Concise total synthesis of (±)-salinosporamide A, (±)-cinnabaramide A, and derivatives via a bis-cyclization process: Implications for a biosynthetic pathway?

Article abstract of DOI:10.1021/ol070616u

4-Alkylidene-β-lactones (hetero ketene dimers) and α-amino acids are useful precursors for total syntheses of the β-lactone-containing proteasome inhibitors salinosporamide A, cinnabaramide A, and derivatives. A key step is a nucleophile-promoted, bis-cyc

Full text of DOI:10.1021/ol070616u

ORGANIC
LETTERS
2007
Vol. 9, No. 11
2143-2146
Concise Total Synthesis of
(±
±
)-Salinosporamide A,
)-Cinnabaramide A, and Derivatives
(
via a Bis-cyclization Process:
Implications for a Biosynthetic
Pathway?
Gil Ma, Henry Nguyen, and Daniel Romo*
Department of Chemistry, Texas A&M UniVersity, P.O. Box 30012,
College Station, Texas 77842-3012
romo@tamu.edu
Received March 15, 2007
ABSTRACT
4-Alkylidene-â-lactones (hetero ketene dimers) and
r-amino acids are useful precursors for total syntheses of the
â-lactone-containing proteasome
inhibitors salinosporamide A, cinnabaramide A, and derivatives. A key step is a nucleophile-promoted, bis-cyclization of keto acids that
simultaneously generates the
of these natural products.
γ-lactam and â-lactone of these natural products. This reaction sequence may have implications for the biosynthesis
Omuralide (1), derived from lactacystin (2),1 the salino-
sporamides, e.g., salinosporamide A (3),2 and the recently
disclosed cinnabaramides, e.g., cinnabaramide A (4)3 are
unique bicyclic â-lactone-containing natural products of
bacterial origin that have recently attracted intense interest
from both synthetic and biological perspectives (Figure 1).
Several synthetic efforts including numerous total syntheses
of omuralide1 and three syntheses of salinosporamide A4
attest to the interest in these novel proteasome inhibitors due
to their highly functionalized [3.2.0] bicyclic core and due
to the validation of this therapeutic target for cancer.5 Recent
crystallographic studies have elucidated fascinating details
regarding inhibition of the 20S proteasome by salinospor-
amide A involving acylation of the active site threonine by
(2) Isolation: (a) Feling, R. H.; Buchanan, G. O.; Mincer, T. J.;
Kauffman, C. A.; Jensen, P. R.; Fenical, W. F. Angew. Chem., Int. Ed.
2003, 42, 355. Analogue synthesis: (b) Macherla, V. R.; Mitchell, S. S.;
Manam, R. R.; Reed, K. A.; Chao, T.-H.; Nicholson, B.; Deyanat-Yazdi,
G.; Mai, B.; Jensen, P. R.; Fenical, W. F.; Neuteboom, S. T. C.; Lam, K.
S.; Palladino, M. A.; Potts, B. C. M. J. Med. Chem. 2005, 48, 3684. (c)
Williams, P. G.; Buchanan, G. O.; Feling, R. H.; Kauffman, C. A.; Jensen,
P. R.; Fenical, W. F. J. Org. Chem. 2005, 70, 6196. (d) Reed, K. A.; Manam,
R. R.; Mitchell, S. S.; Xu, J.; Teisan, S.; Chao, T. H.; Deyanat-Yazdi, G.;
Neuteboom, S. T. C.; Lam, K. S.; Potts, B. C. M. J. Nat. Prod. 2007, 70,
269.
(1) For reviews on lactacystin/omuralide syntheses, see: (a) Corey, E.
J.; Li, W.-D. Z. Chem. Pharm. Bull. 1999, 47, 1. (b) Masse, C. E.; Morgan,
A. J.; Adams, J.; Panek, J. S. Eur. J. Org. Chem. 2000, 2513. For a lead
reference to more recent syntheses, see: (c) Balskus, E. P.; Jacobsen, E.
N. J. Am. Chem. Soc. 2006, 128, 6810.
(3) Stadler, M.; Bitzer, J.; Mayer-Bartschmid, A.; Muller, H.; Benet-
Buchholz, J.; Gantner, F.; Tichy, H.-V.; Reinemer, P.; Bacon, K. B. J. Nat.
Prod. 2007, 70, 246.
10.1021/ol070616u CCC: $37.00
© 2007 American Chemical Society
Published on Web 05/04/2007

Figure 2. Retrosynthetic analysis of salinosporamide A, cinna-
baramide A, and derivatives.
Figure 1. Structures of proteasome inhibitors and a possible
biosynthetic origin for the γ-lactam-fused â-lactone core.
an intramolecular bis-cyclization process (Figure 2, 10 f
9). Attachment of the cyclohexenyl moiety, or other side
chains, would rely on the strategy of Corey developed in
the course of their salinosporamide synthesis on simpler
aldehyde γ-lactam precursors.11 This would entail addition
of a cyclohexenyl zinc reagent to the aldehyde derived from
benzyl ether 9; however, the success of this process and
subsequent manipulations was not guaranteed given the
presence of the â-lactone.12 The keto acid substrate 10 could
be derived from coupling of an R-amino acid 11 and a ketene
dimer 12, the latter serving as a suitable latent equivalent
for a â-ketoester.
Ultimately, we sought the development of an asymmetric
strategy. However, one difficulty to be overcome was the
potential for enolization of the substrate â-ketoamide render-
ing the ketone nonelectrophilic and most importantly, the
possibility of rapid racemization at C2. However, due to the
known conformationally controlled acidity of â-ketoamides
owing to A1,3 strain,13 retention of optical activity appeared
plausible. Herein, we describe the implementation of the first
goal of this strategy; namely, the bis-cyclization process
which has led to concise total syntheses of rac-salinospora-
mide A (3), rac-cinnabaramide A (4), and derivatives.
We began our studies with simple C2-unsubstituted sub-
strates which were readily prepared by coupling of racemic
ketene homodimer 14a14 with N-PMB-glycine benzyl ester
(13a) by the method of Calter,15 which proceeded efficiently
to provide keto acid substrate 17a following hydrogenolysis.
We were pleased to find that bis-cyclization employing
conditions similar to those developed for carbocycles,8 using
4-pyrrolidinopyridine (4-PPY) as a nucleophilic promoter,
proceeded efficiently to give bicyclic-â-lactones 19a-d
(Table 1). However, the C2-unsubstituted ketoamide 17d
gave only 25% yield (entry 4). Without the C2-substitutent,
facile enolization of the ketoamide likely leads to diminished
rates of the initial aldol step. Interestingly, increased dia-
the â-lactone with concomitant cyclization of the incipient
alkoxide with the C13 chloro substituent leading to a
tetrahydrofuran.6 Salinosporamide is currently in phase I
human clinical studies for multiple myeloma.
We previously reported a catalytic, asymmetric intramo-
lecular, nucleophile catalyzed aldol-lactonization (NCAL)
process employing aldehyde acids that allows access to
carbocycle-fused â-lactones,7 and this process was recently
extended to keto acid substrates.8 This methodology was
initially inspired by omuralide which contains such a bicyclic
â-lactone core. Regarding the biosynthesis of these metabo-
lites, one could speculate the joining of an appropriate amino
acid 5 with an activated â-keto ester 6 followed by either
an aldol-lactonization sequence9 or a [2 + 2] cycloaddition
via a ketene intermediate, a mechanism commonly invoked
for related bis-cyclizations (Figure 1).10
Building on our work with carbocycle-fused â-lactones,
we envisioned a concise synthetic strategy to the bicyclic
core of these natural products by simultaneous formation of
the C-C and C-O bonds from a keto acid precursor 10 via
(4) (a) Reddy, L. R.; Saravanan, P.; Corey, E. J. J. Am. Chem. Soc. 2004,
126, 6230. (b) Reddy, L. R.; Fournier, J-F.; Reddy, B. V. S.; Corey, E. J.
Org. Lett. 2005, 7, 2699. (c) Endo, A.; Danishefsky, S. J. J. Am. Chem.
Soc. 2005, 127, 8298. (d) Mulholland, N. P.; Pattenden, G.; Walters, I. A.
S. Org. Biomol. Chem. 2006, 4, 2845. For a study toward salinspor-
amide A, see: (e) Caubert, V.; Langlois, N. Tetrahedron Lett. 2006, 47,
4473.
(5) (a) Voorhees, P. M.; Dees, E. C.; O’Neil, B.; Orlowski, R. Z. Clin.
Cancer Res. 2003, 9, 6316. (b) Rajkumar, S. V.; Richardson, P. G.;
Hideshima, T.; Anderson, K. C. J. Clin. Oncol. 2005, 23, 630. (c) Joazeiro,
C. A. P.; Anderson, K. C.; Hunter, T. Cancer Res. 2006, 66, 7840.
(6) Groll, M.; Huber, R.; Potts, B. C. M. J. Am. Chem. Soc. 2006, 128,
5136.
(7) (a) Cortez, G. S.; Tennyson, R.; Romo, D. J. Am. Chem. Soc. 2001,
123, 7945. (b) Oh, S. H.; Cortez, G. S.; Romo, D. J. Org. Chem. 2005, 70,
2835.
(8) Henry-Riyad, H.; Lee, C. S.; Purohit, V. C.; Romo, D. Org. Lett.
2006, 8, 4363.
(9) While our work was in progress, a related biosynthetic pathway was
proposed: Moore, B. S. International Conference on Marine Natural
Products, Paris, France, Sep 2005, and recently appeared; see: Beer, L. L.;
Moore, B. S. Org. Lett. 2007, 9, 845.
(10) For previous reports of â-lactones from keto acid derivatives via
proposed [2 + 2] mechanisms, see: (a) Boswell, G. A.; Dauben, W. G.;
Ourisson, G.; Rull, T. Bull. Soc. Chim. Fr. 1958, 1598. (b) Kagan, H. B.;
Jacques, J. Bull. Soc. Chim. Fr. 1958, 1600. (c) Brady, W. T.; Gu, Y. Q. J.
Org. Chem. 1988, 53, 1353. (d) Reddy, L. R.; Corey, E. J. Org. Lett. 2006,
8, 1717. For a previous report of an aldol-lactonzation pathway, see: (e)
Merlic, C. A.; Marlog, B. C. J. Org. Chem. 2003, 68, 6056.
(11) For development of a strategy for attachment of the cyclohexenyl
sidechain, see ref 4a.
(12) Jacobsen and coworkers had previously demonstrated the stability
of a related spiro-â-lactone in their studies toward omuralide (see ref 1c).
(13) Evans, D. A.; Ennis, M. D.; Le, T. J. Am. Chem. Soc. 1984, 106,
1154.
(14) (a) Sauer, J. C. J. Am. Chem. Soc. 1947, 69, 2444. (b) Purohit, V.
C.; Richardson, R. D.; Smith, J. W.; Romo, D. J. Org. Chem. 2006, 71,
4549. (c) Duffy, R. J.; Morris, K. A.; Romo, D. J. Am. Chem. Soc. 2005,
127, 16754.
(15) Calter, M. A.; Orr, R. K.; Song, W. Org. Lett. 2003, 5, 4745.
2144
Org. Lett., Vol. 9, No. 11, 2007

We next studied the impact of a C2 substituent during the
bis-cyclization by targeting the synthesis of cinnabaramide
A (4). The synthesis commenced by reductive amination of
commercially available O-benzyl-L-serine with p-anisalde-
hyde (Scheme 2). Subsequent esterification provided the
Table 1. Synthesis of Simplified, C4-Unsubstituted
Salinosporamide/Cinnabaramide Derivatives 19a-d
Scheme 2. Total Synthesis of rac-Cinnabaramide A (4)
entry
R1
% yield (17)a,b % yield (19)b
drc
1
2
3
4
CyCH2
n-hexyl
PhCH2
H
84 (17a)
80 (17b)
72 (17c)
77 (17d)
93 (19a)
90 (19b)
85 (19c)
25 (19d)
2.2:1
2.2:1
2.5:1 (>19:1)d
a Yield is for two steps. b Yields refer to isolated, purified (SiO2) product.
c Determined by 1H NMR analysis of crude reaction mixtures. d Observed
o
diastereomeric ratio (dr) if reaction is allowed to proceed at 25 C for 1.5
d (54% yield). PMB ) p-methoxybenzyl, 4-PPY ) 4-pyrrolidinopyridine,
Cy ) cyclohexyl.
stereoselectivity is obtained during bis-cyclization if the
reaction is performed at 25 °C for extended times (1.5 d) by
selective degradation of the minor diastereomer (confirmed
1
by H NMR reaction monitoring).
A likely mechanistic pathway for this bis-cyclization
building on our related work with carbocycle-fused â-lac-
tones8 involves initial activation of the carboxylic acid as
pyridone ester 20 with modified Mukaiyama reagent 18.
Following transacylation with 4-PPY, deprotonation by
Hu¨nig’s base leads to ammonium enolate 21. Subsequent
net aldol-lactonization via aldolate 22 then provides the
γ-lactam-fused â-lactone 19a with concomitant regeneration
of the nucleophilic promoter, 4-PPY. However, a [2 + 2]
cycloaddition mechanism via an intermediate ketene has not
been excluded at this time.16 The relative configuration of
the major diastereomeric â-lactone 19a was confirmed by
X-ray analysis following cleavage of the PMB group with
ceric ammonium nitrate (CAN) to provide amide 23a
(Scheme 1). Importantly, the relative configuration corre-
sponds to that found in the salinosporamides and the cin-
nabaramides.
protected serine derivative 11a in 58% overall yield (two
steps). The required unsymmetrical ketene dimer 12a was
obtained by heterodimerization of acetyl and octanoyl
chlorides.14a Coupling of ketene dimer 12a with L-N-PMB-
serine 11a gave diastereomeric esters 25 (dr, 1:1), and
subsequent Sn-mediated hydrolysis17 provided acids 26 in
good overall yield. The key bis-cyclization provided dia-
stereomeric â-lactones 27/28 in 45% yield with moderate
diastereoselectivity (dr 3.3:1); however, the major diastere-
omer corresponded to that found in the natural product as
verified by NOE analysis.18 Deprotection of the benzyl ether
enabled separation of the major alcohol diastereomer 29
(79%, dr >19:1), and this was followed by Parikh-Doering
oxidation19 to give an intermediate aldehyde which was used
directly in the next step. Applying the method developed by
Corey, addition of zinc reagent 304a gave alcohol 31 in 57%
yield (two steps, dr 4.7:1). Finally, oxidative cleavage of the
PMB group gave rac-cinnabaramide A (4), which could be
isolated diastereomerically pure in 48% yield. Spectral data
for the synthetic material correlated with the published data.3
Further verification of relative configuration was accom-
plished by X-ray analysis.18
Scheme 1. Proposed Mechanism for the Bis-cyclization and
X-ray Structure of â-Lactone 23a (major diastereomer)
(16) In addition to previous studies with carbocycle-fused â-lactones
which clearly point to participation of the nucleophile in these bis-cyclization
reactions, lower conversions were obtained with less nucleophilic promoters
e.g., dimethylaminopyridine, suggestive of nucleophile involvement in the
rate-determining or prior step.
(17) Furla´m, R. L. E.; Emesto, G.; Mata, E. G.; Masearetti, O. A.
Tetrahedron 1998, 54, 13023.
(18) See the Supporting Information for experimental details.
(19) Parikh, J. P.; Doering, W. E. J. Am. Chem. Soc. 1967, 89, 5505.
Org. Lett., Vol. 9, No. 11, 2007
2145

To further validate the mildness of this strategy, we
targeted the synthesis of salinosporamide A bearing the
required chloro substituent in the keto acid substrate. In this
case, N-PMB serine allyl ester 11b, available in two steps
from serine, was utilized to enable mild ester deprotection
since presumed cyclopropane formation occurred during
attempted saponification of the corresponding keto methyl
ester (not shown, cf. 25) (Scheme 3). Amine 11b was coupled
ether enabled enrichment of the major diastereomer to
6-10:1 upon purification. Modified Moffatt21 oxidation using
1-(1,3-dimethylaminopropyl)-3-ethyl carbodiimide hydro-
chloride (EDCI)22 and dichloroacetic acid23 followed by
addition of zinc reagent 30 gave predominantly two dia-
stereomeric alcohols 37 (dr 3.5:1) in 33% yield (two steps).24
Final deprotection of the PMB group enabled isolation of
diastereomerically pure rac-salinosporamide A, which cor-
related with the published data and the relative configuration
was further confirmed by X-ray analysis.18
In summary, we have developed concise synthetic routes
to rac-salinosporamide A, rac-cinnabaramide A, and simpli-
fied derivatives. This strategy is unique in enabling simul-
taneous construction of both the γ-lactam and fused-â-lactone
found in these metabolites via a bis-cyclization process. The
â-lactone in these systems and the chloro-substituent in
salinosporamide precursors is shown to be tolerant to several
transformations which contributes to the brevity of the
sequence.1c The described bis-cyclization process points to
a logical biosynthetic origin for these intriguing natural
products and raises the interesting question of whether such
a bis-cyclization might be involved in the biosynthesis of
these natural products.9 Further optimization of this process
including mechanistic studies and extension to an enanti-
oselective strategy premised on A1,3-strain in keto acid
precursors, e.g., keto acids 25 and 33, constitute our ongoing
efforts in this area.
Scheme 3. Total Synthesis of rac-Salinosporamide A (3)
Acknowledgment. We thank the NIH (GM069784), the
Welch Foundation (A-1280), and Pfizer for support of these
investigations. We thank Dr. Joe Reibenspies (TAMU) for
X-ray analysis and Prof. Bill Fenical (Scripps Institute of
Oceanography/UC San Diego) for an 1H NMR spectrum of
natural salinosporamide A.
Supporting Information Available: General procedures
for ketene-dimerizations, bis-cyclizations, and subsequent
transformations with characterization data (including 1H and
13C NMR spectra) for â-lactones 3, 4, 19a-d, 23a, 27, 29,
31, 34, 36, and 37 and products 11a,b, 12a,b, 14b, 16a-c,
17a-d, 25, 26, 32, and 33. This material is available free of
charge via the Internet at http://pubs.acs.org.
with heteroketene dimer 12b, readily available in gram
quantities from heterodimerization of acetyl chloride and
commercially available 4-chlorobutanoyl chloride,18 to pro-
vide keto acids 33 following Pd-mediated ester deprotection.
Bis-cyclization provided bicyclic-â-lactones 34/35 in
25-35% yield (dr 2-3:1) favoring the relative configura-
tion found in salinosporamide A.20 Deprotection of the benzyl
OL070616U
(21) Pfitzner, K. E.; Moffatt, J. G. J. Am. Chem. Soc. 1965, 87, 5661.
(22) Coulton, S.; Southgate, R. J. Chem. Soc., Perkin Trans. 1 1992,
961.
(20) Lower yields in this bis-cyclization and that leading to cinnabaramide
A (26 f 27, 28) in comparison with C4-unsubstituted substrates (Table 1)
are clearly a result of increased steric issues during the bis-cyclization. While
no starting material is recovered, two major byproducts have been identified,
A and B (20-30% combined yield).
(23) Akahoshi, F.; Ashimori, A.; Sakashita, H.; Yoshimura, T.; Imada,
T.; Nakajima, M.; Mitsutomi, N.; Kuwahara, S.; Ohtsuka, T.; Fukaya, C.;
Miyazaki, M.; Nakamura, N. J. Med. Chem. 2001, 44, 1286.
(24) Yields in this two-step process are lower due to incomplete oxidation
and inability to purify the aldehyde due to some sensitivity of this
intermediate. Diastereoselectivity is considerably lower than that reported
previously (see refs 4a-c); however, this appears to be highly substrate
dependent given that Danishefsky observed reduced diastereoselectivity with
N-unprotected substrates (ref 4c).
2146
Org. Lett., Vol. 9, No. 11, 2007