Total synthesis of hapalindoles J and U

Article abstract of DOI:10.1021/jo202139k

The total synthesis of d,l-hapalindoles J and U has been accomplished. Hapalindole J was prepared in 11% overall yield over 11 synthetic steps and hapalindole U was prepared in 25% overall yield over 13 synthetic steps from commercially available materials. The route employs a novel silyl ether-based strategy for accessing the 6:5:6:6 ring system of the hapalindoles rapidly and in good yields.

Full text of DOI:10.1021/jo202139k

Article
pubs.acs.org/joc
Total Synthesis of Hapalindoles J and U
Ryan J. Rafferty† and Robert M. Williams*,†,‡
†Department of Chemistry, Colorado State University, 1301 Center Avenue, Fort Collins, Colorado 80523, United States
‡University of Colorado Cancer Center, Aurora, Colorado 80045, United States
S
* Supporting Information
ABSTRACT: The total synthesis of D,L-hapalindoles J and U
has been accomplished. Hapalindole J was prepared in 11%
overall yield over 11 synthetic steps and hapalindole U was
prepared in 25% overall yield over 13 synthetic steps from
commercially available materials. The route employs a novel
silyl ether-based strategy for accessing the 6:5:6:6 ring system
of the hapalindoles rapidly and in good yields.
construction of isotopically labeled putative biosynthetic
pathway metabolites. Herein, we report our synthetic routes
to D,L-hapalindoles J and U, which is readily amenable for
analogues to be synthesized for biosynthetic studies.
INTRODUCTION
■
The hapalindoles were first isolated in 1984 by Moore and
Patterson.1,2 Since that time, the family has expanded to include
29 members, which comprise both tri- and tetracylic variants
(Figure 1). The tetracyclic hapalindoles differ structurally from
We envisioned that hapalindoles J and U could arise from the
oxidation of alcohol 1, followed by reductive amination to
afford the allylic amine, formylation of said amine, and
subjection to dehydration conditions affording the desired
isonitrile (Scheme 1). Tetracycle 1 could be accessed from the
ring closure of tricycle 2, which in turn could arise from the
Lewis acid mediated coupling of TMS-enol ether 3 and
functionalized indole 4. Compound 3 could be accessed via
alcohol protection and enolization of 6, which could be formed
through a Rubottom oxidation following vinyl cuprate addition
to 3-methylcyclohexenone. Functionalized indole 4 could be
accessed from 4-bromoindole (5).
Figure 1. Tetracyclic hapalindoles J and U and ring notation.
their tricyclic counterparts only by the presence of their C4−
C16 bond, with absolute configurations remaining otherwise
mostly identical. Both hapalindole types contain a quaternary
carbon center at C12 and an isonitrile or isothiocyanate at C11.
Additionally, both structural types possess a hydrogen or
alcohol at C10 (R2) and a hydrogen or chlorine at C13 (R1).
To date, a number of the hapalindoles have been conquered
by total synthesis, most in racemic form with a few
enantioselective reports. Of these syntheses, hapalindoles J
and U (Figure 1), isolated in 1986 from the cultured
cyanophyte Hapalosiphon fontinalis, have been reported both
by Natsume (a racemic synthesis) and Baran (hapalindole U;
enantioselectively), respectively. Natsume’s total synthesis of
hapalindole U was accomplished in 20 synthetic steps in an
overall yield of 0.2%,3 and Baran’s elegant 9-step enantiose-
lective effort from commercially available material gave 7%
overall yield.4 Natsume, in addition to hapalindole U, also
reported the total synthesis of hapalindole J in 0.5% overall
yield in 17 synthetic steps.5 Our interest in these prenylated
indole alkaloids is in large measure devoted to studying the
biosynthesis of these agents, and to that end, we required an
efficient and flexible synthetic strategy that would permit the
RESULTS
■
Our route to α-hydroxy-ketone 6 began by subjecting
commercially available 3-methylcyclohexenone (7) to Michael
conditions utilizing vinyl Grignard, CuBr·Me2S, TMEDA, and
TMSCl to generate TMS-enol ether 8, as shown in Scheme 2.
Cleavage of the TMS-enol ether was accomplished with an
acetic acid/water/THF mixture to furnish compound 9.
Subjecting 8 to Rubottom oxidation conditions, according to
related literature precedent,6 failed to give any α-hydroxy-
ketone 6a/6b but did afford 9 in 82% yield. For this reaction,
the workup involved washing with aqueous saturated NaHCO3
until the aqueous layer became colorless. It is thought that
these conditions might not fully remove the copper from the
TMS-enol ether species, ultimately contributing to the
unsuccessful oxidation with mCPBA. Modification of the
workup conditions (washing the crude material sequentially
with a 0.1 M phosphate buffer at pH 7, followed by 1.0 M
EDTA solution) completely removed the copper from the
TMS-enol ether. Ensuing treatment of 8 with mCPBA,
Received: October 16, 2011
Published: November 29, 2011
© 2011 American Chemical Society
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Scheme 1. Retrosynthetic Analysis for Hapalindoles J and U
yield.7 Treatment of 5 with NaH and TBSCl furnished the N-
TBS-protected indole 11 (Scheme 4). Subjecting 11 to
Scheme 2. Accessing 6a,b via a Rubottom Oxidation
Scheme 4. Synthesis of Various Functionalized Indoles
lithium−halogen exchange conditions followed by addition of
acetone furnished 12 in 99% yield with trace amounts of 13
obtained.
Treating a solution of TMS-enol ether 10 and indole 12 in
CH2Cl2 at −78 °C with a 1 M tin(IV) chloride solution
afforded tricycle 14 (Scheme 5). Similar tricycles, with the
followed by TMS cleavage with HF in MeOH, gave a 3:1
diastereomeric mixture of α-hydroxy-ketones 6a:6b. Unfortu-
nately, it was found that the phosphate buffer and EDTA
quench conditions only worked reproducibly on scales of up to
200 mg. On larger scales these conditions preferentially cleaved
the TMS-enol ether to afford ketone 9, and further
modifications failed to circumvent this result.
Scheme 5. Accessing Allylic Alcohol Tetracycle 15
Attention was then directed toward modifying the cuprate
addition (Scheme 3). Slowly adding CuBr·Me2S in HMPA to
Scheme 3. Multigram Route Accessing 6
vinylmagnesium bromide at −78 °C over 5 min, followed by
TMSCl and 7 in THF (1 M) over 30 min, gave the desired
TMS-enol ether in excellent yields. Subjecting this substance to
Rubottom oxidation conditions then afforded 6 in 91% in one
pot (3:1 ratio as in Scheme 2). TMS protection of the
secondary alcohol was accomplished with LHMDS and
TMSCl; sequential treatment with more LHMDS/TMSCl in
one pot then furnished the desired TMS-enol ether 10 in 92%.
This route gives multiple grams of product 10, starting from 7.
Surprisingly, treatment of 6 with more than 2.0 equiv of
LHMDS/TMSCl or excess TMSOTf failed to give 10 directly.
Our route to 12 began by converting 2-bromo-6-nitrotoluene
(not shown) to indole 5 (also commercially available), in high
exception of the α-trimethylsiloxy group, have been closed to
their corresponding tetracycles with BF3·Et2O with N-alkyl
indoles.8 Treating tricycle 14 with BF3·Et2O in CH2Cl2 failed to
give any of the desired tetracycle 15, but rather only the TBS-
deprotected analogue of 16. Given the lability of O/N-TBS
residues to acidic conditions, several acid-mediated conditions
were screened.9 Treating tricycle 14 with 1 N methanolic HCl
(9 equiv) in MeOH afforded the desired tetracycle 15 in 75%
yields and tricycle 16 in 5% yields. Tricycle 16 could be
converted into tetracycle 15 via treatment with an excess of 1 N
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methanolic HCl (40 equiv) in MeOH in 55% yield; the
remaining 45% suffering decomposition. The desired allylic
hydroxy-tetracycle 15 is thereby accessed in 54% yield over six
synthetic steps from commercially available materials.
Scheme 8. Elaboration of 15 into Hapalindole J
Oxidation of 15 with Dess−Martin periodionane in CH2Cl2
gave the corresponding ketone, which was subsequently treated
with TsCl, NEt3, and catalytic DMAP at reflux to furnish
compound 17 in 89% yield over the two synthetic steps
(Scheme 6). Treating 17 with LAH in THF over 12 h afforded
Scheme 6. Elaboration of 15 into Ketone 18
from 15. Isomers 21 and 22 were structurally confirmed via 1H
NMR comparison to the same intermediates within Natsume’s
hapalindole J route.3,5
Subjecting allylic amine 21 to reduction conditions with LAH
in THF allowed for a face-selective reduction of the tetra-
substituted alkene giving access to 23 in 43% yield. Coupling
formic acid to 23 was accomplished with CDMT, catalytic
DMAP, and N-methyl-morpholine in CH2Cl2 affording the
resulting formamide compound (not shown). Treating the
incipient formamide with Burgess reagent in benzene afforded
hapalindole J in 53% yield from 23. Comparison of the 1H and
13C NMR spectra obtained to Natsume’s total synthesis spectra
confirmed the total synthesis of hapalindole J.3,5
the reduced cis-decalin system in a 3:1 ratio favoring the desired
isomer, which was then subjected to Swern oxidation
conditions to afford trans-decalins 18 and 19 (5:1 ratio) in
85% yield. Purification of the oxidized mixture provided the
desired isomer 18, as well as undesired 19, in 71% and 14%
respective yields over two steps from 17. Isomer 18 was
DISCUSSION
■
1
In summary, the total syntheses of D,L-hapalindoles J and U
have been accomplished in 11% over 11 synthetic steps and
25% yield over 13 synthetic steps, respectively. The route
delineated gaining access to hapalindole J, in this body of work,
is six steps shorter, as well as 22-fold higher-yielding, than the
route Natsume employed in accessing the compound
previously, and to date no other total syntheses have been
reported.
structurally confirmed via H NMR comparison to the same
intermediate within Baran’s hapalindole U route.4
Reductive amination of 18 with ammonium acetate and
NaCNBH3 in MeOH gave the desired substance 20 in 79%
yield, with only a 10% yield of the undesired diastereomer
(Scheme 7). Formamide assembly was accomplished by
Scheme 7. Accessing Hapalindole U
From the perspective of the two previous syntheses of
hapalindole U, Natsume’s racemic synthesis of over 20 steps,
with an overall 0.2% yield,3 and the beautiful enantioselective
effort by Baran over nine steps, with an overall 7.5% yield,4 our
work constitutes an efficient approach to the natural product.
Although our strategy is similar to that of Natsume’s route, it
requires eight fewer steps and gives an 80-fold greater yield,
mostly due to the greatly enhanced efficiency of accessing the
functionalized tetracycle core. Current efforts are being directed
toward harnessing this approach to make isotopically labeled
biosynthetic intermediates that we are presently investigating in
the context of the biogenesis of these substances.
treating 20 with formic acid, 2-chloro-4,6-dimethoxy-1,3,5-
triazine (CDMT), catalytic DMAP, and N-methyl-morpholine
in CH2Cl2.4 The resulting intermediate (not shown) was
subsequently dehydrated with Burgess reagent in benzene to
afford hapalindole U in 77% yield from 20 (Scheme 7).
1
Comparison of the H and 13C NMR obtained to Baran’s and
EXPERIMENTAL SECTION
■
1H and 13C spectra were obtained using 300 MHz spectrometer. The
chemical shifts are given in parts per million (ppm) relative to TMS at
δ 0.00 ppm or to residual CDCl3 δ 7.26 ppm for proton spectra and
relative to CDCl3 at δ 77.23 ppm for carbon spectra. IR spectra were
recorded on an FT-IR spectrometer as thin films. Mass spectra were
obtained using a high/low-resolution magnetic sector mass spec-
trometer. All melting points are uncorrected. Flash column
chromatography was performed with silica gel grade 60 (230−400
mesh). Unless otherwise noted, materials were obtained from
commercially available sources and used without further purification.
Natasume’s spectra of the natural and synthetic product
confirmed the total synthesis of hapalindole U.3−5
Accessing hapalindole J was accomplished via compound 15
as shown in Scheme 8. Oxidation of allylic alcohol 15 with
DMP in CH2Cl2 followed by reductive amination with
ammonium acetate and NaCNBH3 in MeOH afforded allylic
amines 21 and 22 (4:1 ratio) in 92% yield. Purification of the
mixture provided the desired isomer 21, as well as undesired
22, in 74% and 18% yields, respectively, over the two steps
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1
Dichloromethane (CH2Cl2), tetrahydrofuran (THF), toluene (PhMe),
N,N-dimethylformamide (DMF), acetonitrile (CH3CN), triethylamine
(Et3N), and methanol (MeOH) were all degassed with argon and
passed through a solvent purification system containing alumina or
molecular sieves.
94% yield (16.3 g). H NMR (300 MHz, CDCl3) δ 7.482 (d, 8.3 Hz,
1H), 7.296 (d, 7.6 Hz, 1H), 7.239 (d, 3.3 Hz, 1H), 7.025 (t, 7.8, 8.1
Hz, 1H), 6.692 (d, 4.1 Hz, 1H), 1.277 (s, 1H), 0.935 (s, 9H), 0.617 (s,
6H); 13C NMR (75 MHz, CDCl3) δ 141.4, 132.2, 131.8, 122.9, 122.5,
114.7, 113.2, 105.3, 26.5, 19.7, 3.7; HMRS (ESI-APCI) [M] calcd for
C14H39NO4SSi, 310.06; found, 310.0619.
rac-2-Hydroxy-3-methyl-3-vinylcyclohexanone (6a:6b). To a
500 mL RBF was added vinylmagnesium bromide (1 M in THF, 106
mL, 105.78 mmol, 1.5 equiv) and cooled to −78 °C to which a
solution of CuBr·Me2S (1.8 g, 7.05 mmol, 0.1 equiv) in HMPA (30
mL, 0.25 M to the CuBr) was added over 10 min and left to stir at the
same temperature. After 30 min, a solution of compound (7) (8 mL,
70.52 mmol, 1.0 equiv) and TMSCl (18 mL, 141 mmol, 2.0 equiv) in
THF (70 mL, 1 M to 7) was added over 30 min and left to stir at the
same temperature. After 3 h NEt3 (40 mL) was added and allowed to
warm to rt, diluted with hexane, washed with H2O (2 × 100 mL) and
once with aqueous NH4Cl, dried over anhydrous MgSO4, and
concentrated to afford crude TMS-enol ether. The product was used
without further purification. The crude material (8.55 g, 40.64 mmol,
1.0 equiv) and NaHCO3 (4.1 g, 48.77 mmol, 1.2 equiv) was added to
DCM (200 mL) and cooled to 0 °C. A solution of purified mCPBA
(8.4 g, 48.77 mmol, 1.2 equiv) in DCM (80 mL) was added dropwise
via addition funnel, and when complete, the reaction was warmed to rt
and stirred for 3 h. The reaction was filtered through a pad of Celite
and the filtrate concentrated. The resultant slurry was taken up in
pentane and filtered through a pad of Celite to remove the solids and
the filtrate was concentrated. The crude material was dissolved in
MeOH (100 mL) to which 48% aq HF (2.95 mL, 81.28 mmol, 2.0
equiv) was added and left to stir for 2 h. Neutralization of the reaction
was accomplished by adding saturated NaHCO3 followed by H2O and
then EtOAc (300 mL) and extraction with additional EtOAc (200
mL). The organic layers were combined over anhydrous Na2SO4,
dried and concentrated to give crude product. The crude material was
purified by flash silica gel chromatography (1:2 EtOAc/hexane) to
afford a yellow oil (3:1 ratio of diastereomers). Mixture of both
4-(2-Hydroxy-2-propyl)-N-TBS-indole (12). Compound (11)
(1.0 g, 3.22 mmol, 1 equiv) was taken up in Et2O (60 mL) and
cooled to −78 °C. To the cooled solution was added tBuLi (3.79 mL,
6.44 mmol, 2 equiv) slowly and left to stir for 15 min to which a
solution of acetone (0.24 mL, 3.22 mmol, 1 equiv) in Et2O (15 mL)
was added. After 45 min the reaction was quenched with aqueous
NH4Cl and extracted with hexanes (x2). The organic layers were
combined, washed with water and brine, dried, and concentrated. The
crude material was purified by flash silica gel chromatography (1:4
EtOAc/hexane) to afford product as an oil in 99% yield (923 mg). 1H
NMR (300 MHz, CDCl3) δ 7.452 (d, 7.8 Hz, 1H), 7.205 (d, 3.3 Hz,
1H), 7.176−7.088 (m, 2H), 6.965 (d, 3.3 Hz, 1H), 1.992 (s, 1H),
1.763 (s, 6H), 0.946 (s, 9H), 0.603 (s, 6H); 13C NMR (75 MHz,
CDCl3) δ 144.2, 139.1, 128.9, 122.4, 121.6, 113.2, 110.1, 101.5, 75.0,
32.8, 30.2, 25.2, −0.8; IR (NaCl film) 3256 cm−1; HMRS (ESI-APCI)
[M+ Na+] calcd for C17H27NNaOSi, 312.48; found, 312.4821.
rac-6-(2-(1-(tert-butyldimethylsilyl)-1H-indol-4-yl)propan-2-
yl)-3-methyl-2-((trimethylsilyl)oxy)-3-vinylcyclohexanone
(14). To a solution of 12 (76 mg, 0.26 mmol, 1.0 equiv) and 285 (141
mg, 0.47 mmol, 1.8 equiv) in DCM (3 mL) at −78 °C was added a
solution of 1 M tin(IV) chloride in DCM (0.34 mL, 0.34 mmol, 1.3
equiv) and left to stir at the same temperature. The reaction was
poured onto a saturated NaHCO3 solution 15 min later and extracted
with DCM. The organic layers were combined, washed with H2O and
brine, dried, and concentrated to afford crude material. The crude
material was purified via flash silica gel chromatography (10% EtOAc
in hexane) to afford product as an oil in 92% yield (119 mg). 1H NMR
(300 MHz, CDCl3) δ 7.39−7.35 (m, 1H), 7.16−7.15 (m, 1H), 7.09−
7.07 (m, 2H), 6.78−6.77 (m, 1H), 5.72−5.62 (m, 1H), 5.09−5.00 (m,
2H), 4.07−4.01 (m, 1H), 3.50−3.33 (m, 1H), 1.65−1.26 (m, 10H),
0.95 (s, 9H), 0.89−0.88 (m, 3H), 0.59 (s, 9H), 0.01 (s, 6H); 13C NMR
(75 MHz, CDCl3) δ 208.1, 146.5, 144.2, 139.7, 130.1, 122.5, 120.1,
114.4, 112.4, 102.3, 98.7, 61.4, 39.5, 32.6, 31.47, 30.2, 26.4, 24.6, 20.1,
19.0, 4.1, −0.9; IR (NaCl film) 1635, 1205 cm−1; HMRS (ESI-APCI)
[M+ Na+] calcd for C29H47NNaO2Si2, 520.30; found, 520.2976.
r ac - ( 9R ) - 6 , 6 , 9 - T r i m e t h y l - 9 - v i n yl - 2 , 6 , 7 , 8 , 9 , 1 0 -
hexahydronaphtho[1,2,3-cd]indol-10-ol (15). To compound 14
(75 mg, 0.15 mmol, 1.0 equiv) was added MeOH (2 mL) followed by
3 N methanolic HCl (3 N HCl in MeOH, 0.5 mL, 1.51 mmol, 9.0
equiv) and left to stir for 5 h. The reaction was poured onto a 1:1 (v:v)
2 N NaOH/DCM (10 mL) and stirred for 1 h and extracted with
DCM. The organic layers were combined, washed with water and
brine, dried over MgSO4, and concentrated. The crude material was
purified by flash silica gel chromatography (1:4 EtOAc/hexane) to
1
diastereomers: H NMR (300 MHz, CDCl3) δ 6.01 (dd) and 5.72
(dd) a total of 1H, 5.14 (m, 2H), 4.07 (dd) and 3.98 (dd) a total of 1
H), 3.60 (d) and 3.51 (d) a total of 1 H), 2.58−2.35 (m) and 2.05−
1.66 (m) a total of 6H), 1.32 (s) and 0.91 (s) a total of 3 H; 13C NMR
(75 MHz, CDCl3) δ 210.8, 145.1, 138.3, 115.8, 112.3, 81.9, 80.5, 60.3,
53.4, 47.6, 47.1, 38.7, 36.3, 34.8, 26.3, 22.1, 21.9, 15.7; IR (NaCl film)
1635, 3253 cm−1; HMRS (ESI-APCI) [M] calcd for C9H14O2, 154.10;
found, 154.1041.
rac-3-Methyl-2-((trimethylsilyl)oxy)-3-vinylcyclohexanone
(10). To a RBF were added THF (26 mL) and compound (6a/b)
(407 mg, 2.64 mmol, 1.0 equiv) and cooled to −78 °C. LHMDS (1 M
in THF, 2.9 mL, 2.9 mmol, 1.1 equiv) was added and left to stir at the
same temperature for 20 min, then brought to 0 °C. After 45 min,
TMSCl (0.44 mL, 3.4 mmol, 1.3 equiv) was added and stirred for an
addition 45 min. LHMDS (1 M in THF, 0.56 mL, 0.56 mmol, 1.1
equiv) was added and left to stir at the same temperature. After 1 h,
TMSCl (0.08 mL, 0.612 mmol, 1.2 equiv) was added, stirred for an
addition 45 min, and quenched with brine. The solution was poured
onto 1:1 (v:v) brine/hexane, extracted with hexanes, dried over
anhydrous Na2SO4, and concentrated to give crude product. The
crude material was purified by a silica plug to afford a light yellow oil.
The material was not thermally stable for long durations, so upon
formation it was carried on to the next reaction (as such, no 13C could
be obtained as decomposed was observed). Mixture of both
diastereomers: 1H NMR (300 MHz, CDCl3) δ 6.02−5.77 (1H),
5.06−4.95 (2H), 4.83−4.80 (1H), 3.54 and 3.42 a total of 1 H, 2.08−
1.14 (6H), 0.99 and 0.97 a total of 3 H, 0.22−0.08 (18 H); IR (NaCl
film) 3253 cm−1.
N-TBS-4-bromo-indole (11). To a solution of THF (600 mL)
and NaH (2.46 g, 61.4 mmol, 1.1 equiv) was added 4-bromoindole (7
mL, 55.81 mmol, 1.0 equiv). After 1 h TBSCl (10.1 g, 66.97 mmol, 1.2
equiv) was added and left to stir for 2 h. The reaction was quenched
with aqueous NH4Cl and extracted with hexanes ( ×3). The organic
layers were combined, washed with water and brine, dried, and
concentrated. The crude material was purified by flash silica gel
chromatography (1:4 EtOAc/hexane) to afford an opaque color oil in
1
afford product. H NMR (300 MHz, CDCl3) δ 7.71 (bs, 1H), 7.3−
76.9 (m, 3H), 6.7 (s, 1H), 5.78 (dd, 18 Hz, 1H), 5.21−4.92 (m, 2H),
4.81−4.65 (m, 1H), 2.51−2.19 (m, 4H), 1.62−1,53 (m, 1H), 1.39 (s,
6H), 1.06−0.76 (m, 3H); 13C NMR (75 MHz, CDCl3) δ 152.1, 150.6,
146.3, 131.2, 134.5, 127.0, 125.3, 111.8, 108.2, 106.4, 74.8, 48.3, 40.1,
38.2, 36.7, 32.1, 29.3, 21.4, 19.1; IR (NaCl, neat) 3210; HMRS (ESI-
APCI) [M + H] calcd for C20H24NO, 294.19; found, 294.1946.
rac-(R)-6,6,9-Trimethyl-2-tosyl-9-vinyl-6,7,8,9-
tetrahydronaphtho[1,2,3-cd]indol-10(2H)-one (17). To DCM
(2 mL) were added compound 15 (55 mg, 0.19 mmol, 1.0 equiv)
and DMP (157 mg, 0.37 mmol, 2.0 equiv) and left to stir for 2 h. The
reaction was quenched with aqueous NaS2O3 (4 mL) and left to stir
for 45 min, washed with aqueous NaHCO3 (×2), once with aqueous
NaS2O3, water, and brine, dried over MgSO4, and concentrated. The
crude material was dissolved in DCM (2 mL) to which TsCl (145 mg,
0.76 mmol, 4 equiv) and DMAP (5 mg, 0.04 mmol, 0.2 equiv) were
added and brought to reflux. After 12 h the reaction was quenched
with saturated NH4Cl and extracted with DCM. The organic layers
were combined, washed with H2O and brine, dried, and concentrated.
The crude material was purified via flash silica gel chromatography
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1
(1:8 EtOAc/hexane) to afford product as an amorphous solid. H
NMR (300 MHz, CDCl3) δ 7.53 (t, 2.1 Hz, 1H), 7.49−7.29 (m, 4H),
7.05−7.02 (m, 2H), 6.99 (s, 1H), 6.31 (dd, 10.5, 16.3 Hz, 1H), 5.11
(dd, 10.4, 16.1 Hz, 1H), 3.92 (dd, 1.0, 11.5 Hz, 1H), 2.43 (s, 3H),
1.92−2.11 (m, 4H), 1.49 (s, 3H), 1.42 (s, 3H), 1.18 (s, 3H); HMRS
(ESI-APCI) [M + H] calcd for C27H28NO3S, 446.18; found, 446.1825.
rac-(6aS,9R,10aS)-6,6,9-Trimethyl-9-vinyl-6,6a,7,8,9,10a-
hexahydronaphtho[1,2,3-cd]indol-10(2H)-one (18). To a solu-
tion of compound 17 (175 mg, 0.39 mmol, 1.0 equiv) in THF (5 mL)
was added LAH (30 mg, 0.79 mmol, 2.0 equiv) and left to stir at rt.
After 14 h Rochelle’s salt (10 mL) was added and left to stir for an
additional 2 h, and the mixture was extracted with DCM. The organic
layers were combined, washed with H2O and brine, dried, and
concentrated. To oxalyl chloride (0.04 mL, 0.47 mmol, 1.2 equiv) in
DCM (1 mL) was added DMSO (0.07 mL, 0.94 mmol, 2.4 equiv) at
−78 °C and left to stir for 25 min at the same temperature, to which
crude alcohol in DCM (1 mL) was added at the same the temperature.
Triethyl amine (0.27 mL, 1.95 mol, 5.0 equiv) was added 20 min later
and allowed to warm to rt. The reaction was diluted with H2O and
extracted with hexane. The organic layers were combined, washed with
H2O and brine, dried, and concentrated. The crude material was
purified via flash silica gel chromatography (1:3 EtOAc/hexane) to
afford product as a white crystalline solid (mp 141−151 °C). 1H NMR
(300 MHz, CDCl3) δ 8.08 (bs, 1H), 7.49 (t, 1.9 Hz, 1H), 7.20−7.15
(m, 2H), 7.03 (s, 1H), 6.21 (dd, 10.7, 17.2 Hz, 1H), 5.15 (dd, 10.7,
17.3 Hz, 1H), 3.92 (dd, 1.0, 11.5 Hz, 1H) 1.92−2.11 (m, 5H), 1.51 (s,
3H), 1.48 (s, 3H), 1.24 (s, 3H); 13C NMR (75 MHz, CDCl3) δ 212.2,
142.8, 139.7, 133.6, 125.3, 122.4, 121.0, 112.3, 112.2, 108.6, 107.9,
51.6, 50.3, 44.1, 38.0, 37.5, 24.7, 24.1, 23.0, 21.3; IR (NaCl, neat)
3402, 3050, 1672; HMRS-FAB [M + H]+ calcd for C20H24NO, 294.18;
found, 294.1847.
116.0, 113.1, 113.0, 112.6, 108.2, 63.1, 43.2, 39.6, 37.4, 33.7, 30.0, 25.4,
24.4, 21.6, 21.0; IR (NaCl neat) 2143, 1642; HRMS-FAB [M + Na]+
calcd for C21H24NaN2, 327.18; found, 327.1846.
D , L -(10S)-6, 6, 9-Trimethyl-9-vinyl-2, 6, 7, 8, 9, 10-
hexahydronaphtho[1,2,3-cd]indol-10-amine (21). To DCM (2
mL) were added compound 15 (55 mg, 0.19 mmol, 1.0 equiv) and
DMP (157 mg, 0.37 mmol, 2.0 equiv) and left to stir for 2 h. The
reaction was quenched with aqueous NaS2O3 (4 mL) and left to stir
for 45 min, washed with aqueous NaHCO3 (×2), once with aqueous
NaS2O3, water, and brine, dried over MgSO4 and concentrated. The
crude material was dissolved in THF (0.5 mL, 0.5 M) and added to a
solution of ammonium acetate (586 mg, 7.6 mmol, 40 equiv) and
NaCNBH3 (119 mg, 1.9 mmol, 10 equiv) in MeOH (70 mL) and left
to stir for 48 h at rt. The reaction was quenched with aqueous
NaHCO3 and extracted with Et2O (×3). The organic layers were
combined and washed with 1 N HCl, and the organic and aqueous
layers were separated. The aqueous layer was brought to above pH 8
with 2 N NaOH and extracted with EtOAc. The organic layers were
combined, washed with brine, dried over anhydrous Na2SO4, and
concentrated. The crude material was filtered through a silica plug and
was used without further purification, to afford product as a crystalline
solid in 66% yield (total yield 73% including compound 22). 1H NMR
(300 MHz, CDCl3) δ7.5−7.2 (m, 3H), 6.7 (s, 1H), 5.78 (dd, 18 Hz,
1H), 5.29−4.87 (m, 2H), 4.92 (br s, 2H), 4.65 (d, 1H), 2.51−2.19 (m,
4H), 1.62 (s, 1H), 1.39 (s, 6H), 1.06 (s, 3H); HMRS-FAB [M + H]+
calcd for C20H24N2, 293.20; found, 293.2017.
D,L-Hapalindole J. Compound 23 (22 mg, 0.07 mmol, 1.0 equiv)
was dissolved in DCM (1.0 mL), to which was added sequentially
formic acid (0.006 mL, 0.15 mmol, 2.0 equiv), 2-chloro-4,6-
dimethoxy-1,3,5-triazine (26 mg, 0.15 mmol, 2.2 equiv), DMAP (0.5
mg, 0.004 mmol, 0.06 equiv), and N-methyl morpholine (0.002 mL,
0.15 mmol, 2.2 equiv). The mixture was stirred for 2 h, diluted with
DCM, and poured onto saturated NaHCO3. The aqueous layer was
extracted with DCM (×5). The organic layers were combined, washed
with 1 N HCl and brine, dried over anhydrous MgSO4, and
concentrated. The crude material was dissolved in benzene (0.01
M), and Burgess reagent (68 mg, 0.29 mmol, 4.0 equiv) was added at
ambient temperature. Upon completion of the reaction, as determined
by TLC, the solvent was removed in vacuo and the crude material was
purified by flash silica gel chromatography (1:4 EtOAc/hexane) to
afford hapalindole J as a white crystalline solid (mp 182−184 °C). 1H
NMR (300 MHz, CDCl3) δ 8.01 (bs, 1H), 7.22−7.14 (m, 2H), 7.03−
7.02 (m, 1H), 6.91 (bt, 1H), 6.05 (dd, 10.7, 17.4 Hz, 1H), 5.19 (d,
10.9, 17.2 Hz, 2H), 4.11 (bd, 1H), 3.31−3.25 (m, 1H), 2.069−1.94
(m, 3H), 1.69−1.67 (m, 2H), 1.50 (s, 3H), 1.45 (s, 3H), 1.21 (s, 3H);
13C NMR 155.9, 146.1, 141.0, 134.0, 124.2, 123.1, 116.5, 113.1, 112.9,
rac-(6aS, 9R,10R,10aS)-6,6,9-Trimethyl-9-vinyl-
2,6,6a,7,8,9,10,10a-octahydronaphtho[1,2,3-cd]indol-10-
amine (20). Compound (18) (100 mg, 0.34 mmol, 1.0 equiv) was
dissolved in THF (0.8 mL, 0.5 M) and added to a solution of
ammonium acetate (1.05 g, 13.6 mmol, 40 equiv), NaCNBH3 (214
mg, 3.4 mmol, 10 equiv) in MeOH (4 mL) and left to stir for 48 h at
rt. The reaction was quenched with aqueous NaHCO3 and extracted
with Et2O (×3). The organic layers were combined and washed with 1
N HCl, and the organic and aqueous layers were separated. The
aqueous layer was brought to above pH 8 with 2 N NaOH and
extracted with EtOAc. The organic layers were combined, washed with
brine, dried over anhydrous MgSO4, and concentrated. The crude
material was filtered through a silica plug, used without further
1
purification to afford product. H NMR (300 MHz, CDCl3) δ 7.99
(bs, 1H), 7.25−6.98 (m, 3H), 6.87 (s, 1H), 6.01 (dd, 10.5, 16.9 Hz,
1H), 5.06 (dd, 10.6, 15.9 Hz, 2H), 3.01 (s, 1H), 2.89 (bs, 1H), 2.21−
2.05 (m, 2H), 1.82−1.53 (m, 2H), 1.49 (s, 3H), 1.39 (s, 3H), 0.98 (s,
3H); HRMS-FAB [M + H]+ calcd for C20H27N2, 295.21; found,
295.2175.
108.1, 62.2, 43.6, 39.1, 37.9, 33.1, 30.1, 25.2, 24.8, 21.1; IR (NaCl,
neat) 2150, 1642; HRMS-FAB [M + Na]+ calcd for C21H24NaN2,
327.18; found, 327.1846.
rac-Hapalindole U. Compound 20 (22 mg, 0.07 mmol, 1.0
equiv) was dissolved in DCM (1.0 mL), to which was added
sequentially formic acid (0.006 mL, 0.15 mmol, 2.0 equiv), 2-chloro-
4,6-dimethoxy-1,3,5-triazine (26 mg, 0.15 mmol, 2.2 equiv), DMAP
(0.5 mg, 0.004 mmol, 0.06 equiv), and N-methyl morpholine (0.002
mL, 0.15 mmol, 2.2 equiv). The mixture was stirred for 2 h, diluted
with DCM ,and poured onto saturated NaHCO3. The aqueous layer
was extracted with DCM (×5). The organic layers were combined,
washed with 1 N HCl and brine, dried over anhydrous MgSO4, and
concentrated. The crude material was dissolved in benzene (0.01 M),
and Burgess reagent (67 mg, 0.28 mmol, 4.0 equiv) was added at
ambient temperature. Upon completion of the reaction, as determined
by TLC, the solvent was removed in vacuo, and the crude material was
purified by flash silica gel chromatography (1:3 EtOAc/hexane) to
afford hapalindole U as a white crystalline solid (mp 239−241 °C). 1H
NMR (300 MHz, CDCl3) δ 8.00 (bs, 1H), 7.19−7.18 (m, 2H), 7.04−
7.03 (m, 1H), 6.90 (bt, 1H), 6.05 (dd, 10.8, 17.3 Hz, 1H), 5.19 (dd,
10.9, 17.4 Hz, 2H), 4.11 (bd, 1H), 3.29−3.26 (m, 1H), 2.07−1.93 (m,
3H), 1.70−1.66 (m, 2H), 1.49 (s, 3H), 1.45 (s, 3H), 1.20 (s, 3H); 13C
NMR (75 MHz, CDCl3) δ 156.1, 145.7, 141.1, 133.9, 125.8, 122.8,
ASSOCIATED CONTENT
■
S
* Supporting Information
1H NMR and 13C NMR spectra for all new compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rmw@lamar.colostate.edu.
ACKNOWLEDGMENTS
■
We gratefully acknowledge financial support from the National
Institutes of Health (Grant RO1GM068011). This paper is
warmly dedicated to Professor Gilbert Stork on the occasion of
his 90th birthday.
523
dx.doi.org/10.1021/jo202139k | J. Org. Chem. 2012, 77, 519−524

The Journal of Organic Chemistry
Article
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