Stereoselective synthesis of deuterated β-cyclohexenylserine, a biosynthetic intermediate of the salinosporamides

Article abstract of DOI:10.1021/ol201120k

A straightforward, highly stereoselective protocol toward the synthesis of deuterium-labeled (2R,3S,4S)-β-cyclohexenylserine has been developed. Key steps are a Nozaki-Hiyama-Kishi reaction generating the stereogenic centers and a ring-closing metathesis

Full text of DOI:10.1021/ol201120k

ORGANIC
LETTERS
2011
Vol. 13, No. 12
3210–3213
Stereoselective Synthesis of Deuterated
β-Cyclohexenylserine, a Biosynthetic
Intermediate of the Salinosporamides§
€
Jan Deska, Saskia Hahn, and Uli Kazmaier*
€
Universitat des Saarlandes, Institut fur Organische Chemie, Im Stadtwald, Geb. C4.2,
€
D-66123 Saarbru€cken, Germany
u.kazmaier@mx.uni-saarland.de
Received April 27, 2011
ABSTRACT
A straightforward, highly stereoselective protocol toward the synthesis of deuterium-labeled (2R,3S,4S)-β-cyclohexenylserine has been
developed. Key steps are a NozakiꢀHiyamaꢀKishi reaction generating the stereogenic centers and a ring-closing metathesis for the
construction of the cyclohexenyl ring system. The labeled amino acid was further activated as an SNAc-ester for feeding experiments.
The proteasome, a hydrolytically active multienzyme
complex, is responsible for the degradation of proteins in
the cell, and it is a key player in the regulation of a wide
range of cell processes.1 Proteasome inhibitors are therefore
interesting candidates for the development of antitumor
agents. Most synthetic inhibitors are peptide-derived
small molecules, interacting with a threonine in the active
center of the proteasome which is involved in the hydrolysis
of the target protein.2 Typical inhibitors are peptidic
R-ketoamides,3 R-ketoaldehydes,4 epoxyketones,5 vinyl-
sulfones,6 or boronic acids.7 Such proteasome inhibitors
are also found widespread in nature, produced by a wide
range of microorganisms. An important structural class
are γ-lactam thioesters and γ-lactam-β-lacton bicycles
respectively. (þ)-Lactacystin, isolated by Omura from
Streptomyces lactacystineus,8 was found to be a good
inhibitor. However, actually, it was not lactacystin
itself showing this effect, but its clasto-form omuralide
(Figure 1).9 The strained β-lactone ring interacts with
the catalytically active threonine, resulting in a covalent
blocking of the enzyme.10 The same structural motive is
also found in the families of the salinosporamides and
cinnabaramides.
§ Dedicated to Prof. Barry M. Trost on the occasion of his 70th birthday.
(1) (a) Goldberg, A. L. Nature 2003, 426, 895–899. (b) Goldberg,
A. L. Biochem. Soc. Trans. 2007, 35, 12–17.
(2) Borissenko, L.; Groll, M. Chem. Rev. 2007, 107, 687–717.
(3) Chatterjee, S.; Dunn, D.; Mallya, S.; Ator, M. A. Bioorg. Med.
Chem. Lett. 1999, 9, 2603–2606.
Salinosporamide was first isolated by Fenical et al. from
the marine actimomycete Salinispora tropica,11 which also
(4) Lynas, J. F.; Harriott, P.; Healy, A.; McKervey, M. A.; Walker,
B. Bioorg. Med. Chem. Lett. 1998, 8, 373–378.
(5) (a) Meng, L.; Mohan, R.; Kwok, B. H. B.; Elofsson, M.; Sin, N.;
Crews, C. M. Proc. Natl. Acad. Sci. U.S.A. 1999, 96, 10403–10408. (b)
Meng, L.; Kwok, B. H. B.; Sin, N.; Crews, C. M. Cancer Res. 1999, 59,
2798–2801.
(8) Omura, S.; Fujimoto, T.; Matsuzaki, K.; Moriguchi, R.; Tanaka,
H.; Sasaki, Y. J. Antibiot. 1991, 44, 113–116.
(9) Dick, L. R.; Cruikshank, A. A.; Grenier, L.; Melandri, F. D.;
Nunes, S. L.; Stein, R. L. J. Biol. Chem. 1996, 271, 7273–7275.
(10) (a) Fenteany, G.; Standaert, R. F.; Lane, W. S.; Choi, S.; Corey,
E. J.; Schreiber, S. L. Science 1995, 268, 726–731. (b) Groll, M.; Ditzel,
L.; Lowe, J.; Stock, D.; Bochtler, M.; Bartunik, H. D.; Huber, R. Nature
1997, 386, 463–471.
(11) Feling, R. H.; Buchanan, G. O.; Minzer, T. J.; Kauffman, C. A.;
Jensen, P. R.; Fenical, W. Angew. Chem. 2003, 115, 369–371. Angew.
Chem., Int. Ed. 2003, 42, 355–357.
(12) 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, 269–276.
(6) Bogyo, M.; McMaster, J. S.; Gaczynska, M.; Tortorella, D.;
Goldberg, A. L.; Ploegh, H. Proc. Natl. Acad. Sci. U.S.A. 1997, 94,
6629–6634.
(7) (a) Adams, J.; Behnke, M.; Chen, S.; Cruickshank, A. A.; Dick,
L. R.; Grenier, L.; Klunder, J. M.; Ma, Y. T. Bioorg. Med. Chem. Lett.
1998, 8, 333–338. (b) Adams, J.; Behnke, M.; Chen, S.; Cruickshank,
A. A.; Grenier, L. R.; Klunder, J. M.; Ma, Y.-T.; Plamondon, L.; Stein,
R. L. Bioorg. Med. Chem. Lett. 1998, 8, 333–338. (c) Palombella, V. J.;
Conner, E. M.; Fuseler, J. W.; Destree, A.; Davis, J. M.; Laroux, F. S.;
Wolf, R. E.; Huang, J.; Brand, S.; Elliot, P. J.; Lazarus, D.; McCormack,
T.; Parent, L.; Stein, R.; Adams, J.; Grisham, M. B. Proc. Natl. Acad.
Sci. U.S.A. 1998, 95, 15671–15676.
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10.1021/ol201120k
Published on Web 05/26/2011
2011 American Chemical Society

Thedevelopmentsinthisfield wereexcellentlycovered by a
recent review by Gulder and Moore.19
Scheme 1. Biosynthesis of Salinosporamide A According to
Moore20
Figure 1. Natural products showing proteasome inhibition
activity.
produces a wide range of closely related secondary meta-
bolites.12 In contrast, the cinnabaramides were found in
the terrestrial strain Streptomyces JS 360.13 Typical repre-
sentatives such as salinosporamide A and cinnabaramide
A inhibit the 20S proteasome in the low nanomolar
range13,14 and are therefore interesting candidates for
antitumor therapies. Salinosporamide is currently in phase
I clinical studies for the treatment of multiple myeloma.15
Therefore, it comes to no surprise that several total
syntheses16 and formal syntheses,17 especially toward sal-
inosporamide A, have been developed during the past few
years.18 The unusual β-cyclohexenylserine as a central
building block was either generated via nucleophilic attack
of cyclohexenyl organometallics or of cyclohexanone en-
olates onto suitable protected serine-derived aldehydes.
The group of Moore also investigated the biosynthesis of
the salinosporamides (Scheme 1).20 In principle, the mole-
cule can be divided into two major building blocks. The
chlorinated activated β-keto thioester A is obtained from a
tetrose via chloroethylmalonyl-CoA, which is coupled with
acetyl-CoA using a polyketide synthase (PKS). The second
building block B, a β-cyclohexenylserine, is obtained via a
modified shikimate pathway. Both building blocks were
coupled on a nonribosomal peptide synthetase (NRPS)
giving C, which undergoes cyclizations toward salinospor-
amide A. In the original publication, a (2R)-configuration
was shown for the unusual amino acid B.20 Later on,
during investigations of the enzymes involved in this
biosynthesis, this configuration was assigned to be 2S.21
€
(13) 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, 246–252.
(14) (a) Fenteany, G.; Standaert, R. F.; Lane, W. S.; Choi, S.; Corey,
E. J.; Schreiber, S. L. Science 1995, 268, 726–731. (b) Groll, M.; Ditzel,
€
L.; Lowe, J.; Stock, D.; Bochtler, M.; Bartunik, H. D.; Huber, R. Nature
1997, 386, 463–471.
Figure 2. Deuterated β-cyclohexenylserine derivatives.
(15) Chauhan, D.; Catley, L.; Li, G.; Podar, K.; Hideshima, T.;
Velankar, M.; Mitsiades, C.; Mitsiades, N.; Yasui, H.; Letai, A.; Ovao,
H.; Berkers, C.; Nicholson, B.; Chao, T.-H.; Neuteboom, S. T. C.;
Richardson, P.; Palladino, M. A.; Anderson, K. C. Cancer Cell 2005, 8,
407–419.
Our group has been involved in the synthesis of unusual
amino acids for several years,22 and for biosynthetic
studies we were interested in a stereoselective synthesis of
a dideuterated cyclohexenylserine derivative for feeding
experiments. We began our investigations shortly after the
proposal by Moore20 in 2007. Therefore, our targets were
the protected (2R,3S,4S)-configured amino acid 1 and the
deprotected activated derivative 2 (Figure 2).
(16) (a) Reddy, L. R.; Saravanan, P.; Corey, E. J. J. Am. Chem. Soc.
2004, 126, 6230–6231. (b) Endo, A.; Danishefsky, S. J. J. Am. Chem. Soc.
2005, 127, 8298–8299. (c) Mulholland, N. P.; Pattenden, G.; Walters,
I. A. S. Org. Biomol. Chem. 2006, 4, 2845–2846. (d) Ma, G.; Nguyen, H.;
Romo, D. Org. Lett. 2007, 9, 2143–2146. (e) Ling, T.; Macherla, V. R.;
Manam, R. R.; McArthur, K. A.; Potts, B. C. M. Org. Lett. 2007, 9,
2289–2292. (f) Mulholland, N. P.; Pattenden, G.; Walters, I. A. S. Org.
Biomol. Chem. 2008, 6, 2782–2789. (g) Takahashi, K.; Midori, M.;
Kawano, K.; Ishihara, J.; Hatakeyama, S. Angew. Chem. 2008, 120,
6340–6342. Angew. Chem., Int. Ed. 2008, 47, 6244–6246. (h) Fukuda, T.;
Sugiyama, K.; Arima, S.; Harigaya, Y.; Nagamitsu, T.; Omura, S. Org.
Lett. 2008, 10, 4239–4242. (i) Nguyen, H.; Ma, G.; Romo, D. Chem.
Commun. 2010, 46, 4803–4805.
(19) Gulder, T. A. M.; Moore, B. S. Angew. Chem. 2010, 122, 9534–
9556. Angew. Chem., Int. Ed. 2010, 49, 9346–9367.
(20) Beer, L. L.; Moore, B. S. Org. Lett. 2007, 9, 845–848.
(17) (a) Caubert, V.; Langlois, N. Tetrahedron Lett. 2006, 47, 4473–
4475. (b) Caubert, V.; Masse, J.; Retailleau, P.; Langlois, N. Tetrahedron
ꢀ
ꢀ
(21) McGlinchey, R. P.; Nett, M.; Eustaquio, A. S.; Asolkar, R. N.;
Lett. 2007, 48, 381–384. (c) Margalef, I. V.; Rupnicki, L.; Lam, H. W.
Tetrahedron Lett. 2008, 64, 7896–7901. (d) Mosey, R. A.; Tepe, J. J.
Tetrahedron Lett. 2009, 50, 295–297. (e) Momose, T.; Kaiya, Y.;
Hasegawa, J.-I.; Sato, T.; Chida, N. Synthesis 2009, 2983–2991.
(18) Shibasaki, M.; Kanai, M.; Fukuda, N. Chem. Asian J. 2007, 2,
20–38.
Fencial, W.; Moore, B. S. J. Am. Chem. Soc. 2008, 130, 7822–7823.
(22) (a) Kazmaier, U. Amino Acids 1996, 11, 283. (b) Deska, J.;
Kazmaier, U. Curr. Org. Chem. 2008, 12, 355–385. (c) Kazmaier, U. In
Frontiers in Asymmetric Synthesis and Application of R-Amino Acids;
Soloshonok, V. A., Izawa, K., Eds.; American Chemical Society: Washington,
DC, 2009; p 157 and references cited therein.
Org. Lett., Vol. 13, No. 12, 2011
3211

Subsequent ring-closing metathesis provided the protected
cyclohexenylserinol 6 in almost quantitative yield.
The allylation product5 wasobtained withhigh 3,4-anti-
diastereoselectivity (92% ds), which can be explained by a
chairlike transition state (Figure 3)26 and is in good agree-
ment with literature precedence.27 Out of the four possible
stereoisomers, only two were observed by HPLC. Ob-
viously, the induced diastereoselectivity from the protected
(S)-serinal was almost perfect.
Scheme 2. Retrosynthesis of Deuterated Cyclohexenylserine
Derivatives
According to Scheme 2, we planned to obtain the
cyclohexenyl ring via a ring-closing metathesis of the
deuterated linear precursor D, accessible by a stereoselec-
tive NozakiꢀHiyamaꢀKishi (NHK) reaction23 of deuter-
ated diene E and protected serinal.
Figure 3. Transition state of the NozakiꢀHiyamaꢀKishi
reaction.
Scheme 3. Stereoselective Synthesis of Protected Cyclohexenyl-
serinol 6
For analytical purposes and to determine the relative 2,3-
stereochemistry, we converted 6 into the corresponding
oxazolidinone 7 by treatment with NaH. The TBDPS
protecting group was not stable under these reac-
tion conditions, and therefore, it was replaced by the
TBDMS group. The ring protons of anti-disubstituted
oxazolidinones typically show coupling constants J of
4ꢀ6 Hz, while J = 7ꢀ9 Hz are found in the correspond-
ing syn derivatives.28 A coupling constant of J = 4.3 Hz
for 7 clearly indicates an anti-configuration at C2 and
C3 of 7, corresponding to a syn substitution pattern of
6. (Figure 4).
The starting point for the synthesis of the dideuterated
dienyl substrate was oct-7-en-2-ynol, easily accessible from
propargylic alcohol and 5-bromopentene.24 Stereoselec-
tive reduction with LiAlH4, followed by hydrolysis with
D2O, gave rise to the dideuterated allyl alcohol 3 in almost
quantitative yield (Scheme 3). This alcohol was converted
into the corresponding phosphate 4 for the subsequent
NHK reaction. The phosphate has been chosen because
phosphates are very convenient allylation reagents, show-
ing a better stability than the corresponding bromides or
even chlorides.25 For the subsequent allylation reaction
protected serinal was prepared fresh by Dibal-H reduction
of the corresponding methyl ester and was used without
further purification. Ininitial experiments using phosphate
4 alone, only low yields of the allylation product 5 were
obtained. The situation changed significantly after addi-
tion of NaI, and the NHK reaction provided dideuterated
homoallyl alcohol 5 in good yield. The corresponding
more reactive allyl iodide is probably formed in situ.
Figure 4. Determination of configuration.
The formation of this stereoisomer can be explained by a
hydrogen bond in the preferred conformation of the serinal
derivative, resulting in a syn periplanar orientation of the
amide and the carbonyl functionality and a nucleophilic
attack of the chromium reagent from the Si-face (Figure 3).
Obviously this attack is highly favored because only the
(26) Ciapetti, P.; Falorni, M.; Taddei, M. Tetrahedron 1996, 52,
7379–7390.
(23) (a) Takai, K.; Kimura, K.; Kuroda, T.; Hiyama, T.; Nozaki, H.
Tetrahedron Lett. 1983, 24, 5281–5284. (b) Jin, H.; Uenishi, J.; Christ,
W. J.; Kishi, Y. J. Am. Chem. Soc. 1986, 108, 5644–5646.
(27) For general reviews on reactions of crotylmetal reagents with
carbonyl, see: (a) Hoffmann, R. W. Angew. Chem. 1982, 94, 569–580.
(b) Yamamoto, Y.; Asao, N. Chem. Rev. 1993, 93, 2207–2293.
(28) Futagawa, S.; Inui, T.; Shiba, T. Bull. Chem. Soc. Jpn. 1973, 46,
3308–3310.
ꢁ
(24) (a) Mendes, C.; Renard, S.; Rofoo, M.; Roux, M.-C.; Rousseau,
G. Eur. J. Org. Chem. 2003, 463–471. (b) Bagutski, V.; Moszner, N.;
Zeuner, F.; Fischer, U. K.; de Meijere, A. Adv. Synth. Catal. 2006, 348,
2133–2147.
(29) (a) Dess, D. B.; Martin, J. C. J. Org. Chem. 1983, 48, 4155–4156.
(b) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277–7287.
(30) Bal, B. S.; Childers, W. E.; Pinnick, H. W. Tetrahedron 1981, 37,
2091–2096.
(25) (a) Yanagisawa, A.; Noritake, J.; Nomura, N.; Yamamoto, H.
Synlett 1991, 251–253. (b) Yanagisawa, A.; Nomura, N.; Yamamoto, H.
Synlett 1991, 513–514. (c) Nowotny, S.; Tucker, C. E.; Jubert, C.;
Knochel, P. J. Org. Chem. 1995, 60, 2762–2772.
3212
Org. Lett., Vol. 13, No. 12, 2011

product in good yield, but it was difficult to remove the
chromium completely.
Scheme 4. Synthesis of Protected Amino Acid 1 and
SNAc-ester 2
Because 1 should be applied to biological systems, we
tried to avoid contamination with traces of toxic heavy
metal. Therefore, we decided to use a metal-free two step
oxidation protocol. Oxidation with DessꢀMartin period-
inane29 gave rise to aldehyde 10, which was further oxi-
dized to 1 with NaClO2 and 2-methyl-2-butene according
to Pinnick.30
For the synthesis of 2, amino acid 1 was coupled with N-
acetylcysteamine to 11 prior to the simultaneous removal
of the remaining protecting groups by trifluoroacetic acid
(TFA).
In conclusion, we have shown that the Nozakiꢀ
HiyamaꢀKishi reaction is a highly suitable tool for
the stereoselective synthesis of cyclohexenylserine deri-
vatives, including deuterium-labeled ones. Further
investigations of the (2S)-configured isomer and other
deuterium-labeled biosynthetic intermediates are cur-
rently in progress.
formation of the anti-oxazolidinone was observed by
NMR.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft and by the Fonds der
Chemischen Industrie.
To complete the syntheses of 1 and 2 the free OH-group
of 6 was protected with a THP group (Scheme 4). Two
diastereomers of 8 were formed as a 1:1 mixture, which are
separable by flash chromatography. Deprotection of the
silyl protecting group with TBAF gave rise to the primary
alcohol 9, which could be oxidized to the required amino
acid 1. In principle, PDC oxidation gave the expected
Supporting Information Available. Analytical and
spectroscopic data of all new products. This material
is available free of charge via the Internet at http://
pubs.acs.org.
Org. Lett., Vol. 13, No. 12, 2011
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