
mmol) was added dropwise at 0 °C, and the reaction mixture was
slowly warmed to room temperature and stirred for 2 h. LC−MS at
this point shows complete conversion of the intermediate to the
desired mass (see [M + H]+ in Figure S2). Aqueous HCl (91.0 μL,
0.37 mmol, 4 M) was added dropwise, and the resulting solution was
stirred for 30 min at room temperature. The mixture was poured into
a separation funnel charged with saturated aqueous NaHCO3 (50
mL) and EtOAc (100 mL). The organic layer was separated, and the
aqueous layer was extracted with EtOAc (50 mL × 2), dried over
MgSO4, and concentrated in vacuo. The resulting crude material was
purified by column chromatography [EtOAc (w/10% MeOH and 5%
Et3N):heptane = 0:1 to 8:2], affording (+)-Cocculidine (3) (13.0 mg,
1
with a 5 mm TCI cryoprobe operating at 600 MHz for H and 151
MHz for 13C. Chemical shifts are reported in parts per million with
respect to the residual solvent signal CDCl3 (1H NMR: δ = 7.26; 13
C
NMR: δ = 77.16) or D2O (1H NMR: δ = 4.79) or CD3OD (1H
NMR: δ = 3.31; 13C NMR: δ = 49.00) or C6D6 (1H NMR: δ = 7.16;
13C NMR: δ = 128.06), as well as using tetramethylsilane as an
internal reference. Peak multiplicities are reported as follows: s =
singlet, d = doublet, t = triplet, q = quartet, p = pentet, dd = doublet
of doublets, td = triplet of doublets, qd = quartet of doublets, dt =
8254
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
1
1
0.046 mmol, 62%) as a white solid. H NMR (600 MHz, C6D6): δ
tentatively assigned. The H NMR shows a presence (less than 5%)
of the other epimer at C-12. The peaks of the major epimer are
reported.
6.97 (d, J = 2.8 Hz, 1H), 6.90 (d, J = 8.3 Hz, 1H), 6.67 (dd, J = 8.3,
2.7 Hz, 1H), 5.41−5.37 (m, 1H), 3.91 (dddd, J = 12.5, 8.7, 6.5, 4.1
Hz, 1H), 3.44 (ddd, J = 14.2, 10.8, 7.0 Hz, 1H), 3.38 (s, 3H), 3.01 (s,
3H), 2.90 (ddd, J = 14.2, 7.7, 2.1 Hz, 1H), 2.83 (td, J = 8.8, 3.2 Hz,
1H), 2.76 (ddd, J = 17.9, 10.8, 7.7 Hz, 1H), 2.63−2.56 (m, 2H), 2.52
(dd, J = 11.3, 4.0 Hz, 1H), 2.34−2.22 (m, 2H), 2.19−2.07 (m, 2H),
1.95 (t, J = 11.8 Hz, 1H). 13C{1H} NMR (151 MHz, C6D6): δ 157.9,
142.0, 140.6, 130.6, 126.0, 117.8, 114.5, 112.0, 74.1, 64.8, 55.7, 54.8,
47.5, 42.2, 41.4, 32.6, 27.7, 21.7. HRMS (ESI) m/z: [M + H]+ Calcd
for C18H24NO2, 286.1802; Found, 286.1803. Rf = 0.18 (silica gel, 7:1
(2S,9aR,13bS)-2-Methoxy-2,3,5,6,9,9a,10,11-octahydro-1H-
indolo[7a,1-a]isoquinolin-12(8H)-one (24′ + 24). An oven-dried 25
mL microwave vial equipped with a stir bar was charged with dione
23 (125 mg, 0.36 mmol) and water (7 mL). Potassium hydroxide
(100 mg, 1.79 mmol) in water (2.3 mL) was added at room
temperature. The vial was capped, and the reaction mixture was
heated to 105 °C (using an oil bath) and stirred for 2 h. At this point,
the starting material was completely consumed, as indicated by TLC.
Potassium hydroxide (300 mg, 5.37 mmol) in water (2.3 mL) was
added. The reaction mixture was heated to 105 °C (using an oil bath)
and stirred for another 3 h. At this point, the intermediate product
was completely consumed, as indicated by TLC. The mixture was
cooled to room temperature and subsequently poured into a
separation funnel charged with saturated aqueous NH4Cl (50 mL),
brine (50 mL), and EtOAc (100 mL). The organic layer was
separated, and the aqueous layer was extracted with EtOAc (100 mL
× 2), dried over MgSO4, and concentrated in vacuo. The resulting
crude material was purified by column chromatography [EtOAc (w/
10% MeOH and 5% Et3N)/heptane = 0:1 to 8:2], affording enone
24′ + 24 as an inseparable 1:2 diastereomeric mixture (76.0 mg, 0.28
mmol, 78% total yield) as a colorless oil. The crude diastereomeric
mixture was proceeded with the reactions below.
(2S,9aS,13bS)-2-Methoxy-2,3,5,6,9,9a,10,11-octahydro-1H-
indolo[7a,1-a]isoquinolin-12(8H)-one (24). An oven-dried 5 mL
microwave vial equipped with a stir bar was charged with enone 24′ +
24 (20.0 mg, 0.073 mmol) and chloroform (1.5 mL). HCl (0.37 mL,
0.46 mmol, 1.25 M) in water methanol was added at room
temperature. The vial was capped, and the reaction mixture was
stirred for 24 h at room temperature. At this point, only one
compound is visible, as indicated by crude 1H NMR. The mixture was
poured into a separation funnel charged with saturated aqueous
NaHCO3 (50 mL), brine (50 mL), and EtOAc (100 mL). The
organic layer was separated, and the aqueous layer was extracted with
EtOAc (100 mL × 2), dried over MgSO4, and concentrated in vacuo.
The resulting crude material was purified by column chromatography
[EtOAc (w/10% MeOH and 5% Et3N)/heptane = 0:1 to 8:2],
affording enone 24 in over 20:1 d.r. (14.8 mg, 0.054 mmol, 74%) as a
25
EtOAc/MeOH). [α]D = +255.9° (c 1.0, MeOH). >99% ee.
25
[Reference 1−[α]D = +260° (c 1.0, MeOH).] mp = 80−82 °C.
[Reference 26−m.p. 86−87]. The H and 13C NMR spectra are in
1
congruent with previously reported spectra.10b
The olifination reaction leading to 10 was optimized in the
following way.
1-(Tert-butyl) 2-Methyl (S)-2-(2,2-dimethoxyethyl)-3-methylene-
pyrrolidine-1,2-dicarboxylate (10). An oven-dried 250 mL round-
bottom flask equipped with a stir bar was charged with sodium
hydride (3.62 g, 91 mmol 60% w/w), methyltriphenylphosphonium
bromide (32.3 g, 90 mmol), and toluene (160 mL). The flask was
equipped with a reflux condenser, and the reaction mixture was
heated to 90 °C (using an oil bath) and stirred for 12 h. The reaction
mixture was allowed to reach room temperature, allowing the solids to
settle. The yellow supernatant was added to a toluene solution (75
mL) of acetal 9 (2.75 g, 8.3 mmol) in an oven-dried 500 mL round-
bottom flask, and the resulting solution was stirred at room
temperature for 12 h. At this point, the starting material was
completely consumed, as indicated by TLC, and the mixture was
poured into a separation funnel charged with water (200 mL) and
EtOAc (300 mL). The organic layer was separated, the aqueous layer
was extracted with EtOAc (150 mL × 3), and the combined organic
phase was dried over MgSO4 and concentrated in vacuo. The resulting
crude material was purified by column chromatography (EtOAc/
heptane = 0:1 to 7:3), affording olefin 10 (2.11 g, 6.42 mmol, 78%) as
a pale yellow oil. For analytical data of olein 10, see ref 10k.
Methyl (10S,11aS)-1-hydroxy-10-methoxy-3,4,7,9,10,11-hexahy-
dro-6H-pyrido[2,1-i]indole-2-carboxylate (18). The title compound
was synthesized, as previously described.10k
1
colorless oil. H NMR (600 MHz, CDCl3): δ 5.86−5.84 (m, 1H),
Methyl (10R,11aS)-1-hydroxy-10-methoxy-3,4,7,9,10,11-hexahy-
dro-6H-pyrido[2,1-i]indole-2-carboxylate (epi-18). The title com-
pound was synthesized, as previously described.10k
5.60−5.56 (m, 1H), 3.36−3.28 (m, 5H), 3.00 (dd, J = 14.5, 4.3 Hz,
1H), 2.98−2.91 (m, 1H), 2.89−2.85 (m, 1H), 2.77−2.71 (m, 1H),
2.57−2.50 (m, 2H), 2.45−2.29 (m, 4H), 2.18−2.12 (m, 1H), 1.99−
1.88 (m, 2H), 1.72−1.63 (m, 2H), 1.46 (t, J = 12.0 Hz, 1H).
13C{1H} NMR (151 MHz, CDCl3): δ 199.6, 164.9, 139.3, 127.2,
119.8, 73.8, 67.1, 56.1, 46.7, 43.1, 37.6, 36.5, 32.9, 31.8, 29.5, 28.7,
The enol-esters 18 and epi-18 were usually carried on without any
purification but were on occasion purified using column chromatog-
raphy [EtOAc (w/10% MeOH and 5% Et3N)/heptane = 0:1 to 8:2].
Methyl (2R,10S,11aS)-10-methoxy-1-oxo-2-(3-oxobutyl)-
1,2,3,4,7,9,10,11-octahydro-6H-pyrido[2,1-i]indole-2-carboxylate
(23). An oven-dried 25 mL microwave vial equipped with a stir bar
was charged with enol-ester 18 (100 mg, 0.36 mmol) and methanol
(7 mL). Et3N (0.25 mL, 1.79 mmol) and methyl vinyl ketone (0.15
mL, 1.79 mmol) were added at room temperature. The vial was
capped, and the reaction mixture was heated to 70 °C (using an oil
bath) and stirred for 14 h. At this point, the starting material was
completely consumed, as indicated by TLC, and the mixture was
cooled to room temperature and concentrated in vacuo. The resulting
crude material was purified by column chromatography [EtOAc (w/
10% MeOH and 5% Et3N)/heptane = 0:1 to 8:2], affording dione 23
26.7. HRMS (ESI) m/z: [M + H]+ Calcd for C17H24NO2, 274.1802;
25
Found, 274.1802. Rf = 0.26 (silica gel, 6:2 EtOAc/MeOH). [α]D
+85.6° (c 0.5, CHCl3). >99% ee.
=
(2S,13bS)-2-Methoxy-2,3,5,6,8,9-Hexahydro-1H-indolo-
[7a,1-a]isoquinolin-12-ol (+)-Cocculine (25). An oven-dried 5
mL microwave vial equipped with a stir bar was charged with enone
24 + 24′ (20.0 mg, 0.073 mmol) and acetonitrile (1.5 mL).
Copper(II) bromide (65.0 mg, 0.29 mmol) was added at room
temperature. The vial was capped, and the reaction mixture was
heated to 40 °C (using an oil bath) and stirred for 5 h. At this point,
the starting material was completely consumed, as indicated by TLC
and LC−MS [see [M + H]+ in Figure S3]. The reaction mixture was
concentrated in vacuo directly purified by column chromatography
[EtOAc (w/10% MeOH and 5% Et3N)/heptane = 0:1 to 8:2],
affording (+)-Cocculine (25) (12.0 mg, 0.044 mmol, 60%) as a white
solid. 1H NMR (600 MHz, CDCl3): δ 7.01 (d, J = 8.3 Hz, 1H), 6.84
(dd, J = 8.3, 2.5 Hz, 1H), 6.66 (d, J = 2.6 Hz, 1H), 5.80−5.77 (m,
1H), 3.81−3.75 (m, 1H), 3.69 (ddd, J = 14.1, 10.0, 7.2 Hz, 1H),
3.43−3.37 (m, 1H), 3.30−3.23 (m, 4H), 3.00 (ddd, J = 17.7, 10.3, 6.3
Hz, 1H), 2.93−2.79 (m, 2H), 2.78−2.70 (m, 1H), 2.63−2.55 (m,
1H), 2.38−2.28 (m, 2H), 2.19−2.12 (m, 1H), 2.08−2.02 (m, 1H).
13C{1H} NMR (151 MHz, methanol-D4): δ 155.7, 136.7, 134.8,
1
(125 mg, 0.36 mmol, >99%) as a colorless oil. H NMR (600 MHz,
CDCl3): δ 5.64−5.60 (m, 1H), 3.68 (s, 3H), 3.54−3.44 (m, 2H), 3.29
(s, 3H), 3.09 (td, J = 9.3, 5.7 Hz, 1H), 3.05−2.98 (m, 2H), 2.87 (ddd,
J = 14.3, 9.7, 6.0 Hz, 1H), 2.72−2.60 (m, 2H), 2.41−2.28 (m, 4H),
2.25−2.21 (m, 2H), 2.13 (s, 3H), 1.97−1.89 (m, 1H), 1.81 (dt, J =
14.3, 4.6 Hz, 1H), 1.45 (t, J = 12.2 Hz, 1H). 13C{1H} NMR (151
MHz, CDCl3): δ 211.0, 207.4, 172.2, 138.6, 120.8, 73.4, 71.9, 58.7,
56.3, 52.5, 48.9, 40.3, 39.2, 36.8, 31.5, 30.3, 30.1, 29.0, 28.1. HRMS
(ESI) m/z: [M + H]+ Calcd for C19H28NO5, 350.1962; Found,
25
350.1965. Rf = 0.72 (silica gel, 7:1 EtOAc/MeOH). [α]D = +16.3°
(c 1.0, CHCl3). >99% ee. The stereochemistry could not be
established through 2D NMR, and the structure is therefore
130.5, 121.9, 121.4, 115.5, 113.3, 73.0, 67.1, 55.2, 47.3, 40.9, 38.9,
8255
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
31.2, 25.9, 20.6. HRMS (ESI) m/z: [M + H]+ Calcd for C17H22NO2,
272.1645; Found, 272.1645. Rf = 0.15 (silica gel, 7:1 EtOAc/
MeOH). [α]D25 = +215.5° (c 1.0, MeOH). >99% ee. (Reference 27)−
NaHCO3 (100 mL) and 2-methyl-THF (100 mL). The organic layer
was separated, the aqueous layer was extracted with EtOAc (100 mL
× 2), and the combined organic phase was washed with brine (200
mL), dried over MgSO4, and concentrated in vacuo. The resulting
crude material was purified by column chromatography [EtOAc (w/
10% MeOH and 5% Et3N)/heptane = 0:1 to 8:2], affording ketone
29 (435 mg, 1.97 mmol, 66%) as a colorless oil. 1H NMR (600 MHz,
CDCl3): δ 5.62−5.57 (m, 1H), 3.45 (ddd, J = 14.6, 13.3, 3.4 Hz, 1H),
3.32 (s, 3H), 3.31−3.26 (m, 1H), 3.03−2.94 (m, 3H), 2.80 (td, J =
13.8, 6.8 Hz, 1H), 2.67 (ddd, J = 12.0, 3.4, 0.9 Hz, 1H), 2.57 (tddd, J
= 18.2, 13.5, 8.2, 4.4 Hz, 2H), 2.41 (ddt, J = 8.7, 6.0, 2.7 Hz, 2H),
2.34 (ddt, J = 14.2, 4.6, 2.0 Hz, 1H), 2.05−2.00 (m, 1H), 1.93 (ddq, J
= 17.2, 9.3, 3.2 Hz, 1H), 1.54 (t, J = 12.2 Hz, 1H). 13C{1H} NMR
(151 MHz, CDCl3): δ 214.6, 137.7, 119.7, 73.6, 73.6, 56.2, 47.3, 43.0,
37.2, 36.4, 31.8, 26.9, 25.9. HRMS (ESI) m/z: [M + H]+ Calcd for
C13H20NO2, 222.1489; Found, 222.1494. Rf = 0.21 (silica gel, 7:1
25
[α]D = +217−218° (c 1.0, MeOH). mp = 207−212 °C (Reference
27)−m.p. 216−217).
(2S,12bS)-2-Methoxy-2,3,5,6,9,11-hexahydro-1H,8H-pyrazolo-
[3′,4′:3,4]pyrido[2,1-i]indol-10-ol (27). An oven-dried 5 mL micro-
wave vial equipped with a stir bar was charged with crude enol-ester
18 (22.0 mg, 0.079 mmol) and 1,4-dioxane (1 mL). Hydrazine-
hydrate (64% wt, 60 μL, 0.79 mmol) was added at room temperature.
The vial was capped, and the reaction mixture was heated to 100 °C
(using an oil bath) and stirred for 72 h. At this point, the starting
material was completely consumed, as indicated by TLC. The
reaction mixture was concentrated in vacuo directly purified by
column chromatography (MeOH/EtOAc = 0:1 to 1:1), affording
1
pyrazole 27 (6.00 mg, 0.02 mmol, 30%) as a clear colorless oil. H
25
NMR (600 MHz, CDCl3): δ 5.57−5.54 (m, 1H), 3.75−3.69 (m, 1H),
3.31 (s, 3H), 3.24−3.20 (m, 2H), 2.94 (td, J = 9.2, 3.2 Hz, 1H),
2.75−2.65 (m, 2H), 2.63−2.58 (m, 1H), 2.47−2.39 (m, 2H), 2.28−
2.22 (m, 2H), 2.09−2.03 (m, 1H), 1.53 (t, J = 11.8 Hz, 1H).
13C{1H} NMR (151 MHz, CDCl3): δ 160.9, 143.7, 139.1, 119.1,
97.5, 74.2, 61.9, 56.3, 46.2, 40.9, 40.2, 31.8, 27.1, 14.0. HRMS (ESI)
EtOAc/MeOH). [α]D = +11.7° (c 1.0, CHCl3). >99% ee.
(10R,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-
i]indol-1(2H)-one (epi 29). An oven-dried 100 mL flask equipped
with a stir bar was charged with bis-ester methyl (6R,7aS)-6-methoxy-
1-(4-methoxy-4-oxobutyl)-1,2,3,5,6,7-hexahydro-7aH-indole-7a-car-
boxylate (191 mg, 0.61 mmol) and toluene (7 mL). Potassium tert-
butoxide (158 mg, 1.41 mmol) was added at room temperature, the
vial was equipped with a reflux condenser, and the reaction mixture
was heated to 95 °C (using an oil bath) and stirred for 1 h. At this
point, the starting material was completely consumed, as indicated by
TLC, and the mixture was cooled to room temperature and was
poured into a separation funnel charged with saturated aqueous
NH4Cl (50 mL), brine (50 mL), and 2-methyl-THF (100 mL). The
organic layer was separated, the aqueous layer was extracted with
EtOAc (100 mL × 2), and the combined organic phase was washed
with brine (200 mL), dried over MgSO4, and concentrated in vacuo.
To an oven-dried 100 mL flask equipped with a stir bar were added
the resulting crude oil and 3N aqueous HCl (15 mL). The flask was
equipped with a reflux condenser under argon, and the reaction
mixture was heated to 110 °C (using an oil bath) for 3 h. At this
point, the starting material was completely consumed, as indicated by
TLC, and the mixture was poured into a separation funnel charged
with saturated aqueous NaHCO3 (50 mL) and 2-methyl-THF (50
mL). The organic layer was separated, the aqueous layer was extracted
with EtOAc (50 mL × 2), and the combined organic phase was
washed with brine (100 mL), dried over MgSO4, and concentrated in
vacuo. The resulting crude material was purified by column
chromatography [EtOAc (w/10% MeOH and 5% Et3N)/heptane =
0:1 to 8:2], affording C3-epi-ketone 29 (74 mg, 0.34 mmol, 55%) as a
+
m/z: [M + H]+ Calcd for C14H20N3O2 , 262.1550; Found, 262.1555
25
Rf = 0.28 (silica gel, 1:1 EtOAc/MeOH) [α]D = +203.8° (c 0.5,
CHCl3).
(2S,13bS)-12-Mercapto-2-methoxy-2,3,5,6,8,9-hexahydro-1H-
pyrimido[4′,5′:3,4]pyrido[2,1-i]indol-10-ol (28). An oven-dried 5 mL
microwave vial equipped with a stir bar was charged with crude enol-
ester 18 (60.0 mg, 0.21 mmol), thiourea (41.0 mg, 0.54 mmol), and
potassium tert-butoxide (120 mg, 0.54 mmol). MeOH (1.6 mL) was
added at room temperature. The vial was capped, and the reaction
mixture was heated to 90 °C (using an oil bath) and stirred for 12 h.
At this point, the starting material was almost completely consumed,
as indicated by TLC. The solution was poured into a separation
funnel containing brine (20 mL) and 2-methyl-THF (20 mL). The
organic layer was separated, and the pH of the water phase was
adjusted to 7, using 1 M HCl before extracting it with 2-methyl-THF
(3 × 20 mL). The combined organic phases were dried over MgSO4
and concentrated in vacuo. The resulting crude was purified by
column chromatography (MeOH/EtOAc = 0:1 to 1:1), affording
1
pyrimidine 27 (3.30 mg, 0.001 mmol, 5%) as a clear colorless oil. H
NMR (600 MHz, CDCl3): δ 8.32 (s, 1H), 5.87−5.83 (m, 1H), 3.69−
3.62 (m, 1H), 3.38 (s, 3H), 3.36−3.26 (m, 2H), 3.18 (dd, J = 14.8,
7.6 Hz, 1H), 3.05−3.00 (m, 1H), 2.82−2.71 (m, 2H), 2.66−2.58 (m,
1H), 2.56−2.49 (m, 1H), 2.46−2.35 (m, 2H), 2.29 (dd, J = 18.4, 6.6
Hz, 1H), 2.18−2.11 (m, 1H), 1.66 (t, J = 12.3 Hz, 1H). 13C{1H}
NMR (151 MHz, CDCl3): δ 173.7, 161.1, 150.0, 136.7, 123.1, 110.1,
72.8, 62.6, 56.6, 46.7, 40.6, 39.3, 31.6, 27.1, 15.3. HRMS (ESI) m/z:
[M + H]+ Calcd for C15H20N3O2S+, 306.1271; Found, 306.1271 Rf =
0.36 (silica gel, 8:2 EtOAc/MeOH) [α]D25 = +184.8° (c 0.3, CHCl3).
(10S,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-
i]indol-1(2H)-one (29). An oven-dried 100 mL flask equipped with a
stir bar was charged with bis-ester 17 (920 mg, 2.96 mmol) and
toluene (33 mL). Potassium tert-butoxide (763 mg, 6.80 mmol) was
added at room temperature, the vial was equipped with a reflux
condenser, and the reaction mixture was heated to 95 °C (using an oil
bath) and stirred for 1 h. At this point, the starting material was
completely consumed, as indicated by TLC, and the mixture was
cooled to room temperature and was poured into a separation funnel
charged with saturated aqueous NH4Cl (50 mL), brine (50 mL), and
2-methyl-THF (100 mL). The organic layer was separated, the
aqueous layer was extracted with EtOAc (100 mL × 2), and the
combined organic phase was washed with brine (200 mL), dried over
MgSO4, and concentrated in vacuo. To an oven-dried 100 mL flask
equipped with a stir bar were added the resulting crude oil and 4N
aqueous HCl (29 mL). The flask was equipped with a reflux
condenser under argon, and the reaction mixture was heated to 110
°C (using an oil bath) for 3 h. At this point, the starting material was
completely consumed, as indicated by TLC, and the mixture was
poured into a separation funnel charged with saturated aqueous
1
colorless oil. H NMR (600 MHz, CDCl3): δ 5.56−5.54 (m, 1H),
3.68−3.65 (m, 1H), 3.42 (td, J = 13.9, 3.5 Hz, 1H), 3.23 (s, 3H),
2.94−2.86 (m, 5H), 2.61−2.50 (m, 2H), 2.42 (dddd, J = 18.5, 10.9,
6.2, 2.2 Hz, 1H), 2.37−2.32 (m, 1H), 2.28−2.18 (m, 2H), 2.07−2.02
(m, 1H), 1.67 (dd, J = 13.4, 2.9 Hz, 1H). 13C{1H} NMR (151 MHz,
CDCl3): δ 212.2, 138.7, 118.0, 73.7, 69.6, 56.6, 46.3, 43.2, 37.3, 35.7,
29.6, 28.8, 27.6. HRMS (ESI) m/z: [M + H]+ Calcd for C13H20NO2,
222.1489; found, 222.1489. Rf = 0.3 (silica gel, 7:1 EtOAc/MeOH).
25
[α]D = +88.6° (c 0.9, CHCl3). >99% ee.
(10S,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-
i]indol-1-yl Trifluoromethanesulfonate (30). An oven-dried 25 mL
microwave vial equipped with a stir bar was charged with ketone 29
(100 mg, 0.46 mmol) and THF (3.8 mL). The mixture was cooled to
−78 °C before KHMDS (1.80 mL, 0.90 mmol, 0.5 M) was added.
The resulting mixture was left stirring at the same temperature for 1.5
h. PhN(OTf)2 (300 mg, 0.84 mmol) in 1 mL THF was added
dropwise before the dry ice bath was replaced with an ice bath. The
mixture was allowed to warm to 0 °C and was stirred at this
temperature for 2 h. At this point, the starting material was completely
consumed, as indicated by TLC, and the mixture was quenched with
ice-cold water and was subsequently poured into a separation funnel
charged with saturated aqueous NH4Cl (50 mL), brine (50 mL), and
EtOAc (100 mL). The organic layer was separated, the aqueous layer
was extracted with EtOAc (100 mL × 2), and the combined organic
phase was washed with brine (200 mL), dried over MgSO4, and
8256
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
concentrated in vacuo. The resulting crude material was purified by
column chromatography [EtOAc (w/10% MeOH and 5% Et3N)/
heptane = 0:1 to 8:2], affording alkenyl triflate 30 (131 mg, 0.37
208.1332; Found, 208.1338. Rf = 0.11 (silica gel, 7:1 EtOAc/
MeOH). [α]D = +11.6° (c 0.5, CHCl3). >99% ee.
25
General Procedure for the Suzuki Coupling. An oven-dried 5
mL microwave vial equipped with a stir bar was charged with alkenyl
triflate 30 or C3-epi-30 (20.0 mg, 0.06 mmol), Pd(dppf)Cl2 DCM
(4.60 mg, 0.006 mmol), and boronic acid or boronic ester (0.10
mmol, 1.7 equiv). Degassed DME (0.6 mL) and NaOH (0.05 mL, 2
M) were added, and the resulting solution was further degassed for 10
min. The vial was capped, and the reaction mixture was submerged in
an oil bath that was preheated to 85 °C. The reaction mixture was
stirred for 75 min. At this point, the starting material was completely
consumed, as indicated by TLC. The mixture was cooled to room
temperature and concentrated in vacuo. The resulting crude material
was directly purified by column chromatography (EtOAc/MeOH =
1:0 to 4:6), affording Suzuki-adducts 32−43 and epi-32−epi-43 as
pale yellow oils. Note: The same elution gradient was used for the
purification of compounds 32−43.
1
mmol, 82%) as a colorless oil. H NMR (600 MHz, CDCl3): δ 5.75
(dd, J = 5.2, 2.8 Hz, 1H), 5.71−5.68 (m, 1H), 3.87 (dddd, J = 12.4,
8.0, 6.7, 4.4 Hz, 1H), 3.37 (s, 3H), 3.18 (ddd, J = 14.6, 11.7, 5.7 Hz,
1H), 3.02−2.90 (m, 3H), 2.76−2.56 (m, 3H), 2.50−2.41 (m, 2H),
2.12 (dt, J = 18.7, 5.4 Hz, 1H), 1.98 (ddtd, J = 17.7, 7.6, 3.6, 2.4 Hz,
1H), 1.52 (t, J = 12.0 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ
152.1, 136.1, 123.3, 118.4 (q, J = 319.7 Hz), 116.8, 73.2, 63.9, 56.4,
46.6, 41.4, 39.8, 31.7, 27.2, 21.0. HRMS (ESI) m/z: [M + H]+ Calcd
for C14H19F3NO4S, 354.0981; Found, 354.0980. Rf = 0.25 (silica gel,
7:1 EtOAc/MeOH). [α]D25 = +58.4° (c 1.0, CHCl3). >99% ee.
(10R,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-
i]indol-1-yl Trifluoromethanesulfonate (epi-30). An oven-dried 5
mL microwave vial equipped with a stir bar was charged with epi-29
(33.0 mg, 0.15 mmol) and THF (1 mL). The mixture was cooled to
−78 °C before KHMDS (0.60 mL, 0.30 mmol, 0.5 M) was added.
The resulting mixture was left stirring at the same temperature for 1.5
h. PhN(OTf)2 (80.0 mg, 0.22 mmol) in 0.5 mL THF was added
dropwise before the dry ice bath was replaced with an ice bath. The
mixture was allowed to warm to 0 °C and was stirred at this
temperature for 2 h. At this point, the starting material was completely
consumed, as indicated by TLC, and the mixture was quenched with
ice-cold water and was subsequently poured into a separation funnel
charged with saturated aqueous NH4Cl (50 mL), brine (50 mL), and
EtOAc (100 mL). The organic layer was separated, the aqueous layer
was extracted with EtOAc (100 mL × 2), and the combined organic
phase was washed with brine (200 mL), dried over MgSO4, and
concentrated in vacuo. The resulting crude material was purified by
column chromatography [EtOAc (w/10% MeOH and 5% Et3N)/
heptane = 0:1 to 8:2], affording C3-epi-alkenyl triflate 30 (44.0 mg,
3-((10S,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-pyrido-
[2,1-i]indol-1-yl)benzonitrile (32). The title compound was derived
from 30 and (3-cyanophenyl)boronic acid to give the desired product
1
(15.0 mg, 0.049 mmol, 86%) H NMR (600 MHz, CDCl3): δ 7.54−
7.51 (m, 1H), 7.42−7.41 (m, 1H), 7.37−7.34 (m, 2H), 5.73−5.69
(m, 1H), 5.61−5.58 (m, 1H), 3.29 (ddd, J = 14.4, 11.8, 5.9 Hz, 1H),
3.09−3.03 (m, 6H), 2.76 (dddd, J = 12.5, 8.8, 6.2, 3.9 Hz, 1H), 2.64−
2.53 (m, 3H), 2.41 (ddd, J = 11.6, 4.0, 0.9 Hz, 1H), 2.10−2.05 (m,
1H), 1.98 (dt, J = 19.2, 5.4 Hz, 1H), 1.82 (ddtd, J = 17.6, 8.9, 3.5, 2.0
Hz, 1H), 1.51 (t, J = 11.9 Hz, 1H). 13C{1H} NMR (151 MHz,
CDCl3): δ 145.0, 141.1, 138.8, 133.5, 132.5, 130.5, 128.8, 128.1,
121.7, 119.0, 112.0, 73.7, 64.6, 56.1, 46.1, 42.8, 40.0, 31.7, 28.2, 19.6.
HRMS (ESI) m/z: [M + H]+ Calcd for C20H23N2O, 307.1805;
Found, 307.1807. Rf = 0.32 (silica gel, 6:2 EtOAc/MeOH). [α]D25
+97.6° (c 1.0, CHCl3). >99% ee.
=
1
0.13 mmol, 84%) as a colorless oil. H NMR (600 MHz, CDCl3): δ
3-((10R,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-pyrido-
[2,1-i]indol-1-yl)benzonitrile (epi-32). The title compound was
derived from epi-30 and (3-cyanophenyl)boronic acid to give the
5.79 (dtd, J = 7.9, 2.5, 1.3 Hz, 1H), 5.76 (dd, J = 5.6, 2.5 Hz, 1H),
3.35 (s, 4H), 3.32 (tdd, J = 10.2, 5.8, 4.6 Hz, 1H), 3.14 (ddd, J = 14.5,
11.5, 5.2 Hz, 1H), 3.00 (td, J = 9.2, 5.5 Hz, 1H), 2.96−2.90 (m, 2H),
2.73 (dddd, J = 18.1, 11.5, 6.3, 2.5 Hz, 1H), 2.67−2.60 (m, 1H),
2.47−2.41 (m, 1H), 2.40−2.34 (m, 1H), 2.27 (ddd, J = 14.0, 5.8, 2.1
Hz, 1H), 2.09−2.04 (m, 1H), 2.04−1.97 (m, 1H), 1.74 (dd, J = 14.0,
10.2 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ 152.2, 137.8,
122.5, 118.3 (q, J = 319.6 Hz), 115.9, 75.0, 63.7, 56.2, 46.5, 39.8, 38.1,
29.4, 27.3, 20.1. HRMS (ESI) m/z: [M + H]+ Calcd for
C14H19F3NO4S, 354.0981; Found, 354.0980. Rf = 0.25 (silica gel,
7:1 EtOAc/MeOH). [α]D25 = +46.3° (c 1.0, CHCl3). >99% ee. Note:
It was not possible to remove all traces of solvent from this compound
without significant decomposition.
1
desired product (6.00 mg, 0.02 mmol, 30%) H NMR (600 MHz,
CDCl3): δ 7.51 (d, J = 7.7 Hz, 1H), 7.45 (s, 1H), 7.40 (d, J = 7.8 Hz,
1H), 7.36−7.30 (m, 1H), 5.74−5.70 (m, 1H), 5.60 (d, J = 6.8 Hz,
1H), 3.31−3.22 (m, 5H), 3.07−2.99 (m, 3H), 2.67−2.51 (m, 2H),
2.44 (dt, J = 14.8, 7.1 Hz, 1H), 2.29 (dd, J = 14.4, 5.4 Hz, 1H), 2.23−
2.16 (m, 1H), 1.90 (dt, J = 18.9, 5.4 Hz, 1H), 1.50−1.43 (m, 1H),
1.09−1.01 (m, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ 143.0,
141.9, 141.0, 133.9, 132.9, 130.5, 128.5, 127.3, 120.4, 119.2, 111.7,
75.2, 64.9, 56.3, 46.2, 39.9, 38.1, 29.6, 28.1, 18.7. HRMS (ESI) m/z:
[M + H]+ Calcd for C20H23N2O, 307.1805; Found, 307.1805. Rf =
0.19 (silica gel, 6:2 EtOAc/MeOH). [α]D25 = +47.2° (c 0.55,
CHCl3). >99% ee.
Methyl (10R,11aS)-1-Hydroxy-10-methoxy-3,4,7,9,10,11-hexahy-
dro-6H-pyrido[2,1-i]indole-2-carboxylate (31). To an oven-dried 25
mL microwave vial equipped with a stir bar were added epi-18 (95.0
mg, 0.34 mmol) and 6N aqueous HCl (10 mL). The vial was capped,
and the reaction mixture was submerged in an oil bath that was
preheated to 115 °C (using an oil bath) and stirred for 16 h. At this
point, the starting material was completely consumed, as indicated by
TLC, and the mixture was poured into a separation funnel charged
with saturated aqueous NaHCO3 (50 mL) and 2-methyl-THF (50
mL). The organic layer was separated, the aqueous layer was extracted
with EtOAc (50 mL × 2), and the combined organic phase was
washed with brine (100 mL), dried over MgSO4, and concentrated in
vacuo. The resulting crude material was purified by column
chromatography [EtOAc (w/10% MeOH and 5% Et3N)/heptane =
0:1 to 8:2], affording lactol 31 (21.0 mg, 0.10 mmol, 30%) as a
(10S,11aS)-1-(3-Fluorophenyl)-10-methoxy-3,4,7,9,10,11-hexa-
hydro-6H-pyrido[2,1-i]indole (33). The title compound was derived
from 30 and (3-fluorophenyl)boronic acid to give the desired product
1
(9.00 mg, 0.050 mmol, 53%) H NMR (600 MHz, CDCl3): δ 7.20
(m, 1H), 6.95−6.89 (m, 2H), 6.84 (dt, J = 10.3, 2.1 Hz, 1H), 5.70
(dd, J = 5.2, 2.6 Hz, 1H), 5.60−5.58 (m, 1H), 3.29 (ddd, J = 14.3,
11.9, 6.0 Hz, 1H), 3.12−3.04 (m, 6H), 2.87 (tdd, J = 12.5, 6.2, 3.9 Hz,
1H), 2.68−2.52 (m, 3H), 2.42 (dd, J = 11.6, 3.9 Hz, 1H), 2.14−2.07
(m, 1H), 1.98 (dt, J = 19.1, 5.5 Hz, 1H), 1.87−1.79 (m, 1H), 1.52 (t,
J = 11.8 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ 162.4 (d, J =
245.5 Hz), 145.8 (d, J = 7.6 Hz), 141.5, 138.7, 129.3 (d, J = 8.5 Hz),
126.7, 124.9 (d, J = 2.7 Hz), 121.7, 115.9 (d, J = 21.3 Hz), 113.7 (d, J
= 21.1 Hz), 73.7, 64.9, 56.0, 46.1, 42.6, 40.1, 31.7, 28.1, 19.6. HRMS
(ESI) m/z: [M + H]+ Calcd for C19H23NOF, 300.1758; Found,
1
25
colorless oil. H NMR (600 MHz, CDCl3): δ 5.61−5.58 (m, 1H),
300.1760. Rf = 0.29 (silica gel, 7:1 EtOAc/MeOH). [α]D = +88.9°
4.32 (dtd, J = 5.5, 2.6, 1.3 Hz, 1H), 3.29 (dt, J = 10.3, 7.9 Hz, 1H),
3.07−2.97 (m, 2H), 2.91 (td, J = 8.3, 1.2 Hz, 1H), 2.74 (s, 1H), 2.68
(dd, J = 10.6, 5.8 Hz, 1H), 2.65−2.56 (m, 1H), 2.54−2.48 (m, 1H),
2.44−2.30 (m, 2H), 2.08 (dtd, J = 13.4, 3.3, 1.5 Hz, 1H), 1.95 (tddd, J
= 13.8, 12.2, 4.7, 3.4 Hz, 1H), 1.73 (td, J = 13.7, 4.0 Hz, 1H), 1.50 (d,
J = 10.6 Hz, 1H), 1.31−1.24 (m, 1H). 13C{1H} NMR (151 MHz,
CDCl3): δ 145.2, 117.1, 102.9, 72.8, 67.9, 49.2, 44.5, 36.6, 35.6, 35.1,
28.3, 16.6. HRMS (ESI) m/z: [M + H]+ Calcd for C13H20NO2,
(c 0.9, CHCl3). >99% ee.
(10S,11aS)-1-(3-(Tert-Butyl)phenyl)-10-methoxy-3,4,7,9,10,11-
hexahydro-6H-pyrido[2,1-i]indole (34). The title compound was
derived from 30 and (3-(tert-butyl)phenyl)boronic acid to give the
1
desired product (10.0 mg, 0.030 mmol, 52%) H NMR (600 MHz,
CDCl3): δ 7.24 (ddd, J = 7.8, 2.1, 1.1 Hz, 1H), 7.17 (t, J = 7.7 Hz,
1H), 7.13 (t, J = 1.9 Hz, 1H), 6.92 (dt, J = 7.4, 1.4 Hz, 1H), 5.69
(ddd, J = 5.1, 2.6, 1.0 Hz, 1H), 5.55 (dd, J = 3.7, 2.2 Hz, 1H), 3.31
8257
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
(ddd, J = 14.3, 11.9, 6.0 Hz, 1H), 3.13 (ddd, J = 10.4, 9.1, 6.5 Hz,
1H), 3.11−3.02 (m, 5H), 2.78 (tdd, J = 12.5, 6.3, 3.9 Hz, 1H), 2.72−
2.64 (m, 1H), 2.57 (dddd, J = 18.9, 11.7, 7.1, 2.6 Hz, 2H), 2.44 (dd, J
= 11.4, 3.9 Hz, 1H), 2.06−2.00 (m, 1H), 1.97 (dt, J = 18.9, 5.4 Hz,
1H), 1.79 (ddtd, J = 17.4, 8.7, 3.6, 1.7 Hz, 1H), 1.45 (t, J = 11.8 Hz,
1H), 1.29 (s, 9H). 13C{1H} NMR (151 MHz, CDCl3): δ 150.5,
143.1, 143.0, 139.5, 127.6, 126.3, 126.2, 125.3, 123.6, 121.0, 73.8,
65.0, 56.0, 46.2, 42.7, 40.3, 34.7, 31.9, 31.4, 28.2, 19.7. HRMS (ESI)
m/z: [M + H]+ Calcd for C23H32NO, 338.2478; Found, 338.2477. Rf
= 0.34 (silica gel, 7:1 EtOAc/MeOH). [α]D25 = +105.0° (c 1.0,
CHCl3). >99% ee.
(10R,11aS)-1-(3-(Tert-Butyl)phenyl)-10-methoxy-3,4,7,9,10,11-
hexahydro-6H-pyrido[2,1-i]indole (epi-34). The title compound was
derived from epi-30 and (3-(tert-butyl)phenyl)boronic acid to give
the desired product (8.80 mg, 0.026 mmol, 42% crude yield) 1H
NMR (600 MHz, CDCl3): δ 7.24 (d, J = 0.9 Hz, 1H), 7.21−7.14 (m,
2H), 6.98 (dt, J = 7.6, 1.4 Hz, 1H), 5.74 (dd, J = 5.5, 2.3 Hz, 1H),
5.64 (d, J = 8.0 Hz, 1H), 3.37−3.30 (m, 2H), 3.28 (s, 3H), 3.22−3.08
(m, 3H), 2.71 (tdd, J = 13.4, 6.4, 3.3 Hz, 1H), 2.59 (dddd, J = 18.7,
11.8, 6.8, 2.3 Hz, 1H), 2.48 (ddd, J = 14.6, 8.0, 4.9 Hz, 1H), 2.34 (dd,
J = 14.2, 3.6 Hz, 1H), 2.17 (dddd, J = 11.9, 7.7, 4.0, 2.3 Hz, 1H), 2.00
(dt, J = 18.7, 5.5 Hz, 1H), 1.61 (dd, J = 14.3, 11.0 Hz, 1H), 1.28 (s,
9H), 1.15−1.07 (m, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ
150.3, 142.8, 140.3, 139.7, 127.6, 126.8, 126.0, 124.8, 124.1, 121.5,
74.9, 66.1, 56.3, 46.2, 40.0, 37.1, 34.7, 31.4, 29.3, 27.9, 18.9. HRMS
(ESI) m/z: [M + H]+ Calcd for C23H32NO, 338.2478; Found,
338.2482. Rf = 0.29 (silica gel, 7:1 EtOAc/MeOH). [α]D25 = +60.9°
(c 0.09, CHCl3). >99% ee. Note: NMR contains 10% of an unknown
impurity.
J = 17.4, 8.6, 3.0 Hz, 1H), 1.53 (t, J = 12.0 Hz, 1H). 13C{1H} NMR
(151 MHz, CDCl3): δ 165.8, 149.7, 146.1, 141.0, 139.0, 138.7, 136.9,
129.5, 124.3, 121.9, 73.7, 64.6, 56.2, 53.1, 46.2, 42.7, 39.9, 31.7, 28.1,
19.7. HRMS (ESI) m/z: [M + H]+ Calcd for C20H25N2O3, 341.1860;
25
Found, 341.1864. Rf = 0.11 (silica gel, 6:2 EtOAc/MeOH). [α]D
+80.3° (c 0.5, CHCl3). >99% ee.
=
Methyl 5-((10R,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-
pyrido[2,1-i]indol-1-yl)picolinate (epi-36). The title compound was
derived from epi-30 and (6-(methoxycarbonyl)pyridin-3-yl)boronic
1
acid to give the desired product (2.70 mg, 0.008 mmol, 13%) H
NMR (600 MHz, CDCl3): δ 8.51 (d, J = 2.3 Hz, 1H), 8.01 (d, J = 8.0
Hz, 1H), 7.66 (dd, J = 8.1, 2.2 Hz, 1H), 5.81−5.79 (m, 1H), 5.61−
5.58 (m, 1H), 4.00 (s, 3H), 3.34−3.24 (m, 1H), 3.23 (s, 3H), 3.07−
3.03 (m, 3H), 2.65−2.54 (m, 2H), 2.48−2.42 (m, 1H), 2.29 (dd, J =
14.3, 5.4 Hz, 1H), 2.23−2.18 (m, 1H), 1.94 (dt, J = 19.1, 5.4 Hz, 1H),
1.50 (dd, J = 14.2, 10.4 Hz, 1H), 1.13−1.08 (m, 1H). 13C{1H} NMR
(151 MHz, CDCl3): δ 165.9, 150.1, 146.2, 141.0, 140.8, 139.8, 137.1,
128.4, 124.2, 120.6, 74.9, 64.6, 56.2, 53.0, 46.1, 39.8, 38.3, 29.7, 28.1,
18.7. HRMS (ESI) m/z: [M + H]+ Calcd for C20H25N2O3, 341.1860;
25
Found, 341.1867. Rf = 0.10 (silica gel, 6:2 EtOAc/MeOH). [α]D
=
not enough material for a reliable measurement.
(10S,11aS)-10-Methoxy-1-(1-methyl-1H-pyrazol-4-yl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (37). The title com-
pound was derived from 30 and (1-methyl-1H-pyrazol-4-yl)boronic
1
acid to give the desired product (12.0 mg, 0.042 mmol, 74%) H
NMR (600 MHz, CDCl3): δ 7.35 (s, 1H), 7.21 (s, 1H), 5.72 (t, J =
3.8 Hz, 1H), 5.61 (dt, J = 4.0, 1.9 Hz, 1H), 3.84 (s, 3H), 3.31−3.21
(m, 2H), 3.16 (s, 3H), 3.06−2.96 (m, 3H), 2.56−2.46 (m, 3H), 2.39
(dd, J = 11.3, 4.2 Hz, 1H), 2.34−2.29 (m, 1H), 1.96−1.90 (m, 2H),
1.49 (t, J = 11.8 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ
140.2, 138.3, 132.5, 127.9, 124.3, 124.2, 120.4, 74.1, 64.2, 56.0, 46.1,
42.6, 39.9, 39.0, 31.9, 27.9, 19.7. HRMS (ESI) m/z: [M + H]+ Calcd
for C17H24N3O, 286.1914; Found, 286.1917. Rf = 0.09 (silica gel, 6:2
(10S,11aS)-1-(Benzo[d][1,3]dioxol-5-yl)-10-methoxy-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (35). The title com-
pound was derived from 30 and benzo[d][1,3]dioxol-5-ylboronic acid
1
to give the desired product (11.0 mg, 0.034 mmol, 60%) H NMR
25
EtOAc/MeOH). [α]D = +44.3° (c 1.0, CHCl3). >99% ee.
(600 MHz, CDCl3): δ 6.70 (d, J = 7.8 Hz, 1H), 6.62−6.57 (m, 2H),
5.93 (dd, J = 8.2, 1.5 Hz, 2H), 5.64 (dd, J = 5.2, 2.6 Hz, 1H), 5.59−
5.54 (m, 1H), 3.26 (ddd, J = 14.3, 11.9, 6.0 Hz, 1H), 3.13 (s, 3H),
3.10−3.00 (m, 3H), 2.98 (dddd, J = 15.6, 8.8, 6.6, 3.5 Hz, 1H), 2.65−
2.50 (m, 3H), 2.41 (dd, J = 11.6, 3.8 Hz, 1H), 2.18−2.12 (m, 1H),
1.94 (dt, J = 18.9, 5.2 Hz, 1H), 1.83 (ddtd, J = 17.4, 8.8, 3.5, 1.8 Hz,
1H), 1.48 (t, J = 11.8 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ
147.0, 146.5, 142.1, 139.0, 137.7, 125.7, 122.5, 121.3, 109.6, 107.8,
101.0, 73.9, 64.9, 56.0, 46.0, 42.6, 40.1, 31.9, 28.2, 19.8. HRMS (ESI)
m/z: [M + H]+ Calcd for C20H24NO3, 326.1751; Found, 326.1754. Rf
= 0.21 (silica gel, 6:2 EtOAc/MeOH). [α]D25 = +104.1° (c 1.0,
CHCl3). >99% ee.
(10R,11aS)-1-(Benzo[d][1,3]dioxol-5-yl)-10-methoxy-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (epi-35). The title
compound was derived from epi-30 and benzo[d][1,3]dioxol-5-
ylboronic acid to give the desired product (11.0 mg, 0.034 mmol,
60%) 1H NMR (600 MHz, CDCl3): δ 6.68−6.66 (m, 2H), 6.64 (dd, J
= 7.8, 1.7 Hz, 1H), 5.91 (dd, 2H), 5.67−5.65 (m, 1H), 5.59−5.56 (m,
1H), 3.29 (s, 3H), 3.29−3.23 (m, 2H), 3.08−2.98 (m, 3H), 2.65−
2.58 (m, 1H), 2.54 (dddd, J = 18.6, 11.7, 6.8, 2.5 Hz, 1H), 2.42−2.36
(m, 1H), 2.28 (ddd, J = 14.1, 5.6, 2.3 Hz, 1H), 2.23−2.17 (m, 1H),
1.87 (dt, J = 18.7, 5.4 Hz, 1H), 1.57 (dd, J = 14.0, 11.0 Hz, 1H),
1.27−1.23 (m, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ 146.8,
146.5, 142.8, 141.2, 135.7, 125.5, 122.8, 120.1, 109.9, 107.6, 100.9,
75.5, 65.3, 56.2, 46.1, 39.9, 37.7, 29.7, 28.1, 18.9. HRMS (ESI) m/z:
[M + H]+ Calcd for C20H24NO3, 326.1751; Found, 326.1754. Rf =
0.20 (silica gel, 6:2 EtOAc/MeOH). [α]D25 = +43.3° (c 0.9, CHCl3).
>99% ee.
(10R,11aS)-10-Methoxy-1-(1-methyl-1H-pyrazol-4-yl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (epi-37). The title
compound was derived from epi-30 and (1-methyl-1H-pyrazol-4-
yl)boronic acid to give the desired product (6.30 mg, 0.022 mmol,
1
33%) H NMR (600 MHz, CDCl3): δ 7.42 (s, 1H), 7.31 (s, 1H),
5.85−5.83 (m, 1H), 5.63−5.61 (m, 1H), 3.82 (s, 3H), 3.37−3.33 (m,
1H), 3.32 (s, 3H), 3.24 (ddd, J = 14.4, 11.5, 5.7 Hz, 1H), 3.00−2.95
(m, 3H), 2.58−2.46 (m, 2H), 2.38 (dddd, J = 14.8, 8.0, 4.3, 2.3 Hz,
1H), 2.32−2.26 (m, 2H), 1.84 (dt, J = 18.9, 5.4 Hz, 1H), 1.72−1.66
(m, 1H), 1.51 (dd, J = 14.0, 11.4 Hz, 1H). 13C{1H} NMR (151 MHz,
CDCl3): δ 142.7, 138.4, 133.8, 127.9, 123.7, 121.8, 119.0, 75.3, 64.5,
56.2, 45.9, 39.9, 39.0, 37.2, 30.1, 28.1, 18.8. HRMS (ESI) m/z: [M +
H]+ Calcd for C17H24N3O, 286.1914; Found, 286.1921. Rf = 0.10
(silica gel, 6:2 EtOAc/MeOH). [α]D25 = +77.1° (c 0.55, CHCl3).
>99% ee.
(10S,11aS)-10-Methoxy-1-(1-methyl-1H-imidazole-5-yl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (38). The title com-
pound was derived from 30 and 1-methyl-5-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-1H-imidazole to give the desired product
(6.00 mg, 0.021 mmol, 37%) 1H NMR (600 MHz, CDCl3): δ 7.26 (s,
1H), 6.53 (s, 1H), 5.59−5.55 (m, 1H), 5.43−5.39 (m, 1H), 3.39 (s,
3H), 3.14 (ddd, J = 14.4, 11.7, 5.9 Hz, 1H), 2.98 (s, 3H), 2.93 (dd, J
= 14.5, 7.1 Hz, 1H), 2.89−2.85 (m, 2H), 2.49−2.32 (m, 4H), 2.16
(dd, J = 11.7, 4.0 Hz, 1H), 2.07 (dd, J = 17.7, 4.6 Hz, 1H), 1.87 (dt, J
= 19.4, 5.4 Hz, 1H), 1.69−1.61 (m, 1H), 1.39 (t, J = 12.0 Hz, 1H).
13C{1H} NMR (151 MHz, CDCl3): δ 138.3, 137.6, 131.8, 130.4,
130.1, 128.7, 121.3, 73.9, 64.8, 56.1, 46.2, 42.6, 40.0, 31.9, 31.7, 27.8,
19.5. HRMS (ESI) m/z: [M + H]+ Calcd for C17H24N3O, 286.1914;
Methyl 5-((10S,11aS)-10-Methoxy-3,4,7,9,10,11-hexahydro-6H-
pyrido[2,1-i]indol-1-yl)picolinate (36). The title compound was
derived from 30 and (6-(methoxycarbonyl)pyridin-3-yl)boronic acid
Found, 286.1914. Rf = 0.1 (silica gel, 6:2 EtOAc/MeOH). [α]D25
+31.9° (c 0.6, CHCl3). >99% ee.
=
1
to give the desired product (5.00 mg, 0.015 mmol, 29%) H NMR
(10S,11aS)-10-Methoxy-1-((E)-4-(trifluoromethyl)styryl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (39). The title com-
pound was derived from 30 and (E)-(3-(trifluoromethyl)styryl)-
boronic acid to give the desired product (11.0 mg, 0.029 mmol, 52%)
1H NMR (600 MHz, CDCl3): δ 7.55 (d, J = 8.1 Hz, 2H), 7.42 (d, J =
(600 MHz, CDCl3): δ 8.51 (d, J = 2.2 Hz, 1H), 8.02 (d, J = 8.1 Hz,
1H), 7.58 (dd, J = 8.1, 2.2 Hz, 1H), 5.81−5.77 (m, 1H), 5.61−5.59
(m, 1H), 4.01 (s, 3H), 3.38−3.29 (m, 1H), 3.12−3.03 (m, 6H), 2.79
(tdd, J = 12.4, 6.1, 3.9 Hz, 1H), 2.64−2.53 (m, 3H), 2.45 (dd, J =
11.6, 4.0 Hz, 1H), 2.13−2.07 (m, 1H), 2.03−2.00 (m, 1H), 1.83 (ddt,
8.0 Hz, 2H), 6.68 (d, J = 16.0 Hz, 1H), 6.60 (d, J = 16.0 Hz, 1H),
8258
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
6.11 (t, J = 4.0 Hz, 1H), 5.69−5.64 (m, 1H), 3.80 (tdd, J = 12.5, 6.3,
4.0 Hz, 1H), 3.33−3.24 (m, 4H), 3.05 (dd, J = 14.5, 7.4 Hz, 1H), 2.98
(m, 1H), 2.89 (m, 1H), 2.63−2.53 (m, 2H), 2.51−2.36 (m, 3H),
2.09−2.02 (m, 1H), 1.99 (dt, J = 19.9, 5.5 Hz, 1H), 1.52 (t, J = 11.8
Hz, 1H). 13C NMR{1H} (151 MHz, CDCl3): δ 141.3, 139.9, 138.0,
132.1, 129.1 (q, J = 32.4 Hz), 126.5, 125.9, 125.7 (q, J = 3.8 Hz),
124.5 (d, J = 271.8 Hz), 124.0, 119.6, 73.8, 64.1, 56.2, 46.1, 42.2, 39.9,
32.1, 27.5, 20.0. HRMS (ESI) m/z: [M + H]+ Calcd for
C22H25F3NO, 376.1883; Found, 376.1891. Rf = 0.33 (silica gel, 6:2
(ESI) m/z: [M + H]+ Calcd for C21H24NOS, 338.1573; Found,
338.1584. Rf = 0.23 (silica gel, 6:2 EtOAc/MeOH). [α]D = +68.7°
(c 0.96, CHCl3). >99% ee.
25
(10R,11aS)-1-(Benzo[b]thiophen-3-yl)-10-methoxy-3,4,7,9,10,11-
hexahydro-6H-pyrido[2,1-i]indole (epi-41). The title compound was
derived from epi-30 and benzo[b]thiophen-3-ylboronic acid to give
the desired product (4.50 mg, 0.013 mmol, 24%) 1H NMR (600
MHz, C6D6): δ 7.79 (d, J = 8.1 Hz, 1H), 7.58 (d, J = 8.0 Hz, 1H),
7.21 (t, J = 7.6 Hz, 1H), 7.13 (s, 1H), 7.08 (t, J = 7.5 Hz, 1H), 5.68−
5.65 (m, 1H), 5.31−5.28 (m, 1H), 3.39−3.33 (m, 1H), 3.23 (ddd, J =
14.3, 11.6, 5.5 Hz, 1H), 3.02 (s, 3H), 3.00−2.95 (m, 1H), 2.87 (td, J
= 10.1, 9.5, 4.5 Hz, 1H), 2.79 (dd, J = 14.5, 6.7 Hz, 1H), 2.54−2.47
(m, 2H), 2.26−2.19 (m, 1H), 2.18−2.13 (m, 1H), 2.10−2.04 (m,
1H), 1.74 (dd, J = 13.9, 10.7 Hz, 1H), 1.51 (dt, J = 18.7, 5.4 Hz, 1H),
1.39−1.34 (m, 1H). 13C{1H} NMR (151 MHz, C6D6): δ 142.0,
140.9, 140.1, 137.0, 136.3, 128.4, 124.5, 124.3, 123.2, 123.0, 122.9,
120.2, 75.7, 65.7, 55.7, 46.5, 40.3, 38.8, 30.0, 28.7, 18.7. HRMS (ESI)
m/z: [M + H]+ Calcd for C21H24NOS, 338.1573; Found, 338.1572.
25
EtOAc/MeOH). [α]D = +85.7° (c 1.0, CHCl3). >99% ee.
(10R,11aS)-10-Methoxy-1-((E)-4-(trifluoromethyl)styryl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (epi-39). The title
compound was derived from epi-30 and (E)-(3-(trifluoromethyl)-
styryl)boronic acid to give the desired product (12.0 mg, 0.032 mmol,
54%) 1H NMR (600 MHz, CDCl3): δ 7.52 (d, J = 8.1 Hz, 2H), 7.41
(d, J = 8.1 Hz, 2H), 6.73 (d, J = 16.1 Hz, 1H), 6.58 (d, J = 16.1 Hz,
1H), 6.14−6.12 (m, 1H), 5.68−5.65 (m, 1H), 3.43 (tt, J = 9.8, 4.9
Hz, 1H), 3.34 (s, 3H), 3.25 (ddd, J = 14.1, 11.2, 5.6 Hz, 1H), 3.03−
2.97 (m, 2H), 2.90 (ddd, J = 10.9, 9.4, 4.5 Hz, 1H), 2.60−2.51 (m,
2H), 2.44−2.38 (m, 1H), 2.32−2.26 (m, 2H), 2.04−1.97 (m, 1H),
1.93 (dt, J = 19.5, 5.5 Hz, 1H), 1.66 (dd, J = 14.2, 10.5 Hz, 1H).
13C{1H} NMR (151 MHz, CDCl3): δ 141.9, 141.2, 139.0, 130.4,
25
Rf = 0.17 (silica gel, 6:2 EtOAc/MeOH). [α]D = +91.6° (c 0.24,
CHCl3). >99% ee.
(10S,11aS)-10-Methoxy-1-(thiophen-3-yl)-3,4,7,9,10,11-hexahy-
dro-6H-pyrido[2,1-i]indole (42). The title compound was derived
from 30 and thiophen-3-ylboronic acid to give the desired product
128.8 (q, J = 32.4 Hz), 127.0, 126.7, 126.5, 126.3, 125.5 (q, J = 3.8
Hz), 124.3 (q, J = 271.7 Hz), 124.1, 118.4, 75.1, 63.6, 56.1, 45.8, 39.8,
37.8, 30.5, 27.6, 19.0. HRMS (ESI) m/z: [M + H]+ Calcd for
C22H25F3NO, 376.1883; Found, 376.1886. Rf = 0.23 (silica gel, 6:2
1
(12.5 mg, 0.044 mmol, 77%) H NMR (600 MHz, CDCl3): δ 7.17
(dd, J = 5.0, 3.0 Hz, 1H), 7.04 (dd, J = 3.0, 1.2 Hz, 1H), 6.94 (dd, J =
5.0, 1.3 Hz, 1H), 5.81−5.79 (m, 1H), 5.60−5.57 (m, 1H), 3.28 (ddd,
J = 14.3, 11.8, 6.1 Hz, 1H), 3.11 (s, 3H), 3.08−2.98 (m, 3H), 2.92
(dddd, J = 12.5, 8.8, 6.2, 3.9 Hz, 1H), 2.64−2.50 (m, 3H), 2.40 (dd, J
= 11.4, 3.9 Hz, 1H), 2.26−2.18 (m, 1H), 1.94 (dt, J = 19.1, 5.4 Hz,
1H), 1.86 (dddd, J = 17.4, 8.7, 3.5, 1.9 Hz, 1H), 1.46 (t, J = 11.8 Hz,
1H). 13C{1H} NMR (151 MHz, CDCl3): δ 144.1, 139.9, 137.2,
128.7, 125.6, 124.6, 121.6, 120.8, 74.1, 64.5, 56.1, 46.1, 43.0, 40.1,
32.0, 28.0, 19.7. HRMS (ESI) m/z: [M + H]+ Calcd for C17H22NOS,
288.1417; Found, 288.1420. Rf = 0.18 (silica gel, 6:2 EtOAc/
25
EtOAc/MeOH). [α]D = +96.5° (c 0.4, CHCl3). >99% ee.
(10S,11aS)-10-Methoxy-1-(1-methyl-1H-indol-3-yl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (40). The title com-
pound was derived from 30 and 1-methyl-3-(4,4,5,5-tetramethyl-
1,3,2-dioxaborolan-2-yl)-1H-indole to give the desired product (17.0
1
mg, 0.051 mmol, 90%) H NMR (600 MHz, CDCl3): δ 7.67 (d, J =
8.0 Hz, 1H), 7.28 (d, J = 8.0 Hz, 1H), 7.21 (t, J = 7.6 Hz, 1H), 7.11
(t, J = 7.4 Hz, 1H), 6.80 (s, 1H), 5.88−5.84 (m, 1H), 5.63−5.58 (m,
1H), 3.74 (s, 3H), 3.43 (ddd, J = 14.3, 11.8, 6.1 Hz, 1H), 3.18−3.07
(m, 3H), 2.90−2.83 (m, 4H), 2.71−2.63 (m, 1H), 2.63−2.54 (m,
2H), 2.51 (dd, J = 11.3, 3.9 Hz, 1H), 2.13 (dt, J = 18.9, 5.4 Hz, 1H),
2.09−2.06 (m, 1H), 1.88−1.80 (m, 1H), 1.48 (t, J = 11.7 Hz, 1H).
13C{1H} NMR (151 MHz, CDCl3): δ 139.7, 136.5, 133.3, 128.1,
126.7, 125.4, 121.8, 120.9, 119.9, 119.4, 116.4, 109.1, 74.1, 65.3, 55.8,
46.2, 42.3, 40.2, 32.9, 32.1, 28.0, 19.7. HRMS (ESI) m/z: [M + H]+
Calcd for C22H27N2O, 335.2118; Found, 335.2123. Rf = 0.24 (silica
25
MeOH). [α]D = +50.2° (c 0.5, CHCl3). >99% ee.
(10R,11aS)-10-Methoxy-1-(thiophen-3-yl)-3,4,7,9,10,11-hexahy-
dro-6H-pyrido[2,1-i]indole (epi-42). The title compound was derived
from epi-30 and thiophen-3-ylboronic acid to give the desired
product (11.0 mg, 0.038 mmol, 45%). 1H NMR (600 MHz, CDCl3):
δ 7.18 (s, 1H), 7.16−7.14 (m, 1H), 7.02 (d, J = 5.0 Hz, 1H), 5.90−
5.88 (m, 1H), 5.63−5.60 (m, 1H), 3.36−3.24 (m, 5H), 3.04−2.98
(m, 3H), 2.60−2.53 (m, 2H), 2.37−2.28 (m, 3H), 1.89 (dt, J = 18.9,
5.5 Hz, 1H), 1.56 (dd, J = 14.3, 11.5 Hz, 1H), 1.53−1.48 (m, 1H).
13C{1H} NMR (151 MHz, CDCl3): δ 142.0, 141.6, 137.9, 128.5,
25
gel, 6:2 EtOAc/MeOH). [α]D = +172.4° (c 1.0, CHCl3). >99% ee.
(10R,11aS)-10-Methoxy-1-(1-methyl-1H-indol-3-yl)-
3,4,7,9,10,11-hexahydro-6H-pyrido[2,1-i]indole (epi-40). The title
compound was derived from epi-30 and 1-methyl-3-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-indole to give the desired
product (11.6 mg, 0.035 mmol, 63%) 1H NMR (600 MHz, CDCl3): δ
7.62 (d, J = 7.9 Hz, 1H), 7.25 (d, J = 3.7 Hz, 1H), 7.19 (t, J = 7.5 Hz,
1H), 7.10−7.07 (m, 2H), 5.99−5.97 (m, 1H), 5.64 (d, J = 7.8 Hz,
1H), 3.72 (s, 3H), 3.42−3.35 (m, 2H), 3.27 (s, 3H), 3.15−3.06 (m,
3H), 2.71−2.60 (m, 2H), 2.40−2.35 (m, 2H), 2.31−2.26 (m, 1H),
2.08−2.02 (m, 1H), 1.57−1.52 (m, 2H). 13C{1H} NMR (151 MHz,
CDCl3): δ 142.0, 136.7, 134.6, 128.7, 126.2, 125.4, 121.7, 120.0,
119.8, 119.3, 114.3, 109.1, 75.2, 65.7, 56.2, 46.1, 40.1, 37.2, 32.9, 30.0,
28.2, 18.9. HRMS (ESI) m/z: [M + H]+ Calcd for C22H27N2O,
335.2118; Found, 335.2118. Rf = 0.17 (silica gel, 6:2 EtOAc/
125.4, 124.3, 121.8, 119.6, 75.3, 64.8, 56.2, 46.0, 39.9, 37.5, 29.8, 28.1,
18.91. HRMS (ESI) m/z: [M + H]+ Calcd for C17H22NOS,
288.1417; Found, 288.1426. Rf = 0.21 (silica gel, 6:2 EtOAc/
MeOH). [α]D25 = +83.9° (c 0.03, CHCl3). >99% ee.
(10S,11aS)-1-((E)-3-Fluorostyryl)-10-methoxy-3,4,7,9,10,11-hexa-
hydro-6H-pyrido[2,1-i]indole (43). The title compound was derived
from 30 and (E)-(3-fluorostyryl)boronic acid to give the desired
product (17.0 mg, 0.052 mmol, 92%) 1H NMR (600 MHz, CDCl3): δ
7.26 (td, J = 8.0, 6.0 Hz, 1H), 7.09 (d, J = 7.7 Hz, 1H), 7.03 (dt, J =
10.3, 2.1 Hz, 1H), 6.94−6.87 (m, 1H), 6.62 (d, J = 15.9 Hz, 1H), 6.51
(d, J = 15.9 Hz, 1H), 6.07 (t, J = 4.0 Hz, 1H), 5.67−5.64 (m, 1H),
3.81 (dddd, J = 12.6, 8.8, 6.4, 4.0 Hz, 1H), 3.26 (s, 4H), 3.05 (dd, J =
14.4, 7.3 Hz, 1H), 2.98 (td, J = 9.0, 3.8 Hz, 1H), 2.89 (td, J = 9.5, 9.1,
6.6 Hz, 1H), 2.62−2.52 (m, 2H), 2.50−2.44 (m, 1H), 2.43−2.36 (m,
2H), 2.08−2.01 (m, 1H), 2.01−1.94 (m, 1H), 1.52 (t, J = 11.8 Hz,
1H). 13C{1H} NMR (151 MHz, CDCl3): δ 163.3 (d, J = 245.0 Hz),
140.2 (d, J = 7.7 Hz), 137.9, 130.8, 130.2 (d, J = 8.3 Hz), 126.3 (d, J =
2.7 Hz), 123.3, 122.4 (d, J = 2.8 Hz), 119.6, 114.2 (d, J = 21.5 Hz),
112.7 (d, J = 21.9 Hz), 73.8, 64.1, 56.2, 46.2, 42.2, 39.9, 32.0, 27.4,
25
MeOH). [α]D = +200.9° (c 0.22, CHCl3). >99% ee.
(10S,11aS)-1-(Benzo[b]thiophen-3-yl)-10-methoxy-3,4,7,9,10,11-
hexahydro-6H-pyrido[2,1-i]indole (41). The title compound was
derived from 30 and benzo[b]thiophen-3-ylboronic acid to give the
1
desired product (9.60 mg, 0.028 mmol, 50%) H NMR (600 MHz,
CDCl3): δ 7.83 (dddd, J = 15.2, 8.0, 1.2, 0.7 Hz, 2H), 7.41−7.31 (m,
2H), 6.99 (s, 1H), 5.84 (dd, J = 4.7, 2.8 Hz, 1H), 5.61 (d, J = 3.3 Hz,
1H), 3.46 (ddd, J = 14.3, 11.9, 6.0 Hz, 1H), 3.22−3.14 (m, 3H),
2.77−2.67 (m, 4H), 2.67−2.57 (m, 2H), 2.56−2.47 (m, 2H), 2.17
(dt, J = 19.2, 5.5 Hz, 1H), 2.08−2.00 (m, 1H), 1.84−1.76 (m, 1H),
1.54 (t, J = 11.8 Hz, 1H). 13C{1H} NMR (151 MHz, CDCl3): δ
139.7, 139.7, 138.3, 136.6, 134.5, 127.5, 124.4, 124.4, 123.5, 122.9,
122.8, 122.2, 73.6, 65.7, 55.9, 46.3, 41.6, 40.2, 32.0, 27.9, 19.4. HRMS
20.0. HRMS (ESI) m/z: [M + H]+ Calcd for C21H25NOF, 326.1915;
25
Found, 326.1923. Rf = 0.35 (silica gel, 6:2 EtOAc/MeOH). [α]D
+41.3° (c 1.0, CHCl3). >99% ee.
=
(10R,11aS)-1-((E)-3-Fluorostyryl)-10-methoxy-3,4,7,9,10,11-hex-
ahydro-6H-pyrido[2,1-i]indole (epi-43). The title compound was
derived from epi-30 and (E)-(3-fluorostyryl)boronic acid to give the
1
desired product (6.50 mg, 0.020 mmol, 35%). H NMR (600 MHz,
8259
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
CDCl3): δ 7.23 (td, J = 8.0, 6.0 Hz, 1H), 7.09 (d, J = 8.0 Hz, 1H),
7.03 (dt, J = 10.4, 2.1 Hz, 1H), 6.88 (td, J = 8.5, 2.6 Hz, 1H), 6.67 (d,
J = 16.1 Hz, 1H), 6.49 (d, J = 16.1 Hz, 1H), 6.10−6.06 (m, 1H),
5.68−5.63 (m, 1H), 3.41 (tt, J = 9.8, 4.7 Hz, 1H), 3.35 (s, 3H), 3.24
(ddd, J = 14.3, 11.4, 5.7 Hz, 1H), 3.02−2.93 (m, 2H), 2.90 (ddd, J =
11.0, 9.4, 4.5 Hz, 1H), 2.61−2.48 (m, 2H), 2.45−2.36 (m, 1H),
2.32−2.24 (m, 2H), 2.08−1.95 (m, 1H), 1.90 (dt, J = 19.4, 5.4 Hz,
1H), 1.64 (dd, J = 14.2, 10.7 Hz, 1H). 13C{1H} NMR (151 MHz,
CDCl3): δ 163.3 (d, J = 245.0 Hz), 142.3, 140.3 (d, J = 7.7 Hz),
139.3, 130.1 (d, J = 8.7 Hz), 129.4, 126.9 (d, J = 2.7 Hz), 123.7, 122.4
(d, J = 2.6 Hz), 118.2, 114.0 (d, J = 21.5 Hz), 112.6 (d, J = 21.6 Hz),
75.4, 63.7, 56.2, 45.9, 39.9, 38.0, 30.6, 27.8, 19.1. HRMS (ESI) m/z:
[M + H]+ Calcd for C21H25NOF, 326.1915; Found, 326.1917. Rf =
0.33 (silica gel, 6:2 EtOAc/MeOH). [α]D25 = +74.4° (c 0.54,
CHCl3). >99% ee.
Pharmacology. Materials. (S)-Nicotine and all chemicals for the
buffers were purchased from Sigma-Aldrich (St. Louis, MO), and
Dulbecco’s Modified Eagle Medium Glutamax-I (DMEM) medium,
serum, Hanks Buffered Salt Solution (HBSS), and antibiotics were
obtained from Invitrogen (Paisley, UK). [3H]epibatidine and the
Fluo-4/AM dye were purchased from PerkinElmer (Waltham, MA)
and Molecular Probes (Eugene, OR), respectively. The stable hα4β2-
(PerkinElmer). The test compounds were tested at least three times at
each receptor.
Ca2+/Fluo-4 Assay. The functional properties of the test
compounds were characterized at the hα4β2-HEK293 and rα3β4-
HEK293 cell lines in the fluorescence-based Ca2+/Fluo-4 assay
performed essentially, as previously described.25b Briefly, the cells
were split into poly-D-lysine-coated black 96-well plates with clear
bottom (6 × 104 cells/well). The following day the culture medium
was aspirated, and the cells were incubated in 50 μL of incubation
buffer (HBSS containing 20 mM HEPES, 1 mM CaCl2, 1 mM MgCl2,
and 2.5 mM probenecid, pH 7.4) supplemented with 6 mM Fluo−4/
AM at 37 °C for 1 h. Then, the buffer was aspirated, the cells were
washed once with 100 μL of incubation buffer, once with 100 μL of
assay buffer (140 mM N-methyl-D-glucamine, 5 mM KCl, 1 mM
MgCl2, 10 mM CaCl2, 10 mM HEPES, and 2.5 mM probenecid, pH
7.4), and then, 100 μL of assay buffer was added to the cells (in the
antagonist experiments, the test compound was added to the buffer at
this point). The 96-well plate was assayed in a FLEXStation3
(Molecular Devices, Crawley, UK) measuring emission [in
fluorescence units (FU)] at 525 nm caused by excitation at 485 nm
before and up to 90 s after addition of 33.3 μL agonist solution in
assay buffer. The compounds were initially tested as agonists at the
two receptors, and none of them displayed significant agonist activity
at concentrations up to 100 μM. The compounds were characterized
as antagonists in duplicate at least three times at the two receptors
using (S)-nicotine ∼ EC80 (range EC70−EC90) as an agonist.
Data Analysis. Data analysis was performed using GraphPad Prism
7.0c (GraphPad Software, Inc., La Jolla, CA). For the saturation
[3H]epibatidine-binding data, specific binding was analyzed with a
nonlinear regression one site-specific binding model given by the
equation: Y = Bmax × X/(Kd + X), where Y is the specific
[3H]epibatidine binding, Bmax is the total amount of binding, X is
the [3H]epibatidine concentration, and Kd is the dissociation
constant. The competition binding data were analyzed with a
nonlinear regression one site-fit log IC50 model given by the following
and hα6/α3β2β3V9′S-HEK293 cell lines were obtained from Dr. Tino
Dyhring (Saniona A/S, Ballerup, Denmark), and the stable rα3β4-
and rα4β4-HEK293 cell lines were obtained from Dr. Kenneth J.
Kellar (Georgetown University Medical Center, Washington, D.C.).2
Cell Culture. The four nAChR-expressing HEK293 cell lines were
cultured in a humidified atmosphere at 37 °C and 5% CO2 in DMEM
supplemented with penicillin (100 U/mL), streptomycin (100 μg/
mL), 5% dialyzed fetal bovine serum, and, for the rα3β4- and rα4β4-
HEK293 cell lines, 1 mg/mL G-418.
[3H]epibatidine-Binding Assay. The binding affinities of the test
compounds were determined at membranes from the hα4β2-, hα6/
α3β2β3V9′S-, rα3β4-, and rα4β4-HEK293 cell lines in a [3H]-
epibatidine competition binding assay, as previously described.25a,b
Cells were collected at ∼90% confluency by scraping into ice-cold
assay buffer (140 mM NaCl, 1.5 mM KCl, 2 mM CaCl2, 1 mM
MgCl2, 25 mM HEPES, pH 7.4), homogenized for 10 s, and
centrifuged for 30 min at 47,810 ×g at 4 °C. The pellet was
resuspended in ice-cold fresh assay buffer, homogenized, and
centrifuged for another 30 min under the same conditions. The
supernatant was decanted, and the cell membranes were stored at −80
°C until use. On the day of the experiment, cell membranes were
resuspended in assay buffer and incubated with [3H]epibatidine and
various concentrations of test compounds at room temperature for 3−
4 h while shaking. In the competition binding experiments,
membranes were incubated with a fixed concentration of [3H]-
epibatidine hα4β2 and hα6/α3β2β3V9′S: ∼30 pM (range: 20−40 pM)
and rα3β4 and rα4β4: ∼150 pM (range: 100−200 pM) in the
presence of various concentrations of the test compounds or (S)-
nicotine in a total reaction volume of 3 mL. In saturation binding
experiments performed in parallel, the membranes were incubated
with 12 different concentrations of [3H]epibatidine (spanning from 1
pM to 30 nM) in the absence (total binding) or presence of 100 μM
(S)-nicotine (non-specific binding). In both saturation and competi-
tion binding experiments, the amount of cell membranes used and
total reaction volumes were adjusted so that the bound/free ratios of
the radioligand always were <10%. Prior to harvesting, Whatman GF/
C filters (PerkinElmer) were treated with 0.2% polyethyleneimine
solution for 30 min. The reaction was terminated by filtration through
the presoaked filters using a Brandell M-48T cell harvester (Alpha
Biotech, London, UK) and washed three times with 5 mL ice-cold
harvesting buffer (0.9% NaCl, 10 mM Tris−HCl, pH 7.4). Filters
were subsequently allowed to dry at room temperature and then
transferred to scintillation vials, and 3 mL OptiFluor (PerkinElmer)
was added to each sample. The filter-retained radioactivity and the
exact [3H]epibatidine concentrations used in the experiments were
determined by liquid scintillation counting using a Tri-Carb 4910 TR
̂
equation: Y = bottom + (top − bottom)/(1 + 10((logIC50 − X)*nH),
where Y is the specific [3H]epibatidine binding, top and bottom are
the plateau values of the curves, X is the test compound
concentration, IC50 is the equilibrium affinity of the test compound,
and nH is the Hill slope. Ki values were calculated from the
determined IC50 values using the Cheng−Prusoff equation [32]: Ki =
IC50/(1 − ([RL]/Kd), where [RL] is the [3H]epibatidine
concentration used for the specific experiment and KD is the
dissociation constant determined in the saturation binding experi-
ments.
The data from the Ca2+/Fluo-4 assay were extracted as the
difference in relative fluorescence units (ΔRFU) between the
maximum fluorescence level measured after the agonist application
and the basal level measured before the agonist application. The
concentration−inhibition relationships of the test compounds as
antagonists were fitted to a nonlinear regression curve fit with variable
̂
slope Y = bottom + (top − bottom)/(1 + 10((logIC50 − X)*nH),
where top and bottom values are plateaus in the units of the response
axis, X is the concentration of the ligand, IC50 is the concentration of
the ligand that gives a response half way between bottom and top, and
nH is the Hill slope.
Molecular Modeling. The modeling study was performed using
the Drug Discovery Suite 2020−3 from Schrodinger Inc.
The structure from a study by Yu et al.6b was processed using the
standard protocol for the protein preparation wizard in standard
settings. A model including the water molecule seen in the X-ray
structure of the AChBP (PDB: 4alx) was also made for docking in the
same way. All ligands were treated with ligprep before docking,
however, in the protonated state of the amine and without generating
other stereoisomers or tautomers. Two grids were generated in Glide
in standard settings: with and without a water molecule included. All
ligands were docked with Glide in both models allowing up to five
poses for each ligand. The best scoring poses in the model including
water are provided in the Supporting Information. Figures were
generated using Pymol 1.8.0.4.
8260
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.joc.1c00707.
ACKNOWLEDGMENTS
■
■
sı
We thank the Medicinal Chemistry Department at H.
Lundbeck A/S for valuable resources and inputs. Drs. Tino
Dyhring (Saniona A/S, Ballerup, Denmark) and Ken Kellar
(Georgetown University Medical Center, Washington, DC)
are thanked for their generous gifts of nAChR-expressing cell
lines.
Experimental details and docking poses of investigated
ligands (PDF)
AUTHOR INFORMATION
Corresponding Author
Jesper L. Kristensen − Department of Drug Design and
Pharmacology, Faculty of Health and Medical Sciences,
University of Copenhagen, Copenhagen 2100, Denmark;
orcid.org/0000-0002-5613-1267;
■
REFERENCES
■
(1) Folkers, K.; Unna, K. Erythrina alkaloids, II. A review, and new
data on the alkaloids of species of the genus Erythrina. J. Am. Pharm.
Assoc. 1938, 27, 693.
(2) Lehman, A. Actions of Erythrina Americana, A Possible Curare
Substitute. J. Am. Pharm. Assoc. 1937, 60, 69.
(3) Garín-Aguilar, M. E.; Luna, J. E.; Soto-Hernández, M.; del Toro,
G. V.; Vázquez, M. M. Effect of crude extracts of Erythrina americana
Mill. on aggressive behavior in rats. J. Ethnopharmacol. 2000, 69, 189.
(4) Shapiro, S.; Baker, A. B. Treatment of paralysis agitans with
dihydro-beta-erythroidine. Am. J. Med. 1950, 8, 153.
(5) Andreasen, J.; Olsen, G.; Wiborg, O.; Redrobe, J. Anti-
depressant-like effects of nicotinic acetylcholine receptor antagonists,
but not agonists, in the mouse forced swim and mouse tail suspension
tests. J. Psychopharmacol. 2009, 23, 797.
(6) (a) Burman, M. S. Therapeutic use of curare and erythroidine
hydrochloride for spastic and dystonic states. Arch. Neurol. Psychiatr.
1938, 41, 307. (b) Yu, R.; Tae, H. S.; Xu, Q.; Craik, D. J.; Adams, D.
J.; Jiang, T.; Kaas, Q. Molecular dynamics simulations of dihydro-β-
erythroidine bound to the human α4β2 nicotinic acetylcholine
receptor. Br. J. Pharmacol. 2019, 176, 2750. (c) Shahsavar, A.;
Kastrup, J. S.; Nielsen, E.Ø.; Kristensen, J. L.; Gajhede, M.; Balle, T.
Crystal Structure of Lymnaea stagnalis AChBP Complexed with the
Potent nAChR Antagonist DHβE Suggests a Unique Mode of
Antagonism. PLoS One 2012, 7, 1.
(7) Majinda, R. R.. In Progress in the Chemistry of Organic Natural
Products; Kinghorn, D. A., Falk, H., Gibbons, S., Kobayashi, J.,
Asakawa, Y., Liu, J.-K., Eds.; Springer: Cham, 2018, pp 95−152.
(8) Reimann, E.. In Progress in the Chemistry of Organic Natural
Products; Herz, W., Falk, H., Kirby, G., Eds.; Springer: Vienna, 2007,
pp 2−56.
(9) Boekelheide, V.; Weinstock, J.; Grundon, M. F.; Sauvage, G. L.;
Angello, E. J. The Structure of β-Erythroidine and its Derivatives. J.
Am. Chem. Soc. 1953, 75, 2550.
Email: jesper.kristensen@sund.ku.dk
Authors
Sebastian Clementson − Molecular Discovery and Innovation,
H. Lundbeck A/S, Valby 2500, Denmark; Department of
Drug Design and Pharmacology, Faculty of Health and
Medical Sciences, University of Copenhagen, Copenhagen
2100, Denmark
Sergio Armentia Matheu − Molecular Discovery and
Innovation, H. Lundbeck A/S, Valby 2500, Denmark;
Department of Drug Design and Pharmacology, Faculty of
Health and Medical Sciences, University of Copenhagen,
Copenhagen 2100, Denmark
Emil Märcher Rørsted − Department of Drug Design and
Pharmacology, Faculty of Health and Medical Sciences,
University of Copenhagen, Copenhagen 2100, Denmark;
orcid.org/0000-0002-2783-6655
Henrik Pedersen − Molecular Discovery and Innovation, H.
Lundbeck A/S, Valby 2500, Denmark
Anders A. Jensen − Department of Drug Design and
Pharmacology, Faculty of Health and Medical Sciences,
University of Copenhagen, Copenhagen 2100, Denmark
Rasmus P. Clausen − Department of Drug Design and
Pharmacology, Faculty of Health and Medical Sciences,
University of Copenhagen, Copenhagen 2100, Denmark;
orcid.org/0000-0001-9466-9431
(10) (a) Parsons, A. F.; Palframan, M. J. Erythrina and Related
Alkaloids. Alkaloids; Cordell, G. A., Ed.; Elsevier: Heslington, York,
2010; Vol. 68, pp 39−81. (b) Heller, S. T.; Kiho, T.; Narayan, A. R.
H.; Sarpong, R. Protic-Solvent-Mediated Cycloisomerization of
Quinoline and Isoquinoline Propargylic Alcohols: Syntheses of
( )-3-Demethoxyerythratidinone and ( )-Cocculidine. Angew.
Chem., Int. Ed. 2013, 52, 11129. (c) Kawasaki, T.; Onoda, N.;
Watanabe, H.; Kitahara, T. Total synthesis of ( )-cocculolidine.
Tetrahedron Lett. 2001, 42, 8003. (d) Funk, R. L.; Belmar, J. Total
synthesis of ( )-isophellibiline. Tetrahedron Lett. 2012, 53, 176.
(e) Jepsen, T. H.; Glibstrup, E.; Crestey, F.; Jensen, A. A.; Kristensen,
J. L. A strategic approach to [6,6]-bicyclic lactones: application
towards the CD fragment of DhβE. Beilstein J. Org. Chem. 2017, 13,
988. (f) Jepsen, T. H.; Jensen, A. A.; Lund, M. H.; Glibstrup, E.;
Kristensen, J. L. Synthesis and Pharmacological Evaluation of DHβE
Analogues as Neuronal Nicotinic Acetylcholine Receptor Antagonists.
ACS Med. Chem. Lett. 2014, 5, 766. (g) Chuang, K. V.; Navarro, R.;
Reisman, S. E. Benzoquinone-derived sulfinylimines as versatile
intermediates for alkaloid synthesis: Total synthesis of (−)-3-
demethoxyerythratidinone. Chem. Sci. 2011, 2, 1086. (h) Blackham,
E. E.; Booker-Milburn, K. I. A Short Synthesis of ( )-3-
Demethoxyerythratidinone by Ligand-Controlled Selective Heck
Cyclization of Equilibrating Enamines. Angew. Chem., Int. Ed. 2017,
56, 6613. (i) Crestey, F.; Jensen, A. A.; Borch, M.; Andreasen, J. T.;
Andersen, J.; Balle, T.; Kristensen, J. L. Design, synthesis, and
biological evaluation of Erythrina alkaloid analogues as neuronal
Paulo Vital − Molecular Discovery and Innovation, H.
Lundbeck A/S, Valby 2500, Denmark; Present
Address: Current address: Department of Research &
Development, BIAL-Portela & Ca, S.A., S. Mamede do
Coronado, Portugal.
Emil Glibstrup − Molecular Discovery and Innovation, H.
Lundbeck A/S, Valby 2500, Denmark
Mikkel Jessing − Molecular Discovery and Innovation, H.
Lundbeck A/S, Valby 2500, Denmark
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.joc.1c00707
Author Contributions
Conceptualization: S.C., P.V., M.J., and J.L.K.; investigation:
S.C., S.A.M., E.M.R., H.P., A.A.J., and R.P.C.; supervision: S.C.,
P.V., E.G., M.J., and J.L.K.; and writing: S.C., A.A.J., R.P.C.,
M.J., and J.L.K.
Funding
This research was financially supported by the Innovation
Fund Denmark (Case no. 7038-00149B) and H. Lundbeck A/
S.
Notes
The authors declare no competing financial interest.
8261
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262

The Journal of Organic Chemistry
pubs.acs.org/joc
Article
nicotinic acetylcholine receptor antagonists. J. Med. Chem. 2013, 56,
9673. (j) Crestey, F.; Jensen, A. A.; Soerensen, C.; Magnus, C. B.;
Andreasen, J. T.; Peters, G. H. J.; Kristensen, J. L. Dual Nicotinic
Acetylcholine Receptor α4β2 Antagonists/α7 Agonists: Synthesis,
Docking Studies, and Pharmacological Evaluation of Tetrahydroiso-
quinolines and Tetrahydroisoquinolinium Salts. J. Med. Chem. 2018,
61, 1719. (k) Clementson, S.; Jessing, M.; Pedersen, H.; Vital, P.;
Kristensen, J. L. Enantioselective Total Synthesis of (+)-Dihydro-β-
erythroidine. J. Am. Chem. Soc. 2019, 141, 8783. (l) Clementson, S.;
Jessing, M.; Vital, P. J.; Kristensen, J. L. Development of a Divergent
Route to Erythrina Alkaloids. Synlett 2020, 31, 327. (m) Umihara, H.;
Yoshino, T.; Shimokawa, J.; Kitamura, M.; Fukuyama, T. Develop-
ment of a Divergent Synthetic Route to the Erythrina Alkaloids:
Asymmetric Syntheses of 8-Oxo-erythrinine, Crystamidine, 8-Oxo-
erythraline, and Erythraline. Angew. Chem., Int. Ed. 2016, 55, 6915.
(11) (a) Fukumoto, H.; Takahashi, K.; Ishihara, J.; Hatakeyama, S.
Total Synthesis of (+)-β-Erythroidine. Angew. Chem., Int. Ed. 2006,
45, 2731.
(12) (a) Trost, B. M.; Schäffner, B.; Osipov, M.; Wilton, D. A. A.
Palladium-Catalyzed Decarboxylative Asymmetric Allylic Alkylation
of β-ketoesters: An Unusual Counterion Effect. Angew. Chem., Int. Ed.
2011, 50, 3548. (b) Trost, B. M.; Crawley, M. L. Asymmetric
Transition-Metal-Catalyzed Allylic Alkylations: Applications in Total
Synthesis. Chem. Rev. 2003, 103, 2921.
(13) Vanrheenen, V.; Kelly, R. C.; Cha, D. Y.; Company, T. U. An
improved catalytic OsO4 oxidation of olefins to cis-1, 2-glycols using
tertiary amine oxides as the oxidant. Tetrahedron Lett. 1976, 17, 1973.
(14) (a) Pappo, R.; Allen, D., Jr.; Lemieux, R.; Johnson, W. Notes−
Osmium Tetroxide-Catalyzed Periodate Oxidation of Olefinic Bonds.
J. Org. Chem. 1956, 21, 478. (b) Li, W.; Chen, Z.; Yu, D.; Peng, X.;
Wen, G.; Wang, S.; Xue, F.; Liu, X.-Y.; Qin, Y. Asymmetric Total
Syntheses of the Akuammiline Alkaloids (−)-Strictamine and
(−)-Rhazinoline. Angew. Chem., Int. Ed. 2019, 58, 6059.
(15) (a) Hartley, R. C.; Li, J.; Main, C. A.; Mckiernan, G. J.
Titanium carbenoid reagents for converting carbonyl groups into
alkenes. Tetrahedron 2007, 63, 4825. (b) Takai, K.; Hotta, Y.;
Oshima, K.; Nozaki, H. Effective methods of carbonyl methylenation
using CH2I2-Zn-Me3Al and CH2Br2-Zn-TiCl4 system. Tetrahedron
Lett. 1978, 19, 2417. (c) Okazoe, T.; Hibino, J.-i.; Takai, K.; Nozaki,
H. Chemoselective methylenation with a methylenedianion synthon.
Tetrahedron Lett. 1985, 26, 5581. (d) Hibino, J.-i.; Okazoe, T.; Takai,
K.; Nozaki, H. Carbonyl methylenation of easily enolizable ketones.
Tetrahedron Lett. 1985, 26, 5579. (e) Tebbe, F. N.; Parshall, G. W.;
Reddy, G. S. Olefin homologation with titanium methylene
compounds. J. Am. Chem. Soc. 1978, 100, 3611. (f) Petasis, N. A.;
Bzowej, E. I. Titanium-mediated carbonyl olefinations. 1. Methylena-
tions of carbonyl compounds with dimethyltitanocene. J. Am. Chem.
Soc. 1990, 112, 6392. (g) Yan, T.-H.; Tsai, C.-C.; Chien, C.-T.; Cho,
C.-C.; Huang, P.-C. Dichloromethane Activation. Direct Methylena-
tion of Ketones and Aldehydes with CH2Cl2 Promoted by Mg/
TiCl4/THF. Org. Lett. 2004, 6, 4961.
(16) (a) Van Staden, L. F.; Gravestock, D.; Ager, D. J. New
developments in the Peterson olefination reaction. Chem. Soc. Rev.
2002, 31, 195. (b) Chen, D.; Liu, H.-M.; Li, M.-M.; Yan, Y.-M.; Xu,
W.-D.; Li, X.-N.; Cheng, Y.-X.; Qin, H.-B. Concise synthesis of
( )-Lingzhiol via epoxy-arene cyclization. Chem. Commun. 2015, 51,
14594.
(17) Kumar, T.; Jarhad, D. B.; Mobin, S. M.; Singh, V. Molecular
complexity and diversity from aromatics. Intramolecular cycloaddition
of cyclohexa-2,4-dienones and sigmatropic shift in excited state: a
unified approach towards synthesis of polycyclic frameworks related
to crotogoudin, conidiogenol, and crinipellins. Tetrahedron 2016, 72,
5377.
(20) (a) Katakam, R.; Anugula, R.; Macha, L.; Batchu, V. R.
Stereoselective synthesis of N-benzyl conduramine F-1, N-benzyl ent-
conduramine E-1, dihydroconduramine F-1 and ent-dihydrocondur-
amine E-1. Tetrahedron Lett. 2017, 58, 559. (b) Scholl, M.; Ding, S.;
Lee, C. W.; Grubbs, R. H. Synthesis and activity of a new generation
of ruthenium-based olefin metathesis catalysts coordinated with 1, 3-
dimesityl-4, 5-dihydroimidazol-2-ylidene ligands. Org. Lett. 1999, 1,
953.
(21) (a) Kuwajiama, I.; Hirokazu, U. Regioselective Arylation of
Silyl Enol Ethers of Methyl Ketones with Aryl Bromides. J. Am. Chem.
Soc. 1982, 104, 6831. (b) Moradi, W. A.; Buchwald, S. L. Palladium-
Catalyzed α-Arylation of Esters. J. Am. Chem. Soc. 2001, 123, 7996.
(c) Carfagna, C.; Musco, A.; Sallese, G.; Santi, R.; Fiorani, T.
Palladium-catalyzed coupling reactions of aryl triflates or halides with
ketene trimethylsilyl acetals. A new route to alkyl 2-arylalkanoates. J.
Org. Chem. 1991, 56, 261. (d) Jørgensen, M.; Lee, S.; Liu, X.;
Wolkowski, J. P.; Hartwig, J. F. Efficient synthesis of alpha-aryl esters
by room-temperature palladium-catalyzed coupling of aryl halides
with ester enolates. J. Am. Chem. Soc. 2002, 124, 12557.
(22) Shang, R.; Ji, D.-S.; Chu, L.; Fu, Y.; Liu, L. Synthesis of α-Aryl
Nitriles through Palladium-Catalyzed Decarboxylative Coupling of
Cyanoacetate Salts with Aryl Halides and Triflates. Angew. Chem., Int.
Ed. 2011, 50, 4470.
(23) Caprioglio, D.; Fletcher, S. P. An alternative synthesis of the
breast cancer drug fulvestrant (Faslodex): Catalyst control over C−C
bond formation. Chem. Commun. 2015, 51, 14866.
(24) Shi, J.; Shigehisa, H.; Guerrero, C. A.; Shenvi, R. A.; Li, C.-C.;
Baran, P. S. Stereodivergent Synthesis of 17-α and 17-β-Aryl Steroids:
Application and Biological Evaluation of D-Ring Cortistatin
Analogues. Angew. Chem., Int. Ed. 2009, 48, 4328.
(25) (a) Jensen, A. A.; Mikkelsen, I.; Frølund, B.; Bräuner-Osborne,
H.; Falch, E.; Krogsgaard-Larsen, P. Carbamoylcholine homologs:
novel and potent agonists at neuronal nicotinic acetylcholine
receptors. Mol. Pharmacol. 2003, 64, 865. (b) Sandager-Nielsen, K.;
Ahring, P. K.; Klein, J.; van Hout, M.; Thaneshwaran, S.; Dos Santos,
A. B.; Jacobsen, T. A.; Amrutkar, D. V.; Peters, D.; Jensen, A. A.;
Kohlmeier, K. A.; Christophersen, P.; Dyhring, T. Characterization of
AN317, a novel selective agonist of α6β2-containing nicotinic
acetylcholine receptors. Biochem. Pharmacol. 2020, 174, 113786.
(26) Inubushi, Y.; Nomura, K.-i.; Miyawaki, M. Studies on the
Alkaloids of Menispermaceous Plants. CXCVII. J. Pharm. Soc. Jpn.
1963, 83, 282.
(27) Yunusov, S. Ob Alkaloidakh Cocculus Laurifolius DC. 1. Zh.
Obshch. Khim. 1950, 20, 368.
(28) Timmermann, D.; Sandager-Nielsen, K.; Dyhring, T.; Smith,
M.; Jacobsen, A.-M.; Nielsen, E.; Grunnet, M.; Christensen, J.; Peters,
D.; Kohlhaas, K.; Olsen, G.; Ahring, P. Augmentation of cognitive
function by NS9283, a stoichiometry-dependent positive allosteric
modulator of α2-and α4-containing nicotinic acetylcholine receptors.
Br. J. Pharmacol. 2012, 167, 164.
(29) Rasmussen, A. H.; Strøbæk, D.; Dyhring, T.; Jensen, M. L.;
Peters, D.; Grunnet, M.; Timmermann, D. B.; Ahring, P. K.
Biophysical and pharmacological characterization of α6-containing
nicotinic acetylcholine receptors expressed in HEK293 cells. Brain
Res. 2014, 1542, 1.
(30) Xiao, Y.; Meyer, E. L.; Thompson, J. M.; Surin, A.; Wroblewski,
J.; Kellar, K. J. Rat α3/β4 subtype of neuronal nicotinic acetylcholine
receptor stably expressed in a transfected cell line: pharmacology of
ligand binding and function. Mol. Pharmacol. 1998, 54, 322.
(31) Xiao, Y.; Baydyuk, M.; Wang, H.; Davis, H. E.; Kellar, K. J.
Pharmacology of the agonist binding sites of rat neuronal nicotinic
receptor subtypes expressed in HEK 293 cells. Bioorg. Med. Chem.
Lett. 2004, 14, 1845.
(18) Schuppe, A. W.; Huang, D.; Chen, Y.; Newhouse, T. R. Total
synthesis of (−)-Xylogranatopyridine B via a Palladium-catalyzed
oxidative stannylation of enones. J. Am. Chem. Soc. 2018, 140, 2062.
(19) Hoffmann, R. W. α-Chiral allylboronates: reagents for
asymmetric synthesis. Pure Appl. Chem. 1988, 60, 123.
8262
https://doi.org/10.1021/acs.joc.1c00707
J. Org. Chem. 2021, 86, 8248−8262