Total synthesis and structure assignment of papuamide B, A potent marine cyclodepsipeptide with anti-HIV properties

Article abstract of DOI:10.1002/anie.200705557

Synthesis in stereo: The first total synthesis of papuamide B, a cyclic peptide isolated from a marine sponge, has been achieved. The configuration of three stereogenic centers of its dienoic acid unit has been established by comparison to a series of ste

Full text of DOI:10.1002/anie.200705557

Communications
DOI: 10.1002/anie.200705557
Natural Product Synthesis
Total Synthesis and Structure Assignment of Papuamide B, A Potent
Marine Cyclodepsipeptide with Anti-HIV Properties**
Weiqing Xie, Derong Ding, Weiwei Zi, Guangyu Li, and Dawei Ma*
Papuamides A (1a) and B (1b) (Scheme 1) are two novel
tide through an amide bond. Several nonproteinogenic a-
cyclic depsipeptides that were isolated from Papua New
amino acid residues are present in the molecule, namely
(2S,3S,4R)-3,4-dimethylglutamine,
(2R,3R)-3-hydroxyleu-
cine, (2S,3R)-2,3-diaminobutanoic acid, and b-methoxytyro-
sine. As the configuration of the b-methoxytyrosine unit was
assigned as 2R,3R by synthetic preparation,[5a,b] the remaining
stereochemical uncertainty of papuamides concerns the
configuration of the three chiral centers in the 2,3-dihy-
droxy-2,6,8-trimethyldeca-(4Z,6E)-dienoic acid unit. As the
1H NMR spectroscopic data of alcohol 2, a degradation
product of papuamide A, is known,[1] we planned to use this
information to determine the stereochemistry of the papua-
mide side chain.
The required (2R,3R)-3-hydroxyleucine and (2R,3R)-b-
methoxytyrosine units, 7 and 5, respectively, were elaborated
from the same l-serine derivative 3 (Scheme 2).[9] Alcohol 3
Scheme 1. Structures of papuamides A and B, as well as their chemical
degradation product 2.
Guinea collections of the marine sponges Theonella mirabilis
and Theonella swinhoei.[1] Both compounds exhibit a strong
inhibitory effect on the infection of human T-lymphoblastoid
cells by HIV-1RF in vitro, with an EC50 value of about
4 ngmLÀ1. Similar biological activities were also observed
for other cyclic depsipeptides isolated from different marine
sponges, namely: callipeltin A,[2] neamphamide A,[3] and
mirabamides A–D.[4] Given this background, it is not surpris-
ing that recent intensive efforts have been directed towards
gaining synthetic access to these natural products.[5–7] These
efforts have led to the successful establishment of the
stereochemistry of some sterogenic centers within this class
of compounds.[5] However, none of these molecules have yet
been assembled. Herein, we report our synthetic strategy
towards papuamide B.[8]
Scheme 2. Reagents and conditions: a) Swern oxidation; b) RMgBr,
THF; c) NaH, MeI, THF; d) Pd(OH)2/C, (Boc)2O, MeOH; e) Fmoc-(S)-
pipecolic acid, TCBC, iPr2NEt, DMAP; f) I2, MeOH; g) Dess–Martin
oxidation; h) NaClO2, NaH2PO4; i) allyl bromide, KHCO3; j) Et2NH,
MeCN. Bn=benzyl, Boc=tert-butyloxycarbonyl, DMAP=4-dimethyl-
aminopyridine, Fmoc=9-fluorenylmethyloxycarbonyl, TBS=tert-butyldi-
methylsilyl, TCBC=trichlorobenzoyl chloride, THP=tetrahydropyran.
Structurally, papuamide B comprises a 22-membered
macrocycle, which is connected to a complex linear tetrapep-
was oxidized to the aldehyde, and the desired anti amino
alcohols 4 were prepared by a Grignard reaction. Alkylation
of 4b with iodomethane and NaH provided amino ether 5. In
a parallel procedure, esterification of Boc-protected 4a with
Fmoc-(S)-pipecolic acid in the presence of TCBC[10] and
DMAP furnished ester 6, which was subsequently depro-
tected by using I2/MeOH, oxidized stepwise to an acid, and
converted into an allyl ester. Removal of the Fmoc protecting
group afforded amino ester 7 in 68% overall yield.
[*] Dr. W. Xie, Dr. D. Ding, W. Zi, Dr. G. Li, Prof. Dr. D. Ma
State Key Laboratory of Bioorganic & Natural Products Chemistry,
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
354 Fenglin Lu, Shanghai 200032 (China)
Fax: (+86)21-6416-6128
E-mail: madw@mail.sioc.ac.cn
Assembly of the macrocycle of papuamide B is outlined in
Scheme 3. Treatment of 5 with TsOH/MeOH selectively
removed the THP group, and subsequent hydrogenolysis over
Pd/C in methanol produced the free amino alcohol, which was
then coupled to dipeptide 8 to afford amide 9. Stepwise
[**] The authors are grateful to the Chinese Academy of Sciences and the
National Natural Science Foundation of China (grant numbers
20632050 & 20621062) for their financial support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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Scheme 4. Reagents and conditions: a) CH3CH2CO2Et, LDA, THF,
À788C; b) HCl, THF, H2O; c) Ac2O, Et3N, DMAP, CH2Cl2, reflux;
d) (Boc)2O, DMAP; e) Pd(OH)2/C, NaHCO3, H2O, tBuOH, H2. LDA=
lithium diisopropylamide.
cyclization and dehydration with Ac2O/Et3N/DMAP,
afforded the unsaturated lactone 15 in 78% yield. Hydro-
genation of 15 under various conditions resulted in an
inseparable mixture of two diastereomers in an approximate
ratio of 2:1. Fortunately, we found that when a second Boc
protecting group was introduced, the resultant substrate 16
could be hydrogenated with Pd(OH)2/C/H2 in tBuOH to give
the desired product 17 in 91% yield, together with its
diastereomer 18 in 9% yield. The stereochemistry of 17 was
confirmed by X-ray structural analysis (see the Supporting
Information).
For the construction of the (2S,3R)-2,3-diaminobutanoic
acid fragment (see 21), aziridine 19 was assembled based on
the method of van der Donk (Scheme 5).[14] Switching the
protecting groups in 19 provided aziridine 20, which was
treated with trimethylsilyl azide in methanol to afford the
required allyl ester 21.
Scheme 3. Reagents and conditions: a) TsOH, MeOH; b) Pd/C, H2;
c) 8, EDC, HOAt, iPr2NEt; d) Dess–Martin oxidation; e) NaClO2,
NaH2PO4; f) 7, DEPBT, iPr2NEt; g) [Pd(PPh3)4], PhNHMe; h) 11,
HATU, iPr2NEt; i) 1. [Pd(PPh3)4], PhNHMe; 2. Et2NH; j) HATU,
iPr2NEt, CH2Cl2, RT. EDC=1-(3-dimethylaminopropyl)-3-ethylcarbodi-
imide hydrochloride, DEPBT=3-(diethoxyphosphoryloxy)-1,2,3-benzo-
triazin-4(3H)-one, HATU=O-(7-azabenzotriazol-1-yl)-N,N,N’,N’-
tetramethyluronium hexafluorophosphate, HOAt=1-hydroxyazobenzo-
triazole, Ts=para-toluenesulfonyl.
oxidation of the primary hydroxy group in 9 provided an acid,
which was then condensed with the amino ester 7 to yield
ester 10. In this case, DEPBT[11] was employed because other
coupling reagents such as HATU and EDCI/HOAt failed to
give satisfactory yields. After cleavage of the allyl ester in 10
with Pd0/N-methylaniline,[12] the free acid was coupled to
dipeptide 11 to give linear peptide 12. Subsequent depro-
tection of 12 with [Pd(PPh3)4]and diethylamine provided a
macrocyclization precursor, which was treated with HATU
and DIPEA in dilute methylene chloride solution (0.002m) to
furnish lactam 13.
As (2S,3S,4R)-3,4-dimethylglutamine is a common frag-
ment of papuamides A and B, callipeltin A, and neamphami-
de A,[1–3] considerable effort has been directed toward its
synthesis.[7] However, the reported synthetic routes suffer
from laborious and inconvenient operations or poor diaste-
reoselectivity. We assumed that the key problem with the
current protocols is the difficulty of opening the pyrrolidinone
rings with ammonia. We therefore envisaged using the more
reactive d-valerolactone 17 to build the required amide part
of this glutamine fragment. The synthesis of 17 is illustrated in
Scheme 4. By employing a known procedure, ketone 14 was
prepared from d-serine in seven steps with an overall yield of
61%.[13] Condensation of 14 with the lithium enolate gen-
erated from ethyl propionate, followed by HCl-mediated
Scheme 5. Reagents and conditions: a) LiOH, then allyl bromide,
KHCO3; b) CF3CO2H, then (Boc)2O, Et3N; c) TMSN3, MeOH, 708C.
TMS=trimethylsilyl, Tr=trityl=triphenylmethyl.
The most complex fragment in the papuamide framework
is the 2,3-dihydroxy-2,6,8-trimethyldeca-(4Z,6E)-dienoic acid
moiety. Our efforts on its elaboration and the determination
of the stereochemistry are summarized in Scheme 6. Sharp-
less asymmetric dihydroxylation of olefin 22 produced diol 23
in quantitative yield and 97% ee, as determined by chiral
HPLC. Hydrogenolysis of 23 and subsequent Swern oxidation
led to aldehyde 24, which was treated with the ylide generated
from 25 to afford diene 26 in 82% yield and 20:1 Z/E
selectivity. The reaction conditions for this step proved to be
very critical, as low yields, poor Z/E selectivity, and racemi-
zation were encountered when other bases such as KOtBu, as
well as other solvent systems such as THF or diethyl ether
were used. Hydrolysis of 26 provided acid 27, which was
condensed with amine 28, generated from l-threonine and
glycine, to furnish (2S,3R,8S)-2, following removal of the
acetonide unit with 60% HOAc. By adopting the same
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Scheme 7. Reagents and conditions: a) CF3CO2H, CH2Cl2; b) [Pd-
(PPh3)4], PhNHMe; c) HATU, iPr2NEt; d) NH3, MeOH; e) 1. oxidation
of 30: 30, RuCl3·xH2O, NaIO4, MeCN, CCl4, H2O, 69%, 2. deprotection
of 13: 13, CF3CO2H, CH2Cl2, then 3. Coupling of intermediates from
steps 1. and 2.: DEPBT, iPr2NEt, 89%.
Scheme 6. Reagents and conditions: a) AD-mix-a, H2NSO2Me,
tBuOH, H2O; b) TsOH, DMP, then Pd(OH)2/C, H2; c) Swern oxida-
tion; d) 25, NaHMDS, THF, HMPA, À788C to RT; e) NaOH, MeOH,
H2O; f) 28, HATU; g) 60% HOAc, 508C. HMDS=hexamethyldisila-
zane, HMPA=hexamethyl phosphoramide.
procedure, and employing AD-mix-a as well as the Z isomer
of olefin 22, we obtained three other isomers, namely:
(2R,3S,8S)-2, (2S,3S,8S)-2, and (2R,3R,8S)-2 (see the
Supporting Information for structures). Additionally, use of
the enantiomer of 28 as the coupling substrate afforded
enantiomers of (2R,3S,8R)-2, (2S,3R,8R)-2, (2R,3R,8R)-2, and
(2S,3S,8R)-2, respectively.
With this selection of stereoisomers at hand, we set out to
establish the stereochemistry of the side chain in papuamides
1
by comparison of the H and 13C NMR spectroscopic data
with that reported for the degradation product 2. Unfortu-
nately, although (2S,3S,8S)-2 and (2R,3R,8S)-2, which are the
enantiomers of (2R,3R,8R)-2 and (2S,3S,8R)-2, gave spectro-
scopic signals which are different to those of 2, the chemical
shift data of the other four isomers were indistinguishable
from the literature data. Thus, we could only rule out the anti
relationship of the 2,3-diol in the side chain.
With all the required fragments in hand, assembly of the
target molecules became our next task. Treatment of lactone
17 with CF3CO2H afforded an amine, which was coupled to
the acid released from allyl ester 21 to furnish amide 29 in
71% yield (Scheme 7). As expected, ring opening of 29 with
ammonia in methanol proceeded smoothly to provide amide
30 in 70% yield. After oxidation of 30 with RuCl3·H2O/NaIO4
to give the carboxylic acid, coupling with the deprotected
amine of the macrocycle 13 was carried out, resulting in the
key intermediate 31.
Scheme 8. Reagents and conditions: a) (NH4)2Ce(NO3)6, borate buffer,
MeCN, THF, 608C; b) TBSCl, imidazole, DMF; c) aq. NaOH, MeOH;
d) H-Gly-l-Thr(TBS)-O-allyl, BOP, iPr2NEt, MeCN; e) 1. Deprotection of
32: [Pd(PPh3)4], PhNHMe, 2. Deprotection of 31: 31, AlCl3, CH2Cl2,
then 3. Coupling of intermediates from steps 1. and 2.: DEPBT,
iPr2NEt; f) TAS-F; g) Me3P, H2O, THF. BOP=benzotriazol-1-yloxytris-
(dimethylamino)phosphonium, TAS-F=tris(dimethylamino)sulfur
(trimethylsilyl)difluoride.
The final stages of the total synthesis of papuamide B and
its isomers are depicted in Scheme 8. Removal of the
acetonide moiety of diene 26 with (NH4)2Ce(NO3)6 (ceric
ammonium nitrate, CAN) [15] provided the deprotected diol in
satisfactory yield. Notably, alternative deprotection condi-
tions such as TsOH/MeOH and 60% HOAc gave consid-
erably lower yields owing to the poor stability of the resulting
diol. After selective protection of the secondary alcohol with
TBSCl and subsequent ester hydrolysis with aqueous NaOH
in methanol, coupling to H-Gly-l-Thr(TBS)-O-allyl then
afforded ester 32. After cleavage of the allyl ester of 32 under
palladium catalysis, the resulting acid was coupled to the
amine liberated from 31 to furnish an amide, which was
subjected to deprotection with TAS-F and reduction with
trimethylphosphine/water to afford 33a in 22% overall yield.
By using the same procedure described above, we elaborated
three other isomers (33b–33d) from the corresponding
stereoisomers of 26. Much to our surprise, none of these
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isomers had 1H NMR spectroscopic data identical to that
previously reported for papuamide B. This finding indicated
that a structural revision of this natural product was required.
As the 1H NMR spectra of 33a and 33c are very similar to
that of natural papuamide B (see Supporting Information),
and major differences arose only from the signals of two
protons adjacent to the two amino groups of the (2S,3R)-2,3-
diaminobutanoic acid segment (assigned by TOCSY studies),
we concluded that the stereochemistry of this part of the
molecule might have been misassigned. Thus, we decided to
elaborate the other three isomers of 33a by varying the
stereochemistry of the 2,3-diaminobutanoic acid part. The
required building block 38 was prepared from the enantiomer
of 19 by the procedure depicted in Scheme 5, while the two
anti isomers 36 and 37 were obtained through a slightly
different reaction sequence (Scheme 9), because direct
hydrolysis of 34 led to decomposition.
Scheme 9. Reagents and conditions: a) CF3CO2H, then (Boc)2O, Et3N;
b) LiOH, MeOH, H2O, then allyl alcohol, DCC, DMAP; c) TMSN3,
MeOH, 708C. DCC=1,3-dicyclohexylcarbodiimide.
Scheme 10. Synthesis of papuamide B and its three isomers.
Starting from the azides 36–38, three isomers of 33a were
assembled following the same procedure reported for the
transformation of azide 21 to 33a (Scheme 10). Fortunately,
by comparison, macrolactam 40 gave 1H NMR spectroscopic
data very similar to that of natural papuamide B, thus
suggesting that the diaminobutanoic acid segment might
indeed have a 2S,3S configuration. In order to unambiguously
confirm the stereochemistry of this natural product, we next
introduced different side chains to the revised core structure.
Accordingly, when the same sequence was used as that for the
elaboration of 26 to lactam 33, product 43 was obtained from
2S,3R,8R ester 42 in about 7% overall yield. Much to our
delight, 1H NMR spectra obtained for synthetic 43 were
found to be identical to the literature data of papuamide B.[16]
Although we found some difference in optical rotation
([a]2D1 = + 23.8 (c = 0.37, MeOH) (lit. [a]2D1 = + 12.9 (c = 0.13,
MeOH)) and 13C NMR spectroscopic data,[16] we nevertheless
propose that our synthetic material 43 is natural papuamide B
and that this natural product has a side chain with 2S,3R,8R
configuration; therefore the originally assigned 2S,3R stereo-
chemistry in the 2,3-diaminobutanoic acid segment should be
revised to 2S,3S. This conclusion is further supported by the
fact that the 2R,3S,8S isomer of 43 generated from the
2R,3S,8S isomer of 42 gave 1H NMR spectroscopic data
remarkably different from to that of natural papuamide B.
In conclusion, a convergent route to natural papuamide B
has been developed, which features stereoselective assembly
of its dienoic acid fragment and an efficient elaboration of the
(2S,3S,4R)-3,4-dimethylglutamine residue through a 3,4,5-
trisubstituted d-valerolactone. The latter approach will be of
great benefit for the total synthesis of other anti-HIV cyclic
depsipeptides such as callipeltin A and neamphamide A.
Accompanying the total synthesis, the configuration of three
sterogenic centers of its 2,3-dihydroxy-2,6,8-trimethyldeca-
(4Z,6E)-dienoic acid unit was established, and the stereo-
chemistry of its 2,3-diaminobutanoic acid segment was
revised. This information should be of benefit for fully
establishing the structures of related cyclidepsipeptides like
papuamide A[1] and mirabamides A–D.[4] This synthesis opens
the door for elaboration of related natural products and their
analogues, which would prompt the structure–activity rela-
tionship (SAR) studies of these potent anti-HIV agents.
Further investigations in this area are being conducted within
our research group and will be reported in due course.
Received: December 5, 2007
Published online: March 3, 2008
Keywords: anti-HIV activity · natural products · peptides ·
.
structure elucidation · total synthesis
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Notably, our 1H NMR spectroscopic data were all identical with
those reported except for a less than d = 0.03 ppm difference at
d = 4.82 and 0.59 ppm. However, for 13C NMR spectroscopic
data we found a d = 1.36 ppm difference for the signal at d =
173.06, while the other data are indistinguishable from those
reported. Considering the appearance of many additional peaks
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