Total synthesis of the chlorine-containing hapalindoles K, A, and G

Article abstract of DOI:10.1002/anie.201100957

A common approach: Overall brevity in the synthesis of the title compounds (see scheme) is achieved by a sequential electrophilic aromatic substitution of indole and a regio- and diastereoselective intermolecular Diels-Alder cycloaddition to establish the key precursora - a neopentyl chloride. A stereoselective Ritter reaction and dehydration are used for the installation of the isonitrile unit. Copyright

Full text of DOI:10.1002/anie.201100957

DOI: 10.1002/anie.201100957
Natural Products
Total Synthesis of the Chlorine-Containing Hapalindoles K, A,
and G**
Aroop Chandra and Jeffrey N. Johnston*
Indole alkaloids of the terrestrial blue-green algae[1] are
structurally diverse natural products and have elicited a broad
response in the form of new chemical methods and prepara-
tions. This enterprise is driven, in part, by the diverse
biological activity that is often associated with metabolites
of cyanobacteria resident in these algae.[2] Beginning with the
syntheses of (non-chlorine-containing) hapalindoles J and M
by Muratake and Natsume,[3] fischerindole,[4] hapalindole,[3,5]
Scheme 1. Structures of hapalindoles K, A, and G.
welwitindolinone,[4,6] and ambiguine[7] natural products have
been accessed by total synthesis.[8] The chlorine-containing
congeners further increase the functional and stereochemical
diversity of this natural product class, but solutions for their
synthesis are few in number. Fukuyama and Chen reported
the first synthesis of a hapalindole that contains a chiral
secondary alkyl chloride (hapalindole G, 3).[5g] The syn
relationship between the chlorine and the methyl groups
was established by a cyclopropane-ring-opening reaction with
lithium chloride. In 2005, Baran and co-workers reported the
syntheses of fischerindoles I and G.[4] The configuration at
C13 of these linear tetracycles is diastereomeric to that in 3,
and was also constructed through a series of stereospecific
transformations, including an epoxide-ring-opening reac-
tion.[4] The same neopentyl chloride is contained within
welwitindolinone A, and the solution reported by Wood and
co-workers involved a chloronium-induced [1,2]-methyl shift
to establish the anti relationship between methyl and
chlorine.[6a,b] Equally elegant was the preparation of (+)-
welwitindolinone A from (ꢀ)-fischerindole I by Baran and
co-workers.[4,7]
ments for accessing hapalindoles 1–3 are more directly
addressed by construction of the tricyclic ring system through
an acylation/alkylation protocol in two steps (Scheme 2a).
The Et2AlCl-mediated Friedel–Crafts acylation of indole with
a-methyl tiglic acid chloride provided adduct 4 in excellent
yield.[10] An extensive screening of Lewis acids to effect the
desired C4–C16 bond formation was unsuccessful, as a retro-
Friedel–Crafts acylation that returned indole was predom-
inant in most cases. However, we discovered that the desired
tricyclic ketone 6 could be obtained by the treatment of 4 with
We report herein the first total synthesis of hapalindoles K
(1) and A (2), and a formal synthesis of hapalindole G (3;
Scheme 1). We used 1) a twofold electrophilic aromatic
substitution (EAS) reaction of indole with a-methyl tiglic
acid chloride, 2) a demanding intermolecular [4+2] cyclo-
addition, and 3) a late-stage Ritter reaction. These key steps
provided the convergency and functional group installation
needed to deliver each target in 15 steps or less.
In an earlier report, we applied a rhodium(II)-catalyzed
cyclohexannulation to construct the ABC ring system of
ambiguine G, which bears a substituent at C1.[9] The require-
[*] A. Chandra, Prof. Dr. J. N. Johnston
Department of Chemistry & Vanderbilt Institute of
Chemical Biology, Vanderbilt University
Nashville, TN 37235-1822 (USA)
E-mail: jeffrey.n.johnston@vanderbilt.edu
Homepage: http://www.johnstonchemistry.org
Scheme 2. Diene and dienophile synthesis. NOESY correlations are
shown for compound 9. DIBALH=diisobutylaluminium hydride,
DIEA=N,N-diisopropylethylamine, DMAP=4-dimethylaminopyridine,
TBS=tert-butyldimethylsilyl, Tf=trifluoromethylsulfonyl, Ts=toluene-
4-sulfonyl.
[**] This work was supported by the NIH (GM 063557).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100957.
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Communications
a molten AlCl3–NaCl solution.[11] The temperature was very
that resulted from C C bond formation, but the Mukaiyama
ꢀ
critical to the regioselective outcome of this cyclization step,
since optimal temperatures (117–1208C) provided 6 as the
major product, but elevated temperatures led predominantly
to the corresponding regioisomer 5. On a larger scale,
purification was reserved until after the sulfonamide forma-
tion, since the undesired compound 5 (containing a fischer-
indole backbone) was unreactive under these conditions.
Ketone 7 was subsequently converted to its enol triflate[12] and
treated with Zn(CN)2 and [Pd(PPh3)4] to give the a,b-
unsaturated nitrile in excellent yield.[13] Nitrile reduction
with DIBALH was followed by treatment of the resulting enal
8 with TBSOTf and Et3N. This sequence furnished the
requisite diene 9 with the desired geometry.
aldol product 12 was formed as the major product. It should
be noted that 12 could be recycled to enal 8 upon treatment
with TiCl4 with little overall loss of material. Many titanium
and aluminum Lewis acids favored the formation of enal 8
(Table 1, entries 3–5). However, use of ethyl aluminum
dichloride successfully improved selectivity and conversion
to 11 (Table 1, entry 6), and these conditions could be further
optimized to obtain the Diels–Alder cycloadduct in 59%
yield of isolated product (Table 1, entry 7). The yield,
combined with the analysis of the crude reaction mixture by
1H NMR spectroscopy, suggests a high degree of regio- and
diastereoselectivity, and an additional amount (15%) of 8 and
12 could be isolated.
With diene 9 and dienophile 10a (Scheme 2b) in hand,
different strategies were evaluated to effect the desired
intermolecular Diels–Alder cycloaddition.[14] Attempts to
carry out the cycloaddition under thermal conditions led to
a slow desilylation of 9 to form the corresponding enal 8, as
well as the decomposition of the dienophile through an
undefined pathway. The Lewis acid promoted Diels–Alder
cycloaddition between diene 9 and b-chloromethacrolein
10a[15] was also investigated, but the Mukaiyama aldol
product predominated in all attempts.
With the tetracyclic core of the hapalindoles in place, our
attention focused on elaboration of the cyclohexene ring.
Reduction of the ketone, and in situ treatment of the alcohol
with triflic anhydride and pyridine provided alkene 13 in 56%
yield (Scheme 3). When using the purified alcohol, evidence
In order to slow down the 1,2-addition pathway, we
evaluated dienophile 10b as a substitute for b-chloro-a-
methyl acrolein (Scheme 2b). A range of Lewis acids were
examined to promote the cycloaddition, as partially outlined
in Table 1.
Scheme 3. Completion of the neopentyl chloride subunit. Py=pyridine,
TBAF=tetrabutylammonium fluoride.
Table 1: Lewis acid promoted intermolecular Diels–Alder cycloaddition.
for the production of a Grob fragmentation product was
found, which may have contributed to the slightly reduced
yield. Desilylation using TBAF led to allylic alcohol 14.
ꢀ
At this stage, efforts to construct the C11 N bond using an
established five-step protocol were unsuccessful.[4,5g] Decom-
position of the starting material was observed, perhaps
influenced by the presence of the chloride and C10–C15
unsaturation. However, exposure of 14 to H2SO4–AcOH
provided acetate 15a in excellent yield and good diastereo-
selectivity (d.r. = 7:1; Scheme 4). The relative stereochemistry
was assigned by chemical correlation of 15a to alcohol 14.
Furthermore, the high degree of retention of configuration is
consistent with the approach of acetic acid to the intermediate
allylic cation along an axial trajectory. The coupling constants
associated with the chloro-substituted axial methine (J = 9
and 5.4 Hz) support the assignment of the acetate to an axial
position. Application of a Ritter reaction to 14 using TMSCN
as a nitrogen source provided formamide 15b as a single
diastereomer.[16] The formation of an allylic cation and an
axial approach of the nucleophile again rationalize the
diastereoselectivity, which was further confirmed by the
observation of a NOESY cross-peak (in Scheme 5). A
second possible source of stereocontrol is the proximity of
the equatorial position to C2 of the indole ring, as an
unfavorable steric interaction occurs. Further investigation
revealed that silyl ether 13 was a suitable Ritter substrate,
Entry
Lewis acid
T [8C]
t [h]
Ratio of
Yield of
11/12/8[a]
11 [%][b]
1
2
3
4
5
6
7
TMSOTf
TiCl4
Ti(iOPr)4
Et2AlCl
Me3Al
ꢀ78
2
0.5
12
12
12
1
1:0:1
1:3:1
0:0:1
0:0:1
0:0:1
3:1:0
13:1:1
18
24
-
-
-
ꢀ20
0
0
0
EtAlCl2
EtAlCl2
ꢀ20
36
59
[c]
ꢀ78!ꢀ20
3
[a] Determined by analysis of the crude reaction mixture by 1H NMR
spectroscopy. [b] Yield of isolated product. [c] Use of toluene instead of
CH2Cl2 as the solvent led to 11 in 54% yield.
The use of trimethylsilyl triflate provided the first
evidence of cycloaddition, but the reaction suffered from
low conversion and formation of enal 8 (Table 1, entry 1).
However, it appeared that a single regio- and diastereomer of
the desired Diels–Alder adduct 11 was formed. The use of
titanium tetrachloride improved the conversion to products
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ꢀ
Scheme 4. The Ritter reaction as the key to set the C11 N bond.
TMS=trimethylsilyl.
Scheme 6. Preparation of 3. Alloc=allyloxycarbonyl, LiHMDS=lithium
hexamethyl disilazide, THF=tetrahydrofuran.
precursor—neopentyl chloride 11—by using a convergent,
stereocontrolled approach. A second new development in this
overall approach is the use of the Ritter reaction to establish
ꢀ
the C10 N bond stereoselectively. This aspect was key to the
brevity of each synthesis.
Scheme 5. Preparation of 1 and 2. NOESY correlations are shown for
compound 15b.
Received: February 7, 2011
Revised: May 30, 2011
Published online: July 7, 2011
which provided 15b in a comparable yield. Subsequently, the
synthesis of 1 could be accomplished in two straightforward
synthetic operations which included deprotection of the tosyl
group and isonitrile formation (Scheme 5).
Keywords: cycloaddition · indole alkaloids · natural products ·
Ritter reaction · total synthesis
.
Compound 2 exhibits two additional chiral centers at C10
and C15 in the cis-decalin ring system (rings C and D), which
is possibly of higher energy than the corresponding ring
system in hapalindole G (3). The potential complication of
the neopentyl chloride notwithstanding, we chose to examine
the reductive approach of Muratake and Natsume in this
context.[17] Exposure of formamide 15b to lithium aluminum
hydride provided the desired cis-fused ring system as
formamide 18 (Scheme 5). Additionally, alcohol 17 and
detosylated indole 16 could be isolated. Treatment of
formamide 18 with phosgene then provided 2.
Modification of this sequence can also provide access to
the trans-fused hapalindoles such as compound 3. Reduction
of allylic alcohol 14, again using the protocol of Muratake and
Natsume, afforded alcohol 19 (Scheme 6). Dess–Martin
periodinane mediated oxidation[18] of alcohol 19 was followed
by protection of the indole nitrogen as allyl carbamate 21.
Epimerization at C10 was effected by triethylamine to lead to
trans-decalin 22. This intermediate was prepared by
Fukuyama and Chen in the total synthesis of (ꢀ)-hapal-
indole G.[5g]
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