An efficient synthesis of lactacystin β-lactone

Article abstract of DOI:10.1002/anie.200453843

A key step in the synthesis of lactacystin β-lactone (3), an inhibitor of the 20 S proteasome, was the ammonia-free reductive aldol reaction of pyrrole 1 to form 2 with complete anti selectivity. This route to 3 takes just 13 steps (14% overall yield) and allows the late-stage stereoselective introduction of a methyl group at C4, which is crucial for the production of analogues. Boc = tert-butoxycarbonyl.

Full text of DOI:10.1002/anie.200453843

Angewandte
Chemie
Total Synthesis
An Efficient Synthesis of Lactacystin b-Lactone**
Timothy J. Donohoe,* Herman O. Sintim,
Leena Sisangia, and John D. Harling
Proteasomes are proteins within cells that are responsible for
protein degradation. The 20S proteasome, a 700-kDa protein
that consists of 14 distinct subunits, is implicated in the
ubiquitin proteasome pathway (UPP).[1] UPP, present in all
eukaryotic cells, is essential for the normal turnover of
cellular proteins and for the removal of damaged or misfolded
[*] Dr. T. J. Donohoe, Dr. H. O. Sintim, L. Sisangia
Department of Chemistry, Chemistry Research Laboratory
University of Oxford
Mansfield Road, Oxford, OX1 3TA (UK)
Fax: (+44)1865–275–708
E-mail: timothy.donohoe@chem.ox.ac.uk
Dr. J. D. Harling
GlaxoSmithKline, Medicines Research Centre
Gunnels Wood Road, Stevenage, SG1 2NY (UK)
[**] We thank the EPSRC and GlaxoSmithKline for funding this project.
AstraZeneca, Pfizer, and Novartis are thanked for generous
unrestricted funding.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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DOI: 10.1002/anie.200453843
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Communications
proteins.[1] Another vital role of UPP is the processing and
degradation of regulatory proteins that control cell growth,
transcriptional activation, and metabolism.[1] In view of this
significant biological role, the 20S proteasome has become an
important biological target in many drug-discovery programs.
Omura et al.[2,3] reported the isolation and characteriza-
tion of (+)-lactacystin (1) in 1991 and showed it to be a novel
Scheme 2. Retrosynthetic analysis of 2.
Herein we describe a short alternative approach to (Æ )-
lactacystin b-lactone (2) through a diastereoselective reduc-
tive aldol reaction of Boc-protected pyrrole 8 that we have
recently developed.[23] The reaction of pyrrole 8 in the
presence of MgBr2 led to an anti selectivity greater than
20:1 (Scheme 3): This selectivity has its origins in the
formation of Z enolate 9.[23] Subsequent protection of the
anti aldol adduct (Æ )-10 as an acetate following a standard
protocol proceeded to yield 11 (Scheme 3).
g-lactam produced by a culture broth of Streptomyces sp. OM-
6519. As lactacsytin inhibits the 20S proteasome, there has
been a flurry of synthetic approaches towards the synthesis of
this interesting molecule and analogues thereof.[4–10]
Key work by Corey, Schreiber, and co-workers,[11,12]
Huber and co-workers,[13] and Dick et al.[14] clearly defined
the mechanism of inhibition displayed by (+)-lactacystin (1).
These investigations showed that (+)-lactacystin (1) is, in fact,
a prodrug for (+)-lactacystin b-lactone (2), formed by the loss
of N-acetylcysteine (Scheme 1). Once inside a cell, the lactam
2 then acylates the proteasome, causing inhibition.
Scheme 3. Reagents and conditions: a) (Boc)2O, DMAP, Et3N, CH2Cl2;
b) Li, DBB, THF, (MeOCH2CH2)2NH, MgBr2, isobutyraldehyde;
c) Ac2O, pyridine, DMAP. Boc=tert-butoxycarbonyl, DMAP=4-dime-
thylaminopyridine, DBB=4,4’-di-tert-butylbiphenyl.
Scheme 1. (+)-Lactacystin (1) is a prodrug for (+)-lactacystin b-lactone (2).
Once inside a cell, 2 acylates the 20S proteasome, causing inhibition.
Structure–activity relationship (SAR) studies by Corey
and co-workers[11,15–21] and by Adams and co-workers[9]
showed that the SAR requirements for proteasome inhibition
were rather stringent. There is an absolute requirement for
the b-lactone ring to be present, and the stereochemical
fidelity at C2, C3, and C6 cannot be compromised without
losing biological activity. The isopropyl group attached to C6
is also important for inhibition. Thus, the only replaceable
group is the methyl group at C4.[10] All the syntheses of
lactacystin b-lactone (2) reported so far introduce the methyl
group at C4 at a relatively early stage; therefore, these
strategies are not ideal for the easy production of ana-
logues.[9,22] Our work addresses this issue and a retrosynthetic
analysis of such an approach is shown in Scheme 2.
A key step in our synthesis was the diastereoselective
dihydroxylation of 11 (Scheme 4). Treatment of 11 with
catalytic OsO4 and NMO (3 equiv) in acetone/water (4:1)
afforded diol 12 as a single diastereoisomer in an average
yield of 65%. However, by changing the dihydroxylation
conditions to those reported by Poli[24] (cat. OsO4,
Me3NO·2H2O (3 equiv) in CH2Cl2), the diol 12 was produced
in an excellent yield of 95%. The stereochemistry of the diol
12 was proven by a two-step conversion into the crystalline
cyclic sulfate 13, the structure of which was determined by X-
ray diffraction analysis.[25] The X-ray crystal structure of 13
reveals that the isopropyl group effectively shields one face of
the ring. Importantly, reagents therefore approach 11 from
the face syn to the ester functionality.
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Angewandte
Chemie
(Scheme 5). Gratifyingly, the major alkylation adduct 17 was
formed with a similar selectivity to that of the dihydroxylation
step (see above) and had the requisite stereochemistry at C4
for lactacystin b-lactone (2). The stereochemistry of both 17
and 18 was assigned by NOE studies.[28] Finally, cleavage of
the tert-butoxycarbonyl group of 17 with TFA in CH2Cl2 led to
the formation of lactam 19 in quantitative yield. Basic
hydrolysis of the ethyl ester gave acid 20, which was used
without purification to give 2 (Scheme 6). The spectroscopic
data of compound 2 was identical to that reported in the
literature.
Scheme 4. Reagents and conditions: a) cat. OsO4, Me3NO·2H2O,
CH2Cl2; b) 1. SOCl2, Et3N, CH2Cl2; 2. cat. RuCl3·xH2O, NaIO4, MeCN/
CCl4/H2O.
Two crucial steps of our total synthesis were the regiose-
lective deoxygenation at C4 and subsequent diastereoselec-
tive (syn) introduction of the methyl group at C4. To this end,
several strategies were investigated. The C4-OH of diol 12
was selectively converted into a bromide or iodide by
selective mesylation and SN2 displacement of the mesylate
with lithium bromide or zinc iodide. Unfortunately, the
resulting halide functionalities could not be displaced with
any nucleophile that we examined.[26] An alternative
approach was therefore sought. A selective Mitsunobu
reaction led to the conversion of the C4-OH functionality of
12 into iodide 14 (Scheme 5). The regioselectivity observed in
the Mitsunobu reaction was as expected because displace-
ment of the C3 (neopentyl) hydroxy group is slow. The
resulting iodide 14 was deiodinated through a recently
reported method for producing (catalytic) indium hydride
in situ.[27] The use of indium hydride instead of the traditional
tributyltin hydride obviated the need for extensive purifica-
tion of the product. Next, the C3-OH functionality of 15 was
protected with a triethylsilyl group (TES), the product was
oxidized with catalytic RuO4 to form a lactam, and the TES
group was then removed to furnish 16.
Scheme 6. Reagents and conditions: a) CF3CO2H, CH2Cl2; b) NaOH
(aqueous 0.5m); c) BOPCl, Et3N, CH2Cl2. BOPCl=bis(2-oxo-3-oxazoli-
dyl)phosphinic chloride.
In conclusion, a short synthesis of (Æ )-lactacystin b-
lactone (2) was completed in only 13 steps starting from
commercially available pyrrole 7. The final product was
isolated in 14% overall yield. The advantage of our strategy is
centered around the introduction of the methyl group at C4 at
a late stage of the synthesis, thereby making our route easily
amenable to the production of analogues.
The second key step then followed when we introduced
the methyl group at C4 with LDA (2 equiv) and methyl iodide
Received: January 26, 2004 [Z53843]
Keywords: aldol reaction · heterocycles ·
.
natural products · reduction · total synthesis
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