Total synthesis of the proposed structure for spirofungin B: A reassignment of the stereochemistry

Article abstract of DOI:10.1021/ol049873e

(Equation presented) The total synthesis of the proposed structure for spirofungin B (2) is described. The data for the synthetic material did not compare with that for the natural product leading to the conclusion that the structure 2 assigned for spirof

Full text of DOI:10.1021/ol049873e

ORGANIC
LETTERS
2004
Vol. 6, No. 6
1041-1044
Total Synthesis of the Proposed
Structure for Spirofungin B: A
Reassignment of the Stereochemistry
Shannon D. Zanatta, Jonathan M. White, and Mark A. Rizzacasa*
School of Chemistry, The UniVersity of Melbourne, Victoria 3010, Australia
masr@unimelb.edu.au
Received January 21, 2004
ABSTRACT
The total synthesis of the proposed structure for spirofungin B (2) is described. The data for the synthetic material did not compare with that
for the natural product leading to the conclusion that the structure 2 assigned for spirofungin B is incorrect. Analysis of the NMR data
reported for spirofungins A and B as well as related spiroketals allowed for the reassignment of the stereochemistry of spirofungin B to be
that corresponding to 15-epi-spirofungin A (27).
The spirofungins A (1) and B (2) were isolated from a
Streptomyces strain Tu¨ 4113 collected in the Otway National
Park, Australia.1 Compounds 1 and 2 possess a similar 6,6-
spiroketal core to reveromycin A (3)2 and have the same
C1-C10 triene acid and C20-24 diene acid segments;
however, there is no succinate half ester at C18 and the C18
substituent is a methyl group. The spirofungins show high
inhibition activity against yeast and moderate activity against
some fungi with a MIC of 15 mg mL-1 against Candida
albicans.1
Compounds (1) and (2) were isolated as a ∼4:1 mixture,
respectively. Although they possessed slightly different
retention times on reverse phase HPLC, they were not
separated and the structural elucidation was conducted on
this mixture. The structure of the major isomer spirofungin
A (1) was assigned using various NMR techniques with the
It was proposed1 that the spiroketal core of 1 possessed
minor component spirofungin B (2) identified as a diaste-
the conformation A as depicted in Figure 1, with the oxygens
reoisomer of 1, epimeric at both C18 and C19. The absolute
(maximum anomeric stabilization) and the C19 diene acid
configurations of 1 and 2 are suggested by analogy to the
side chain in axial orientations while the C18 methyl group
reveromycins.
is equatorial. In compound 2, the C19 side chain is now
equatorial while the C18 methyl group is oriented axial as
(1) Holtzel, A.; Kempter, C.; Metzger, J. W.; Jung, G.; Groth, I.; Fritz,
shown in proposed conformer B. This was supported by the
vicinal coupling constant measured between H19 and H18,
the C18 change in chemical shift, and the ROE observed
between H19 and H11.
T.; Fiedler, H.-P. J. Antibiot. 1998, 51, 699.
(2) Takahashi, H.; Osada, H.; Koshino, H.; Kudo, T.; Amano, S.;
Shimizu, S.; Yoshihama, M.; Isono, K. J. Antibiot. 1992, 45, 1409.
Takahashi, T.; Osada, H.; Koshino, H.; Sasaki, M.; Onose, R.; Nakakoshi,
M.; Yoshihama, M.; Isono, K. J. Antibiot. 1992, 45, 1414.
10.1021/ol049873e CCC: $27.50 © 2004 American Chemical Society
Published on Web 02/21/2004

could be constructed from a coupling13 between the anion
derived from alkyne 5 and Weinreb amide 6 followed by
alkyne saturation and acid induced deprotection and spiroket-
al formation.14
The route to the nominal structure for spirofungin B began
with the known optically pure alcohol 710 (Scheme 2). Sily-
Scheme 2 a
Figure 1.
The natural occurrence of spirofungin B (2) is curious
since it possesses a different absolute configuration at C19
to that from both 1 and the related reveromycins. Several
groups have reported approaches3,4 to the spiroketal systems
of spirofungins A (1) and B (2) as well as reveromycin A
(3)5,6,7 which has also succumbed to total synthesis.8,9 In this
paper, we report the total synthesis of the structure 2 pro-
posed for the minor component spirofungin B which led to
the reassignment of the stereochemistry of this natural
product.
A brief retrosynthetic analysis of 2 is shown in Scheme
1. The key bond disconnections are similar to those utilized
a Reagents and conditions: (a) TESCl, imidazole, DMF, rt; (b)
cat. OSO4, H2O/THF, then NaIO4; (c) Ph3PdCHCON(OMe)Me,
CH2Cl2; (d) H2, Pd/C; (e) Ph3P, CBr4, CH2Cl2, then n-BuLi (2
equiv), THF, -78 °C; (f) n-BuLi, THF, -78 to 0 °C, cool to -78
°C then add 6 in THF; (g) Lindlar catalyst, H2, EtOAc, rt; (h) PPTS,
MeOH/CH2Cl2; (i) TBSCl, imidazole, DMF, rt.
Scheme 1
lation of 7 gave ether 8 which was smoothly converted into
Weinreb amide 6 by oxidative alkene cleavage, Wittig
extension, and hydrogenation.
Coupling fragment 5 was synthesized from known alcohol
915 by protection, alkene cleavage, and Corey-Fuchs16 al-
kyne formation. Treatment of the alkyne 5 with n-BuLi fol-
lowed by addition of the amide 6 and subsequent complete
saturation using H2 and Lindlar catalyst gave the correspond-
ing ketone 10 in excellent yield without any benzyl group
removal. Compound 10 was subjected to acid deprotection
and spiroketalization by treatment with PPTS in MeOH to
give 4 as the only spiroisomer. Some of the primary TBS
group was cleaved in this sequence but this was easily
reinstalled by subjection of the crude product to resilylation.
With the spiroketal fragment 4 in hand we next appended
the C3-C10 side chain as outlined in Scheme 3.
for our total synthesis of reveromycin B.10 The C2-C3 and
C8-C9 bonds could be formed via stabilized Wittig reactions
while the C4-C5 bond might be formed by a reagent-con-
trolled asymmetric syn-aldol reaction.11 Finally, the C21-C22
bond could be installed using a Stille cross-coupling reac-
tion.12 This leads to the key spiroketal intermediate 4 which
Debenzylation of 4 followed by oxidation17 and homolo-
gation using the Bestmann protocol18 gave alkyne 11. TBAF
induced desilylation then afforded the alcohol 12 which
fortunately was crystalline and an X-ray crystal structure19
(3) Shimizu, Y.; Kiyota, H.; Oritani, T. Tetrahedron Lett. 2000, 41, 3141.
(4) Shimizu, T.; Kusaka, J.; Ishiyama, H.; Nakata, T. Tetrahedron Lett.
2003, 44, 4965.
(5) Shimizu, T.; Kobayashi, R.; Osako, K.; Osado, H.; Nakata, T.
Tetrahedron Lett. 1996, 37, 6755.
(6) Drouet, K. E.; Ling, T.; Tran. H. V.; Theodorakis, E. A. Org. Lett.
2000, 2, 207.
(7) El Sous, M.; Rizzacasa, M. A. Tetrahedron Lett. 2000, 41, 8591.
(8) Shimizu, T.; Masuda, T.; Hiramoto, K.; Nakata, T. Org. Lett. 2000,
2, 2153.
(9) El Sous, M.; Ganame, D.; Tregloan, P. A.; Rizzacasa, M. A.
Manuscript in preparation.
(13) Nahm, S.; Weinreb, S. M. Tetrahedron Lett. 1981, 22, 3815.
(14) For the synthesis of spiroketals, see: (a) Perron, F.; Albizati, K. F.
Chem. ReV. 1989, 89, 1617. (b) Mead K. T.; Brewer; B. N. Curr. Org.
Chem. 2003, 7, 227.
(15) Nicolaou, K. C.; Patron, A. P.; Ajito, K.; Richter, P. K.; Khatuya,
H.; Bertinato, P.; Miller, R. A.; Tomaszewski, M. J. Chem. Eur. J. 1996,
2, 847.
(10) Cuzzupe, A. N.; Hutton, C. A.; Lilly, M. J.; Mann, R. K.; McRae,
K. J.; Rizzacasa, M. A.; Zammit, S. C. J. Org. Chem. 2001, 66, 2382.
(11) Nagao, Y.; Hagiwara, Y.; Kumagi, T.; Ochiai, M.; Inoue, T.;
Hashimoto, K.; Fujita, E. J. Org. Chem. 1986, 51, 2391.
(12) Stille, J. K. Angew. Chem., Int. Ed. Engl. 1986, 25, 508.
(16) Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 3769.
(17) Dess, D. B.; Martin, J. C. J. Am. Chem. Soc. 1991, 113, 7277.
(18) Mu¨ller, S.; Liepold, B.; Roth, G. J.; Bestmann, J. Synlett 1996, 521.
(19) Crystallographic data have been deposited with the Cambridge
Crystallographic Centre as supplementary publication CCDC-224793.
1042
Org. Lett., Vol. 6, No. 6, 2004

Scheme 3 a
Scheme 4 a
a Reagents and conditions: (a) H2, Pd/C, EtOAc; (b) Dess-Mar-
tin periodinane, CH2Cl2, rt; (c) MeOH, K2CO3, (MeO)2P(O)C(N2)-
C(O)Me; (d) TBAF, THF, rt; (e) Ph3PdC(Me)CHO, toulene, reflux;
(f) Ph3PdCHCO2Me, toulene, reflux; (g) DiBALH, CH2Cl2, -78
°C; (h) Oxazolidine-2-thione 15, Sn(OTf)2, N-ethylpiperidine, -40
°C, CH2Cl2, then add 14, -78 °C; (i) NaBH4, THF/H2O.
a Reagents and conditions: (a) Bu3SnH, Pd(Ph3P)2Cl2, CH2Cl2,
O °C; (b) iodide 19, Pd2(dba)3, TFP, NMP, 55 °C, 5 h; (c) TBSCl,
imidazole, DMF; (d) HF‚pyrdine/pyrdine, THF, 0 °C; (e) Dess-
Martin periodinane, CH2Cl2, rt; (f) Ph3PdCHCO2Tmse, CH2Cl2,
rt. (g) TBAF, DMF, rt, 24h.
catalysis20 proceeded in good yield. The stannane was then
cross-coupled21 with the known iodide 1910 (Tmse ) tri-
methylsilylethyl) to cleanly provide ester 20 with the
C20-C24 side chain fully installed.
was obtained for this intermediate confirming the stereo-
chemistry (Figure 2). Oxidation of alcohol 12 and dual Wittig
Protective group manipulation gave 21 which upon oxida-
tion and Wittig reaction provided the protected precursor 22.
Exposure of 22 to excess TBAF in DMF caused global
deprotection to give the nominal structure for spirofungin B
(2) which was purified by reverse phase HPLC. Unfortu-
nately, the physical data obtained for synthetic 2 did not
compare with that reported for spirofungin B.1 Furthermore,
1
the H NMR spectrum of an authentic spirofungin mixture
measured on the same instrument did not match our
synthesized material. In particular, the chemical shift for H19
in the natural product (4.76 ppm) was vastly different to that
found for compound 2 (4.29 ppm). In addition to other shift
differences in the 1H NMR spectrum, the chemical shift for
C15 in the 13C NMR spectrum of 2 was also measurably
different to the literature value (Natural spirofungin B: 97.7
ppm. Synthetic 2: 96.2 ppm.) These differences lead to the
obvious conclusion that the structure proposed for spirofun-
gin B (2) is incorrect.
Figure 2. X-ray structure of spiroketal 12.
An investigation of the literature revealed a possible
alternative for the structure of spirofungin B. In their syn-
thesis of the spiroketal fragment of spirofungin A, Shimizu
and co-workers4 obtained a mixture of spiroketals 23 and
24 (ratio 1.53:1) isomeric at the spiro center via a thermo-
dynamic approach similar to that utilized above (Figure 3).
extensions gave the ester 13 in good yield. Adjustment of
the oxidation level in 13 gave aldehyde 14 which underwent
an asymmetric aldol reaction with the tin enolate derived
from oxazolidine-2-thione 15.10,11 This sequence of reactions
which provided syn-aldol adduct 16 was high yielding and
easily conducted on large scale. Auxiliary removal by treat-
ment of 16 with NaBH4 in wet THF gave diol 17.
(20) Zhang, H. X.; Guibe´, F.; Balavoine, G. J. Org. Chem. 1990, 55,
1857.
(21) Farina, V.; Krishnan, B. J. Am. Chem. Soc. 1991, 113, 9585.
The final steps to compound 2 are outlined in Scheme 4.
Conversion of alkyne 17 into vinyl stannane 18 using Pd
Org. Lett., Vol. 6, No. 6, 2004
1043

Figure 4.
The results cited above strongly suggest that the structure
for spirofungin B is in fact 15-epi-spirofungin A (27) (Figure
4). The absolute configurations at C18 (S) and C19 (R) in
spirofungin B are identical to that for spirofungin A (1) while
the spiro carbon is epimeric (15R). Spirofungin B therefore
has the conformation B′ as shown which is consistent with
the NMR data presented. It is not unreasonable to suggest
that biosynthetically, spirofungin A (1) could give rise to
27 by simple spiroketal isomerization in analogy to the
spiroisomers of the pectenotoxins.25 Efforts toward the
synthesis of spirofungin A (1) and its C15 epimer 27 are in
progress and will be reported in due course.
Figure 3. Related spiroketals (spirofungin numbering).
This clearly demonstrates that isomerization of the spiro-
fungin A spiroketal is possible under acidic conditions (due
to the axial orientation of the C19 substituent) to afford a
spiroketal with one less anomeric stabilization.22 The chemi-
cal shift for the H19 in the preferred isomer is upfield from
the corresponding shift in the minor isomer where the axial
H19 is deshielded by the axial oxygen23 and the opposite
change in chemical shift is evident for H11 (Figure 3).
Comparable chemical shift differences are also observed for
H11 and H19 in spirofungins A and B themselves (see A
and B′, respectively, in Figure 4). Another example can be
found in the synthesis of the spiroketal fragment of revero-
mycin A (3). Theodorakis and co-workers6 also obtained a
mixture of spiroketal isomers 25 and 26 in a ratio of 1.5:1
respectively by acid induced spiroketalization. The chemical
shift differences (Figure 3) for the H11 and H19 protons for
each isomer are similar to those cited above. In addition,
the C15 chemical shifts for each isomer differ in a manner
similar to that for spirofungin A (95.7 ppm) verses B (97.7
ppm). This is consistent with the trend of a slight downfield
shift of the spiro carbon in going from ax-ax to ax-eq oxygen
orientations.24 An NOE was also observed between H19 and
H11 which was also seen in spirofungin B (Figure 1).
Acknowledgment. We are indebted to Professors Hans-
Peter Fiedler and Gu¨nther Jung (Tu¨bingen University) for
providing NMR spectra and an authentic sample of spiro-
fungins A and B. This work was funded by the Melbourne
Research Grants Scheme.
Supporting Information Available: Characterization
data for compounds 2, 4-6, 10, 12, 13, 16, 17, and 20-22
as well as NMR spectra of 2 and authentic spirofungins A
and B. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL049873E
(23) Ireland, R. E.; Daub, J. P. J. Org. Chem. 1983, 48, 1303.
(24) Pothier, N. Goldstein, Deslongchamps, P. HelV. Chim. Acta 1992,
75, 604.
(22) Deslongchamps, P. Rowan, D. D.; Potheir, N.; Sauve´, G.; Sanders,
J. K. Can. J. Chem. 1981, 59, 1105.
(25) Sasaki, K.; Wright, J. L. C.; Yasumoto, T. J. Org. Chem. 1998, 63,
2475.
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Org. Lett., Vol. 6, No. 6, 2004