Total Synthesis of Cytospolide D and Its Biomimetic Conversion to Cytospolides M, O, and Q

Article abstract of DOI:10.1021/acs.orglett.6b02193

A total synthesis of cytospolide D, starting from l-glutamic acid, is described. The critical macrolactonization to the 10-membered lactone containing an (E)-configured double bond was successfully achieved by Shiina esterification. Conversion of cytospol

Full text of DOI:10.1021/acs.orglett.6b02193

Letter
pubs.acs.org/OrgLett
Total Synthesis of Cytospolide D and Its Biomimetic Conversion to
Cytospolides M, O, and Q
Gunnar Ehrlich* and Christian B. W. Stark*
Department of Chemistry, Institute of Organic Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, 20146 Hamburg,
Germany
S
* Supporting Information
ABSTRACT: A total synthesis of cytospolide D, starting from
L-glutamic acid, is described. The critical macrolactonization to
the 10-membered lactone containing an (E)-configured double
bond was successfully achieved by Shiina esterification.
Conversion of cytospolide D to its bicyclic derivatives M, O,
and Q was accomplished under mild conditions, lending
support to the proposed biosynthetic hypothesis.
he first five members (1−5 in Figure 1) of the group of
enones of 4 and 5, while the bicyclic cytospolide M (6) is
presumably derived from 4 by epoxidation and subsequent
transannular epoxide opening. The γ-lactone 9 can be traced back
to cytospolide M (6) via a transesterification (Scheme 1). Since
T
cytospolides were isolated by Zhang et al. from extracts of
the fungus Cytospora sp. growing on leaves of the evergreen shrub
Ilex canariensis.1 Cytospolides A−D (1−4) share the same carbon
skeleton and only differ in the acetylation pattern of the two
hydroxy groups (Figure 1). Interestingly, the C-2 epimer of 4,
Scheme 1. Proposed Biosynthetic Relationship between
Cytospolides M, O, Q, and D
the first publication, especially the more bioactive members of the
cytospolide family, namely E (5) and P (8), have attracted
significant research interest,3−5 which has culminated in the
publication of several total syntheses.6−9
As part of our interest in biomimetic natural product
chemistry,10 we were aiming to establish a synthetic route toward
cytospolide D (4) and to investigate its conversion into its higher
oxygenated derivatives M (6), O (7), and Q (9), all of which have
not been synthesized yet. Our retrosynthetic analysis of
cytospolide D (4) is based on the retro-macrolactonization of 4
to give seco-acid 10 with a protected hydroxy group at C-3 as a
precursor (Scheme 2). We envisaged the 2,3-anti relationship of
the C-2 methyl group and the hydroxy group at C-3 to be
Figure 1. Selected structures of cytospolides.
cytospolide E (5), was also isolated from the same natural source.
The inversion of the C-2 methyl group in cytospolide E (5) to the
(2S)-configuration was found to significantly increase its activity
against the A-549 tumor cell line. Shortly after the first
publication, Zhang et al. reported on the discovery of 13 closely
relatednatural products(cytospolides F−Qanddecytospolides A
and B) isolated from extracts of the same fungus cultivated on
biomalt agar medium.2 Here, cytospolides P (8) and Q (9)
emerged as the most potent congeners with activity against
different cancer cell lines.
Except for the decytospolides, all newly isolated metabolites
possess a more highly oxygenated carbon skeleton. Biosyntheti-
cally, cytospolides O(7)andP(8)arebelievedtobeformedbyan
intra- or intermolecular oxa-Michael addition to the respective
Received: July 25, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b02193
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When switching to a nonchelating TBS group in the α-position or
altering the reaction conditions (THF as solvent and
pentyllithium as reagent) the yields markedly decreased and the
selectivity only changed to 2:1 for the 4,5-anti diol 15. We
therefore investigated the inversion at C-5 by Mitsunobu
esterification15 with acetic acid.21 Here, we observed a decline
inthediastereomericratiofromover95:5forthe4,5-syndiol13to
5:1 for the 4,5-anti product and a low yield of 33%. The decreased
diastereomeric ratio can be explained by a partial direct
esterification process by acyloxyphosphonium salts occurring in
competition with the anticipated inversion.22 As neither a change
in selectivity in the Grignard addition nor the one-step inversion
via Mitsunobu esterification led to an acceptable diastereose-
lectivity, we next tried to invert the C-5 hydroxy group by means
of an oxidation−reduction sequence. Oxidation of 13 to α-alkoxy
ketone 14 proceeded in high yields with Dess-Martin period-
inane.19 For the selective reduction of α-alkoxy ketones with
chelating protecting groups, zinc borohydride has found ample
use.23 In our case, the reduction provided 4,5-anti diol 15 in ahigh
diastereoselectivity of 94:6. The C-5 hydroxy group was then
protectedasacetateinordertoallowforaconcomitantcleavageof
acetate and benzoate in the final stages of the synthesis prior to
macrolactonization. Next, the primary TBS ether was cleaved and
alcohol 19 was oxidized to aldehyde 20. For the Wittig
homologation we decided to use (triphenylphos-
phoranylidene)-acetaldehyde.12 Although its reactivity with
aldehyde 20 is quite low as compared to ethyl (triphenylphos-
phoranylidene)-acetate, the synthesis can be shortened by two
subsequent transformations. The low reactivity of this stabilized
ylide required prolonged heating up to 50 °C for 3 d. The
conversion was monitored by running the reaction in CDCl3 and
1H NMR analysis at intervals.
Scheme 2. Retrosynthetic Analysis of Cytospolide D
established by an anti-aldol addition of enal 11 using Paterson’s
method.11 The enal moiety in 11 can be assembled by Wittig
homologation using a stabilized ylide already bearing the
aldehyde functionality.12 For installation of the pentyl side
chain we proposed a Grignard addition of pentylmagnesium
bromide to α-chiral aldehyde 12. In the case of a chelating
protecting group such as PMB, the diastereoselectivity can be
assumed according to the Cram-chelate model13 which would
give rise to the undesired 4,5-syn diol 13. Conceivable means to
invert the stereochemistry may be provided either by switching
the mechanism to the polar Felkin model with nonchelating
protection groups14 or by a subsequent Mitsunobu reaction15 of
4,5-syndiol13. Aldehyde12 shouldbeaccessible from (2S)-1,2,5-
pentanetriol (16).
The synthesis commenced with (2S)-1,2,5-pentanetriol (16)
obtained from L-glutamic acid.16,17 In order to differentiate the
hydroxy groups we installed the 1,2-p-methoxybenzylidene
acetal, leaving the C-5 hydroxy group unprotected (Scheme 3).
At C-5 we decided to introduce a TBS ether. Next PMP acetal 17
was opened with DIBAl-H at 0 °C in toluene.18 Under these
conditions the ratio of regioisomers was determined as 10:1 in
favor of the desired secondary PMB ether 18. Oxidation of 18 to
the corresponding aldehyde 12 with Dess-Martin periodinane19
proceededwitha76%yield. Whenusingbasicreactionconditions
as for instance with the Swern protocol,20 diminished yields were
obtained. Next, we investigated the diastereoselectivity of the
Grignard addition. Thus, aldehyde 12 was reacted with
pentylmagnesium bromide in diethyl ether, resulting in the
exclusive formation of the 4,5-syn diol 13 in good yield. The
configuration of diol 13 was assigned by NMR after deprotection
of the PMB group and installation of an acetonide (see the
Supporting Information for details).
To complete the assembly of the carbon skeleton we applied
the Paterson anti-aldol addition with (2S)-benzoyloxy-3-
pentanone (21)11 to establish the desired (2R,3R) configuration
of carboxylic acid 10 (Scheme 4). The C-3 hydroxy group of aldol
product 22 was protected as TBS ether. The chiral auxiliary was
thencleavedbyreductionoftheketonewithsodiumborohydride,
saponification of the benzoate as well as the acetate, and
subsequent glycol cleavage24 of the resulting 2,3-diol. The
ensuing aldehyde was directly oxidized to seco-acid 10 using
sodium chlorite.25,26
Having synthesized this macrolide precursor 10, we next
investigated the macrolactonization. A similar seco-acid with a
different set of protecting groups (PMB atC-3 and Bn at C-8) had
been cyclized in 40% yield by Kamal et al.9 using the Yamaguchi
Theexclusiveformationofthe4,5-syndiol13isindicativeofthe
fact that the Grignard addition follows the Cram-chelate model.
Scheme 3. Synthesis of Aldehyde 11
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Scheme 4. Synthesis of seco-Acids 10 and 23 and Investigations into the Macrolactonization
Scheme 5. Synthesis of Cytospolide D and its Biomimetic Conversion to Cytospolides M, O, and Q
esterification.27 However, when we screened these conditions
with our substrate, the desired 10-membered macrolide was only
formed in very low yields of less than 5%. Neither prolonging the
period of addition up to 16 h nor variation of the temperature or
concentrations significantly improved the outcome. The low
tendency of the mixed anhydride to form the 10-membered
macrolidemay, inadditiontotheobviousringstrain, beattributed
to an unfavorable steric repulsion between the C-2 methyl group
and the large TBS ether at C-3 (Scheme 4). In this respect it is
worth noting that during the synthesis of cytospolide P (8) by
Raju et al.6 the C-2 epimeric seco-acid with TBS at C-3 and lacking
the double bond was cyclized under Yamaguchi conditions in a
good yield of 76%. In order to decrease steric congestion and
increase the conformational flexibility of the seco-acid, we
screened smaller protecting groups at C-3. Notably, even small
protecting groups such as MOM or acetate did not lead to
significant amounts of the 10-membered macrolide. Therefore,
we next investigated the cyclization of the unprotected dihydroxy
acid 23. When we applied the Yamaguchi protocol to dihydroxy
acid 23, macrolactone 24 was not produced at all. Instead, 23
appears to undergo an acylation of the allylic alcohol. To suppress
this undesired esterification, we probed the Shiina lactonization
using 2-methyl-6-nitrobenzoic anhydride.28 After optimization
wefoundthatadditionofdihydroxy acid23over36htoasolution
of 2-methyl-6-nitrobenzoic anhydride and DMAP at 20 °C led to
formation of the desired macrolide 24 in 21% yield, a reaction
which may actually proceed through a β-lactone intermediate.
Notably,inalltheseexperiments, diene25,formedbydehydrative
decarboxylation,29 was observed as a byproduct. The predom-
inant formation of the (E/E)-isomer of this side product leads toa
syn-eliminationasamechanisticrationalewhichisagainindicative
of the formation of a β-lactone intermediate.30 When dichloro-
methane was used as the solvent instead of toluene, 25 was
producedexclusivelyandnomacrolide24wasdetected. Similarly,
the use of Mukaiyama’s reagent31 in dichloromethane gave diene
25 as the only product. Finally, the PMB group in 24 was
deprotected with DDQ to give cytospolide D (4) in a good yield
(Scheme 5). The 1H NMR spectrum of synthetic cytospolide D
(4)showedasecondsetofsignalswithabout15%intensity. Inthe
course of the isolation of the natural product Zhang et al. noted
the same additional set of signals at almost identical relative
intensity and were able to establish the existence of two
conformers.
For the biomimetic conversion of cytospolide D (4) to
cytospolide M (6) we investigated a diastereoselective
epoxidation.32 When 4 was reacted with mCPBA at −20 °C
overnight, a clean and highly diastereoselective conversion to the
epoxide was observed. Under these slightly acidic reaction
C
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(6) Suman, P.; Raju, B. C. Org. Biomol. Chem. 2014, 12, 3358−3361.
(7) Chatterjee, S.; Guchhait, S.; Goswami, R. K. J. Org. Chem. 2014, 79,
7689−7695.
(8) Rej, R. K.; Kumar, R.; Nanda, S. Tetrahedron 2015, 71, 3185−3194.
(9) Kamal, A.; Balakrishna, M.; Venkat Reddy, P.; Rahim, A.
Tetrahedron: Asymmetry 2014, 25, 148−155.
(10) (a) Schmidt, J.; Stark, C. B. W. Org. Lett. 2012, 14, 4042−4045.
(b) Giera, D. S.; Stark, C. B. W. RSC Adv. 2013, 3, 21280−21284.
(c) Schmidt, J.; Stark, C. B. W. J. Org. Chem. 2014, 79, 1920−1928.
(d) Schmidt, J.; Khalil, Z.; Capon, R. J.; Stark, C. B. W. Beilstein J. Org.
conditions the epoxide instantaneously underwent partial
cyclization33 to give cytospolide M (6), though the reactive
conformer presumably has to accommodate several pseudoaxial
substituents. AdditionofacatalyticamountofCSAcompletedthe
formation of the natural [6.2.1]-bicycle. To further convert the
10-membered lactone 6 into the 5-membered γ-lactone 9, we
tried different conditions for saponification. Lactone 6 showed
remarkable stability toward 1 N HCl and 1 N NaOH, but finally
withpotassiumtrimethylsilanolate34 lactone6couldbeopenedto
the carboxylate. After aqueous workup and concentration in
vacuo, a spontaneous recyclization to cytospolide Q (9) occurred.
However, under these basic reaction conditions, we also observed
partial epimerization. For the synthesis of cytospolide O (7) we
utilized an oxa-Michael addition strategy. Macrolide 24 was
oxidizedtoenone26andthendeprotected togive27. Contraryto
the epoxide of 4, enone 27 did not undergo a spontaneous oxa-
Micheal addition, but after formation of the alkoxide with
LiHMDS, cytospolide O (7) was produced in 63% yield.
In summary we established a synthetic route toward
cytospolide D (4). Significant hurdles emerging during the
macrolactonization could be circumvented by omitting a
protecting group at C-3 and through use of Shiina’s method.
However, the low yield of 21% underlines the difficulties in
closing a 10-membered ring containing an E-double bond. The
first total syntheses of the higher oxygenated cytospolides M (6),
O (7), and Q (9) were successfully achieved using a strategy
reflecting the putative biosynthetic pathways based on
cytospolide D (4).
Chem. 2014, 10, 1228−1232. (e) Muller, J. M.; Stark, C. B. W. Angew.
̈
Chem., Int. Ed. 2016, 55, 4798−4802.
(11) (a) Paterson, I.; Wallace, D. J.; Velazquez, S. M. Tetrahedron Lett.
1994, 35, 9083−9086. (b) Paterson, I.; Wallace, D. J.; Cowden, C. J.
Synthesis 1998, 1998, 639−652.
(12) (a) Staab, H. A.; Sommer, N. Angew. Chem. 1962, 74, 294.
(b) Schobert, R.; Gordon, G. J. Science of Synthesis 2004, 27, 973−1070.
(13) (a) Cram, D. J.; Elhafez, F. A. A. J. Am. Chem. Soc. 1952, 74, 5828−
5835. (b) Cram, D. J.; Wilson, D. R. J. Am. Chem. Soc. 1963, 85, 1245−
1249. (c)Reetz,M.T.Acc. Chem.Res.1993,26,462−468.(d)Mengel,A.;
Reiser, O. Chem. Rev. 1999, 99, 1191−1223.
́
(14) (a) Cherest, M.; Felkin, H.; Prudent, N. Tetrahedron Lett. 1968, 9,
2199−2204. (b) Anh, N. T.; Eisenstein, O. Tetrahedron Lett. 1976, 17,
155−158. (c) Anh, N. T. Top. Curr. Chem. 1980, 88, 145−162.
(15) (a) Kurihara, T.; Nakajima, Y.; Mitsunobu, O. Tetrahedron Lett.
1976, 17, 2455−2458. (b) Mitsunobu, O. Synthesis 1981, 1981, 1−28.
(16) Larcheveque, M.; Lalande, J. Tetrahedron 1984, 40, 1061−1065.
(17) Kuehnert, S. M.; Maier, M. E. Org. Lett. 2002, 4, 643−646.
(18) Takano, S.; Akiyama, M.; Sato, S.; Ogasawara, K. Chem. Lett. 1983,
12, 1593−1596.
(19) (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.
(20) Mancuso, A. J.; Swern, D. Synthesis 1981, 1981, 165−185.
(21) Initially, we intended to make use of the good dr of 13 and
synthesized the C-9 epimer of seco-acid 10 aiming at a late stage inversion
via Mitsunobu macrolactonization. Unfortunately, the desired macrolide
was not formed under these conditions.
(22) Ahn, C.; Correia, R.; DeShong, P. J. Org. Chem. 2002, 67, 1751−
1753.
(23) (a) Nakata, T.; Tanaka, T.; Oishi, T. Tetrahedron Lett. 1983, 24,
2653−2656. (b) Takahashi, T.; Miyazawa, M.; Tsuji, J. Tetrahedron Lett.
1985, 26, 5139−5142.
ASSOCIATED CONTENT
* Supporting Information
■
S
TheSupportingInformationisavailablefreeofchargeontheACS
Publications website at DOI: 10.1021/acs.orglett.6b02193.
General experimental procedures, spectroscopic data (1H,
13C, IR) and HRMS of all compounds (PDF)
AUTHOR INFORMATION
Corresponding Authors
■
(24) Schmidt, A.-K. C.; Stark, C. B. W. Synthesis 2014, 46, 3283−3308.
(25) Lindgren, B. O.; Nilsson, T. Acta Chem. Scand. 1973, 27, 888−890.
(26) Bal, B. S.; Childers, W. E., Jr.; Pinnick, H. W. Tetrahedron 1981, 37,
2091−2096.
*E-mail: ehrlich@chemie.uni-hamburg.de.
*E-mail: stark@chemie.uni-hamburg.de.
Notes
(27) Inanaga, J.; Hirata, K.; Saeki, H.; Katsuki, T.; Yamaguchi, M. Bull.
Chem. Soc. Jpn. 1979, 52, 1989−1993.
The authors declare no competing financial interest.
(28) Shiina, I.; Kubota, M.; Oshiume, H.; Hashizume, M. J. Org. Chem.
2004, 69, 1822−1830.
ACKNOWLEDGMENTS
We thank A.-K. Sassnau, I. Heing genannt Becker, R. Koll and J.
Spottel (University of Hamburg) for their experimental
contribution to this work. We are also grateful to the Fonds der
■
(29) Mulzer, J.; Bruntrup, G. Chem. Ber. 1982, 115, 2057−2075.
̈
(30) Luengo, J. I.; Koreeda, M. Tetrahedron Lett. 1984, 25, 4881−4884.
(31) (a) Mukaiyama, T.; Usui, M.; Saigo, K. Chem. Lett. 1976, 5, 49−50.
(b) Mukaiyama, T. Angew. Chem., Int. Ed. Engl. 1979, 18, 707−721.
(32) For early work on diastereoselective epoxidations using macro-
cyclic stereocontrol, see: (a) Still, W. C.; Romero, A. G. J. Am. Chem. Soc.
1986, 108, 2105−2106. (b) Vedejs, E.; Buchanan, R. A.; Watanabe, Y. J.
Am. Chem. Soc. 1989, 111, 8430−8438.
̈
ChemischenIndustrie(FCI)forfinancialsupportofthisresearch.
REFERENCES
■
(1) Lu, S.; Kurtan
́
, T.; Yang, G.; Sun, P.; Man
́
di, A.; Krohn, K.; Draeger,
(33) A similar instantaneous transannular epoxide opening was
observed during the synthesis of Englerin A: Radtke, L.; Willot, M.;
S.; Schulz, B.; Yi, Y.; Li, L.; Zhang, W. Eur. J. Org. Chem. 2011, 2011,
5452−5459.
Sun, H.; Ziegler, S.; Sauerland, S.; Strohmann, C.; Frohlich, R.;
̈
́ ́
(2) Lu, S.; Sun, P.; Li, T.; Kurtan, T.; Mandi, A.; Antus, S.; Krohn, K.;
Habenberger, P.; Waldmann, H.; Christmann, M. Angew. Chem., Int.
Draeger, S.; Schulz, B.; Yi, Y.; Li, L.; Zhang, W. J. Org. Chem. 2011, 76,
9699−9710.
Ed. 2011, 50, 3998−4002.
(34) Laganis, E. D.; Chenard, B. L. Tetrahedron Lett. 1984, 25, 5831−
(3) Vadhadiya, P. M.; Rout, J. K.; Ramana, C. V. Tetrahedron 2015, 71,
9088−9094.
5834.
(4) Yadav, J. S.; Pandurangam, T.; Suman Kumar, A.; Adi Narayana
Reddy, P.; Prasad, A. R.; Subba Reddy, B. V.; Rajendraprasad, K.;
Kunwar, A. C. Tetrahedron Lett. 2012, 53, 6048−6050.
(5) Rej, R. K.; Nanda, S. Eur. J. Org. Chem. 2014, 2014, 860−871.
D
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