Stereoselective synthesis of 2,6-trans-tetrahydropyran via primary diamine-catalyzed oxa-conjugate addition reaction of α,β-unsaturated ketone: Total synthesis of psymberin

Article abstract of DOI:10.1021/ol2024289

The total synthesis of psymberin was achieved employing a readily available chiral epoxide to prepare two of the three subunits in the natural product. The key reaction was a highly stereoselective organocatalytic oxa-conjugate addition reaction of α,β-un

Full text of DOI:10.1021/ol2024289

ORGANIC
LETTERS
2011
Vol. 13, No. 21
5816–5819
Stereoselective Synthesis of 2,6-trans-
Tetrahydropyran via Primary Diamine-
Catalyzed Oxa-Conjugate Addition
Reaction of r,β-Unsaturated Ketone:
Total Synthesis of Psymberin
Seong Rim Byeon,†,§ Heekwang Park,† Hyoungsu Kim,‡ and Jiyong Hong*,†
Department of Chemistry, Duke University, Durham, North Carolina 27708,
United States, and College of Pharmacy, Ajou University, Suwon 443-749, Korea
jiyong.hong@duke.edu
Received September 7, 2011
ABSTRACT
The total synthesis of psymberin was achieved employing a readily available chiral epoxide to prepare two of the three subunits in the natural
product. The key reaction was a highly stereoselective organocatalytic oxa-conjugate addition reaction of R,β-unsaturated ketone catalyzed by
primary diamine for the synthesis of the 2,6-trans-tetrahydropyran embedded in psymberin.
The marine cytotoxin psymberin (1, Scheme 1) is a
potent inhibitor of cancer cell proliferation1 and has
attracted considerable interest from a number of synthetic
groups.2ꢀ4 It possesses the psymberic acid side chain, a 2,6-
trans-3,3-dimethyl tetrahydropyran-4-ol, and a dihydro-
isocoumarin. One of the synthetic challenges it presents is
the stereoselective formation of the 2,6-trans-tetrahydro-
pyran subunit. Due to their poor thermodynamic stability,
the stereoselective synthesis of 2,6-trans-tetrahydropyrans
remains a great challenge.5,6 Displacement of acetate with
TMSCN,2a,3c configuration-dependent spirodiepoxide
opening,2b PhI(OAc)2-mediated cyclization,2c intramole-
cular cylization of epoxy alcohols,2d,f addition of enolsi-
lane to oxocarbenium ion,2e and conjugate addition of
vinylmagnesium bromide to dihydropyranone3f have been
utilized for the synthesis of the 2,6-trans-tetrahydropyran
in the natural product. Even though the intramolecular
† Duke University.
‡ Ajou University.
§ Current address: Center for Neuro-Medicine, Brain Science Institute,
Korea Institute of Science and Technology, Seoul 136-791, Korea.
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r
10.1021/ol2024289
Published on Web 10/11/2011
2011 American Chemical Society

Scheme 1. Retrosynthetic Plan for Psymberin (1)
Scheme 2. Synthesis of Hydroxy Ketone 5
that it would be challenging to stereoselectively synthesize
2,6-trans-3,3-dimethyl tetrahydropyran 3 in a substrate-
controlled manner. Instead, we decided to explore the organo-
catalytic oxa-conjugate addition reaction to prepare 3 in a
reagent-controlled manner. We anticipated that the key
intermediates 3 and 4 could be derived from the common
chiral epoxide 6.
conjugate addition of an oxygen nucleophile to R,β-
unsaturated carbonyl compounds (oxa-conjugate addition
reaction) is a direct and efficient way to tetrahydropyrans,
the oxa-conjugate addition reaction has never been used
for the synthesis of 1.
The synthesis of psymberin (1) started with the prepara-
tion of hydroxy ketone 5 (Scheme 2). Coupling7b,8 of allyl
alcohol 89,10 (8 steps from the commercially available 2,2-
dimethyl-1,3-propanediol) with the readily available chiral
epoxide 710,11 (3 steps from the commercially available
(S)-(þ)-glycidyl benzyl ether) provided 9. Hydrolysis of
the dithiane group of 9, stereoselective reduction to 1,3-syn
diol 11 (dr >20:1),12 and MnO2-oxidation of the second-
ary alcohol 11set the stage for the key organocatalytic oxa-
conjugate addition reaction.
With an interest in facilitating access to biologically
important natural products with tetrahydropyrans,7
we designed our retrosynthetic plan for 1 relying on the
oxa-conjugate addition reaction of R,β-unsaturated ke-
tone 5 catalyzed by primary diamine for the stereoselective
synthesis of 2,6-trans-3,3-dimethyl tetrahydropyran 3 em-
bedded in 1 (Scheme 1). Because of our previous report on
the stereoselective synthesis of 2,6-cis-3,3-dimethyl tetra-
hydropyrans through the tandem oxidation/oxa-Michael
reaction of R,β-unsaturated aldehyde,7d we envisioned
Due to the steric congestion of ketones in iminium
formation, we attempted the oxa-conjugate addition
reaction catalyzed by primary amine (Table 1).13ꢀ15 The
organocatalytic oxa-conjugate addition reaction of 5 in the
presence of (1R,2R)-1,2-diphenylethane-1,2-diamine (A)16
and TFA smoothly proceeded but afforded the undesired
2,6-cis-3,3-dimethyl tetrahydropyran 12 as a single diaster-
eomer (entry 1). When HOAc was used in the reaction as
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Org. Lett., Vol. 13, No. 21, 2011
5817

Table 1. Synthesis of 2,6-trans-3,3-Dimethyl Tetrahydropyran 3
through the Organocatalytic Oxa-Conjugate Addition Reaction
Figure 1. Proposed transition states for the oxa-conjugate addi-
tion reaction catalyzed by (1R,2R)-1,2-diphenylethane-1,2-di-
amine (A = acid).
a dramatic effect on the stereochemical outcome of the
reaction and encouraged us to test various acids to further
improve the stereoselectivity of the organocatalytic oxa-
conjugate addition reaction. The use of a strong acid such as
(1S)-10-camphorsulfonic acid (CSA) provided 12 as a single
diastereomer (entry 7), suggesting that oxa-conjugate addi-
tion reaction might proceed via an acid-catalysis mechan-
ism. Aliphatic acids did not change the stereoselectivity
(entries 8 and 9). Finally, when 5 was treated with the
combination of A and 9-anthracenecarboxylic acid (E) in
EtOAc, the organocatalytic oxa-conjugate addition reac-
tion proceeded smoothly to provide 3 with high stereose-
lectivity and yield (dr = 10:1, 92%, entry 13).
Our proposed rationale for the stereochemical outcome
as a function of the steric bulkiness of acid is that the use of
acid with steric bulkiness enhances the steric repulsions
in the less favorable dimeric conformer I and increases
the population of conformer II (Figure 1).16a,19 Even
though the organocatalytic conjugate addition of a carbon
nucleophile to R,β-unsaturated ketones has been explored
for a direct and efficient way to form CꢀC bonds, to the
catalyst
acid
temp yield
entry (equiv)
(equiv)
solvent time (%)a
drb
1
A (0.4)
A (0.4)
A (0.2)
A (0.4)
B (0.4)
C (0.4)
TFA (10)
HOAc (10)
HOAc (10)
HOAc (1)
toluene 0 °C
16 h
90
80
89
93
12 only
2
toluene 0 °C
17 h
3:1
3
toluene 0 °C
46 h
3:1
4
toluene 0 °C
24 h
3:1
5
HOAc (0.8) toluene 25 °C 68
1:2
44 h
6
HOAc (5)
toluene 0 °C
79
99
95
94
87
89
92
92
78
1:3
72 h
7
A (0.4) (1S)-10-CSA toluene 0 °C
(1) 14 h
4-pentenoic toluene 0 °C
acid (10) 18 h
A (0.4) palmitic acid toluene 0 °C
(1) 46 h
benzoic acid toluene 0 °C
(1) 16 h
1-naphthoic toluene 0 °C
12 only
3.5:1
3:1
8
A (0.4)
9
10
11
12
13
14
15
A (0.4)
A (0.4)
A (0.4)
A (0.4)
D (0.4)
none
2:1
(14) For recent examples of the conjugate addition reaction of
ketones catalyzed by primary amines, see: (a) Zhang, G.; Wang, Y.;
Zhang, W.; Xu, X.; Zhong, A.; Xu, D. Eur. J. Org. Chem. 2011, 2142–
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Angew. Chem., Int. Ed. 2011, 50, 1–6.
3:1
acid (1)
47 h
toluene 0 °C
46 h
E (1)
6:1
E (1)
E (1)
E (1)
EtOAc 0 °C
48 h
10:1
10:1
EtOAc 0 °C
48 h
EtOAc 0 °C
48 h
NRc NAd
a Combined yield of the isolated 3 and 12. b The diastereomeric ratio
(3:12) was determined by integration of the 1H NMR of the crude
product. c No reaction. d Not applicable.
(15) For a recent example of the aza-conjugate addition reaction of
ketones catalyzed by primary amines, see: Gogoi, S.; Zhao, C.-G.; Ding,
D. Org. Lett. 2009, 11, 2249–2252.
an alternative acid source, the stereoselectivity was reversed
to provide 2,6-trans-3,3-dimethyl tetrahydropyran 3 as the
major diastereomer (dr = 3:1, entries 2ꢀ4). Other primary
amines such as 9-amino-9-deoxy-epi-cinchonine (B)17 or
thiourea (C)18 gave 12 as the major diastereomer (entries 5
and 6). These results implied that the acid used in the
organocatalytic oxa-conjugate addition reaction might have
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5239–5242. (b) Bartoli, G.; Bosco, M.; Carlone, A.; Pesciaioli, F.;
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5818
Org. Lett., Vol. 13, No. 21, 2011

Scheme 3. Aldol Coupling of 2,6-trans-Tetrahydropyran 3 and
Dihydroisocoumarin 2
Scheme 4. Synthesis of Pivalate Mixed Anhydride of Psymberic
Acid 21 and Completion of the Synthesis of Psymberin (1)
Coupling of 18 with 21 followed by exhaustive deprotection
of the silyl protecting groups completed the synthesis of 1.2d
In summary, the total synthesis of psymberin (1) was
accomplished in 24 steps (in the longest sequence from
2,2-dimethyl-1,3-propanediol) enlisting the oxa-conjugate
addition reaction. The organocatalytic oxa-conjugate addi-
tion reaction of R,β-unsaturated ketone 5 promoted by
primary diamine was explored for the stereoselective synthe-
sis of 2,6-trans-3,3-dimethyl tetrahydropyran 3. It was
demonstrated that the stereochemical outcome of the reac-
tion depends on the steric bulkiness of acid used as an
additive. It is noteworthy that two of the three subunits in
1 were derived from the readily avaible chiral epoxide 6.
best of our knowledge, this is the first report for the
organocatalytic oxa-conjugate addition reaction of an
alcohol nucleophile to R,β-unsaturated ketones promoted
by primary diamine.20 It would be an effective way to
construct β-hydroxy carbonyl compounds.
Acknowledgment. This work was supported by Duke
University. We are grateful to the NCBC (Grant No. 2008-
IDG-1010) for funding of NMR instrumentation and to
the NSF MRI Program (Award ID No. 0923097) for
funding of mass spectrometry instrumentation.
With the key 2,6-trans-3,3-dimethyl tetrahydropyran 3 in
hand, we next examined the coupling of 2,6-trans-3,3-di-
methyl tetrahydropyran 3 and dihydroisocoumarin 22d,3c,21
through the 1,2-syn-aldol reaction (Scheme 3). TBS-protec-
tion of 3 followed by the aldol addition of 13 to 2 (PhBCl2,
i-Pr2NEt, CH2Cl2)2a,d,f provided the desired β-hydroxy ke-
tone 14 as a single diastereomer. 1,3-syn-Reduction (dr =
6:1)2a,f followed by lactonization under acidic conditions
afforded 15 with loss of the C22 SEM group. TBS-protection,
Bn-deprotection, and oxidation provided the corresponding
carboxylic acid 17. Curtius rearrangement following the
procedure previously reported by Smith and co-workers
smoothly proceeded to give hemiaminal 18.2d
The pivalate mixed anhydride of psymeric acid 21 was
prepared from the common epoxide 6 (Scheme 4). SEM-
protection, addition of isopropenylmagnesium bromide,
methylation, and Bn-deprotection proceeded smoothly to
provide 20. Oxidation to carboxylic acid and formation of
pivalate mixed anhydride proceeded smoothly to afford 21.2d
Supporting Information Available. General experimental
procedures including spectroscopic and analytical data along
1
with copies of H and 13C NMR spectra. This material is
available free of charge via the Internet at http://pubs.acs.org.
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5819