Biomimetic synthesis of Cbz-(S)-dolaphenine

Article abstract of DOI:10.1016/j.tetlet.2012.07.027

A new route to Cbz-(S)-dolaphenine, a recurring element in bioactive peptidic natural products, has been implemented, which closely parallels the biogenetic pathway. Cyclodehydration of 11 to yield thiazoline 2 allows for a Ni(0)-promoted decarbonylative aromatization to provide the thiazole framework with retention of stereochemistry.

Full text of DOI:10.1016/j.tetlet.2012.07.027

Tetrahedron Letters 53 (2012) 4989–4993
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Tetrahedron Letters
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Biomimetic synthesis of Cbz-(S)-dolaphenine
⇑
Pablo García-Reynaga, Michael S. VanNieuwenhze
Department of Chemistry, Indiana University, 800 E. Kirkwood Ave., Bloomington, IN 47405, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A new route to Cbz-(S)-dolaphenine, a recurring element in bioactive peptidic natural products, has been
implemented, which closely parallels the biogenetic pathway. Cyclodehydration of 11 to yield thiazoline
2 allows for a Ni(0)-promoted decarbonylative aromatization to provide the thiazole framework with
retention of stereochemistry.
Received 20 June 2012
Revised 4 July 2012
Accepted 6 July 2012
Available online 16 July 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Thiazole
Biomimetic
Thiazoline
Cyclodehydration
Decarbonylation
The marine shell mollusk Dollabella auricularia is the source of a
wealth of thiazole-containing chemical agents with exceptional
cytotoxic ability.1,2 One of the most pharmaceutically promising
members of this family of bacteriocins has been dolastatin 10, re-
cently reaching Phase II clinical trials (Fig. 1).3,4 First isolated by
Pettit and coworkers,5 dolastatin 10 exerts its cytotoxic activity
by inhibiting microtubule assembly, binding tubulin at a site
distinct from other antimitotic agents.6,7 It additionally exhibits
potent proapoptotic effects in some drug-resistant cancer cell
lines.8 As expected, derivatives of dolastatin 10 have also shown
promise as medicinal agents,9 with additional evidence suggesting
that they may be used in combination therapy given their synergis-
tic activity with largazole, an HDAC inhibitor.10
widely distributed across the range of products isolated from mar-
ine bacteria (Fig. 1).
By virtue of their common descarboxythiazole motif, dolastatin
10 and its congeners represent a unified class of compounds with
medicinal and biological appeal. Enzymatic decarboxylations are
widespread in nature,12–14 and occur through different mecha-
nisms, although examples of oxidative decarboxylations for the
production of enamide-type products are known.15,16 It has been
postulated that, in the case of barbamide, BarJ is responsible for
the oxidative decarboxylation of a thiazoline-4-carboxylic acid
intermediate as the final step to N-methyl-dolaphenine biogenesis.
Given the putative generation of an enethiolate intermediate dur-
ing aminovinylcysteine (AviMeCys) biosynthesis,17 the mechanism
for oxidative decarboxylation in barbamide and dolastatin 10 may
follow a similar pathway.18
Not surprisingly, related heterocyclic compounds also display
strong medicinal potential. Virenamide B, for example, exhibits
strong cytotoxicity with IC50 values of 5
l
g/mL against P388,
Due to its widespread occurrence in the abovementioned series
of medicinally promising metabolites, numerous syntheses of the
dolaphenine subunit have been reported.19–25 By far the greatest
synthetic challenge to the synthesis of dolaphenine has been the
ability to secure the stereochemical purity of the stereocenter adja-
cent to the C-2 position of the thiazole unit, either by its introduc-
tion with chiral auxiliaries or by its retention, starting from
enantiomerically pure substrates. The majority of the successful
approaches have been geared toward the former given the facile
lability of this stereocenter.26
Most notably, Shioiri and Hamada have reported various studies
on the synthesis of this unit.19,20 Traditional approaches, such as
condensation of aldehydes with aminothiols, followed by oxida-
tion of the resulting thiazolidine with excess manganese dioxide
returned low yields of the desired product along with considerable
degradation of stereochemical integrity (77% ee). Excessive loss of
A549, HT29, and CV1 cell lines. Perhaps the most outstanding
member of this class is bottromycin A2, which has shown excep-
tional activity against methicillin-resistant Streptococcus aureus
(MRSA, MIC = 1.0
lg/mL) and vancomycin-resistant Enterococci
(VRE, MIC <1.0 g/mL). Bottromycin A2 inhibits protein biosynthe-
l
sis by blocking the binding of aminoacyl tRNA to the A site on the
50S ribosome.11
The product of hybrid PKS–NRPS machinery, the structure of
dolastatin 10 incorporates acetyl as well as peptidic frameworks,
including the presence of the descarboxythiazole functionality,
dolaphenine (Doe). This functionality, as well as its variants, is
⇑
Corresponding author. Tel.: +1 812 856 7545; fax: +1 812 855 8300.
E-mail address: mvannieu@indiana.edu (M.S. VanNieuwenhze).
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.07.027

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P. García-Reynaga, M. S. VanNieuwenhze / Tetrahedron Letters 53 (2012) 4989–4993
O
O
S
S
H
N
H
N
H
N
HN
N
O
N
N
O
N
H
Me2N
N
NH
O
O
OMe O
Ph
NH
O
OMe
S
O
Dolastatin 10
N
Bottromycin A2
Doe
O
N
N
O
S
Ph
O
N
S
H
O
N
H
H
N
N
N
Cl
HN
O
N
N
Cl
Cl
N
H
O
O
Ph
O
NH
Ph
NH
Barbamide
O
Virenamide B
S
Ph
O
CONH2
O
H
S
H
N
Dolastatin 3
N
N
N
Me
O
O
Ph
Dolastatin 18
Figure 1. Dolastatin 10 and related cyanobacterial metabolites.
stereochemical purity (56% ee) was also observed with the modi-
fied Hantzch method, which had served well previously to produce
optically pure 4-carboxy thiazoles.27
Collectively, these examples illustrate that there is still a need
for an efficient and general synthesis of the dolaphenine unit. Gi-
ven our desire to evaluate the oxidative decarboxylation step lead-
ing to the descarboxythiazole motif in the abovementioned family
of natural products, we envisioned that a route paralleling the bio-
synthetic pathway would best serve our purpose. This would
potentially have the additional advantage of deriving chirality from
favor of the undesired diastereomer. While it was possible to sep-
arate these diastereomers, the lability of this compound toward
epimerization was further illustrated by the fact that the pure dia-
stereomers reverted to the same 1:1 diastereomeric mixture upon
standing in chloroform or silica, as observed by HPLC and NMR
analysis.
Nevertheless, the diastereomeric mixture of 2 allowed for the
assessment of the oxidative decarbonylation leading to dolaphe-
nine. In the event, we were pleased to observe the formation of
the requisite double bond expected under the Pd-catalyzed reac-
tion conditions developed earlier, albeit in low yield. Increasing
the reaction temperature did not improve yields despite the obser-
vation of complete consumption of the starting material.30 Addi-
tionally, the near zero optical rotation of the product, rac-1,
confirmed epimerization had indeed taken place at the Phe
stereocenter.
In light of these observations, we turned our attention to
improving the synthesis of thioester 2 by means of installing the
thioester functionality prior to thiazoline formation, with the
expectation that we would find less erosion of stereochemical
integrity. Thus, thioester 8 was cleanly prepared in 2 steps from
acid 3 in 89% yield (Scheme 3).
Kelly’s conditions were initially investigated given their estab-
lished reputation for providing excellent retention of diastereo-
meric purity during the cyclodehydration process. Thus, the
thioester was subjected to the action of Ph3PO–Tf2O at À18 °C for
2 h (Scheme 3). Unfortunately, a diastereomeric mixture of thiazo-
line 2 (ca. 1.1:1) was again observed in addition to low yields of the
desired product. Furthermore, increasing reaction temperatures
gave similar ratios of diastereomers, without improvement in
yield, along with an increasing amount of undesired byproducts.
In order to assess whether the steric hindrance of the trityl protect-
ing group hindered the cyclodehydration step, we prepared the
2,4,6-trimethoxybenzyl (Tmob)-protected cysteinyl dipeptide 10.
Unfortunately, similar reactivity and diastereomer ratios were ob-
served when 10 was subjected to Kelly’s conditions.
the widely accessible pool of L and D amino acids, providing the rest
of the dolaphenine congeners via simple amino acid substitution.
As outlined in Scheme 1, we envisioned a direct pathway to the
installation of the thiazole unit via an oxidative decarbonylation
reaction, employing a strategy developed recently in our group
for the synthesis of enamides from amino acids.28 The requisite
thiazoline, 2, was then envisioned to arise through sequential con-
densation reactions between appropriately protected phenylala-
nine and cysteine subunits. The major obstacle to this approach
was anticipated to be the control of stereochemical integrity dur-
ing these condensation events.
Our investigation began with assessing the stereoconservative
construction of activated thiazoline 2, evaluating first the thioeste-
rification of thiazoline acid 7 as the leading strategy. In this aim,
coupling of Cbz-Phe-OSu (3) with H-Cys(Trt)-OH, followed by
esterification of the resulting acid with allyl bromide secured
dipeptide 5 (Scheme 2). Thiazoline formation was subsequently ef-
fected through Kelly’s conditions, employing the catalyst resulting
from the combination of Ph3PO and Tf2O,29 which afforded thiazo-
line allyl ester 6 as a single diastereomer in good yield (67%, 3
steps). Cleavage of the allyl ester protecting group cleanly pro-
duced the requisite acid, 7, which was ready for transformation
into thioester 2.
Thioesterification of 7, without compromising stereochemical
integrity proved to be problematic, however. In the event, activa-
tion of the acid with DCC produced the desired thioester function-
ality, albeit with significant amounts of epimerization of the
stereocenter alpha to C-2 of the thiazoline moiety. Monitoring of
the thioesterification reaction by HPLC revealed that extensive epi-
merization had occurred within 1 h of reaction time (1.3:1 dr) in
Since the Ph3PO–Tf2O system results in the generation of a
strongly acidic reaction solution—TfOH is generated to cleave the
thiol protecting group prior to the cyclodehydration step—we were
prompted to examine a stepwise approach to heterocycle forma-

P. García-Reynaga, M. S. VanNieuwenhze / Tetrahedron Letters 53 (2012) 4989–4993
Ph
CbzHN
4991
OSu
OH
oxidative
SPh
O
S
S
decarbonylation
3
condensation
CbzHN
CbzHN
N
O
N
TrtS
H2N
Ph
Ph
1
2
O
4
Scheme 1. Synthetic route to Z-(S)-dolaphenine.
1) H-Cys(Trt)-OH, THF,
STrt
Ph
KHCO3 (aq), rt, 14 h
O
Ph3PO, Tf2O
CbzHN
OAllyl
OSu
N
H
CbzHN
2) AllylBr, Cs2CO3, DMF,
4 h, 95% (2 steps)
CH2Cl2, -18 oC, 2 h,
71%
O
O
Ph
3
5
S
DCC, PhSH
S
Pd(PPh3)4, PhSiH3
CbzHN
Ph
OH
CbzHN
Ph
OAllyl
N
CH2Cl2,
N
CH2Cl2, rt, 30 min
quantitative
O
0
oC to rt, 74%
O
S
7
6
CuMeSal
Pd(OAc)2, (OEt)3P
S
CbzHN
CbzHN
Ph
SPh
N
N
Dioxane, rt, 18 h, 9%
O
Ph
rac-1
2, dr=1.3:1
Scheme 2. Synthesis of rac-1.
1) H-Cys(Trt)-OH, THF,
KHCO3 (aq), rt, 14 h
STrt
O
O
With this result in hand, we proceeded to examine the decarb-
onylation of 2 to (S)-dolaphenine using the milder conditions
employing Ni(0) as the catalyst. In the event, we were delighted
to observe that the major diastereomer 2 provided Z-(S)-dolaphe-
nine in 88% yield after stirring in a solution of Ni(PPh3)4 and CuTC
at room temperature over 22 h, while the minor diastereomer 12
provided Z-(R)-dolaphenine (Scheme 4). Importantly, the optical
rotation of this product proved to be in excellent agreement with
that of the pure substrate.19 This further illustrates the efficiency
of these conditions in providing for the mild decarbonylation of
thioester substrates.
In an attempt to mitigate epimerization of the thiazoline inter-
mediate by installing the required oxidation state prior to cyclode-
hydration, we carried out decarbonylation of thioester 10 to give
the Tmob-protected enethiol 14 in modest yields (Scheme 5).
Unfortunately, our preliminary attempts to induce the subsequent
cyclodehydration with Ph3PO–Tf2O did not yield the desired
thiazole.
In summary, we have developed an efficient synthesis of Z-(S)-
dolaphenine in a total of 5 steps with an overall yield of 52%. The
route is adaptable to the synthesis of dolaphenine congeners via
substitution of phenylalanine for other amino acids. In addition,
the results presented in this manuscript further demonstrate the
utility of our recently developed method for oxidative decarbony-
lation of thioester precursors.28 Furthermore, the intermediates
produced during the course of this work can be used as probes to
study the enzymes responsible for oxidative decarboxylation in
the dolaphenine (and related) biosynthetic pathway(s). Results to-
ward this end will be presented in due course.
CbzHN
CbzHN
SPh
OSu
N
H
2) PhSH, DCC, CH2Cl2,
rt, 89%
O
Ph
Ph
3
8
S
Ph3PO, Tf2O
CH2Cl2, -18 oC, 2 h, 14%
CbzHN
Ph
SPh
N
O
2, dr=1.1:1
Scheme 3. Cyclodehydration of thioester 8.
tion. Thus, Tmob-protected cysteinyl peptide 10 was treated with
10% TFA/CH2Cl2 using triethylsilane as a cation capture reagent
to smoothly provide the pure and isolated thiol, 11. Upon treat-
ment of this thiol with Ishihara’s dehydration catalyst,31 we were
delighted to find that, under optimized conditions, the desired
product 2 could be isolated in 76% yield, along with minimal
amounts of the undesired diastereomer 12 (Scheme 4). Notably,
it was observed that with extended reaction times, the initially
formed thiazoline 2 steadily epimerized. Furthermore, catalyst
loading correlated strongly with both the yield and optical purity
of the resulting product. In particular, employing less than 10% cat-
alyst loadings yielded significantly increased amounts of the unde-
sired diastereomer (ca. 86% yield, 1:1 dr after 1 h of reaction time).
Although higher catalyst loadings were not examined, our observa-
tions suggest that further increases in catalyst loading may pro-
duce better results.

4992
P. García-Reynaga, M. S. VanNieuwenhze / Tetrahedron Letters 53 (2012) 4989–4993
Ph
O
Ph
O
O
H
N
H-Cys(Tmob)-OH
PhSH, PyBOP
CH2Cl2, rt, 86%
OSu
CbzHN
OH
STmob
CbzHN
KHCO3/THF,
95%
3
9
Ph
O
Ph
O
H
H
N
Et3SiH, 10% TFA/CH2Cl2,
rt, 15 min, 94%
N
CbzHN
SPh
CbzHN
SPh
O
O
SH
STmob
11
10
S
S
10% MoO2(2eqn)2
PhMe, reflux, 15 min
CbzHN
Ph
SPh
N
CbzHN
Ph
SPh
N
O
O
12, 12%
2, 76%
1.5 equiv Ni(PPh3)4,
1.2 equiv CuTC
Dioxane, rt, 22 h
86%
1.5 equiv Ni(PPh3)4,
1.2 equiv CuTC
Dioxane, rt, 22 h
88%
S
S
CbzHN
CbzHN
N
N
Ph
Ph
Z-(S)-dolaphenine (1)
Z-(R)-dolaphenine (13)
[α]D= -21.3, c 1.0, MeOH
[α]D= +19.3, c 1.0, MeOH
Scheme 4. Stereoretentive synthesis of Z-(S)-Doe.
Ph
TmobS
O
Ni(COD)2,
O
S
Ph3PO, Tf2O
H
N
PPh3, CuTC
CbzHN
CbzHN
CbzHN
SPh
N
N
H
Ph
CH2Cl2, -18 oC,
2 h
Dioxane, 50 oC,
2.5 h, 38%
O
STmob
Ph
10
1
14
Scheme 5. Attempted cyclodehydration of enamide 14.
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Acknowledgments
Financial support was provided by the NIH (AI 059327) and
Indiana University is gratefully acknowledged. P.G.-R. thanks Eli
Lilly and Company for fellowship support. We also thank Profes-
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Supplementary data (Experimental procedures and character-
ization data for all new compounds) associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.tetlet.2012.07.027.
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