A1,3-strain enabled retention of chirality during bis-cyclization of β-ketoamides: Total synthesis of (-)-salinosporamide A and (-)-homosalinosporamide A

Article abstract of DOI:10.1039/c0cc00607f

A concise, enantioselective synthesis of the Phase I anticancer agent, (-)-salinosporamide A, is described. The brevity of the described strategy stems from a key bis-cyclization of a β-keto tertiary amide, accomplished on gram scale, which retains optical purity enabled by A^1,3-strain rendering epimerization slow relative to the rate of bis-cyclization. The versatility of the strategy for derivative synthesis is demonstrated by the synthesis of (-)-homosalinosporamide A.

Full text of DOI:10.1039/c0cc00607f

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A1,3-strain enabled retention of chirality during bis-cyclization
of b-ketoamides: total synthesis of (ꢀ)-salinosporamide A and
(ꢀ)-homosalinosporamide Aw
Henry Nguyen, Gil Maz and Daniel Romo*
Received 26th March 2010, Accepted 22nd April 2010
First published as an Advance Article on the web 25th May 2010
DOI: 10.1039/c0cc00607f
A concise, enantioselective synthesis of the Phase I anticancer
agent, (ꢀ)-salinosporamide A, is described. The brevity of the
described strategy stems from a key bis-cyclization of a b-keto
tertiary amide, accomplished on gram scale, which retains
optical purity enabled by A1,3-strain rendering epimerization
slow relative to the rate of bis-cyclization. The versatility of the
Fig. 1 Structures of natural occurring bicyclic-b-lactone proteasome
inhibitors.
strategy for derivative synthesis is demonstrated by the synthesis
of (ꢀ)-homosalinosporamide A.
Inhibitors of the human 20S proteasome are of continued
intense interest due to their potential as anticancer therapeutics
and the recent FDA approval of bortezomib (Velcade) validates
the proteasome as a target for cancer chemotherapy.1 Salinos-
poramides (salino A/B, 1a–b) are unique bicyclo [3.2.0]
b-lactone-containing natural products of marine bacterial
origin isolated by Fenical and coworkers2 from the marine
actinomycete, Salinispora tropica (Fig. 1). Salino A is a potent
nanomolar inhibitor of the proteasome and is currently in
Phase I clinical studies for multiple myeloma having shown
potential in mouse models toward several cancers when
administered intravenously despite the potentially labile
b-lactone.3 Salinosporamide A and congeners, which bear
close structural similarities to the terrestrial metabolites, the
cinnabaramides (e.g. cinnabaramide A, 2) and lactacystin-b-
lactone (omuralide),4 have been targets for total5 and formal6
syntheses, structure–activity studies,7 biosynthetic engineering,8
and crystallographic studies with the 20S proteasome.9 The
latter studies revealed an intriguing mode of action for salino
A involving O-acylation of the N-terminus threonine of the
proteasome by the b-lactone with concomitant cyclization of
the incipient alkoxide with the C13 chloro substituent leading
to a tetrahydrofuran. Herein we disclose a versatile and
concise, enantioselective strategy to both (ꢀ)-salino A and
(ꢀ)-homosalino A made possible by the A1,3-strain of b-keto
tertiary amides10 which enables retention of optical purity
during a key bis-cyclization process that simultaneously forms
the g-lactam and fused b-lactone core.
Fig. 2 Enantioselective strategy to salino A and derivatives.
delivers the bicyclic-b-lactone pharmacophore 4 in a single
operation from acyclic precursors and the optical purity of the
b-lactone would reflect the diastereomeric purity of b-keto-
amide 5, derived from acylation of serine derivatives with
racemic ketene dimers 7. The C4 stereocenter in b-ketoamide 5
is lost during the bis-cyclization process, thus the integrity of
the C2 stereocenter dictates the optical purity of b-lactone 4
which we predicted would be preserved by A1,3 strain.
The synthesis of salino A began with reductive amination of
commercially available (R)-O-benzyl serine (8) (99% ee) with
p-anisaldehyde, followed by esterification to provide allyl ester
(+)-9 (Scheme 1). After extensive optimization by careful control
of reaction temperature and time, the desired ester (+)-9 could
be obtained with minimal racemization (98% ee) through these
two steps in 79% yield.y The required unsymmetrical ketene
dimer (ꢁ)-7a was obtained by heterodimerization of acetyl
chloride and 4-chlorobutanoyl chloride following the procedure
of Sauer.z11 Acylation of serine derivative (+)-9 with this ketene
dimer under microwave conditions gave diastereomeric b-keto-
amides 10a/10a0 (dr 1 : 1) in 80% yield. Separation of the
diastereomers by MPLC provided the required (R,R)-b-keto-
amide 10a (45%, dr 30 : 1, 98% ee). Conditions were identified
to recycle the undesired diastereomer, (R,S)-10a0, via epimeriza-
tion (98% yield, dr 1 : 1) of the C2 but not the C4 stereocenter
thus allowing an effective resolution of ketene dimer (ꢁ)-7a and
greater material throughput.8
Building on our previously reported racemic synthesis of
salino A,5e a key step in our strategy is a diastereoselective,
nucleophile-promoted, bis-cyclization (Fig. 2). This process
Department of Chemistry, Texas A&M University, College Station,
TX 77840, USA. E-mail: romo@tamu.edu
w Electronic supplementary information (ESI) available: Experimental
procedures and all 1H/13C NMR spectra. CCDC 775264. See DOI:
10.1039/c0cc00607f
z Current address: Lundbeck Research USA, Inc., 215 College Rd,
Paramus, NJ 07652, USA.
ꢂc
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Table
2
Varying conditions for the bis-cyclization leading to
b-lactone (ꢀ)-3a including a gram scale synthesis
% Recovered
ketoacid
Entry
Time/h
% Yielda,b
% ee
dr
1
1.5
3.5
3.5
35
60
52
92
88
90
7 : 1
4 : 1
5 : 1
28
10
11
2
Scheme 1 Reagents and conditions: (a) p-anisaldehyde, MeOH, 0 1C,
8 h; NaBH4, MeOH, 12 h, ꢀ10 1C (97%); (b) p-TsOH, allyl alcohol,
PhH, 100 1C, 8 h (82%); (c) (ꢁ)-7a, ClCH2CH2Cl, pyridone, mW,
50 1C, 2 h (80%); (d) p-TsOH, MeOH : EtOAc (1 : 4), 45 1C, 48 h
(98%).
3c
a
b
Yield for 2 steps (deprotection and bis-cyclization). Yields refer to
c
isolated, purified b-lactones. This run was performed on gram scale.
g- and d-substituted substrates, pointing to the inductive
effects and resultant increased acidity imposed by these
b-heteroatoms.
Towards the required ketoacid substrate for bis-cyclization,
mild and brief exposure of b-ketoamide (R,R)-10a to
Pd(0)-mediated deprotection provided keto acid (R,R)-5 with
only slight erosion of the diastereomeric purity (Table 2). At
the outset, it was unclear whether the A1,3-strain induced
Towards optimization of the synthesis of b-lactone 3a
required for salino A, reaction parameters were optimized
extensively including alternative carboxylic acid activating
agents. Ultimately we found that mesyl chloride at lower
temperature and less polar solvents led to dramatic increases
in both diastereoselectivity (7 : 1) and retained enantiopurity
of bicyclic b-lactone 3a (92% ee, 35% yield, 28% recovered
keto acid, Table 2, entry 1). In addition, a yield of 60% was
obtained with longer reaction times under these conditions,
however this led to reduced diastereoselectivity and enantio-
purity (dr 4 : 1, 88% ee, Table 2, entry 2). Importantly, this
reaction could be performed on gram scale with comparable
diastereoselectivity and retention of enantiopurity (52%, dr 5 : 1,
90% ee, Table 2, entry 3).
conformational bias in b-keto tertiary amides (B4 kcal molꢀ1
)
would be sufficient to avoid epimerization of this center in the
time frame and under the basic conditions of the bis-cyclization.
As expected, applying bis-cyclization conditions that were
previously developed for the racemic series to keto acid
(R,R)-5 (dr 29 : 1, 98% ee) led to similar yields and diaster-
eoselectivity (40% yield, dr 2.5 : 1),5e however erosion of
optical purity of adduct 3a was observed (85% ee). Towards
exploring the impact of the chlorine atom on the acidity of the
C2-proton and the versatility of this strategy for derivative
synthesis, we studied variations of the C2-substituent by
employing other heteroketene dimers for the synthesis of
ketoamide substrates 5b–c (see ESIw for details). Bis-cyclization
of ketoacid 5b bearing a g-siloxy group (Table 1, entry 2) led to
similar results as ketoacid 5a bearing a g-chloro substituent,
Completion of the salino A synthesis entailed hydro-
genolysis, which facilitated separation of the minor diastereomer
3a0 from the bis-cyclization, to provide alcohol (ꢀ)-12a in 75%
yield (Scheme 2). Modified Moffatt oxidation12 and addition of
zinc reagent 135a gave diastereomeric alcohols (dr 11 : 3 : 1 : 1)
in 62% yield (2 steps) and final PMB deprotection of the mixture
of diastereomers led to pure (ꢀ)-salinosporamide A following
recrystallization (syn. [a]D ꢀ71.3; lit. [a]D ꢀ72.9).
however the d-chloro ketoacid 5c led to
a significant
increase in yield (Table 1, entry 3). These results are mirrored
by the 1H NMR chemical shifts of the C2 protons of
b-ketoamides 10a–c, which show a Dd of 0.4 ppm between
The synthesis of homosalino A (14), a derivative with the
potential to form a tetrahydropyran upon acylation of the
proteasome, followed an identical route as for salinosporamide
A but using heteroketene dimer (ꢁ)-7c with serine derivative
(+)-9 to provide the homologous ketoester (ꢀ)-10c (94% ee,
dr 25 : 1) after MPLC separation (Scheme 3). Ester deprotection
and subsequent bis-cyclization provided the bicyclic-b-lactone
(ꢀ)-3c/3c0 in 60% yield (dr 3.5 : 1, 2 steps). Subsequent
Table 1 Variation of the C2-side chain: inductive effects on efficiency
of bis-cyclization with b-ketoacids 5a–c
R1
b-Lact.
% Yielda,b
dHa, dHb
c
Entry
1
2
3
(CH2)2Cl
(CH2)2OTBS
(CH2)3Cl
3a/3a0
3b/3b0
3c/3c0
40
42
62
3.92, 3.94
3.92, 3.95
3.52, 3.57
Scheme 2 Reagents and conditions: (a) H2, Pd/C THF, 23 1C, 12 h
(75%); (b) (i) EDCl, Cl2CHCO2H, DMSO/PhMe, 23 1C, 5 h;
(ii) 2-cyclohexenylzinc chloride (13), THF, ꢀ78 1C, 4 h (62%, dr =
11 : 3 : 1 : 1); (c) CAN, MeOH/H2O, 0 1C, 6 h (43%, dr 15 : 1).
a
b
Yield for 2 steps (deprotection and bis-cyclization). Yields refer to
c
isolated, purified b-lactones. Chemical shift of C2 protons (Ha/b) of
diastereomeric ketoamide esters 10a–c.
ꢂc
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Notes and references
y Optical purities were determined by chiral HPLC analysis. See ESI.w
z While ketene dimer (ꢁ)-7a was obtained in low statistical yield
(13%, 52% of theoretical), its synthesis is readily performed on
multigram scale. See ESI.w
8 Chiral HPLC analysis verified that epimerization occurred only at
C2. See ESI.w
** C2-Epimers of salinosporamides have been isolated during large-
scale fermentation of S. tropica (see ref. 7a).
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Scheme
3
Reagents and conditions: (a) (ꢁ)-7c, pyridone,
1); (b) Pd(PPh3)4,
ClCH2CH2Cl, mW, 53 1C, 2 h (80%, dr 1
:
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following recrystallization: 52%, 89% ee).
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hydrogenolysis again enabled separation of the minor
diastereomer to give alcohol (ꢀ)-12c in 62% yield (82% ee)
and recrystallization led to enrichment of enantiopurity to
89% ee (52% yield). Using an identical sequence as described
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In summary, we have developed a concise, 9-step enantio-
selective route to (ꢀ)-salinosporamide A from O-benzyl serine.
The key bis-cyclization of a b-ketoamide, amenable to gram
scale, constructs both the g-lactam and the fused-b-lactone in
one operation contributing to the brevity of the synthesis. The
ability of the described b-keto tertiary amide substrates to
maintain stereochemical integrity by virtue of A1,3 strain raises
the intriguing question of how such stereochemical integrity is
maintained with b-keto secondary amides, known salino A
precursors, prior to and during related biosynthetic cycliza-
tions or if a dynamic kinetic resolution is operative.** The
flexibility of the described strategy derives from the versatility
of bicyclic cores e.g. 3a–c obtained from alternative ketene
dimers 7 which enable variation of the C2 side chain as
demonstrated in the synthesis of (ꢀ)-homosalinosporamide
A. Furthermore, the strategy allows for changes in the C4 side
chain by addition of various organometallic reagents, notably
in the presence of the b-lactone. The synthesis of additional
hypothesis-driven salino A derivatives by the described
strategy and their bioactivity will be reported in due course.
We thank the NIH (GM069784) and the Welch Foundation
(A-1280) for support. We thank Dr Wei Zhang for initially
preparing b-ketoamide 10b and Dr Joe Reibenspies (TAMU)
for X-ray analysis.
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ꢂc
This journal is The Royal Society of Chemistry 2010
Chem. Commun., 2010, 46, 4803–4805 | 4805