Enantiospecific synthesis of (+)-alstonisine via a stereospecific osmylation process

Article abstract of DOI:10.1021/np800269k

The first enantiospecific total synthesis of (+)-alstonisine has been accomplished from D-tryptophan methyl ester 13 in 12% overall yield (in 17 reaction vessels). A diastereospecific osmylation process has been employed as a key step to convert indole 18 into spirocyclic oxindole 19. Mechanistic studies of the stereoselective osmylation of the 2,3-indole double bond of indole alkaloids has been carried out. Compelling evidence for the intramolecular delivery of OsO₄ via N_b-complexation was obtained for the osmylation process. The correct structure of (+)-alstonisine (1) was determined by NOE spectroscopic experiments and further confirmed by single-crystal X-ray analysis.

Full text of DOI:10.1021/np800269k

J. Nat. Prod. 2008, 71, 1431–1440
1431
Enantiospecific Synthesis of (+)-Alstonisine via a Stereospecific Osmylation Process1
Jie Yang,† Xiangyu Z. Wearing,† Philip W. Le Quesne,‡ Jeffrey R. Deschamps,§ and James M. Cook*,†
Department of Chemistry, UniVersity of Wisconsin-Milwaukee, Milwaukee, Wisconsin 53201, Department of Chemistry, Northeastern
UniVersity, Boston, Massachusetts 02115, and NaVal Research Laboratory, Code 6030, Washington, D.C. 20375
ReceiVed May 1, 2008
The first enantiospecific total synthesis of (+)-alstonisine has been accomplished from D-tryptophan methyl ester 13 in
12% overall yield (in 17 reaction vessels). A diastereospecific osmylation process has been employed as a key step to
convert indole 18 into spirocyclic oxindole 19. Mechanistic studies of the stereoselective osmylation of the 2,3-indole
double bond of indole alkaloids has been carried out. Compelling evidence for the intramolecular delivery of OsO4 via
Nb-complexation was obtained for the osmylation process. The correct structure of (+)-alstonisine (1) was determined
by NOE spectroscopic experiments and further confirmed by single-crystal X-ray analysis.
Oxindole alkaloids are an important group of monoterpene
alkaloids that contain the spirocyclic oxindole nucleus.2 It is still
unclear whether oxindole alkaloids serve a specific function in plant
species or are simply present as indole alkaloid catabolites, but
biogenetic considerations suggest that the indole alkaloid alstonerine
(5) may serve as a precursor to alstonisine (1).3 Oxindole alkaloids
are often associated with significant pharmacological activity.4 The
Gardneria oxindole alkaloids such as chitosenine (7) and alkaloid
I (9) exhibit short-lived inhibitory activity in ViVo of ganglionic
transmission in both rats and rabbits.4g The spirocyclic oxindole
10, prepared by Sakai and co-workers as an analogue of 9, has
been employed in a formulation known to inhibit ulcers.6–9 The
bisindole gardmultine (11), isolated from Gardneria multifora, has
been shown to display antitumor activity.5,6 The alkaloids 7 and 9
are the two monomeric bases of bisindole 11.5 A number of
alkaloids from Alstonia angustifolia have been reported to possess
potent antimalarial activity;4b–f however, none of the Alstonia
oxindoles 1-4 have been evaluated biologically in detail because
of the paucity of isolated material.
Alstonisine (1), the first macroline-related oxindole alkaloid, was
isolated by Elderfield and Gilman from Alstonia muelleriama
Domin in 1972.10 The original structure and relative configuration
of this alkaloid were established through single-crystal X-ray
analysis by Nordman et al.11 However, the absolute configuration
reported for this molecule was chosen to agree with that deduced
for ajmalicine and resulted in an incorrect representation of the
structure of alstonisine (1).11 The structures of alstonisine (1) and
Nb-demethylalstophylline oxindole (3) illustrated by Wong et al.
would on this basis, presumably, also be incorrect.4f The absolute
configuration of alstonisine (1) was later determined by a biomi-
Figure 1. Examples of oxindole alkaloids.
metic transformation of alstonisine (1) into talpinine;3 however,
with previously reported biogenetic proposals.3,13,14 The Gardneria
alkaloids 7-12 also contain the spirocyclic oxindole system present
in 6, but the configuration at the spirocyclic C(7) atom is opposite
the proposed structure of alstonisine (1). The structure of gard-
multine (11) was deduced by chemical and spectroscopic evidence6,8
and concurrently established by single-crystal X-ray analysis.7 The
structure of alstonisine (1) [especially the configuration at the
spirocenter C(7)], however, has not been unambiguously established
to date.11,15–17 Therefore, development of an efficient approach to
that of the synthesis of both series of oxindole alkaloids would not
only provide sufficient quantities of synthetic material for structure
determination [i.e., for alstonisine (1)] and for biological screening
but also provide a general route for the synthesis of other related
oxindole alkaloids or their antipodes.
direct confirmation of the configuration at the spirocenter had not
been reported. This constituted the principal interest in the
enantiospecific total synthesis of this alkaloid. After this initial
report, alstonisine (1) was also found in Alstonia angustifolia and
A. macrophylla.12,4f Other macroline-related oxindole alkaloids have
been isolated from A. macrophylla Wall, including alstonal (2),4f
Nb-demethylalstophylline oxindole (3),13 and 16-hydroxy-Nb-dem-
ethylalstophylline oxindole (4).14 All of these macroline-related
oxindole alkaloids 1-4 contain the 8-azabicyclo[3.2.1]nonane
substructure represented in oxindole 6. The structures of oxindole
alkaloids 2 and 3 have been determined by NOE spectroscopic
experiments and are believed to be correct, as they correlate well
* To whom correspondence should be addressed. E-mail: capncook@
uwm.edu. Tel: +1-414-229-5856. Fax: +1-414-229-5530.
† University of Wisconsin-Milwaukee.
Herein we report the enantiospecific total synthesis of alstonisine
(1). This work has culminated in the establishment of the correct
absolute configuration of alstonisine (1). It also confirmed the
‡ Northeastern University.
§ Naval Research Laboratory, Code 6030.
10.1021/np800269k CCC: $40.75
2008 American Chemical Society and American Society of Pharmacognosy
Published on Web 07/09/2008

1432 Journal of Natural Products, 2008, Vol. 71, No. 8
Yang et al.
Scheme 1
Scheme 2. 19
Scheme 3. 29a,d
correct structure of alstonisine (1) in agreement with that proposed
earlier based on biogenetic grounds.3 In addition, an osmylation
process that provides entry into either spirocyclic oxindole, dia-
stereomeric at C-7, has been developed in the course of this work.
As shown in the retrosynthetic analysis in Scheme 1, the synthetic
protocol rested on the readily available (-)-Nb-benzyl tetracyclic
ketone 14, prepared from ester 13 in greater than 98% ee.18
Results and Discussion
The synthesis began with the readily available (-)-Na-methyl
tetracyclic ketone 14 prepared on 300 g scale from ester 13 (>98%
ee).18 Ester 13 had been converted into 16 via an enantiospecific,
stereospecific sequence (Scheme 2). This synthesis (Scheme 2)
provided the first stereocontrolled entry into the correct chirality of

Enantiospecific Synthesis of (+)-Alstonisine
Journal of Natural Products, 2008, Vol. 71, No. 8 1433
Scheme 4
alstonisine at C-3, C-5, C-15, and C-16.18,19 The regiospecific
oxyselenation of olefin 16 using N-phenylselenophthalimide20,21 in
CH2Cl2/MeOH in the presence of p-TSA at 0 °C for 20 h provided
the isomeric mixture of selenoacetals 17a in high yield. This mixture
of selenoacetals was directly treated with NaIO4 in THF/MeOH/H2O
solution at 0 °C for 10 h without separation to afford 17 in 90% yield
as a 4:1 mixture of Z/E isomers (Scheme 2).19 Hydroboration of the
mixture of olefins represented by 17 with BH3 ·THF complex in THF
at 0 °C for 14 h, followed by H2O2-mediated oxidation, provided the
corresponding isomeric alcohols 18a, which were directly converted
into the intermediate ketoacetal 18 under the modified Swern oxidation
conditions of Zhang22 (-78 to -10 °C, 1.5 h). With the ketoacetal
18 in hand, it was decided to approach the synthesis of alstonisine (1)
in a stereocontrolled manner to establish the correct chirality at C-7.
26 (required for Gardneria oxindoles 7-12) or oxindole 27 (needed
for Alstonia oxindoles 1-4). Earlier work in our group had initiated
investigation of the indole-oxindole conversion using Sharpless ligated
osmylation reagents and demonstrated excellent diastereoselectivity.17
In the absence of the Sharpless ligands, oxindole 27 was the major
diastereomer (91:9 27:26) in this process; in a similar case with OsO4/
py, the OsO4 was believed to form a reversible complex with pyridine
in the reaction mixture. When this osmium reagent coordinated with
the Nb-piperidine nitrogen atom of 14 and formed the OsO4 complex
in the equatorial configuration, attack on the indole 2,3-double bond
readily occurred from the convex face to afford the intermediate osmate
ester 29 (see Scheme 4). The complexation of the OsO4 with the Nb-
nitrogen atom to increase the reaction rate and provide enhanced
diastereoselectivity postulated by Peterson17 was in complete agreement
with the work of Donohoe et al.30a–c in TMEDA/OsO4-directed
dihydroxylations. Surprisingly, in the presence of the Sharpless ligand,
the opposite diastereomer 26 was formed in overwhelming preponder-
ance (93:7 26:27). It is difficult to rationalize this result on the basis
of simple collision interactions, and in this context it is also remarkable
that reaction of (()-14 with the Sharpless reagents led to no differences
between the enantiomers of 14 in the ratios of products.17 However,
it was felt that the osmium complexed with the Sharpless ligand and
could not coordinate with the Nb-piperidine nitrogen atom of 14; the
attack on the indole 2,3-double bond occurred from the concave face
and formed the intermediate osmate ester 28. It is believed the OsO4/
Sharpless ligand complex was too strong to be replaced with an Nb-
nitrogen/osmium complex. In addition, because of nitrogen inversion
at the Nb-benzyl position, presumably, the benzyl group occupied the
equatorial position a portion of the time, inhibiting approach of the
OsO4/Sharpless ligand complex from the convex face.31 It appears
that the diastereoselectivity in the case of 26 rested on the size of the
ligands. When 1.5 equiv of OsO4 was employed at room temperature
for 3 days with pyridine, quinuclidine,30a–d or the Sharpless ligands,
the ratio of 26:27 went from 1:1, to 3:1, to 9:1.17 Obviously, the small
Recently, a number of successful approaches have emerged for the
construction of spirocyclic oxindoles,23 such as Mannich reactions,24
anionic routes,25 aryl radical cyclizations,26 intramolecular Heck
reactions,27 oxidations of indole double bonds,28 osmylations,17,22,29
and the use of NBS or t-BuOCl.15,16 Earlier, Fu et al.29a,d had reported
that treatment of Nb-benzyl tetracyclic monoketal 22 with OsO4/py,
followed by periodate oxidation, provided oxindole 25 (Scheme 3).
This conversion occurred with complete diastereoselectivity with the
correct configuration relative to alstonisine (1). Esmond et al.29e had
also observed a similar formation of an oxindole during dihydroxylation
of a key intermediate with OsO4/pyridine in the biomimetic synthesis
of macroline.
A proposed pathway for this transformation is depicted in Scheme
3.17 The attack of the osmium reagent on the indole 2,3-double bond
has occurred from the less hindered convex face to afford the
intermediate bisosmate ester 23, which reacted with NaIO4 to furnish
the diol-aldehyde 24. The ensuing pinacol rearrangement of diol 24
that followed afforded the oxindole 25.29a,d
Since the intermediate ketone 14 was available on large scale, it
was employed to investigate the conversion of 14 into either oxindole

1434 Journal of Natural Products, 2008, Vol. 71, No. 8
Yang et al.
Scheme 5
size of the pyridine ligand did not promote discrimination between
the diastereomeric faces of the 2,3-double bond of substrate 14 by the
attacking osmium reagent. Moreover, when the bulky Sharpless ligands
were employed, the attack of the osmium complex occurred prefer-
entially from the concave face without regard to asymmetry in the
pendent ligand.17
Nb-methyl oxindole (chitosenine stereochemistry) was formed in 36%,
40%, or 66% yield, respectively. In contrast to the Nb-benzyl group
of 14, which occupies the axial position,31 the smaller Nb-methyl
substituent in ketone 36a and in the other macroline-related indoles is
believed to preferentially occupy the equatorial position of the D ring
(see 36a, Scheme 5).32 As a result, the attack of the osmium reagent
would be hindered by the Nb-methyl group of the piperidine nitrogen.
Furthermore, the lone pair of electrons of the piperidine nitrogen atom
would not be available at the equatorial position for the formation of
the equatorial OsO4 complex. Hence the ligation and subsequent attack
of the osmium reagent could only approach the indole moiety of 36a
from the concave face (Scheme 5) in the absence or presence of the
pyridine or other ligands.17 The pinacol rearrangement that followed
afforded the spirocyclic oxindole 37 with complete diastereoselectivity.
The chirality at C(7) of Nb-methyloxindole 37 is identical to that of
26.
Finally, compelling evidence for the Nb-complexation/intramo-
lecular delivery of OsO4 to provide 27 was obtained when the
hydrochloride salt of ketone 14 was employed as the substrate in
the osmylation process (Scheme 5). The lone pair of electrons of
the Nb-piperidine nitrogen atom were not available for coordination
with OsO4 in this hydrochloride salt 38. Consequently, OsO4
preferentially attacked the indole 2,3-double bond from the concave
face in refluxing THF (39, Scheme 5). Oxindole 26, with the
chitosenine stereochemistry, was obtained in 72% yield (26:27, 9:1).
It was now clear that when the lone pair of electrons of the Nb-
nitrogen atom are not available to form the Nb-nitrogen/OsO4
complex, attack of OsO4 occurred from the concave face to provide
the chitosenine stereochemistry (Scheme 5).
In order to increase the yield of the oxindole and further improve
the diastereoselectivity of osmylation for the generation of 27, ketone
14 was converted into ketal 31 to block the concave face of the 2,3-
indole double bond. In this fashion, it was felt that the OsO4 could
approach the indole moiety of 31 only from the convex face (30,
Scheme 4). In this manner, ketal oxindole 32 was obtained as a single
isomer in 80% yield. Removal of the ketal function in 32 under mild
acidic conditions provided the desired diastereomer at C(7) in oxindole
27 as a single diastereomer in almost quantitative yield. This ketoox-
indole was identical to 27 obtained from ketone 14 with OsO4/py.
The importance of the Nb-piperidine nitrogen atom for the
complexation/intramolecular delivery of OsO4 in this process was
further investigated. Hence the Nb-benzoyl ketone 33 was chosen
as a substrate for this process. The Nb-benzoyl group of substrate
33 was approximately the same size as the benzyl group in ketone
14, but the lone pair of electrons of the piperidine nitrogen atom
were delocalized into the carbonyl group of the amide function to
form 34 and not readily available to coordinate with OsO4 (Scheme
5). Execution of this process failed to convert the Nb-benzoyl ketone
33 into Nb-benzoyloxindole 35 or its diastereomer. Only the starting
ketone 33 was recovered from this sequence. This example
demonstrated that the nitrogen atom of amides could not coordinate
with osmium and the attack of the indole 2,3-double bond would
not occur from the convex face; moreover, the OsO4 was not
reactive enough in this system at room temperature to react with
the substrate 33 even from the concave face of the indole 2,3-double
bond without previous ligation to a nitrogen function.17
It also should be noted that when an electron-rich ligand (DMAP)
was used to replace pyridine in the previous example in the OsO4/
pyridine process (Scheme 4), the yield of the oxindole decreased from
66% to only 15%, although the alstonisine stereochemistry in oxindole
27 was produced as the major isomer in 86% de. Presumably, the
OsO4 formed a strong complex with DMAP (due to its electron-rich
character) and reactivity with the nucleophilic indole double bond via
In addition, when the Na-methyl, Nb-methyl tetracyclic ketone was
subjected to oxidation with osmium tetraoxide, osmium tetraoxide/
pyridine, or osmium tetraoxide/DHQ-CLB, only one diastereomeric

Enantiospecific Synthesis of (+)-Alstonisine
Journal of Natural Products, 2008, Vol. 71, No. 8 1435
Scheme 6
very good diastereoselectivity in excellent yield. In view of the
cost and toxicity of osmium tetraoxide, various Sharpless AD
ligands were employed with a catalytic amount of OsO4 via the
Sharpless osmylation process.33 However, these attempts failed and
only starting material was recovered. The failure of the catalytic
dihydroxylation process, presumably, was derived from the slow
hydrolysis of the osmate ester intermediate, in agreement with
similar findings reported by Donohoe et al.,30a–c which disrupted
the metal-mediated catalytic cycle.
Alternative routes to generate the configuration at the spirocenter
required for the Gardneria or Alstonia alkaloids have been explored
with tert-butylhypochlorite (t-BuOCl). Earlier,16 it had been demon-
strated that the isomeric spiro[pyrrolidine-3,3′-oxindole] system with
the alstonisine stereochemistry could be obtained stereospecifically
upon reaction of the Na-H, Nb-benzoyl tetracyclic ketone with t-BuOCl/
Et3N, followed by treatment with acetic acid. In this same approach,
when Na-H, Nb-benzyl tetracyclic ketone 40a was reacted directly with
t-BuOCl, the oxindole 41, which was structurally related to chitosenine,
was obtained in 93% yield in diastereospecific fashion. In contrast,
when the Na-H, Nb-H tetracyclic ketone 42 was employed, oxindole
43, which was structurally related to alstonisine, was diastereospecifi-
cally formed (Scheme 6). The structure of oxindole 41 was confirmed
by single-crystal X-ray analysis,15,31a and the NMR spectroscopic data
of the oxindole 43 were in agreement with the reported values for
racemic 43.16 Oxindole 41 was also converted into 26 by Na-
methylation in 93% yield (Scheme 6).
As illustrated in Scheme 7, the 100% diastereoselectivity obtained
here was a result of the difference in reactivity of tert-butylhypochlorite
with the Nb-benzyl vs Nb-H tetracyclic ketone. In contrast to the Na-
methyl, Nb-benzyl tetracyclic ketone 14, whose Nb-benzyl group
preferably occupied the axial position of the Nb-piperidine nitrogen
atom,31 the Nb-benzyl group in the Na-H, Nb-benzyl tetracyclic ketone
40a series, presumably, occupied the equatorial position (see 40c,
Scheme 7) and, therefore, blocked one face of the 2,3-indole double
bond. This promoted attack by Cl+ from the concave face. In the
subsequent pinacol-type rearrangement the migrating group must attack
from the face opposite the C-Cl bond to provide oxindole 41.
Meanwhile, in the case of the Na-H, Nb-H ketone 42, the concave face
was more hindered when compared to Nb-benzyl 40a (Scheme 7). In
indole 42, the 2,3-indole double bond was presumably attacked from
the convex face to form oxindole 43, with the alstonisine stereochem-
istry.3 It was of interest to attempt the t-BuOCl oxidation with the
corresponding Na-methyl tetracyclic ketones 14 and 33. However, this
method failed for conversion of Na-methyl derivatives 14 and 33 into
the corresponding oxindoles, as these Na-methyl analogues will not
readily undergo oxidation of the indole 2,3-double bond with t-BuOCl.
Presumably, formation of the indolenine intermediate iminium ion
similar to 44 in Scheme 7 was too high in energy in the Na-methyl
series.
Figure 2. Crystal structure of oxindole 41. Displacement ellipsoids
are at the 50% level.
the Nb-piperidine nitrogen atom was reduced. Any remaining (dis-
sociated) OsO4 attacked 14 from the convex face of the indole 2,3-
double bond following the usual coordination with the Nb-piperidine
nitrogen atom and furnished 27 (15% yield).
As illustrated in Schemes 4 and 5, ketooxindole 26 could be
obtained via the Sharpless osmylation process from ketone 14 with
Scheme 7

1436 Journal of Natural Products, 2008, Vol. 71, No. 8
Yang et al.
Scheme 8
Scheme 9
With this information in hand, the ketoacetal 18 was treated with a
premixed OsO4 solution (THF/py, 5:1) at room temperature for 3 days,
followed by reductive workup with aqueous NaHSO3, the diol of which
underwent pinacol rearrangement to provide the spirocyclic oxindole
19 as the sole diastereomer in 81% yield (Scheme 8). As described
above, it was felt that the 100% diastereoselectivity of this conversion
was a result of coordination between OsO4 and the Nb-nitrogen atom
of the piperidine ring 48 (Scheme 8), which delivered the intramo-
lecular attack of the osmium reagent from the convex face of ketoacetal
18 and furnished the osmate ester 49. The osmate ester 49 that resulted
was converted into a cis-diol intermediate, which underwent pinacol
rearrangement to offer oxindole 19.
After the diastereomer 19 was obtained, attention turned to the
process for Nb-debenzylation and elimination of CH3OH to complete
the enantiospecific synthesis of alstonisine (1). As illustrated in Scheme
8, oxindole 19 was first treated with NaOH to eliminate the elements
of methanol and provided the Nb-benzylalstonisine 20. Various attempts
were made to execute the catalytic Nb-debenzylation process; however,
they failed to generate the compound alstonisine (1) from 20. The
failure of these attempts prompted the search for alternative routes for
the synthesis of alstonisine (1) from 19. A catalytic amount of 10%
Pd/C in ethanol in the presence of hydrogen was employed to convert
19 into 21; however, this conversion failed probably due to steric
hindrance in the 8-azabicyclo[3.2.1]nonane system (6, Figure 1).31a
Since oxindole 41 was readily available on a large scale, it was
decided to take advantage of model reactions. As shown in Scheme
9, the benzyl group of 41 was successfully removed by hydrogenolysis
when 2 equiv of Pearlman’s catalyst (Pd(OH)2/C)34 was employed
with hydrogen. Consequently, the treatment of Nb-benzyl derivative
19 with 2 equiv of Pearlman’s catalyst in the presence of H2 permitted
successful removal of the benzyl function to provide Nb-H analogue
21. Base-mediated elimination of the elements of methanol was then
carried out to furnish alstonisine (1) in 86% (two steps) yield. It should
be feasible to execute the reduction and then add NaOH to the crude
mixture to reduce this to a one-pot process.
(Nujol mulls) 1690, 1645, doublets at 1615 and 1605 cm-1] of 1 were
in excellent agreement with the published values for the natural product
alstonisine.4f,10,11 However, since the possibility existed that the
,36
sample from A. macrophylla4f required for literature 13C NMR
comparisons may not be identical to that from A. muelleriana obtained
much earlier by Elderfield et al.,10 a mixed sample was employed for
13C NMR data comparison. A 13C NMR spectrum of a mixed sample
[1.5 mg of synthetic 1 and 1.5 mg of authentic alstonisine isolated
from A. muelleriana] contained only 20 signals, which indicated that
the synthetic compound was unambiguously identical to the natural
product and (Vida infra) to that isolated from A. macrophylla.4f
Selected NOE experiments were then carried out on synthetic
alstonisine (1) (Figure 3). Irradiation of H-9 effected enhancement of
H-15 by 20% and vice versa, which would not have been observed in
the other diastereomer at C-7, thus confirming the configuration of
the spirocenter in alstonisine as the same in the structure proposed
earlier based on biomimetic grounds3 and also found in Nb-demethy-
lalstophylline oxindole by Atta-ur-Rahman.13,14 The other NOEs,
shown in Figure 3, permitted the complete assignment of the
stereochemistry of alstonisine (1). The structure of alstonisine (1) was
further confirmed by single-crystal X-ray analysis. Therefore, the
original structure of alstonisine reported by Nordman (X-ray crystal-
The NMR (1H NMR in Table 1 and 13C NMR in Table 2) data and
physical properties including the specific rotation [R]25 ) +197 (c
D
1.0 in EtOH) [lit.4 [R]25D ) +200 (c 1.0 in EtOH)] and IR spectrum
[(Nujol mulls) 1691, 1646, doublets at 1619 and 1610 cm-1) [lit.10

Enantiospecific Synthesis of (+)-Alstonisine
Journal of Natural Products, 2008, Vol. 71, No. 8 1437
4f
1
Table 1. Comparison of the H NMR Data for Synthetic Alstonisine (1) to that Obtained from A. macrophylla
position
natural 14f
synthetic 1
3
5
6
6
9
10
11
12
14
14
15
16
17
17
18
3.15-3.20(1 H, m)
3.68 (1 H, d, J ) 7 Hz)
2.19 (1 H, d, J ) 14 Hz)
3.20 (1 H, m)
3.70 (1 H, d, J ) 6.9 Hz)
2.20 (1 H, d, J ) 13.4 Hz)
2.53 (1 H, dd, J ) 13.5, 7.5 Hz)
6.86 (1 H, d, J ) 7.8 Hz)
7.30 (1 H, td, J ) 7.6, 1.2 Hz)
7.34 (1 H, td, J ) 7.6, 1.2 Hz)
8.26 (1 H, d, J ) 7.7 Hz)
1.53-1.58(1 H, m)
2.24-2.26 (1 H, m)
3.39 (1 H, dt, J ) 11.8, 6.0 Hz)
1.96 (1 H, m)
4.25 (1 H, ddd, J ) 11.0, 4.0, 1.5 Hz)
4.45 (1 H, t, J ) 11.0 Hz)
2.25 (3 H, s)
2.52 (1 H, dd, J ) 14, 7 Hz)
6.86 (1 H, dd, J ) 7, 1.5 Hz)
7.30 (1 H, dd, J ) 7, 1.5
7.32 (1 H, dd, J ) 7, 1.5 Hz)
8.25 (1 H, dd, J ) 7, 1.5 Hz)
1.47-1.58 (1 H, m)
Hz)
2.20-2.33(1 H, m)
3.35-3.41 (1 H, m)
1.90-2.00 (1 H, m)
4.26 (1 H, ddd, J ) 11, 4, 1.5 Hz)
4.45 (1 H, t, J ) 11 Hz)
2.24 (3 H, s)
21
7.62 (1 H, s)
7.63 (1 H, s)
Na-methyl
3.19 (3 H, s)
3.21 (3 H, s)
Table 2. Comparison of the 13C NMR Data for Synthetic
Alstonisine (1) to that Obtained from A. macrophylla35
carbon natural 135 synthetic 1
natural 135 synthetic 1
atom
(δ ppm)
(δ ppm) carbon atom (δ ppm)
(δ ppm)
2
3
181.9
63.4
182.6
64.0
13
14
143.6
30.6
144.2
31.2
5
55.9
56.5
15
23.8
24.4
6
41.5
42.1
16
36.5
37.2
7
56.5
57.1
17
68.2
68.5
8
9
10
11
12
128.7
125.1
122.8
127.5
107.5
129.2
125.8
123.6
128.2
108.2
18
19
20
24.5
25.2
196.0
121.3
157.2
25.8
196.8
121.9
157.8
26.4
21
Na-methyl
Figure 4. Crystal structure of alstonisine, 1. Displacement ellipsoids
are at the 30% level. See the Cambridge Structural Database entry
ALKCAM01 for complete details.
Figure 3. Selected NOE interactions of alstonisine, 1.
lography) in 1963 was the enantiomer of natural alstonisine (1). The
ORTEP (Figure 4) of synthetic alstonisine (1) was in agreement with
the structure of alstonisine proposed on biogenetic grounds.3
that the asymmetry in the OsO4/Sharpless ligand complex has provided
entry into the chitosenine (7) stereochemistry (26, Scheme 4) in high
de. This, presumably, is because the OsO4/Sharpless ligand complex
is irreversible under the conditions employed here, and the fit with
indole 14 is better from the concave face. Although it is tempting to
speculate that this preferred asymmetric fit is observed from the
concave face in the OsO4/Sharpless ligand process, it must be
remembered that identical results were obtained with both types of
Sharpless ligands (DHQ-CLB vs DHQD-CLB and (DHQ)2PHAL vs
(DHQD)2PHAL). Additional work in this area is required to fully
understand these phenomena. The first enantiospecific total synthesis
In summary, mechanistic studies of the stereoselective osmylation
of the 2,3-indole double bond of indole alkaloids has been carried out.
Compelling evidence for the intramolecular delivery of OsO4 via Nb-
complexation was obtained in the osmylation process. While it is easy
to understand the preferred attack of indole 14 (convex face) with either
OsO4/THF or OsO4/py/THF to provide the alstonisine (1) stereochem-
istry (27, Scheme 4),17,30a–d the situation is much more complicated
in the presence of the Sharpless ligands, on the basis of the results of
the achiral DMAP experiment (15% yield, 27:26, 86% de). It is clear

1438 Journal of Natural Products, 2008, Vol. 71, No. 8
Yang et al.
3.90-4.06 (m, 5H), 6.86 (t, J ) 7.0 Hz, 1H), 7.03-7.41 (m, 7H), 7.53 (d,
J ) 7.71 Hz, 1H); 13C NMR (75.5 MHz, CDCl3) δ 17.5, 27.0, 28.9, 49.9,
57.3, 58.0, 64.3, 64.4, 106.5, 108.7, 108.8, 118.1, 118.7, 120.7, 126.6, 126.8,
128.1, 128.7, 133.4, 137.0, 139.0; EIMS (m/e, relative intensity) 390 (M+,
100); HREIMS m/z 391.2017 (M + H) (calcd for C24H26N2O3, 391.2022
(M + H)); anal. C, 72.55; H, 6.74; N, 6.97, calcd for C24H26N2O3 ·1/
2H2O: C, 72.16; H, 6.81; N, 7.01.
of (+)-alstonisine (1) has been accomplished from D-tryptophan methyl
ester 13 in 12% overall yield (in 17 reaction vessels). The stereospecific
conversion of D-(+)-tryptophan methyl ester 13 into tetracyclic ketone
14 was carried out in two reaction vessels on 300 g scale. The
ketoacetal 18 was prepared from 14 by following the steps employed
in the improved synthesis of alstonerine.19 A diastereospecific osmy-
lation process has been investigated and employed as a key step to
convert ketoacetal 18 into spirocyclic oxindole 19. This could serve
as a general approach for synthesis of other macroline-related oxin-
doles. The Nb-benzyl group of 19 was successfully removed by treating
it with 2 equiv of Pearlman’s catalyst in the presence of H2, followed
by base-mediated elimination to provide 1. The absolute structure of
(+)-alstonisine was determined by NOE spectroscopic experiments
and further confirmed by single-crystal X-ray analysis. The original
structure of alstonisine reported by Nordman (X-ray crystallography)
in 1963 therefore was the enantiomer of natural alstonisine (1). In
addition, the structures of the two oxindoles in the report of Wong et
al.4f were illustrated incorrectly. It is important to note that experiments
with t-BuOCl also provide stereospecific entry into the chtosenine
stereochemistry or the alstonisine stereochemistry depending on the
presence or absence of the Nb-benzyl group. Future work now rests
on the synthesis of Gardneria alkaloids [diastereomic at C-7 with
respect to 1], since a route to the chitosenine stereochemistry 26 has
also been developed in high de.
p-TSA/py-Mediated Transfer of the Ketal Group from the Ketal
Oxindole 32 to Provide Ketooxindole 27. To a round-bottom flask (50
mL) that contained a solution of ketal oxindole 32 (110 mg, 0.3 mmol) in
aqueous acetone (20 mL, 50%) was added freshly prepared p-TSA/py
complex (50 mg). The resulting solution was heated to reflux for 24 h.
Analysis by TLC (Si gel, EtOAc/hexanes, 3:7) indicated the disappearance
of the ketal oxindole 32. The above reaction mixture was cooled to rt and
then neutralized with an aqueous solution of NaHCO3. The aqueous layer
was separated and then extracted with CH2Cl2/MeOH (9:1, 3 × 20 mL).
The combined organic layers were washed with brine (3 × 20 mL) and
dried (K2CO3). The solvent was removed under reduced pressure, and the
resulting residue was purified by flash chromatography (Si gel, EtOAc/
hexanes, 1:4) to provide the ketooxindole 27 (292 mg, 80%), the
spectroscopic data of which were identical to the published values.17
27: 1H NMR (500 MHz, CDCl3) δ 1.75 (m, 1H), 2.09 (d, J ) 13.9 Hz,
1H), 2.19 (m, 1H), 2.60 (m, 1H), 2.69 (m, 1H), 2.91 (dd, J ) 13.9, 7.7
Hz, 1H), 3.19 (s, 3H), 3.45 (d, J ) 6.1 Hz, 1H), 3.72 (d, J ) 7.6 Hz, 1H),
4.01 (d, J ) 12.8 Hz, 1H), 4.24 (d, J ) 12.8 Hz, 1H), 6.79 (d, J ) 7.7 Hz,
1H), 7.02 (t, J ) 7.6 Hz, 1H), 7.11 (d, J ) 7.4 Hz, 1H), 7.22 (t, J ) 7.5
Hz, 1H), 7.26 (t, J ) 7.6 Hz, 1H), 7.31 (t, J ) 7.5 Hz, 2H), 7.48 (m, 2H);
CIMS (m/z, relative intensity) 347 (M+ + 1, 100).
Experimental Section
Reaction of Na-Methyl, Nb-Benzyl Tetracyclic Ketone Hydro-
chloride Salt 38 with Osmium Tetraoxide to Provide Ketooxindole
26 as the Major Diastereomer. The Na-methyl, Nb-benzyl, tetracyclic
ketone 14 (512 mg, 1.55 mmol) was dissolved in anhydrous ethanolic HCl
(5%, 20 mL), after which the solvent was removed under reduced pressure.
The solid was washed with cold dry ether a few times and then dissolved
in THF (25 mL). To this solution was added a solution of osmium
tetraoxide (394 mg, 1.55 mmol) in THF (15 mL) at 0 °C. The resulting
black-colored mixture was allowed to stir at rt for 2 h and then at reflux
for 3 days. The reaction mixture was cooled to 0 °C before it was
neutralized with an aqueous solution of NH4OH (10%), and then a solution
of NaHSO3 (800 mg) in H2O (10 mL) was added. The resulting mixture
was stirred at rt for 4 h. Water (50 mL) was added, and the mixture was
extracted with CHCl3 (3 × 400 mL). The combined organic extracts were
washed with brine (50 mL), dried (Na2SO4), and concentrated under
reduced pressure. Flash chromatography (Si gel, EtOAc/hexanes, 1:4)
furnished 386 mg (72% of a 9:1 mixture of diastereomeric oxindoles 26:
27).
General Experimental Procedures. Reagent and solvent purifica-
tion, workup procedures, and analyses were in general performed as
described previously.19 The ketoacetal intermediate 18 was prepared
according to the published procedure.19
Protection of Na-Methyl, Nb-Benzyl, Tetracyclic Ketone 14 with
Ethylene Glycol to Provide Ketal 31. To a round-bottom flask (50 mL)
that contained a solution of Na-methyl, Nb-benzyl, tetracyclic ketone 14
(2.0 g, 0.6 mmol) in benzene (20 mL) were added ethylene glycol (5 mL)
and p-TSA (0.2 g, 7 mmol). The resulting solution was heated to reflux
for 24 h in the presence of a DST. After examination by TLC (Si gel,
EtOAc/hexanes, 3:1) indicated the disappearance of ketone 14, the reaction
mixture was allowed to cool to rt and then poured into a cold aqueous
solution of cold NH4OH (10%, 20 mL). The aqueous layer was separated
and extracted with benzene (3 × 20 mL). The combined organic layers
were washed with brine (2 × 20 mL) and dried (K2CO3), and the solvent was
removed under reduced pressure. The residue that resulted was purified by
crystallization (MeOH/CH2Cl2, 10:1) to provide the ketal 31 (1.8 g, 95%).
31: 1H NMR (300 MHz, CDCl3) δ 1.67-1.76 (m, 3H), 2.14-2.20 (m,
1H), 2.22-2.29 (m, 1H), 2.43 (d, J ) 13.5 Hz, 1H), 2.97 (s, 1H),
3.04-3.16 (m, 1H), 3.21 (s, 3H), 3.27 (d, J ) 7.39 Hz, 1H), 3.75-3.81
(m, 1H), 3.81-3.92 (m, 2H), 4.01-4.03 (m, 1H), 4.04 (d, J ) 12.5 Hz,
1H), 4.36 (d, J ) 12.9 Hz, 1H), 6.74 (d, J ) 7.7 Hz, 1H), 7.06 (t, J ) 7.5
Hz, 1H), 7.23-7.27 (m, 2H), 7.32 (t, J ) 7.2 Hz, 2H), 7.45 (d, J ) 7.6
Hz, 1H), 7.77 (d, J ) 7.4 Hz, 1H); 13C NMR (75.5 MHz, CDCl3) δ 19.7,
26.2, 28.5, 39.0, 51.9, 54.8, 60.01, 63.3, 64.9, 65.4, 106.9, 108.1, 122.3,
123.6, 126.6, 127.3, 127.9, 129.2, 138.1, 139.8, 141.9, 177.7; EIMS (m/z,
relative intensity) 374 (M+, 56), 273 (100); HREIMS, m/z 375.2070 (M
+ H) (calcd for C24H26N2O2, 375.2073 (M + H)); anal. C, 76.74; H, 7.15;
N, 7.48, calcd for C24H26N2O2: C, 76.98; H, 7.00; N, 7.48.
Major diastereomer 26: 1H NMR (500 MHz, CDCl3) δ 2.21 (dd, J
) 14.5, 9.7 Hz, 1H), 2.29 (m, 1H), 2.42 (dd, J ) 13.7, 0.7 Hz, 1H), 2.44
(dd, J ) 17.5, 8.6 Hz, 1H), 2.54 (dd, J ) 13.7, 7.3 Hz, 1H), 3.16 (br d, J
) 4.0 Hz, 1H), 3.23 (s, 3H), 3.55 (dt, J ) 17.5, 9.8 Hz, 1H), 3.70 (br d,
J ) 7.3 Hz, 1H), 3.87 (d, J ) 13.0 Hz, 1H), 4.05 (d, J ) 13.0 Hz, 1H),
6.81 (d, J ) 7.4 Hz, 1H), 7.12 (t, J ) 7.4 Hz, 1H), 7.27 (t, J ) 7.4 Hz,
1H), 7.28 (t, J ) 7.4 Hz, 1H), 7.32 (t, J ) 7.4 Hz, 2H), 7.43 (d, J ) 7.4
Hz, 2H), 7.85 (d, J ) 7.4 Hz, 1H); CIMS (m/z, relative intensity) 347
(M+ + 1, 100). The spectral data for the major isomer 26 were identical
to that of an authentic sample.17
Diastereospecific Conversion of the Na-H, Nb-Benzylketone 40
into the Na-H, Nb-Benzylketooxindole 41 via tert-Butyl Hypochlorite.
To a round-bottom flask (100 mL) that contained a stirred solution of Na-
H, Nb-benzylketone 40 (816 mg, 2.5 mmol) and Et3N (0.28 g, 1.1 equiv)
in CH2Cl2 (30 mL) was added the freshly prepared tert-butyl hypochlorite16
(0.36 g, 0.4 mL, 3 mmol) at 0 °C. The resulting reaction mixture was
stirred at 0 °C for 12 h. The solvent was removed under reduced pressure,
and the resulting solid was dissolved in a solution of MeOH/10% AcOH
(1:1, 20 mL). The resulting reaction mixture was heated to reflux for 2 h,
and the solvent was removed under reduced pressure. The residue was
dissolved in EtOAc (50 mL), and the organic layer was washed with an
aqueous solution of NaHCO3 (20 mL), brine (2 × 20 mL), and dried
(Na2SO4). After removal of the solvent under reduced pressure, the crude
product was purified by flash chromatography (Si gel, EtOAc/hexanes,
3:7) to provide the Na-H, Nb-benzylketooxindole 41 (770 mg, 93%).
41: mp 203-204 °C; [R]25D +182.3 (c 0.95, CHCl3); IR (KBr) 3236,
Reaction of Ketal 31 with Osmium Tetraoxide to Provide Ketal
Oxindole 32. To a round-bottom flask (100 mL) was charged a lightly
yellow-colored solution of OsO4 (266 mg, 1.0 mmol) in dry THF (5 mL)
and pyridine (2 mL, freshly distilled), and this was stirred at rt for 2 h. To
this solution was added the solution of ketal 31 (350 mg, 1.0 mmol) in
dry THF (20 mL) and pyridine (freshly distilled, 1.5 mL) at 0 °C. The
resulting black-colored mixture was stirred at rt for 72 h under an
atmosphere of Ar. An aqueous solution of NaHSO3 (0.6 g in 3 mL of
H2O) was then added, and the slurry that resulted was stirred at rt for 5 h.
The reaction mixture was diluted with CH2Cl2 (50 mL), and the aqueous
layer was separated and then extracted with CH2Cl2/MeOH (4:1, 5 × 50
mL). The combined organic layers were dried (K2CO3), and the solvent
was removed under reduced pressure. The residue that resulted was purified
by flash chromatography (Si gel, EtOAc/hexanes, 1:4) to provide the ketal
oxindole 32 (292 mg, 80%).
1
32: 1H NMR (300 MHz, CDCl3) δ 1.51-1.57 (m, 2H), 1.57-1.68 (m,
3H), 2.07-2.31 (m, 1H), 2.87-3.14 (m, 3H), 3.57 (s, 3H), 3.70 (s, 3H),
1707, 1676, 1466 cm-1; H NMR (300 MHz, CDCl3) δ 2.14-2.28 (m,
2H), 2.31-2.48 (m, 3H), 3.11 (d, J ) 3.0 Hz, 1H), 3.41(dt, J ) 7.7, 8.5

Enantiospecific Synthesis of (+)-Alstonisine
Journal of Natural Products, 2008, Vol. 71, No. 8 1439
Hz, lH), 3.50 (d, J ) 7.3 Hz, lH), 3.80 (dd, J ) 4.7, 14.5 Hz, 2H), 6.72
(d, J ) 6.1 Hz, lH), 7.03 (t, J ) 6.1 Hz, lH), 7.15-7.33 (m, 6H), 7.67 (d,
J ) 6.0 Hz, lH), 8.05 (s, lH); 13C NMR (75.5 MHz, CDCl3) δ 23.7, 34.3,
39.6, 51.7, 55.7, 65.3, 67.5, 109.0, 122.9, 124.0, 127.3, 127.9, 128.4, 128.8,
137.5, 137.6, 139.0, 179.4, 212.5; EIMS (m/z, relative intensity) 332 (M+,
26), 304 (80), 173 (100); HREIMS m/z 333.1609 (M + H) (calcd for
C21H20N2O2, 333.1603 (M + H)); anal. C, 74.80; H, 5.89; N, 8.18, calcd
for C21H20N2O2 ·1/4H2O: C, 74.87; H, 6.13; N, 8.32.
Diastereospecific Conversion of the Na-H, Nb-H Ketone 42 into
the Na-H, Nb-H Ketooxindole 43 via tert-Butyl Hypochlorite. To a
round-bottom flask (50 mL) that contained a stirred solution of Na-H, Nb-H
ketone 42 (113 mg, 0.5 mmol) and Et3N (0.1 g, 1.2 equiv) in CH2Cl2 (20
mL) was added the freshly prepared tert-butyl hypochlorite16 (72 mg, 80
µL, 0.6 mmol) at 0 °C. The resulting reaction mixture was stirred at 0 °C
for 12 h. The solvent was removed under reduced pressure, and the resulting
solid was dissolved in a solution of MeOH/10% AcOH (1:1, 15 mL). The
resulting solution was heated to reflux for 2 h, and the solvent was removed
under reduced pressure. The residue was dissolved in EtOAc (20 mL),
and the organic layer was washed with an aqueous solution of NaHCO3
(10 mL), brine (2 × 10 mL), and dried Na2SO4. After removal of the
solvent under reduced pressure, the crude oxindole was purified by flash
chromatography (Si gel, EtOAc/hexanes, 3:5) to provide the Na-H, Nb-H
ketooxindole 43 (97 mg, 80%), the spectroscopic data of which were
identical to the published values.15,16
Na-Methylation of Na-H, Nb-Benzylketooxindole 41 to Provide
Na-Methyl, Nb-Benzylketooxindole 26. To a round-bottom flask (50
mL) that contained a suspension of NaH (70 mg, 2.5 mmol) in THF (5
mL) was added the solution of Na-H, Nb-benzylketooxindole 41 (160 mg,
0.5 mmol) in THF (20 mL) at 0 °C. The resulting reaction mixture was
allowed to stir at rt for 2 h and cooled to 0 °C. Methyl iodide (180 mg,
1.8 mmol) was added, and the resulting solution was allowed to stir at 0
°C for 6 h. Analysis by TLC indicated the disappearence of 41. The reaction
was quenched by addition of CH3OH (0.5 mL) and was then neutralized
with an aqueous solution of NH4Cl. The aqueous layer was separated and
extracted with EtOAc (3 × 20 mL). The combined organic layers were
washed with brine (2 × 10 mL) and dried (K2CO3). The solvent was
removed under reduced pressure, and the resulting residue was subjected
to a wash column (Si gel, EtOAc/hexanes, 2:8) to provide the Na-methyl,
Nb-benzylketooxindole 26 (159 mg, 93%), which was identical to 26 from
the osmylation of ketone 14.
1H), 4.85 (d, J ) 2.6 Hz, 1H), 6.65 (d, J ) 7.3 Hz, 1H), 6.80 (t, J ) 10.8
Hz, 1H), 7.16-7.34 (m, 6H), 7.46 (d, J ) 4.5 Hz, 1H); 13C NMR (75.5
MHz, CDCl3) δ 23.3, 26.4, 26.8, 29.0, 40.4, 42.8, 54.9, 55.6, 55.8, 55.9,
58.7, 59.5, 67.3, 98.2, 107.5, 122.7, 124.0, 127.5, 127.8, 128.3, 128.7, 129.3,
129.4, 138.5, 139.5, 142.2, 178.55, 208.6; EIMS (m/z, relative intensity)
460 (M+, 10.0), 301 (38.0), 228 (42.0), 258 (30.0), 196 (30.0), 170 (100.0),
146 (100.0), 130 (40.0), 106 (41.4). This material was used directly in the
next step.
Base-Mediated Elimination of the Elements of Methanol from
Ketoacetal 19 to Provide Nb-Benzylastonisine 20. To a round-bottom
flask (10 mL) that contained a stirred solution of the ketoacetal oxindole
19 (6 mg, 0.013 mmol) in CH3OH (2 mL) was added an aqueous solution
of NaOH (2 N, 1 mL) at 0 °C. The resulting reaction mixture was allowed
to stir at rt for 12 h and then was cooled to 0 °C. The above solution was
neutralized with an aqueous solution of saturated NH4Cl (2 mL), and the
aqueous layer was extracted with CH2Cl2 (3 × 3 mL). The combined
organic extracts were washed with brine (2 × 2 mL) and dried (Na2SO4).
After the solvent was removed under reduced pressure, the residue was
purified by flash chromatography (Si gel, EtOAc/hexanes, 3:7) to provide
Nb-benzylalstonisine 20 (5.2 mg, 90%).
20: IR (KBr) 1650, 1625 cm-1; 1H NMR (300 MHz, CDCl3) δ 0.81 (t,
J ) 5.0 Hz, 1H), 1.46-1.73 (m, 1H), 1.91 (m, 1H), 2.12 (m, 1H), 2.17
(dd, J ) 6.2, 3.8 Hz, 1H), 2.21 (s, 3H), 2.36 (d, J ) 12.6 Hz, 1H), 2.46
(dd, J ) 16.5, 7.2 Hz, 1H), 3.05 (m, 1H), 3.15 (s, 3H), 3.64 (d, J ) 6.9
Hz, 1H), 3.89-3.94 (m, 1H), 3.99 (d, J ) 3.9 Hz, 1H), 4.15 (d, J ) 12.6
Hz, 1H), 4.29-4.36 (m, 1H), 6.63 (d, J ) 8.1 Hz, 1H), 6.81 (t, J ) 7.6
Hz, 1H), 7.10 (t, J ) 7.8 Hz, 1H), 7.20-7.33 (m, 5H), 7.44 (d, J ) 7.5
Hz, 1H); 13C NMR (75.5 MHz, CDCl3) δ 14.0, 22.6, 24.5, 26.0, 26.4,
31.5, 39.3, 42.2, 54.9, 55.7, 58.5, 67.5, 67.7, 107.0, 121.87, 121.9, 123.4,
127.1, 127.3, 128.3, 128.9, 137.7, 138.8, 142.3, 155.9, 177.6, 197.5; HRMS
m/z 428.2125 (calcd for C27H28N2O3, 428.2110).
Debenzylation of Na-H, Nb-Benzyl Ketooxindole 41 to Provide
Na-H, Nb-H Spirooxindole 50. To a round-bottom flask (25 mL) that
contained a solution of Na-H, Nb-benzyl ketooxindole 41 (100 mg, 0.31
mmol) in absolute EtOH (10 mL) was added 2 equiv (20 wt %) of
Pd(OH)2/C. The resulting slurry was allowed to stir at rt under 1 atm of
H2 for 6 h. Examination of the mixture by TLC (EtOAc/hexanes, 1:1)
indicated the disappearance of starting material 41 and the appearance of
a new component, 50. The catalyst was filtered from the medium and
washed with EtOH (5 × 10 mL). The combined filtrates were concentrated
under reduced pressure. The residue was purified by flash chromatography
(Si gel, CHCl3/MeOH, 1:19) to provide the Na-H, Nb-H spirooxindole 50
(69 mg, 90%).
Reaction of Ketoacetal 18 with Osmium Tetraoxide to Provide
(3S,3′S,4aR,6S,9S,9aR)-4-Acetyl 3,4,4a,5,6,8,9,9a-octahydro-3-meth-
oxy-1′-methyl-10-(phenylmethyl)spiro[cyclohepta[c]pyran]-6,9-imino-
7(1H),3′-[3H]indol-2′(1′H)-one (19). The ketoacetal 18 was prepared
according to the published procedure, the spectroscopic data of which were
1
50: FTIR 3419, 1637 cm-1; H NMR (300 MHz, CDCl3) δ 1.90 (m,
2H), 2.10 (m, 2H), 2.28 (dd, J ) 14.1, 7.2 Hz, 1H), 2.39 (dd, J ) 14.0,
1.4 Hz, 1H), 3.40 (s, 1H), 3.75 (m, 1H), 4.13 (m, 1H), 6.98 (d, J ) 7.7
Hz, 1H), 7.10 (td, J ) 7.6, 0.9 Hz, 1H), 7.29 (td, J ) 7.7, 0.9 Hz, 1H),
7.55 (d, J ) 7.6 Hz, 1H); 13C NMR (75.5 MHz, CDCl3) δ 24.7, 25.8,
33.1, 56.2, 59.3, 61.6, 67.5, 109.9, 122.3, 124.3, 128.07, 128.4, 142.3,
184.1; CIMS (m/z, relative intensity) 244 (M+ + 1, 100).
19
1
identical to the published values. 18: H NMR (300 MHz, CDCl3) δ
1.77-1.84 (m, 2H), 1.93 (s, 3H), 2.28 (m, 1H), 2.35-2.44 (m, 2H), 2.55
(dt, J ) 12.9, 4.0 Hz, 1H), 2.98 (d, J ) 7.0 Hz, 1H), 3.20 (dd, J ) 9.5,
7.0 Hz, 1H), 3.35-3.40 (m, 1H), 3.41 (s, 3H), 3.49 (s, 3H), 3.53 (s, 2H),
3.90 (t, J ) 3.2 Hz, 1H), 4.38 (t, J ) 11.6 Hz, 1H), 5.02 (q, J ) 3.4 Hz,
1H), 7.07 (t, J ) 7.8 Hz, 1H), 7.14 (t, J ) 6.8 Hz, 1H), 7.20-7.27 (m,
6H), 7.46 (d, J ) 7.6 Hz, 1H); 13C NMR (75.5 MHz, CDCl3) δ 22.7,
26.2, 27.4, 28.2, 28.9, 42.9, 51.7, 52.8, 54.7, 55.4, 57.4, 59.8, 98.3, 106.7,
108.9, 117.7, 118.6, 120.6, 126.4, 126.8, 128.1, 128.5, 133.8, 137.0, 139.5,
205.1; CIMS (m/z, relative intensity) 445 (M+ + 1, 100). This material
was used directly in the next step.
Debenzylation of (3S,3′S,4aR,6S,9S,9aR)-4-Acetyl-3,4,4a,5,6,8,9,9a-
octahydro-3-methoxy-1′-methyl-10-(phenylmethyl)spiro[cyclohep-
ta[c]pyran]-6,9-imino-7(1H),3′-[3H]indol-2′(1′H)-one (19) Followed
by Base-Mediated Elimination of the Elements of Methanol to
Provide Alstonisine (1). To a round-bottom flask (10 mL) that contained
a solution of Na-methyl, Nb-benzyl ketoacetal oxindole 19 (40 mg, 0.087
mmol) in absolute EtOH (5 mL) was added 2 equiv of 20 wt % Pd(OH)2/C
(Pearlman’s catalyst; 122.1 mg). The resulting slurry was allowed to stir
at rt under 1 atm of H2 for 5 h. Examination of the mixture by TLC (EtOAc/
hexanes, 1:1) indicated the disappearance of starting material and the
appearance of a new component. The solvent was removed after the catalyst
was filtered and washed with EtOH (5 × 10 mL). This Nb-H ketoacetal
oxindole intermediate 21 was directly employed for the next step without
further purification. To a round-bottom flask (10 mL) that contained a
stirred solution of the above intermediate in CH3OH (5 mL) was added
an aqueous solution of NaOH (2 N, 4 mL) at 0 °C. The resulting reaction
mixture was allowed to stir at rt for 2 h. The above solution was neutralized
with an aqueous solution of saturated NH4Cl (5 mL), and the aqueous
layer was extracted with CH2Cl2 (3 × 5 mL). The combined organic
extracts were washed with brine (2 × 5 mL) and dried (Na2SO4). After
the solvent was removed under reduced pressure, the residue was purified
by flash chromatography (Si gel, EtOAc/hexanes, 2:3) to provide (+)-
alstonisine (1) (25.2 mg, 86%).
To a round-bottom flask (100 mL) was charged a light yellow-colored
solution of OsO4 (114.4 mg, 0.45 mmol) in dry THF (15 mL) and pyridine
(3 mL, freshly distilled), which had been prestirred at rt for 2 h. To this
solution was added at 0 °C the solution of ketoacetal 18 (200 mg, 0.45
mmol) in dry THF (15 mL) and pyridine (3 mL, freshly distilled). The
resulting black-colored mixture was stirred at rt for 72 h under an
atmosphere of Ar. An aqueous solution of NaHSO3 (1.6 g in 8 mL of
H2O) was then added at 0 °C, and the resulting slurry was allowed to stir
at rt for 5 h. The reaction mixture was diluted with CH2Cl2 (30 mL), and
the aqueous layer was separated and then extracted with CH2Cl2/MeOH
(4:1, 5 × 15 mL). The combined organic layers were dried (K2CO3), and
the solvent was removed under reduced pressure. The resulting residue
was purified by flash chromatography (Si gel, EtOAc/hexanes, 1:4) to
provide the ketoacetal oxindole 19 as a white solid (167.5 mg, 81%).
19: IR (KBr) 1702, 1610 cm-1 1H NMR (300 MHz, CDCl3) δ
;
1.35-1.38 (ddd, J ) 13.9, 5.7, 2.0 Hz, 1H), 2.10 (m, 1H), 2.24-2.30 (m,
1H), 2.29 (s, 3H), 2.39 (m, 2H), 2.69 (m, 1H), 2.98 (m, 1H), 3.11 (s, 3H),
3.30-3.40 (m, 3H), 3.45 (s, 3H), 3.59 (m,1H), 4.05 (m, 1H), 4.20 (m,

1440 Journal of Natural Products, 2008, Vol. 71, No. 8
Yang et al.
1: mp 169-170 °C (lit.10 mp 168-169 °C); [R]25 +197 (c 1.0 in
J.; Sakai, S. Chem. Pharm. Bull. 1990, 38, 573. (e) Bascop, S. I.;
Sapi, J.; Laronze, J. Y.; Levy, J. Heterocycles 1994, 38, 725. (f) von
Nussbaum, F.; Danishefsky, S. J. Angew. Chem., Int. Ed. 2000, 39,
2175.
D
EtOH), [lit.10 [R]25D +200 (c 1.0 in EtOH)]; IR (Nujol mulls) 1691, 1646,
doublets at 1619, 1610 cm-1, [lit.10 Nujol mulls, 1690, 1645, doublets at
1
1615 and 1605 cm-1]; H NMR (500 MHZ, CDCl3) δ 1.53-1.58 (m,
(25) Fleming, I.; Loreto, M. A.; Michael, J. P.; Wallace, I. H. M.
Tetrahedron Lett. 1982, 23, 2053.
1H), 1.96 (m, 1H), 2.20 (d, J ) 13.4 Hz, 1H) 2.25 (s, 3H), 2.24-2.26 (m,
1H), 2.53 (dd, J ) 13.5, 7.5 Hz, 1H), 3.20 (s, 1H), 3.21 (s, 3H), 3.39 (dt,
J ) 11.8, 6 Hz, 1H), 3.70 (s, 1H), 4.25 (ddd, J ) 11.0, 4.0, 1.5 Hz, 1H),
4.45 (t, J ) 11.0 Hz, 1H), 6.86 (d, J ) 7.8 Hz, 1H), 7.30 (td, J ) 7.6, 1.2
Hz, 1H), 7.34 (td, J ) 7.6, 1.1 Hz, 1H), 7.63 (s, 1H), 8.26 (d, J ) 7.7 Hz,
1H); 13C NMR (75.5 MHz, CDCl3) δ 24.4, 25.2, 26.4, 31.2, 37.3, 42.1,
56.5, 57.1, 64.0, 68.5, 108.2, 121.9, 123.6, 125.8, 128.2, 129.3, 144.2, 157.8,
182.6, 196.8; EIMS (m/z, relative intensity) 338 (M+, 38), 192 (9), 179
(55), 160 (100), 136 (57), 118 (18). The spectral data for this material
were identical to that reported for natural alstonisine.3
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Acknowledgment. X-ray crystallographic studies were supported under
NIDA contact Y1-DA6002. We thank J. Flippen-Anderson and the Office
of Naval Research for the crystal structure of 1, as well as NIDA and
NIMH, for support of this work. This paper is dedicated to Professor Atta-
ur-Rahman for his seminal contributions to natural products chemistry.
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Guller, R.; Borschberg, H.-J. Tetrahedron: Asymmetry 1992, 3, 1197.
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Aimi, N.; Yamaguchi, K.; Yamanaka, E. J. Chem. Soc., Perkin Trans.
1 1982, 12, 1257. (c) Sakai, S.; Aimi, N.; Yamaguchi, K.; Yamanaka,
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K.; Moore, P. R.; Waring, M. J.; Winter, J. J.; Helliwell, M.;
Newcombe, N. J; Stemp, G. J. Org. Chem. 2002, 67, 7946. (c)
Donohoe, T. J.; Churchill, G. H.; Wheelhouse, K. M. P.; Glossop,
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ReV. 1980, 80, 187.
(31) (a) Wearing, X. Z. Ph.D. Thesis, University of Wisconsin-Milwaukee,
2004. The Nb-benzyl group of 14 was believed to be favored in the axial
position (with respect to ring D) according to the previous results from a
study (NMR) in the Na-Me, Nb-Bn tetracyclic series 14. (b) In earlier
work, a single-crystal X-ray analysis of an Nb-benzyltetracyclic derivative
also indicated that the benzyl group rested in the axial position of the D
ring in the crystal: Zhang, L. H.; Trudell, M. L.; Hollinshead, S. P.; Cook,
J. M. J. Am. Chem. Soc. 1989, 111, 8263. In solution this has been
supported by analysis of NOE studies. The concomitant complexation
of osmium at the equatorial position (with respect to ring D) of indole
51 facilitated intramolecular attack of the osmium reagent to furnish
osmate ester 29 (Scheme 4) upon heating at reflux. If complexation
occurred at the axial position (with respect to the D ring) of indole 52
to give a complex, the intramolecular delivery of the osmium reagent
to the 2,3-indole double bond would be unlikely; see ref 17.
Supporting Information Available: 1H and 13C NMR spectra for 1,
18, 19, 31, 32, 41, and 50; HSQC for 50; NOE and NOESY spectra of 1;
and the X-ray crystal parameters for alstonisine (1) and compound 41.
These materials are available free of charge via the Internet at http://
pubs.acs.org.
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NP800269K