Concise, asymmetric total synthesis of spirotryprostatin A

Article abstract of DOI:10.1016/j.tet.2004.07.047

The structurally intriguing cell-cycle inhibitor spirotryprostatin A has been synthesized utilizing an azomethine ylide dipolar cycloaddition reaction as the key step. This pentacyclic alkaloid contains a prenylated tryptophan-derived oxindole moiety that

Full text of DOI:10.1016/j.tet.2004.07.047

Tetrahedron 60 (2004) 9503–9515
Concise, asymmetric total synthesis of spirotryprostatin A
*
Tomoyuki Onishi, Paul R. Sebahar and Robert M. Williams
Department of Chemistry, Colorado State University, 1301 Center Avenue, Fort Collins, CO 80523, USA
Received 13 February 2004; revised 19 July 2004; accepted 20 July 2004
Abstract—The structurally intriguing cell-cycle inhibitor spirotryprostatin A has been synthesized utilizing an azomethine ylide dipolar
cycloaddition reaction as the key step. This pentacyclic alkaloid contains a prenylated tryptophan-derived oxindole moiety that has been
created in a regiocontrolled and stereocontrolled manner in a single step.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
chain, three or four stereogenic centers which pose
formidable synthetic challenges, and the enamide moiety
in the case of spirotryprostatin B. These issues have been
addressed by various strategies and have culminated in the
total synthesis of spirotryprostatin B (2) by the groups of
Williams,8 Danishefsky,9 Ganesan,10 Overman,11 Fuji12 and
Carreira.13,14 On the other hand, only one total synthesis of
spirotryprostatin A using the classical halohydrin to
oxindole spiro-ring-forming contraction sequence has
been reported thus far by Danishefsky.15 We previously
described the total synthesis of spirotryprostatin B (2) using
a stereochemically distinct three-component asymmetric
azomethine ylide [1,3]-dipolar cycloaddition reaction.8
Herein, we report a concise asymmetric total synthesis of
spirotryprostatin A (1).16
The spirotryprostatins,1 tryprostatins2 and cyclotryprosta-
tins3 represent a promising class of antimitotic arrest agents.
Isolated from Aspergillus fumigatus, spirotryprostatin A (1,
Fig. 1) and spirotryprostatin B (2) were shown to completely
inhibit the progression of cells at concentrations greater than
253 and 34.4 mM, respectively.1 The spirotryprostatins are
characterized by a unique spiro-oxindole substituted cis-
prolyl-prolyl-diketopiperazine that is prenylated at C-18.
The detailed mechanism of action by which these
substances inhibit microtubule assembly is presently not
known and studies to discover the target of these natural
products have been hampered by the small quantities of
these substances that can be conveniently isolated from the
producing organism. Despite their relatively modest bio-
logical activity relative to other members of this family, the
spirotryprostatins have nonetheless garnered the most
attention due to their intriguing molecular structures.
2. Results and discussion
2.1. Initial synthetic route to spirotryprostatin A
Since the isolation of the natural products in 1996,
numerous groups have embarked on research programs
directed towards the total synthesis of spirotryprostatins A
and B. Various research groups have focused their efforts on
the development of synthetic methodology of just the spiro-
oxindole pyrrolidine portion of the natural products. Recent
approaches include [5C2]-cycloaddition of enantiomeri-
cally pure h3-pyridinyl molybdenum complexes,4 directed
radical cyclizations,5 ring expansion of cyclopropanes by
aldimines6 and iodide ion-induced rearrangement of [(N-
aziridinomethylthio)methylene]-oxindoles.7 In addition to
the generation of the spiro-oxindole quaternary carbon, a
total synthesis endeavor must contend with the prenyl side-
In contemplating the synthesis of spirotryprostatin A (1), it
was envisioned that the core pyrrolidine ring could be
formed through an asymmetric [1,3]-dipolar cycloaddition
Keywords: Spirotryprostatin A; Dipolar cycloaddition; Azomethine ylide.
* Corresponding author. Tel.: C1-970-491-6747; fax: C1-970-491-5610;
e-mail: rmw@chem.colostate.edu
Figure 1. Structures of spirotryprostatins A and B.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.07.047

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T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
similar to that employed in our spirotryprostatin B
synthesis.8 Spirotryprostatin A (1) differs from spirotry-
prostatin B (2) in that it is saturated at C-8 and C-9 and is
substituted at C-6 by a methoxy group whereas spirotry-
prostatin B (2) is absent of functionality in the aromatic ring
and contains the characteristic C-8, C-9-enamide moiety.
The enamide of spirotryprostatin B was installed via a
Barton-modified Hunsdiecker reaction through an oxidative
decarboxylation of a carboethoxy group that was introduced
at C-8 in the initial dipolar cycloaddition reaction. First, we
attempted to apply the same strategy, which was developed
for spirotryprostatin B, to the total synthesis of spirotry-
prostatin A (Scheme 1). This approach would have to
account for the substitution of the aromatic ring and the
formation of the fourth stereogenic center. Thus, [1,3]-
dipolar cycloaddition with methoxy-substituted oxindolyli-
dene acetate 3 would yield spiro-oxindole pyrrolidine
amino acid 6 upon reductive cleavage of the chiral auxiliary.
Coupling of 6 to L-proline benzyl ester and concomitant
cyclization would afford diketopiperazine 7. Deprotection
of the carboxyl group followed by a Barton-modified
Hunsdiecker reaction as deployed previously, would result
in the formation of enamide 8. Palladium-catalyzed
reduction of the olefin was reasonably expected to occur
from the least hindered face opposite the isopropylidene
group and the aromatic ring to cis-diketopiperazine. Final
acid-catalyzed elimination of methanol would then afford
spirotryprostatin A (1).
reaction with (carbethoxymethylene)triphenyl phosphorane
then afforded 6-methoxy-ethyl oxindolylidene acetate 9.
Crystallization of the product mixture afforded only the
desired E-isomer. Addition of dipolarophile 9 to the
azomethine ylide derived from morpholinone 4 and
aldehyde 5 yielded cycloadducts 10 and 11 (Scheme 2).
The reaction proceeded in only modest yields (60%) and
afforded the two products as a w1:1 mixture. It was
suspected that the yield and selectivity were a result of the
poor solubility of ethyl oxindolylidene acetate (9) in toluene
at room temperature. It was reasoned that this slowed the
cycloaddition down relative to the analogous system used
quite successfully in our spirotryprostatin B synthesis,
allowing for a competing pathway via formation and
cycloaddition of an incipient unsaturated azomethine ylide
that results in the formation of the unsaturated cycloadduct
(11) to prevail.8 It was speculated that an increase in the
lipophilicity of the dipolarophile might aid in the solubility
of this species and suppress formation of the unsaturated
azomethine ylide. Therefore, the tert-butyl ester 3 was
synthesized which proved to be readily soluble in toluene at
room temperature. Subjecting 3 to the standard reaction
conditions for the [1,3]-dipolar cycloaddition resulted in an
improved yield (70%) and an increase in the ratio (ca. 2:1)
of the desired cycloadduct 12 over the unsaturated
cycloadduct 13.
Construction of the diketopiperazine began with palladium-
catalyzed hydrogenolysis of cycloadduct 12 (Scheme 3).
The resulting amino acid 619 was coupled without
purification to L-proline benzyl ester with BOP as the
activating agent. Reduction of the resulting dipeptide-
benzyl ester, followed by intramolecular cyclization
afforded diketopiperazine 7 in 39% yield over three steps.
This is in contrast to the 69% yield observed for the
In the synthesis of spirotryprostatin B, ethyl oxindolylidene
acetate was synthesized via Wittig reaction of commercially
available 1H-indole-2,3-dione (isatin) and the requisite
stabilized ylide. For spirotryprostatin A, 6-methoxy-isatin17
was not commercially available and necessitated prep-
aration from m-anisidine by a Sandmeyer reaction.18 Wittig
Scheme 1. Initial strategy to spirotryprostatin A (1).

T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
9505
Scheme 2. [1,3]-Dipolar cycloaddition with 6-methoxy-alkyl oxindolylidene acetate 9 and 3.
analogous sequence employed in the spirotryprostatin B
synthesis.8 It is not currently understood why substitution of
the aromatic ring or exchange of the ethyl ester for a tert-
butyl ester caused such a decrease in the overall yield.
Completion of the synthesis of spirotryprostatin A required
hydrolysis of the ester functionality and the Barton-
modified Hunsdiecker reaction to afford enamide 8. The
tert-butyl ester 7 was hydrolyzed using trifluoroacetic acid
in yields ranging from 42–58%. However, subjecting the
resulting carboxylic acid 15 to the same oxidative
decarboxylation conditions employed successfully in the
spirootryprostatin B synthesis failed to provide enamide 8.
Kochi-type conditions (Pb(OAc)4; and thermal or photolytic
cleavage of a benzophenone oxime ester) were also
unsuccessful. Attempted reductive decarboxylation con-
ditions resulted in decomposition of the starting material.
Attempts to affect either the oxidative or the reductive
decarboxylation at an earlier stage in the synthesis were
similarly unsuccessful. The complications in the elimination
of the carboxyl group and the relatively lower yields
observed for the previous steps warranted exploration of a
new strategy.
2.2. Revised synthetic route to spirotryprostatin A
A new approach, one that avoided the problematic oxidative
decarboxylation step, was eventually devised as outlined in
Scheme 4. Since it is not necessary to install a carboxyl
group as a precursor for the saturated pyrrolidine moiety, the
strategy revolved around formation of 6-methoxy-3-meth-
ylene-1,3-dihydro-indol-2-one (16). Elimination of the
carboalkoxy group from the dipolarophile at the outset
would alleviate the problems associated with its ultimate
removal. If the synthesis of this dipolarophile could be
accomplished, [1,3]-dipolar cycloaddition with 4 and 5
would generate a cycloadduct 17 that would have the correct
configuration at the adjacent C-3 quaternary and C-18
stereogenic centers. However, based on the established
facial selectivity of azomethine ylide reactions derived from
4, the a-proton (C-9) would need to be epimerized before
elaboration to the diketopiperazine 19. This was anticipated
to be a non-trivial operation since, as our spirotryprostatin B
synthesis had shown, the thermodynamic instability of the
trans-diketopiperazines in this structural family resulted in
the facile epimerization of the prolyl-stereogenic center.8
Finally, elimination of the tertiary methyl ether of 19 would
afford the natural product (1).
Recently, Horvath and co-workers reported that azomethine
ylides generated from silylaminonitriles and 3-methyle-
neindolin-2-one react to give the corresponding cycloadduct
in 70% yield.20 However, the dipolarophile was generated
by flash vacuum pyrolysis and did not seem compatible with
the synthesis of the methoxy-substitued derivative we
required. After extensive exploration, we found that the
Peterson olefination,21 which has proven to be an efficient
method for the generation of terminal olefins, afforded a
suitable method for the formation of 16. As shown in
Scheme 5, addition of trimethylsilylmethyllithium to 6-
methoxy-isatin17 (20) afforded tertiary alcohol 21 in 85%
yield. The exo-methylene species 16 could be prepared in
situ by the treatment of 21 with trifluoroacetic acid at 0 8C.
Compound 16 proved to be an unstable species that was not
isolable due to polymer formation upon concentration.
Thus, after neutralization with triethylamine, the reaction
mixture containing crude 16 was directly and rapidly used
for the cycloaddition. By the addition of 4 and 5 to the
Scheme 3. Elaboration to diketopiperazine 7 and attempted decarboxyla-
tion of 15.

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T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
Scheme 4. Revised strategy to spirotryprostatin A.
resultant crude mixture of 16 thus prepared, the [1,3]-
dipolar cycloaddition proceeded rapidly to give a mixture of
cycloadducts (17 and 22). We were unable to detect the
generation of alternate regio- or diastereoisomers as
products in the crude reaction mixture. In initial attempts
performed at room temperature, the ratio of products
unfortunately heavily favored the methanol elimination
product 22. Although a strategy utilizing 22 as a potential
intermediate was explored, the olefin and oxindole func-
tionalities proved incompatible with the conditions required
to remove the chiral auxiliary.
since the yield would be influenced by amount of the
unstable intermediate 16; however, the yield for the desired
product 17 was not improved. Next, we evaluated the effect
of washing the reaction mixture after neutralization with
triethylamine. First, the cycloaddition was performed with-
out washing to give the methanol elimination product 22
exclusively (Table 1). When the reaction mixture was
washed with saturated aqueous sodium bicarbonate solution
or saturated aqueous citric acid solution, these attempts also
afforded 22 exclusively. However, when just water was, the
cycloaddition gave the desired compound 17 in 24% yield
along with 42% of 22. 1H NMR studies were then conducted
to decipher at what stage during the reaction methanol was
being eliminated. After mixing aldehyde 5 and 0.83 equiv of
4 for 5 min in C6D6 at room temperature, we observed the
generation of a significant amount (O50%) of 3-methyl-2-
This initially discouraging result prompted us to carefully
explore the cycloaddition to elucidate crucial factors to
control the reaction and suppress formation of 22. First, the
amount of the oxindole 21 was increased from 1.5 to 2 equiv
Scheme 5. [1,3]-Dipolar cycloaddition with methylene indolinone 16: an initial attempt.

T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
Table 1. Optimization studies for the azomethine ylide dipolar cycloaddition
9507
Entry
Washing
Temperature (8C)
Yield
17
22
1
2
3
4
5
No washing
H2O and sat. NaHCO3
H2O and sat. citric acid
H2O
H2O
rt
rt
rt
ND
ND
ND
24
61
40
46
42
20
rt
K15–0 8C
44
N.D.: not detected (i.e. !trace).
butenal whereas 4 remained intact. This indicated that
elimination of methanol from 5 proceeds rapidly at room
temperature. In an attempt to obviate the elimination before
the dipolar cycloaddition reaction, the cycloaddition was
then performed at 0 8C. Under these conditions, the desired
cycloadduct 17 was isolated in 44% yield as a major product
along with 20% of 22.
remove HMPA (a by-product from BOP-mediated coup-
ling) from the product since diketopiperazine 24 proved to
be quite hydrophilic. Additionally, it was observed that
some of the product (less than 18%) were partitioned into
the aqueous layer in the presence of HMPA.
Next, diketopiperazine 24 was subjected to treatment with
p-TsOH-H2O in refluxing toluene to give 9-epi-spirotry-
prostatin A (25) in 44% yield along with the olefin isomer of
9-epi-spirotryprostatin A (26, 3%). The relative configur-
ation of 25 was confirmed by NOESY to be a trans-fused
diketopiperazine. Unexpectedly, only a trace amount (2%)
of the cis-fused substance (spirotryprostatin A, 1), which
could be isolated from 26 by HPLC, was generated. We
were unable to detect the generation of 9,12-bis-epi-
spirotryprostatin A. This result is in stark contrast to that
observed for our spirotryprostatin B synthesis, in which
epimerization readily occurred at the prolyl-stereogenic
center (C-12) to give the thermodynamically more stable
cis-fused diketopiperazine.8
The regiochemistry of 17 was ascertained by the doublet of
doublets observed in the 1H NMR spectrum for the a proton
(to become C-9) and the relative configuration was
confirmed by NOESY. As anticipated, these data indicated
that the cycloadduct possesses the incorrect relative
stereochemistry at C-9 (spirotryprostatin numbering).
However, a strategy utilizing 17 as an intermediate would
potentially be warranted for further investigation since
catalytic hydrogenation can be used for removal of the
chiral auxiliary. Actually, amino acid 23 was cleanly
produced from catalytic hydrogenation of 17 using
Pd(OH)2 as a catalyst in quantitative yield (Scheme 6).
First, we attempted epimerization of the a-proton (C-9) after
conversion to the trans-diketopiperazine 24 since, during
the last methanol elimination step, we anticipated that the
prolyl-stereogenic centers (C-9 or C-12) could be epimer-
ized to give a mixture of cis-diketopiperazines (i.e.
spirotryprostatin A (1) and its bis-epimer at C-9 and C-
12), which are thermodynamically more stable than the
corresponding trans-diketopiperazines for cyclic anhy-
drides of proline.8,22 To obtain diketopiperazine 24,
compound 23 was directly coupled with L-proline benzyl
ester and BOP as the activating agent to give the
corresponding dipeptide. Reduction of the benzyl ester
followed by BOP-mediated cyclization afforded diketopi-
perazine 24, a useful precursor of 9-epi-spirotryprostatin A,
in 21% yield from 23. The modest yield seemed to be
associated with the difficulty of the isolation procedure to
We next turned to examining the epimerization of the a-
proton of 23 in the presence of an aldehyde and acid.23
Butyraldehyde (0.5 equiv) and 23 were dissolved in CD3-
COOD and the mixture was heated to 65 8C (Scheme 7). It
was observed by 1H NMR that the a-proton of the amino acid
was gradually exchanged for deuterium and that thermo-
dynamic epimerization had occurred. After conversion into
the corresponding methyl ester by the treatment with
TMSCHN2, these diastereomers could be isolated by PTLC.
By substituting acetic acid for CD3COOD, 23 was
epimerized to give an inseparable diastereomeric mixture
of amino acids (29, Scheme 8). Separation by PTLC was
possible only after conversion to the pentacyclic substances
24 and 19 by the following three-step sequence.8 Coupling

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T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
Scheme 6. Synthesis of 9-epi-spirotryprostatin A (25).
of 29 with L-proline benzyl ester in the presence of BOP
afforded a diastereomeric mixture of dipeptides that was
used without purification for the next reaction. Reduction of
the benzyl ester followed by WSC-mediated cyclization
afforded the cis-fused product 19 (9% from 23), which has
the correct relative and absolute configuration for the
synthesis of spirotryprostatin A, plus the trans-fused
substance (24, 10% from 23), which was converted into 9-
epi-spirotryprostatin A as shown in Scheme 6. The highly
hydrophilic character of compounds 19 and 24 have resulted
in lower yields due to loss of material in the aqueous work-
up procedure.
Scheme 7. Epimerization of the a-proton of 23 in the presence of butyraldehyde and acetic acid.

T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
9509
Scheme 8. Elaboration to cis-fused diketopiperazine 19.
Finally, 19 was subjected to treatment with p-TsOH-H2O in
refluxing toluene to give spirotryprostatin A (1) in 43%
yield along with tertiary alcohol 30 (31%) (Scheme 9).
Aiming to improve the yield, anhydrous camphorsulfonic
acid, was used instead as a protic acid; however, elimination
of methanol was not observed under these conditions.
were activated by heating for three minutes at the highest
setting in a microwave followed by cooling under argon. All
reactions requiring anhydrous conditions were performed
under a positive pressure of argon using oven-dried
glassware (120 8C) that was cooled in a dessicator, unless
stated otherwise. Column chromatography was performed
on Merck silica gel Kiesel 60 (230–400 mesh).
1
3. Conclusion
Mass spectra were obtained on Fisons VG Autospec. H
NMR, 13C NMR, HSQC and NOE experiments were
recorded on a Varian 300 or 400 MHz spectrometer. Spectra
were recorded in CDCl3 and chemical shifts (d) were given
In summary, a concise asymmetric total synthesis of
spirotryprostatin A utilizing the asymmetric azomethine
ylide [1,3]-dipolar cycloaddition reaction of methylene
indolinone 16 has been achieved. The synthesis recorded
herein requires only twelve steps (seven steps in the longest
linear sequence) from commercially available reagents. The
effects of compound 25, 9-epi-spirotryprostatin A, on the
cell cycle and microtubule assembly will be reported
separately.
1
in ppm and were relative to CHCl3. Proton H NMR were
tabulated in the following order: multiplicity (s, singlet; d,
doublet; t, triplet; q, quartet; and m, multiplet), coupling
constant in hertz, and number of protons. When appropriate,
the multiplicity of a signal is denoted as ‘br’ to indicate the
signal was broad. IR spectra were recorded on a Perkin–
Elmer 1600 series FT-IR spectrometer. Optical rotations
were determined with a Rudolph Research Autopol III
automatic polarimeter referenced to the D-line of sodium.
4. Experimental
4.1. General
4.1.1. tert-Butyl (6-methoxy-2-oxo-1,2-dihydroindol-3-
ylidene)acetate (3). To an oven-dried 25 mL round bottom
flask with stir bar was added 6-methoxy isatin (0.50 g,
2.8 mmol) and carbo-tert-butoxy triphenylphosphylidene
(1.15 g, 3.1 mmol). An oven-dried condensor was attached
and the system flushed with argon. Dimethoxyethane
(30 mL) was added via syringe and the system heated to
Unless otherwise noted, materials were obtained from
commercially available sources and used without purifi-
cation. Toluene was freshly distilled from calcium hydride.
Diethyl ether and tetrahydrofuran were freshly distilled
˚
from sodium benzophenone ketyl. 3 A Molecular sieves
Scheme 9. The final stage: elimination of methanol to give spirotryprostatin A (1).

9510
T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
reflux with stirring. Heating continued for 14 h followed by
filtering through a pad of celite. The solution was then
evaporated to dryness and recrystallized from methanol to
yield 3 (0.42 g, 54%) as an orange solid.
77.9, 82.1, 97.1, 106.8, 119.3, 119.9, 126.1, 126.9, 127.7,
128.0, 128.3, 128.6, 129.3, 136.2, 136.6, 140.8, 142.4,
160.6, 167.7, 171.9, 177.6; IR (NaCl/neat) 1730, 1632 cmK1
;
HRMS (FABC) calcd for C36H39O6N2 (m/z) 595.2808, found
(m/z) 595.2804; NOE data: irradiation of H9 enhanced H7
(2.02%) and H6 (1.31%).
1H NMR (300 MHz, CDCl3) d CHCl3: 1.56 (s, 9H), 3.84 (s,
3H), 4.30 (q, JZ7.5 Hz, 2H), 6.43 (d, JZ2.1 Hz, 1H), 6.53
(dd, JZ2.1, 8.7 Hz, 1H), 6.65 (s, 1H), 8.50 (d, JZ8.7 Hz,
1H), 9.25 (brs, 1H); 13C NMR (75 MHz, CDCl3) d CHCl3:
28.5, 55.9, 81.7, 97.1, 108.0, 113.7, 121.6, 130.9, 137.3,
145.5, 163.4, 165.5, 171.0; IR (NaCl/neat) 3219, 1726,
1700; HRMS (FABC) calcd for C15H17O4N (m/z)
275.1157, found (m/z) 275.1156.
4.1.3. Spiro[3H-indole-3,30-pyrrolidine]-40,50-dicar-
boxylic acid, 1,2-dihydro-6-methoxy-20-(2-methoxy-2-
methylpropyl)-2-oxo, 40-tert-butyl ester, 50-metyl ester,
(20S,3S,40R,50R) (14). Cycloadduct 12 was added to a
sealable pressure tube and dissolved in 1:1 THF/EtOH. The
solvent was purged with argon for 5 min and palladium
dichloride (1.0 equiv) was added. The tube was sealed and
flushed with H2 before finally pressurizing to 70 PSI. The
reaction was stirred for 36 h and then filtered through celite
to remove the palladium catalyst. Concentration afforded a
viscous oil which was triturated with freshly distilled diethyl
ether to afford the crude amino acid 6 as a white solid. For
characterization purposes, a small amount of 6 was
converted to the methyl ester. The carboxylic acid 6 was
dissolved in 1:1 CH2Cl2/MeOH. To the solution was added
2 M (trimethylsilyl)diazomethane in hexane until a yellow
color persisted. The reaction was stirred 5 min and then
concentrated under reduced pressure. The residue was
purified by preparative thin layer chromatography (silica
gel, 1:1 hexane/AcOEt) to give 14 as a white amorphous
solid.
4.1.2.
Spiro[3H-indole-3,70(60H)-[1H]pyrrolo[2,1-
c][1,4]oxazine]-80-carboxylic acid, 1,2,30,40,80,80a-hexa-
hydro-6-methoxy-60-(2-methoxy-2-methylpropyl)-10,2-
dioxo-30-40-diphenyl-, tert-butyl ester, (3S,30S,40R,60S,80
R,80aR) (12) and Spiro[3H-indole-3,70(60H)-[1H]pyr-
rolo[2,1-c][1,4]oxazine]-80-carboxylic acid, 1,2,30,40,
80,80a-hexahydro-6-methoxy-60-(2-methyl-1-propenyl)-
10,2-dioxo-30-40-diphenyl-, tert-butyl ester, (3S,30S,40R,
60S,80R,80aR) (13). To a flame-dried 100 mL round bottom
flask with stir bar was added 3 (0.60 g, 2.2 mmol), (5R,6S)-
2,3,5,6-tetrahydro-5,6-diphenyl-1,4-oxazin-2-one (0.40 g,
˚
1.5 mmol) and 3 A molecular sieves (2.5 g). An oven-
dried condensor was attached and the system flushed with
argon. Distilled toluene (25 mL) was added via syringe
followed by the addition of 3-methoxy-3-methyl butanal
(0.22 g, 1.8 mmol) via syringe. The reaction mixture was
kept at room temperature for 14 h while stirring. The
reaction was then filtered through a pad of celite with
toluene as the eluent and the resulting solution was
evaporated under reduced pressure. Column chromatog-
raphy with 3:1 hexane/AcOEt furnished cycloadduct 12
(0.44 g, 45%) and cycloadduct 13 (0.25 g, 25%).
[a]2D5ZK17.2 (CHCl3, cZ0.64); 1H NMR (400 MHz,
CDCl3) d CHCl3: 0.89 (d, JZ13.6 Hz, 1H), 0.98 (s, 9H),
1.00 (s, 3H), 1.10 (s, 3H), 1.17–1.21 (m, 1H), 3.09 (s, 4H),
3.64 (d, JZ6.4 Hz, 2H), 3.76 (s, 6H), 4.49 (brs, 1H), 6.43 (d,
JZ2.0 Hz, 1H), 6.48 (d, JZ8.4 Hz, 1H), 7.25 (s, 1H), 7.78
(brs, 1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 21.8,
24.0, 24.9, 28.0, 30.1, 39.8, 44.8, 48.9, 55.7, 57.0, 60.4,
60.5, 61.6, 74.5, 82.6, 97.4, 106.9, 116.6, 129.2, 143.1,
160.9, 162.9, 166.2, 168.7, 181.1; IR (NaCl/neat) 1724,
1662 cm-1; HRMS (FABC) calcd for C24H35O7N2 (m/z)
463.2444, found (m/z) 463.2444.
Compound 12. A white solid: [a]2D5Z86.3 (CHCl3, cZ
0.63); H NMR (400 MHz, CDCl3) d CHCl3: 0.97 (s, 9H),
1
1.03 (s, 3H), 1.05 (s, 3H), 1.14 (dd, JZ1.6, 16.4 Hz, 1H),
1.63 (dd, JZ1.6, 16.4 Hz, 1H), 3.03 (s, 3H), 3.76 (s, 3H),
3.80 (d, JZ7.2 Hz, 1H), 3.92 (d, JZ1.6 Hz, 1H), 4.50 (d,
JZ7.2 Hz, 1H), 4.99 (d, JZ3.2 Hz, 1H), 6.34 (d, JZ
3.2 Hz, 1H), 6.45–6.49 (m, 2H), 7.00 (d, JZ8.8 Hz, 1H),
7.12–7.38 (m, 8H), 7.39 (d, JZ8.8 Hz, 1H), 8.10 (brs, 1H);
13C NMR (100 MHz, CDCl3) d CHCl3: 22.6, 26.0, 27.5,
44.3, 49.7, 55.2, 55.7, 56.0, 56.2, 60.0, 64.8, 73.5, 75.9,
82.0, 97.1, 107.2, 119.9, 125.1, 127.3, 127.4, 128.4, 128.5,
129.5, 136.7, 137.5, 142.6, 160.8, 167.7, 172.1, 178.2; IR
(NaCl/neat) 1733, 1628; HRMS (FABC) calcd for
C37H43O7N2 (m/z) 627.3070, found (m/z) 627.3074; NOE
data: irradiation of H7 enhanced H5 (1.12%) and H9
(2.21%).
4.1.4. Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-
2(3H),30-[3H]indole]-1-carboxylic acid, 10,20,5a,6,7,8,
10,10a-octahydro-60-methoxy-3-(2-methoxy-2-methyl-
propyl)-20,5,10-trioxo-, tert-butyl ester, (1R,2S,3S,5a-
S,10aR) (7). Cycloadduct 12 (389 mg, 0.62 mmol) was
added to a sealable pressure tube and dissolved in 1:1 THF/
EtOH (7.8 mL). The solvent was purged with argon for
5 min and palladium dichloride (109 mg, 0.62 mmol) was
added. The tube was sealed and flushed with H2 before
finally pressurizing to 70 PSI. The reaction was stirred for
36 h and then filtered through celite to remove the palladium
catalyst. Concentration afforded a viscous oil which was
triturated with freshly distilled diethyl ether to afford the
crude amino acid 6 as a white amorphous solid. To a 50 mL
round-bottom flask that contained the crude amino acid 6
was added BOP reagent (0.30 g, 0.68 mmol) and L-proline
benzyl ester hydrochloride (0.16 g, 0.68 mmol). The flask
was flushed with argon, 15 mL of acetonitrile was added and
the reaction mixture cooled to 0 8C. With stirring,
triethylamine (0.19 mL, 1.3 mmol) was added dropwise
and the solution allowed to warm to room temperature and
stir for 8 h. The solvent was then evaporated, replaced with
Compound 13. White amorphous solid: [a]2D5ZK12.7
1
(CHCl3, cZ0.29); H NMR (400 MHz, CDCl3) d CHCl3:
0.99 (s, 9H), 1.44 (s, 3H), 1.67 (s, 3H), 3.76 (s, 3H), 3.93 (d,
JZ7.2 Hz, 1H), 4.33 (d, JZ3.2 Hz, 1H), 4.41 (d, JZ
9.2 Hz, 1H), 4.48 (d, JZ9.2 Hz, 1H), 4.76 (d, JZ7.2 Hz,
1H), 6.03 (d, JZ3.2 Hz, 1H), 6.42 (d, JZ2.0 Hz, 1H), 6.49
(dd, JZ2.0, 8.2 Hz, 1H), 7.08 (d, JZ7.2 Hz, 1H), 7.15–7.24
(m, 10H), 7.52 (brs, 1H); 13C NMR (100 MHz, CDCl3) d
CHCl3: 18.9, 26.3, 27.5, 54.7, 55.7, 57.0, 59.3, 60.2, 68.8,

T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
9511
10 mL of ethyl acetate, washed with 1 M HCl (2!2.5 mL),
H2O (1!2.5 mL), 5% NaHCO3 (2!2.5 mL), sat. brine sol.
(1!1 mL), dried over Na2SO4, filtered and evaporated to
yield the crude dipeptide as a brown foam which was
taken on crude. To the foam was added a stir bar and
ethanol (10 mL). Argon was bubbled through for 5 min
and 10% Pd/C (0.04 g) was added. The system was
flushed with H2 and a balloon of H2 was attached. The
solution was stirred vigorously for 1.5 h and then
filtered through celite, evaporated and placed on high
vacuum overnight. To the crude mixture was added a
stir bar, BOP reagent (0.27 g, 0.62 mmol) and aceto-
nitrile (5 mL). Triethylamine (0.086 mL, 0.62 mmol)
was added dropwise and the reaction was allowed to
stir for 8 h at which time the solvent was evaporated.
Purification via column chromatography with 75:20:5
CH2Cl2/AcOEt/iPrOH afforded 7 (127 mg, 39%) as a
white amorphous solid.
[1H]pyrrolo[2,1-c][1,4]oxazine], 1,2,30,40,80,80a-hexahy-
dro-6-methoxy-60-(2-methyl-1-propenyl)-10,2-dioxo-30-
40-diphenyl-, (3S,30S,40R,60S,80aR) (22). To a suspension
of oxyindole 21 (199 mg, 0.750 mmol) in toluene (10 mL)
was added trifluoroacetic acid (97 ml, 1.25 mmol) at once
at 0 8C. After stirring for 15 min at 0 8C, triethylamine
(174 ml, 1.25 mmol) was added at once. After stirring for
additional 5 min at 0 8C, water (6.1 mL) and toluene
(10 mL) were added and the mixture was filtrated through
celite. The residue was washed by toluene (2.5 mL). The
organic layer was separated by phase separation and was
dried over anhydrous sodium sulfate (2.5 g) and anhy-
drous magnesium sulfate (2.5 g) for 20 min. The drying
reagents were removed by filtration and the residue was
washed with toluene (1 mL). To the combined toluene
˚
solution were added 0.5 g of activated 3 A molecular
sieves, (5R,6S)-2,3,5,6-tetrahydro-5,6-diphenyl-1,4-oxazin-
2-one (127 mg, 0.5 mmol), 3-methyl-3-methoxybutanal
(70 mg, 0.6 mmol) and toluene (4 mL) at K15 8C. After
stirring for 25 h at 0 8C, the mixture was filtrated through
celite to remove the sieves and concentrated in vacuo. The
product was purified by thin layer chromatography (eluted
with 1:2 hexane/AcOEt) to give 17 (115 mg, 44%) and 22
(51 mg, 20%).
[a]2D5ZK57.3 (CH2Cl2, cZ1.1); 1H NMR (400 MHz,
CDCl3) d CHCl3: 1.05 (s, 3H), 1.15 (s, 3H), 1.21 (s, 9H),
1.70 (dd, JZ4.0, 18.4 Hz, 2H), 1.78 (quint, JZ8.4 Hz, 1H),
1.85–1.96 (m, 1H), 1.96–2.08 (m, 1H), 2.15 (dd, JZ9.6,
14.0 Hz, 1H), 2.49 (quint, JZ6.0 Hz, 1H), 2.95 (s, 3H), 3.43
(d, JZ9.6 Hz, 1H), 3.39 (ddd, JZ3.6, 10.0, 13.6 Hz, 1H),
3.76 (s, 3H), 3.89 (dt, JZ8.0, 12.4 Hz, 1H), 4.24 (dd, JZ
6.0, 11.6 Hz, 1H), 4.80 (dd, JZ4.4, 9.6 Hz, 1H), 4.96 (d,
JZ10.0 Hz, 1H), 6.45 (d, JZ2.0 Hz, 1H), 6.47 (dd, JZ2.0,
8.4 Hz, 1H), 7.13 (d, JZ8.4 Hz, 1H), 8.01 (brs, 1H); 13C
NMR (75 MHz, CDCl3) d CHCl3: 12.9, 20.7, 23.1, 23.7,
29.1, 38.4, 43.8, 47.9, 53.3, 56.1, 59.3, 59.6, 60.1, 60.6,
73.5, 109.6, 121.1, 123.6, 126.3, 128.5, 141.0, 161.8, 165.2,
Compound 17.
A pale yellow amorphous solid:
[a]2D4ZK46.0 (CHCl3, cZ1); 1H NMR (400 MHz,
CDCl3) d CHCl3: 1.02 (s, 3H), 1.09 (s, 3H), 1.36 (dd,
JZ5.6, 15.6 Hz, 1H), 1.61 (dd, JZ2.1, 15.6 Hz, 1H), 2.44
(dd, JZ8.0, 12.6 Hz, 1H), 2.69 (dd, JZ10.4, 12.6 Hz, 1H),
3.04 (s, 3H), 3.81 (s, 3H), 4.01 (dd, JZ2.1, 5.6 Hz, 1H),
4.46 (dd, JZ8.0, 10.4 Hz, 1H), 4.85 (d, JZ2.6 Hz, 1H),
6.29 (d, JZ2.6 Hz, 1H), 6.55 (d, JZ2.1 Hz, 1H), 6.57 (dd,
JZ2.1, 8.4 Hz, 1H), 6.99–7.05 (m, 2H), 7.13–7.33 (m, 9H),
8.35 (brs, 1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 23.5,
25.0, 41.4, 44.5, 49.1, 55.5, 56.5, 56.7, 59.2, 64.6, 73.3,
76.4, 97.3, 107.4, 122.6, 125.5, 125.7, 127.2, 127.6, 128.0,
128.3, 128.8, 136.8, 136.8, 141.6, 160.2, 172.2, 179.2; IR
(NaCl/neat) 1726, 1631, 1505, 1271, 1238, 1193, 1148, 755,
698 cmK1; HRMS (FABC) calcd for C32H35O5N2 (m/z)
527.2546, found (m/z) 527.2540.
168.8, 179.5; IR (NaCl/neat) 3244, 1763, 1667, 1665 cmK1
;
HRMS (FABC) calcd for C28H38O7N3 (m/z) 528.2710,
found (m/z) 528.2714.
4.1.5. 3-Hydroxy-6-methoxy-3-trimethylsilyl-1,3-dihy-
droindole-2-one (21). To a suspension of 6-methoxy-1H-
indole-2,3-dione (3.22 g, 20 mmol) in tetrahydrofuran
(200 mL) was gradually added a 1.0 M solution of
(trimethylsilyl)methyllithium in pentane (50 mL,
50 mmol) at K78 8C. After stirring for 3 h at K78 8C,
saturated aqueous ammonium chloride solution was added.
The product was extracted with ethyl acetate and dichlor-
omethane. The combined organic layer was dried over
anhydrous sulfate and concentrated to give 21 (4.49 g, 85%)
as a white solid. The compound 21 was used for the next
reaction without further purification.
Compound 22.
A pale yellow amorphous solid:
[a]2D4ZK36.4 (CHCl3, cZ0.55); 1H NMR (400 MHz,
CDCl3) d CHCl3: 1.54 (s, 3H), 1.74 (s, 3H), 2.48 (dd, JZ
8.4, 13.0 Hz, 1H), 2.75 (dd, JZ9.6, 13.0 Hz, 1H), 3.80 (s,
3H), 4.27 (d, JZ3.0 Hz, 1H), 4.52 (d, JZ9.4 Hz, 1H), 4.54
(dd, JZ8.4, 9.6 Hz, 1H), 4.70 (d, JZ9.4 Hz, 1H), 6.18 (d,
JZ3.0 Hz, 1H), 6.48 (d, JZ2.3 Hz, 1H), 6.56 (dd, JZ2.3,
8.4 Hz, 1H), 7.02–7.07 (m, 2H), 7.15–7.33 (m, 9H), 7.75
(brs, 1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 18.6,
26.2, 40.0, 55.5, 56.3, 56.6, 60.2, 67.5, 97.2, 107.2, 120.8,
122.1, 125.6, 125.7, 127.4, 127.9, 128.0, 128.4, 128.9,
136.2, 136.4, 139.9, 141.2, 160.1, 168.5, 171.9, 178.3; IR
(NaCl/neat) 3261, 1723, 1631, 1504, 1453, 1193, 1153, 755,
698 cm-1; HRMS (FABC) calcd for C31H31O4N2 (m/z)
495.2284, found (m/z) 495.2267.
1H NMR (400 MHz, CDCl3) d CHCl3: K0.22 (s, 9H), 1.49
(d, JZ10.2 Hz, 1H), 1.53 (d, JZ10.2 Hz, 1H), 2.65 (s, 1H),
3.79 (s, 3H), 6.45 (d, JZ1.8 Hz, 1H), 6.56 (dd, JZ1.8,
6.3 Hz, 1H), 7.23 (d, JZ6.3 Hz, 1H), 7.88 (brs, 1H); 13C
NMR (100 MHz, CDCl3) d CHCl3: K1.1, 28.3, 55.5, 75.5,
97.5, 107.5, 123.4, 125.4, 141.1, 161.2, 180.6; IR (NaCl/
neat) 3388, 1714, 1634, 1507, 1351, 1251, 1151, 1125,
840 cm-1; HRMS (FABC) calcd for C13H19O3NSi (m/z)
265.1134, found (m/z) 265.1132.
4.2. Confirmation of the relative configuration of 17 by
NOESY
4.1.6.
Spiro[3H-indole-3,70(60H)-[1H]pyrrolo[2,1-
c][1,4]oxazine], 1,2,30,40,80,80a-hexahydro-6-methoxy-60-
(2-methoxy-2-methylpropyl)-10,2-dioxo-30-40-diphenyl-,
(3S,30S,40R,60S,80aR) (17) and Spiro[3H-indole-3,70(60H)-
NOE’s were obrserved between 0the proton at position 4 of
the oxyindole and the proton at 8 a, between the proton at 60

9512
T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
and the proton at 30, and between the proton at 60 and the
proton at 40.
extracted with ethyl acetate and was purified by preparative
thin layer chromatography (silica gel, 10:1 CH2Cl2/MeOH)
to give 24 (25.6 mg, 21%) as a colorless amorphous solid.
[a]2D5ZK77.0 (CHCl3, cZ0.2); 1H NMR (400 MHz,
CDCl3) d CHCl3: 1.02 (s, 3H), 1.19 (s, 3H), 1.76–1.86 (m,
2H), 1.87–2.08 (m, 3H), 2.41–2.53 (m, 3H), 2.93 (s, 3H),
3.40 (ddd, JZ3.6, 9.4, 12.4 Hz, 1H), 3.80 (s, 3H), 3.94 (dt,
JZ12.4, 8.3 Hz, 1H), 4.22 (dd, JZ5.4, 11.4 Hz, 1H), 4.61
(t, JZ8.8 Hz, 1H), 4.81 (t, JZ6.8 Hz, 1H), 6.47 (s, 1H),
6.54 (d, JZ8.2 Hz, 1H), 7.09 (d, JZ8.2 Hz, 1H), 7.58 (brs,
1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 21.7, 23.2,
24.7, 30.0, 40.6, 40.7, 44.7, 48.5, 54.8, 55.5, 58.8, 60.7,
61.0, 74.4, 97.5, 106.7, 121.3, 125.9, 142.0, 160.3, 163.6,
165.8, 181.0; IR (NaCl/neat) 2927, 1718, 1653, 1506, 1456,
1343, 1303, 1194, 1157 cm-1; HRMS (FABC) calcd for
C23H30O5N3 (m/z) 428.2185, found (m/z) 428.2193.
4.2.1. Spiro[3H-indole-3,30-pyrrolidine]-50-carboxylic
acid, 1,2-dihydro-6-methoxy-20-(2-methoxy-2-methyl-
propyl)-2-oxo, (20S,3S,50R) (23). To a solution of com-
pound 17 (640 mg, 1.22 mmol) in dry tetrahydrofuran
(6.2 mL) and methanol (6.2 mL) was added 20 wt%
palladium hydroxide on carbon (215 mg). After stirring
for 77 h at room temperature under H2 atmosphere, the
reaction mixture was filtrated through celite and concen-
trated in vacuo. The residue was washed by ethyl acetate
(6 mL, 3 times) to give 23 (424 mg, 100%) as an off-white
solid.
4.2.3. Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-
2(3H),30-[3H]indole], 10,20,5a,6,7,8,10,10a-octahydro-60-
methoxy-3-(2-methyl-1-propenyl)-20,5,10-trioxo-,
(2S,3S,5aS,10aR) (25; 9-epi-spirotryprostatin A),
Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-2(3H),30-
[3H]indole], 10,20,5a,6,7,8,10,10a-octahydro-60-methoxy-
3-(2-methyl-2-propenyl)-20,5,10-trioxo-, (2S,3S,5aS,
10aR) (26) and Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyr-
azine-2(3H),30-[3H]indole], 10,20,5a,6,7,8,10,10a-octa-
hydro-60-methoxy-3-(2-methyl-1-propenyl)-20,5,10-
trioxo-, (2S,3S,5aS,10aS) (1; spirotryprostatin A). To a
solution of compound 24 (33.9 mg, 0.0793 mmol) in
toluene (2.4 mL) were added p-toluenesulfonic acid
[a]2D3ZK18.4 (MeOH, cZ1); 1H NMR (400 MHz,
CD3OD) d MeOH: 1.17 (s, 3H), 1.20 (s, 3H), 1.29 (dd,
JZ2.0, 14.8 Hz, 1H), 1.72 (dd, JZ9.9, 14.8 Hz, 1H), 2.49
(dd, JZ9.0, 13.1 Hz, 1H), 2.57 (dd, JZ9.0, 13.1 Hz, 1H),
3.24 (s, 3H), 3.85 (s, 3H), 4.19 (dd, JZ2.0, 9.9 Hz, 1H),
4.51 (t, JZ9.0 Hz, 1H), 6.62 (d, JZ2.0 Hz, 1H), 6.70 (dd,
JZ2.0, 8.4 Hz, 1H), 7.32 (d, JZ8.4 Hz, 1H); 13C NMR
(100 MHz, CD3OD) d MeOH: 22.2, 25.0, 40.8, 42.6, 49.6,
56.0, 59.1, 61.3, 63.7, 75.1, 98.9, 108.4, 121.3, 125.8, 144.2,
162.5, 172.3, 178.6; IR (NaCl/neat) 2968, 1716, 1633, 1600,
1507, 1456, 1346, 1193, 1156 cmK1; HRMS (FABC) calcd
for C18H25O5N2 (m/z) 349.1764, found (m/z) 349.1778.
˚
(15.1 mg, 0.0793 mmol) and 144 mg of activated 3 A
molecular sieves. After stirring for 5 h at 110 8C, the
mixture was allowed to cool to room temperature. After the
addition of sodium bicarbonate, the product was extracted
with ethyl acetate and was purified by preparative thin layer
chromatography (silica gel, 18:5:2 CH2Cl2/AcOEt/iPrOH)
to give 25 (13.8 mg, 44%) and a mixture of 26 and 1. Both
compounds could be isolated by reversed-phase high
performance liquid chromatography eluting with a gradient
of 25% MeCN in H2O to give 26 (0.9 mg, 3%) and 1
(0.6 mg, 2%).
4.2.2. Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-
2(3H),30-[3H]indole], 10,20,5a,6,7,8,10,10a-octahydro-60-
methoxy-3-(2-methoxy-2-methylpropyl)-20,5,10-trioxo-,
(2S,3S,5aS,10aR) (24). To a solution of 23 (143 mg,
0.291 mmol), L-proline benzyl ester hydrochloride
(84.5 mg, 0.349 mmol) and triethylamine (97.3 ml,
0.698 mmol) in acetonitrile (2.9 mL) was added BOP
(153 mg, 0.349 mmol) at 0 8C. After stirring for 11 h at
room temperature, the mixture was concentrated in vacuo.
After the addition of 1 M hydrochloric acid, the product was
extracted with ethyl acetate. The organic layer was washed
by sat. NaHCO3 aq. and brine, and was dried over
anhydrous sodium sulfate. The solvent was removed in
vacuo and the residue was dissolved in ethanol (7.4 mL) and
methanol (3.7 mL). To a solution was added 10 wt%
palladium on carbon (20 mg). After stirring for 5 h at
room temperature under H2 atmosphere, the reaction
mixture was filtrated through celite and concentrated in
vacuo. To the residue were added triethylamine (31.8 ml,
0.228 mmol) and acetonitrile (3.4 mL). To the mixture was
added BOP (90.7 mg, 0.207 mmol). After stirring for 10 h at
room temperature, the mixture was concentrated in vacuo.
After the addition of 1 M hydrochloric acid, the product was
Compound 25. A colorless amorphous solid: [a]2D5ZC30.0
1
(CHCl3, cZ0.4); H NMR (400 MHz, CDCl3) d CHCl3:
1.42 (s, 3H), 1.67 (s, 3H), 1.85–2.10 (m, 2H), 2.30 (dt, JZ
5.5, 8.2 Hz, 2H), 2.51 (dd, JZ9.0, 13.4 Hz, 1H), 2.94 (dd,
5.2, 13.4 Hz, 1H), 3.57 (ddd, JZ3.5, 8.2, 11.6 Hz, 1H), 3.68
(dt, JZ11.6, 8.2 Hz, 1H), 3.79 (s, 3H), 4.24 (t, JZ8.2 Hz,
1H), 4.62 (dd, JZ5.2, 9.0 Hz, 1H), 5.11 (s, 2H), 6.46 (d, JZ
2.3 Hz, 1H), 6.51 (dd, JZ2.3, 8.3 Hz, 1H), 7.01 (d, JZ
8.3 Hz, 1H), 7.88 (brs, 1H); 13C NMR (100 MHz, CDCl3) d
CHCl3: 18.1, 23.3, 25.7, 27.8, 35.9, 45.7, 54.2, 55.5, 58.9,
60.0, 61.6, 97.0, 106.9, 119.1, 119.6, 125.7, 138.4, 142.1,
160.3, 165.6, 166.1, 179.7; IR (NaCl/neat) 1720, 1662,
1598, 1506, 1426, 1342, 1310, 1278, 1193, 1156 cmK1
;
HRMS (FABC) calcd for C22H26O4N3 (m/z) 396.1923,
found (m/z) 396.1915.
Compound 26. A colorless amorphous solid: [a]2D5ZC38.8
1
(CHCl3, cZ0.08); H NMR (400 MHz, CDCl3) d CHCl3:
1.65 (s, 3H), 1.85–2.09 (m, 2H), 2.13 (dd, JZ7.7, 13.1 Hz,
1H), 2.19–2.35 (m, 2H), 2.43 (dd, JZ7.8, 13.2 Hz, 1H),

T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
9513
2.79 (dd, JZ7.8, 13.2 Hz, 1H), 2.85 (dd, JZ7.7, 13.1 Hz,
1H), 3.51–3.69 (m, 2H), 3.80 (s, 3H), 4.18 (s, 1H), 4.25 (t,
JZ8.0 Hz, 1H), 4.55 (s, 1H), 4.60 (t, JZ7.8 Hz, 1H), 4.65
(t, JZ7.7 Hz, 1H), 6.46 (d, JZ2.4 Hz, 1H), 6.56 (dd, JZ
2.4, 8.4 Hz, 1H), 7.09 (d, JZ8.4 Hz, 1H), 7.40 (brs, 1H); IR
(NaCl/neat) 2924, 1718, 1662, 1507, 1457, 1429, 1341,
1310, 1157 cm-1; HRMS (FABC) calcd for C22H26O4N3
(m/z) 396.1923, found (m/z) 396.1919.
(NaCl/neat) 2972, 2359, 1711, 1633, 1558, 1506, 1457,
1341, 1194, 1153, 1020, 771 cmK1; HRMS (FABC) calcd
for C19H27O5N2 (m/z) 363.1920, found (m/z) 363.1923.
4.3.2. 50-d1-Spiro[3H-indole-3,30-pyrrolidine]-50-car-
boxylic acid, 1,2-dihydro-6-methoxy-20-(2-methoxy-2-
methylpropyl)-2-oxo, methyl ester, (20S,3S,50R), (d1-27)
and 50-d1-Spiro[3H-indole-3,30-pyrrolidine]-50-car-
boxylic acid, 1,2-dihydro-6-methoxy-20-(2-methoxy-2-
methylpropyl)-2-oxo, methyl ester, (20S,3S,50S), (d1-28).
Amino acid 23 (20.3 mg, 0.0583 mmol) and butyraldehyde
(2.60 ml, 0.0292 mmol) were dissolved in d4-acetic acid
(0.7 mL). After stirring for several hours at 65 8C, it was
obrserved by NMR that a-proton of the amino acid was
converted to deuterium and that epimerization of the wrong
chiral center proceeded. After stirring for 31 h at 65 8C, the
mixture was concentrated in vacuo. The residue was
dissolved in methanol (0.7 mL) and 2 M (trimethylsilyl)-
diazomethane in hexane (146 ml, 0.292 mmol) was added.
After stirring for 5 h at room temperature, the mixture was
concentrated in vacuo. After the addition of aqueous sodium
bicarbonate solution, the product was extracted with ethyl
acetate and was purified by preparative thin layer chroma-
tography (silica gel, 1:3 hexane/AcOEt) to give d1-27
(6.6 mg, 31% (2 steps)) and d1-28 (3.2 mg, 15% (2 steps)).
Compound 1. A colorless amorphous solid: see below.
4.3. Confirmation of the relative configuration of 25 by
NOESY
A NOE was obrserved between the proton at position 4 of
the oxyindole and the olefinic proton of the 2-methyl-1-
propenyl group. A NOE was also obrserved between the
proton at position 4 of the oxyindole and the proton at
position 9.
Compound d1-27. A pale yellow amorphous solid:
[a]2D5ZK17.6 (CHCl3, cZ0.25); 1H NMR (400 MHz,
CDCl3) d CHCl3: 0.96 (d, JZ14.0 Hz, 1H), 1.05 (s, 3H),
1.13 (s, 3H), 1.28 (dd, JZ8.8, 14.0 Hz, 1H), 2.31 (d, JZ
12.6 Hz, 1H), 2.50 (d, JZ12.6 Hz, 1H), 3.12 (s, 3H), 3.73
(d, JZ8.8 Hz, 1H), 3.78 (s, 3H), 3.80 (s, 3H), 6.45 (d, JZ
2.3 Hz, 1H), 6.55 (dd, JZ2.3, 8.2 Hz, 1H), 7.35 (d, JZ
8.2 Hz, 1H), 7.54 (brs, 1H); 13C NMR (100 MHz, CDCl3) d
CHCl3: 24.2, 25.6, 39.7, 41.2, 49.3, 52.4, 55.5, 57.3, 57.7,
62.4, 74.3, 97.0, 107.0, 123.9, 125.8, 140.9, 159.9, 175.7,
179.5; IR (NaCl/neat) 1710, 1630, 1505, 1461, 1270, 1244,
1193, 1153, 1122 cm-1; HRMS (FABC) calcd for
C19H25DO5N2 (m/z) 363.1904, found (m/z) 363.1906.
4.3.1. Spiro[3H-indole-3,30-pyrrolidine]-50-carboxylic
acid, 1,2-dihydro-6-methoxy-20-(2-methoxy-2-methyl-
propyl)-2-oxo, methyl ester, (20S,3S,50R) (27). To a
solution of compound 17 (101 mg, 0.193 mmol) in dry
tetrahydrofuran (1 mL) and methanol (1 mL) was added
palladium dichloride (34.2 mg, 0.193 mmol). After stirring
for 54 h at room temperature under H2 atmosphere, the
reaction mixture was filtrated and concentrated in vacuo.
The residue was dissolved in methanol (1.9 mL) and to the
solution was added 2 M (trimethylsilyl)diazomethane in
hexane (483 ml, 0.965 mmol). After stirring for 2 h at room
temperature, a few drops of acetic acid were added and the
reaction mixture was concentrated in vacuo. After the
addition of aqueous sodium bicarbonate solution, the
product was extracted with ethyl acetate and was purified
by preparative thin layer chromatography (silica gel, 1:2
hexane/AcOEt) to give 27 (26.2 mg, 37% (2 steps)) as a pale
yellow solid.
Compound d1-28. A pale yellow amorphous solid:
[a]2D5ZK18.6 (CHCl3, cZ0.167); 1H NMR (400 MHz,
CDCl3) d CHCl3: 1.02 (d, JZ14.4 Hz, 1H), 1.08 (s, 3H),
1.10 (s, 3H), 1.31 (dd, JZ9.2, 14.4 Hz, 1H), 2.13 (d, JZ
13.8 Hz, 1H), 2.76 (d, JZ13.8 Hz, 1H), 3.09 (s, 3H), 3.54
(d, JZ9.2 Hz, 1H), 3.77 (s, 3H), 3.79 (s, 3H), 6.44 (d, JZ
2.4 Hz, 1H), 6.54 (dd, JZ2.4, 8.0 Hz, 1H), 7.24 (brs, 1H),
7.41 (d, JZ8.0 Hz, 1H); 13C NMR (100 MHz, CDCl3) d
CHCl3: 24.3, 25.4, 38.0, 40.6, 41.2, 49.2, 52.3, 55.5, 57.4,
65.0, 74.1, 97.0, 107.1, 123.4, 125.7, 140.9, 159.8, 175.7,
180.1; IR (NaCl/neat) 1722, 1630, 1506, 1463, 1275, 1194,
1155, 1123, 1077 cmK1; HRMS (FABC) calcd for
C19H25DO5N2 (m/z) 363.1904, found (m/z) 363.1910.
[a]2D5ZK32.0 (CHCl3, cZ1); 1H NMR (400 MHz, CDCl3)
d CHCl3: 0.96 (dd, JZ2.0, 14.3 Hz, 1H), 1.05 (s, 3H), 1.13
(s, 3H), 1.28 (dd, JZ9.7, 14.3 Hz, 1H), 2.30 (dd, JZ9.0,
13.1 Hz, 1H), 2.50 (dd, JZ6.8, 13.1 Hz, 1H), 3.11 (s, 3H),
3.72 (dd, JZ2.0, 9.7 Hz, 1H), 3.77 (s, 3H), 3.80 (s, 3H),
4.12 (dd, JZ6.8, 9.0 Hz, 1H), 6.46 (d, JZ2.1 Hz, 1H), 6.54
(dd, JZ2.1, 8.1 Hz, 1H), 7.32 (d, JZ8.1 Hz, 1H), 7.80 (brs,
1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 24.3, 25.6,
39.8, 41.2, 49.2, 52.3, 55.5, 57.4, 57.7, 62.4, 74.3, 96.9,
107.0, 124.0, 125.7, 141.0, 159.8, 175.8, 179.7; IR
4.3.3. Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-
2(3H),30-[3H]indole], 10,20,5a,6,7,8,10,10a-octahydro-60-
methoxy-3-(2-methoxy-2-methylpropyl)-20,5,10-trioxo-,
(2S,3S,5aS,10aR) (24) and Spiro[1H,5H-dipyrrolo[1,2-
a:10,20-d]pyrazine-2(3H),30-[3H]indole], 10,20,5a,6,7,8,10,
10a-octahydro-60-methoxy-3-(2-methoxy-2-methylpro-
pyl)-20,5,10-trioxo-, (2S,3S,5aS,10aS) (19). To a solution
of compound 23 (58.2 mg, 0.167 mmol) in acetic acid
(1.7 mL) was added n-butyraldehyde (7.4 mL, 0.083 mmol).

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T. Onishi et al. / Tetrahedron 60 (2004) 9503–9515
After stirring for 6 h at 60 8C, the reaction mixture was
concentrated in vacuo. The residue was dissolved in
methanol (1.7 mL) and 37% hydrochloric acid (17 ml,
0.20 mmol) was added. The mixture was concentrated to
give a pale yellow solid. To the solid were added L-proline
benzyl ester hydrochloride (61 mg, 0.251 mmol), triethyla-
mine (88.5 ml, 0.635 mmol) and acetonitrile (1.7 mL). To
the mixture was added BOP (95 mg, 0.216 mmol) at 0 8C.
After stirring for 55 h at room temperature, the mixture was
concentrated in vacuo. After the addition of 1 M hydro-
chloric acid (7.5 mL), the product was extracted with ethyl
acetate (15 mL, 3 times). The organic layer was washed by
sat. NaHCO3 aq. (15 mL) and brine (15 mL), and was dried
over anhydrous sodium sulfate. The solvent was removed in
vacuo and the residue was dissolved in ethanol (3 mL),
methanol (1.5 mL) and 37% hydrochloric acid (15 ml,
0.18 mmol). To a solution was added 10 wt% palladium
on carbon (33 mg). After stirring for 34 h at room
temperature under H2 atmosphere, the reaction mixture
was filtrated through celite and concentrated in vacuo. To
the residue were added triethylamine (51.2 ml, 0.368 mmol)
and acetonitrile (3.4 mL). To the mixture was added 1-(3-
Compound 1. A colorless amorphous solid: [a]2D5ZK30.5
(CHCl3, cZ0.2); H NMR (400 MHz, CDCl3) d CHCl3:
1
1.17 (s, 3H), 1.64 (s, 3H), 1.86–2.08 (m, 2H), 2.20–2.39 (m,
2H), 2.38 (dd, JZ7.2, 13.5 Hz, 1H), 2.60 (dd, JZ10.7,
13.5 Hz, 1H), 3.50–3.68 (m, 2H), 3.79 (s, 3H), 4.27 (t,
8.4 Hz, 1H), 4.77 (d, JZ9.0 Hz, 1H), 4.99 (dd, JZ7.2,
10.7 Hz, 1H), 5.02 (d, JZ9.0 Hz, 1H), 6.41 (d, JZ2.4 Hz,
1H), 6.49 (dd, JZ2.4, 8.5 Hz, 1H), 6.92 (d, JZ8.5 Hz, 1H),
7.48 (brs, 1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 18.0,
23.7, 25.5, 27.4, 34.4, 45.2, 55.5, 58.5, 60.2, 60.2, 61.0,
96.6, 106.7, 118.7, 121.4, 127.3, 138.4, 141.6, 160.4, 167.1,
168.2, 180.5; IR (NaCl/neat) 2924, 1716, 1669, 1653, 1635,
1507, 1457, 1419, 1340, 1157 cmK1; HRMS (FABC) calcd
for C22H26O4N3 (m/z) 396.1923, found (m/z) 396.1909.
Compound 30. A colorless amorphous solid: [a]2D5ZK25.0
1
(CHCl3, cZ0.1); H NMR (400 MHz, CDCl3) d CHCl3:
0.62 (s, 3H), 1.11 (s, 3H), 1.79 (dd, JZ4.3, 15.4 Hz, 1H),
1.92 (dd, JZ4.3, 15.4 Hz, 1H), 1.89–2.10 (m, 2H), 2.16–
2.40 (m, 2H), 2.46 (dd, JZ8.7, 13.6 Hz, 1H), 2.65 (dd, JZ
8.7, 13.6 Hz, 1H), 3.60 (dd, JZ5.4, 8.2 Hz, 2H), 3.79 (s,
3H), 4.29 (t, JZ8.2 Hz, 1H), 4.36 (t, JZ4.3 Hz, 1H), 4.45
(brs, 1H), 4.87 (t, JZ8.7 Hz, 1H), 6.46 (d, JZ2.4 Hz, 1H),
6.56 (dd, JZ2.4, 8.4 Hz, 1H), 6.97 (d, JZ8.4 Hz, 1H), 7.40
(brs, 1H); 13C NMR (100 MHz, CDCl3) d CHCl3: 23.6,
27.0, 27.7, 31.5, 33.9, 43.3, 45.2, 55.5, 59.0, 59.4, 61.0,
68.7, 77.2, 97.5, 107.1, 120.4, 126.3, 142.0, 160.7, 168.1,
168.3, 181.4; IR (NaCl/neat) 2923, 2850, 1717, 1683, 1652,
1636, 1507, 1456, 1433, 1158 cm-1; HRMS (FABC) calcd
for C22H28O5N3 (m/z) 414.2029, found (m/z) 414.2020.
dimethylpropyl)-3-ethylcarbodiimide
hydrochloride
(38.4 mg, 0.200 mmol). After stirring for 123 h at room
temperature, the mixture was concentrated in vacuo. After
the addition of 1 M hydrochloric acid (10 mL), the product
was extracted with ethyl acetate (20 mL, 3 times) and was
purified by preparative thin layer chromatography (silica
gel, 100:7.5 CH2Cl2/MeOH) to give 24 (7.2 mg, 10%) and
19 (6.3 mg, 9%).
Compound 24. A colorless amorphous solid: see above.
Compound 19. A colorless amorphous solid: [a]2D5ZK60.0
Acknowledgements
1
(CHCl3, cZ0.33); H NMR (400 MHz, CDCl3) d CHCl3:
This work was supported by the National Science
Foundation (Grant CHE 0202827). We are grateful to
Ajinomoto Co. Inc., for financial support (to T.O.). Mass
spectra were obtained on instruments supported by the
National Institutes of Health Shared Instrumentation Grant
GM49631. We are grateful to Dr. Hiroyuki Osada of the
RIKEN institute, Japan, for providing spectra of natural
spirotryprostatin A. We thank Dr. Kunisuke Izawa of
Ajinomoto Co. Inc. for helpful discussions.
0.93 (s, 6H), 1.86–2.24 (m, 5H), 2.30–2.66 (m, 3H), 2.83 (s,
3H), 3.57 (dd, JZ5.4, 8.2 Hz, 2H), 3.79 (s, 3H), 4.20–4.27
(m, 2H), 4.77 (t, JZ8.8 Hz, 1H), 6.43 (s, 1H), 6.54 (d, JZ
8.4 Hz, 1H), 7.04 (d, JZ8.4 Hz, 1H), 7.64 (brs, 1H); 13C
NMR (100 MHz, CDCl3) d CHCl3: 23.3, 23.6, 24.9, 27.9,
30.0, 34.9, 40.8, 45.0, 48.7, 55.5, 58.4, 59.8, 61.4, 74.0,
97.1, 106.5, 121.4, 126.3, 142.0, 160.2, 166.7, 168.1, 181.4;
IR (NaCl/neat) 1716, 1665, 1633, 1506, 1461, 1343, 1193,
1157, 732 cm-1; HRMS (FABC) calcd for C23H30O5N3
(m/z) 428.2185, found (m/z) 428.2193.
4.3.4. Spiro[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-
2(3H),30-[3H]indole], 10,20,5a,6,7,8,10,10a-octahydro-60-
methoxy-3-(2-methyl-1-propenyl)-20,5,10-trioxo-, (2S,
3S,5aS,10aS) (1; spirotryprostatin A) and Spiro
[1H,5H-dipyrrolo[1,2-a:10,20-d]pyrazine-2(3H),30-
[3H]indole], 10,20,5a,6,7,8,10,10a-octahydro-60-
methoxy-3-(2-hydroxy-2-methylpropyl)-20,5,10-trioxo-,
(2S,3S,5aS,10aS) (30). To a solution of compound 19
(6.3 mg, 0.0147 mmol) in toluene (0.5 mL) were added p-
toluenesulfonic acid (2.8 mg, 0.0147 mmol) and 75 mg of
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