Diastereoselective synthesis of the 5-hydroxy-pyrrolidinone amino acid of the microsclerodermins and model studies for an end-game strategy for microsclerodermin B

Article abstract of DOI:10.1016/j.tetlet.2016.12.045

The first diastereoselective synthesis of the 5-hydroxy-pyrrolidinone amino acid common to eight members of the microsclerodermin family is presented. Our strategy involves formal hydration of an unsaturated precursor via the use of a two-step hydroxybromination-debromination protocol; this procedure provides exclusively the requisite 4,5-cis-pyrrolidinone. Furthermore model studies are presented that indicated the potential viability of this hydration strategy in the context of a synthesis of microsclerodermin B.

Full text of DOI:10.1016/j.tetlet.2016.12.045

Accepted Manuscript
Diastereoselective synthesis of the 5-hydroxy-pyrrolidinone amino acid of the
microsclerodermins and model studies for an end-game strategy for microscler-
odermin B
Christian Winter, Robert D.C. Pullin, Timothy J. Donohoe
PII:
S0040-4039(16)31692-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.12.045
TETL 48457
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Accepted Date:
6 December 2016
19 December 2016
Please cite this article as: Winter, C., Pullin, R.D.C., Donohoe, T.J., Diastereoselective synthesis of the 5-hydroxy-
pyrrolidinone amino acid of the microsclerodermins and model studies for an end-game strategy for
microsclerodermin B, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.12.045
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Diastereoselective synthesis of the 5-hydroxy-
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pyrrolidinone amino acid of the microsclerodermins
and model studies for an end-game strategy for microsclerodermin B
Christian Winter, Robert D. C. Pullin and Timothy J. Donohoe
NH
late-stage
O
O
hydration?
CO2H
O
HN
HO
O
HN
H
Pyrr
N
N
N
OH
O
HN
O
HN
O
OH O
NH
H
OH
OH
OH
N
cis-selective?
N
H
functional group
compatibility?
O
MeO

1
Tetrahedron Letters
Diastereoselective synthesis of the -hydroxy-pyrrolidinone amino acid of the
microsclerodermins and model studies for an end-game strategy for
microsclerodermin B
Christian Winter, Robert D. C. Pullin and Timothy J. Donohoea
aDepartment of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford, OX1 3TA, UK
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
The first diastereoselective synthesis of the -hydroxy-pyrrolidinone amino acid common to
eight members of the microsclerodermin family is presented. Our strategy involves formal
hydration of an unsaturated precursor via the use of a two-step hydroxybromination-
debromination protocol; this procedure provides exclusively the requisite 4,5-cis-pyrrolidinone.
Furthermore model studies are presented that indicated the potential viability of this hydration
strategy in the context of a synthesis of microsclerodermin B.
Available online
Keywords:
Amino acids
Cyclic peptides
Stereoselective synthesis
Peptide coupling
Metathesis
2009 Elsevier Ltd. All rights reserved.
Introduction
The microsclerodermins (assigned as variants A-M) represent
an intriguing class of cyclic peptide natural product. First
reported in 1994, microsclerodermins A-I were isolated by
Faulkner from the marine sponges Microsclerodema sp. and
Theonella sp.,1 with Li later isolating J and K from the sponge
Microscleroderma herdmani.2 Notably Müller recently reported
the isolation of microsclerodermins L and M from terrestrial
myxobacteria Chondromyces, Jahnella and Sorangium;3
furthermore with a proposal of their biosynthetic origin
which showed that the related natural products pedein A and B
also derive from the same pathway.4 The microsclereodermin
family displays both antifungal activity and cytotoxicity against
various cell lines.1-5 Structurally, the microsclerodermins share a
common 23-membered cyclic hexapeptide motif. Of the six
aminoacid residues, three are conserved; glycine (Gly), sarcosine
(Sar) and -amino--hydroxybutanoic acid (GABOB), with
differences arising in the nature of a substituted tryptophan (Trp),
a complex polyhydroxylated -amino acid and a -amino--
lactam (pyrrolidinone, Pyrr) residue, which is found either in an
unsaturated ‘dehydrated’ form (E, J and K) or a ‘hydrated’ -
hydroxy form (ten remaining members) with a 4,5-cis relative
configuration in all cases (Figure 1).
Figure 1. Microsclerodermin B (1) and the constituent pyrrolidinone
amino acids of the family. The relative stereochemistry of the cis-
pyrrolidinone amino acid in microsclerodermin A has not been
determined.
The microsclerodermins have been the focus of continued
synthetic work since their first isolation. Most reports have
focused on the preparation of the polyhydroxylated -amino acid
residues;6 indeed we have reported the synthesis of the
stereopentad -amino acid AMMTD (present in A, B, J and K).7
However, fewer studies have focused around the pyrrolidinone
amino acid. In 2003 Ma reported the total synthesis of
microsclerodermin E,8 which possesses an unsaturated
pyrrolidinone residue and whilst early studies by Shioiri and
Hamada demonstrated the synthesis of an isolated -
———
 Corresponding author. Tel.: +44-1865-275649; fax: +44-1865-275708; e-mail: timothy.donohoe@chem.ox.ac.uk

2
Tetrahedron
hydroxypyrrolidinone amino acid,9 they found that it easily
dehydrated, in line with previous observations made by
Faulkner.1 As part of our efforts we recently disclosed the total
synthesis of microsclerodermin J and dehydromicrosclerodermin
B, along with the structural reassignment of the C4 pyrrolidinone
stereocentre in both natural products and concurrently the
adjacent C5 stereocentre in microsclerodermin B.10 Whilst we so
far been unsuccessful in our attempts to prepare
microsclerodermin B (1) itself, our synthetic strategy was
designed to incorporate a late-stage hydration of the unsaturated
pyrrolidinone amino acid II of dehydromicrosclerodermin B in
order to reveal the sensitive -hydroxypyrrolidinone
I
(Scheme 1) at a late stage. Details of the synthesis of the
-hydroxypyrrolidinone amino acid and supporting model studies
for a synthesis of microsclerodermin B are now reported.
Scheme 2. Synthesis of dipeptide 5. TMSE = (trimethylsilyl)ethyl
All our attempts to hydrate the dipeptide 5 with either
Brønsted or Lewis acids failed. Therefore, we sought to react the
constituent enamide with a separate electrophile, which could
later be removed, in aqueous solution to effect a formal
hydration. Halogenation, in particular bromination, was an
attractive option. We were delighted to find that direct treatment
of 5 with 2.5 equivalents of either NBS (Table 1, entry 1) or
TBCD (entry 2) in THF/H2O cleanly gave a mono-brominated
bromohydrin, 6 in good yield (57% and 81% respectively) and in
short reaction times (<1 h). Further experimentation revealed that
bromine in CH2Cl2/H2O could also be employed to deliver the
bromohydrin in similar yield (76%) (entry 3). Interestingly only
the mono-brominated hydroxypyrrolidinone was observed in
these reactions, despite the fact that 2.5 equivalents of
brominating agent was used. In all cases a single diastereoisomer
(unassigned at this stage) was observed.
Scheme 1. Proposed hydration strategy to prepare the -
hydroxypyrrolidinone of microsclerodermin B
Results and discussion
Because of the reported ease with which the -
hydroxypyrrolidinone dehydrated we reasoned that in order to
incorporate this residue into a viable total synthesis, hydration of
the unsaturated pyrrolidinone should be conducted as late as
possible in any synthetic sequence. As a result we felt it prudent
to first prepare a suitable model system in which to test the
validity of this strategy. We selected the unsaturated sarcosine-
pyrrolidinone dipeptide 5, where the N-acetamide would provide
a
simple model of the -amino acid present in the
Table 1. Hydroxybromination of dipeptide 5
microsclerodermins. Preparation of the unsaturated pyrrolidinone
amino acid 3 commenced from aspartic acid derivative 2,
originally accomplished with a modification of chemistry
reported by Ma8 and Shioiri (route 1)9 but later via a new route
involving a Blaise reaction (route 2),11 which we then utilised in
the
synthesis
of
microsclerodermin
J
and
dehydromicrosclerodermin B.10 While the second route had the
advantage of allowing the preparation of acid 3 in enantiopure
form, the first route (giving racemic material in our hands) was
used in this model study. Protecting group manipulation then
allowed preparation of the sarcosine-pyrrolidinone dipeptide 5
through coupling of acid 3 with sarscosine (trimethylsilyl)ethyl
ester (4) using HBTU. Removal of the Boc group and acylation
with acetic anhydride delivered dipeptide 5 in high overall yield
(Scheme 2).
Entry
Bromine sourcea
Solvent
Yield of 6 /%b
1
2
3
NBS
TBCD
Br2
THF/H2O
THF/H2O
CH2Cl2/H2O
57
81
76
a 2.5 equivalents used. b Isolated yields after purification by flash
column chromatography; NBS = N-bromosuccinimide, TBCD =
2,4,4,6-tetrabomo-2,5- cyclohexadien-1-one.
With bromohydrin 6 in hand our attention then turned to
debromination and subsequent establishment of the
stereochemistry of the pyrrolidinone product (Scheme 3). After
some optimisation we found that treatment of the bromohydrin 6
with tributyltin hydride and catalytic amounts of triethylborane as
an initiator,12 could cleanly abstract the bromine atom at low
temperature to furnish the desired -hydroxypyrrolidinone 7 in
1
excellent yield (90%).13 Elevated temperature H NMR studies
revealed the hydroxypyrrolidinone
7
to be
a
single
diastereoisomer.14 Furthermore, nOe studies then revealed the
desired 4,5-cis configuration of the substituents on the
pyrrolidinone ring. The removal of the bromide constitutes the
first diastereoselective synthesis of this -hydroxypyrrolidinone
amino acid of the microsclerodermins,9 with dipeptide 7 being
represented in eight family members.

3
yields with these derivatives) revealed, as suspected, that when
styrene derivative 8a was exposed to these conditions none of the
desired bromohydrin was observed, and competing bromination
could be detected on the styrene moiety itself (Table 2, entry 1).
Pleasingly both alkyne 8b (entry 2) and olefin 8c (entry 3)
underwent clean and regioselective hydroxybromination in good
yield (65% and 73% respectively), albeit with elongated reaction
times (~10 h). For comparison, unsubstituted derivative 8d gave
a comparable yield (61%) (entry 4). We were delighted to find
that all three compounds then underwent efficient (~90%)
radical-mediated debromination under our optimised conditions
(as long as the amount of tributyltin hydride was kept at ≤1.1
equivalents in the case of the olefin) to reveal the desired
functionalised -hydroxypyrrolidinones 9b-d.
Scheme 3. Radical-mediated debromination of bromohydrin 6 and
nOe spectroscopy correlations to determine the pyrrolidinone 4,5-
cis-diastereoselectivity
The 4,5-cis-diastereoselectivity that was observed in the two-
step hydration could be rationalised by two different pathways.
Firstly, thermodynamic control could be operative, forming the
more stable (OH-NH cis) diastereoisomer by reversible opening
of the hemi-aminal ring. As evidence against this route, both
Shiori9 and ourselves15 have observed diastereoisomeric hemi-
aminals from simpler model systems; these clearly had not
equilibrated to the cis diastereisomer. Secondly, the cis
configuration could be reached by considering the role of the 4-
NHAc functional group (Scheme 4). Treatment of the
unsaturated pyrrolidinone with TBCD or NBS could form an
N-acyliminium, III. This can subsequently be trapped
intramolecularly by the adjacent N-HAc group to afford a
dihydrooxazolium ion IV that may be subsequently hydrolysed
by attack at the acyl carbonyl to yield the 4,5-cis-pyrrolidinone.16
Clearly, more experimental data is required to distinguish
between the two pathways, and it would then be interesting to
consider the consequences of such reactivity on the biosynthesis
of this amino acid.
Entry
R
Step i. /%a
Step ii. /%a
b
1
2
3
4
a (E)-CH=CH(4-MeOC6H4)
−
NA
90
b C≡CH
c CH=CH2
d H
65
73
61
90
91
a Isolated yields after purification by flash column chromatography;
b Undesired bromination of the aromatic ring was observed.
Table 2. Hydroxybromination and debromination of
microsclerodermin dipeptide mimics 8a-d
Our final task as part of our model studies was to derivatise
either alkyne 9b or olefin 9c to the corresponding
p-
methoxystyrene, as present in AMMTD. We believed that
terminal olefin 9c appeared the best candidate, through the use of
a cross metathesis (CM) reaction.17 After some experimentation
we found that olefin 9c underwent efficient CM with
4-methoxystyrene at room temperature via treatment with three
portions of Grubbs I catalyst and an excess of the styrene.
Pleasingly these conditions provided a high yield of the desired
coupled styrene 10 (80%), exclusively as the (E)-isomer (3J =
15.6 Hz) with no signs of any dehydration of the sensitive
-hydroxypyrrolidinone functionality (Scheme 5). For
completeness an elevated temperature ROESY experiment was
conducted which revealed 10 to be a single 4,5-cis-diasteoisomer
(as expected, in line with previous observations, see ESI for
details).
Scheme 4. Proposed mechanism to account for the observed 4,5-cis-
diastereoselectivity of the two-step formal hydration
Since the -amino acid of microsclerodermins A and B,
AMMTD, contains an electron rich p-methoxy styrene moiety we
were concerned that this moiety would not be compatible with
our two-step hydroxybromination-debromination strategy. If the
hydration proved incompatible, we required a functional group in
a late-stage intermediate that could subsequently be manipulated
to the styrene as a final step in any total synthesis. We felt that
either an alkyne or an olefin would be able to serve this purpose.
Accordingly we set out to test this hypothesis and prepared a
series of four dipeptides functionalised with long-chain acids (to
mimic AMMTD), with either a 4-methoxystyrene, alkyne, or
terminal olefin, along with an unsubstituted variant for
comparison (8a-d) (see ESI for details). Treatment of these
derivatives with NBS in THF/H2O (TBCD also gave comparable

4
Tetrahedron
8.
9.
Zhu, J.; Ma, D. Angew. Chem. Int. Ed. 2003, 42, 5348-5351.
(a) S. Sasaki, Y. Hamada, T. Shioiri, Synlett 1999, 4, 453-455. (b)
T. Shioiri, S. Sasaki, Y. Hamada, Arkivoc 2003, 103-122; (c)
Hamada and Shioiri prepared an isolated -hydroxypyrrolidinone
amino acid with a 3:1 diastereomeric ratio.
Scheme 5. (E)-Selective cross metathesis of olefin 9c with 4-
methoxystyrene in the presence of the 5-hydroxypyrrolidinone
amino acid
At this stage we believed this end-game model could pave a
way, not only for the first total synthesis of microsclerodermin B,
but the first synthesis of any microsclerodermin that possesses
the -hydroxypyrrolidinone residue. At this stage, however, we
have been unable to accomplish this goal when this methodology
10. Melikova, E. Y.; Pullin, R. D. C.; Winter, C.; Donohoe, T. J.
Angew. Chem. Int. Ed. 2016, 55, 9753-9757.
11. Hoang, C. T.; Bouillere, F.; Johannesen, S.; Zulauf, A.; Panel, C.;
Pouilhes, A.; Gori, D.; Alezra, V.; Kouklovsky, C. J. Org. Chem.
2009, 74, 4177-4187.
12. Renaud, P.; Beauseigneur, A.; Brecht-Forster, A.; Becattini, B.;
Darmency, V.; Kandhasamy, S.; Montermini, F.; Ollivier, C.;
Panchaud, P.; Pozzi, D.; Scanlan, E. M.; Schaffner, A.-P.; Weber,
V. Pure Appl. Chem. 2007, 79, 223-233.
was applied to
a late stage intermediate on route to
dehydromicrosclerodermin B;18 further experimentation will be
required to achieve this aim.
13. Using AIBN as an initiator required elevated temperatures, which
led to dehydration; using Pd/C under hydrogen also led to
dehydration.
Conclusion
14. Amide rotamers are present at room temperature.
15. T. J. Donohoe, unpublished results.
16. For an example see: Jha, R.; Davis, J. T. Carbohydrate Res. 1995,
227, 125-134.
17. Chatterjee, A. K.; Choi, T.-L.; Sanders, D. P.; Grubbs, R. H. J.
Am. Chem. Soc., 2003, 125, 11360-11370.
18. See reference 10 for details; the unsaturated pyrrolidinone starting
material was consumed but only undefined and inseparable side
products were obtained.
In summary, we have reported the first diastereoselective
synthesis of the sensitive -hydroxypyrrolidinone amino acid
common to eight microsclerodermin family members. Through a
two-step hydroxybromination-debromination procedure we were
able to prepare the amino acid as part of a dipeptide and as the
requisite 4,5-cis-disastereoisomer. Further experimentation
revealed this pyrrolidinone to be stable to cross metathesis
conditions and this factor allowed the introduction of a 4-
methoxystyrene unit as found in the -amino acid of
microsclerodermin B.
Acknowledgments
We thank the German Academic Exchange Service (DAAD)
(CW) and the EPSRC, EP/I010343/1 (RDCP) for funding. Choon
Boon Cheong is thanked for assistance with NMR spectroscopy.
Supplementary data
Supplementary data associated with this article can be found,
in the online version, at
References and notes
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Faulkner, D. J. Tetrahedron 2000, 56, 3679-3685.
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(a) Zhang, X.; Jacob, M. R.; Rao, R. R.; Wang, Y. H.; Agarwal, A.
K; Newman, D. J.; Khan, I. A; Clark, A. M.; Li, X. C. Res. Rep.
Med. Chem. 2012, 2, 7-14. (b) Zhang, X.; Jacob, M. R.; Rao, R.
R.; Wang, Y. H.; Agarwal, A. K; Newman, D. J.; Khan, I. A;
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Hoffmann, T.; Müller, S.; Nadmid, S.; Garcia, R; Müller, R. J.
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(a) Wright, A. E.; Pomponi, S. A.; Longley, R. E.; Isbrucker, R.
A. US6384187B1, 2002 (b) Guzmán, E. A.; Maers, K.; Roberts,
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6.
(a) Sasaki, S.; Hamada, Y.; Shioiri, T. Tetrahedron Lett. 1997, 38,
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Lett. 1999, 40, 3187-3190. (e) Chandrasekhar, S.; Sultana, S. S.
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5
Diastereoselective synthesis of the 5-hydroxy-pyrrolidinone amino acid of
the microsclerodermins and model studies for an end-game strategy for
microsclerodermin B
Christian Winter, Robert D. C. Pullin and Timothy J. Donohoe*
Department of Chemistry, University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford,
OX1 3TA (UK); Tel.: +44-1865-275649; Fax: (+44) 1865 275708
timothy.donohoe@chem.ox.ac.uk
Highlights
Diastereoselective synthesis of amino acids found in the microsclerodermin natural products
A pyrrolidinone amino acid was made by alkene hydration in two steps
Bromination and low temperature dehalogenation gives a single diastereoisomer of a sensitive hemiaminal
containing amino acid.