Stereo-controlled synthesis of prelasalocid, a key precursor proposed in the biosynthesis of polyether antibiotic lasalocid A

Article abstract of DOI:10.1016/j.tetlet.2007.12.003

In the biosynthesis of a polyether ionophore antibiotic, lasalocid A, the cyclic ether skeleton composed of a tetrahydrofuran linked to a tetrahydropyran could be constructed by oxidative cyclization of linear dodecaketide diene precursor. Hence, we hypothesized a prelasalocid having (E,E)-trisubstituted olefins as the dodecaketide biosynthetic precursor. A stereo-controlled synthetic route to the prelasalocid has been devised in a highly convergent manner entailing installation of a variety of substituents at the trisubstituted olefins.

Full text of DOI:10.1016/j.tetlet.2007.12.003

Available online at www.sciencedirect.com
Tetrahedron Letters 49 (2008) 1021–1025
Stereo-controlled synthesis of prelasalocid, a key precursor proposed
in the biosynthesis of polyether antibiotic lasalocid A
Akira Migita, Yoshihiro Shichijo, Hiroki Oguri *, Mami Watanabe,
Tetsuo Tokiwano, Hideaki Oikawa *
Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
Received 9 November 2007; revised 28 November 2007; accepted 3 December 2007
Available online 5 December 2007
Abstract
In the biosynthesis of a polyether ionophore antibiotic, lasalocid A, the cyclic ether skeleton composed of a tetrahydrofuran linked to
a tetrahydropyran could be constructed by oxidative cyclization of linear dodecaketide diene precursor. Hence, we hypothesized a prel-
asalocid having (E,E)-trisubstituted olefins as the dodecaketide biosynthetic precursor. A stereo-controlled synthetic route to the prela-
salocid has been devised in a highly convergent manner entailing installation of a variety of substituents at the trisubstituted olefins.
Ó 2007 Elsevier Ltd. All rights reserved.
Keywords: Lasalocid; Polyether antibiotics; Biosynthesis; Trisubstituted olefin; Aldol coupling
Polyether antibiotics exhibit a selective antimicrobial
activity against Gram-positive bacteria and have been used
for the control of chicken coccidiosis as well as the growth
promotion in animal husbandry.1 Lasalocid A2 and isola-
salocid,3 isolated from the Streptomyces lasaliensis, are
polyether ionophore antibiotics and have distinct cyclic
ether skeletons on the right side of the molecules, a tetra-
hydrofuran linked to a tetrahydropyran and two adjacent
tetrahydrofuran rings, respectively (Scheme 1). A hypo-
thetical biosynthetic mechanism has been proposed, which
involves stereoselective epoxidations of the dodecaketide
linear precursor followed by sequential cyclizations with
divergent modes of ring-closure leading to co-production
of lasalocid A and isolasalocid.3 This insightful hypothesis
was supported by feeding experiments4 with isotopically
labeled precursors including [1-13C,1-18O]-short chain fatty
acids, indicating that two oxygen atoms on the ether rings
could be derived from molecular oxygen by the corres-
ponding epoxidation of a (E,E)-diene precursor, prelasalocid
*
Corresponding authors. Tel.: +81 11 706 2622; fax: +81 11 706 3448.
E-mail addresses: oguri@sci.hokudai.ac.jp (H. Oguri), hoik@sci.
Scheme 1. Hypothetical biosynthetic pathway of lasalocid A.
hokudai.ac.jp (H. Oikawa).
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.12.003

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A. Migita et al. / Tetrahedron Letters 49 (2008) 1021–1025
(1). These data and the studies on monensin biosynthesis
brought a unified theory, Cane–Celmer–Westley hypothe-
sis for polyether formation.5 Recently, Cambridge group
has identified biosynthetic gene cluster of a representative
polyether antibiotic monensin6 and succeeded isolation of
a putative triene intermediate analog from gene-disruption
mutant.7 To date, however, an enzymatic conversion of
triene or triepoxide has not been reported. To gain mecha-
nistic insights into the polyether formation in lasalocid
biosynthesis based on the enzymatic conversions of
linear substrate possessing either dienes or epoxides, we
have initiated to develop a synthetic route to prelasalocid
1. Herein, we describe the synthesis of 1 based on a
convergent strategy that features stereo-controlled con-
structions of trisubstituted olefins and an anti-selective
aldol coupling.
To obtain mechanistic insight between substrate speci-
ficity and enzymatic polyether formation, it is necessary
to develop a flexible route for synthesizing various sub-
strate analogs. The retrosynthetic disconnections of prela-
salocid 1 are outlined in Scheme 2. Since degradation of
benzyl ester of lasalocid A through the retro-aldol frag-
mentation at C11–C12 was reported to provide the optically
pure aldehyde as a left segment, we employed a convergent
strategy that unifies aldehyde 28 and ethylketone 3 by an
anti-selective aldol coupling. The key right segment 3
would be assembled by B-alkyl Suzuki–Miyaura coupling9
of alkylborane 4 and iodoolefin 5 installing the C18–C19 tri-
substituted olefin. The alkylborane part 4 would be synthe-
sized by syn-selective aldol coupling between C14 and C15.
For the synthesis of iodoolefin 5 having the pair of trisub-
stituted olefins, we devised a flexible and stereo-controlled
synthetic route that entails diversification of olefin geo-
metry as well as the introduction of variety of substituents
at C18, C22, and C23.
Synthesis of 5 commenced with the protection of com-
mercially available 4-pentyn-1-ol 6 as a silyl ether (Scheme
3). The palladium-catalyzed coupling of a terminal acetyl-
ene, TMSI and Et2Zn stereoselectively produced the 2,2-
disubstituted (E)-vinylsilane 7 in 45% yield.10 The regio-
and stereo-selectivities of the conversion are supported by
a mechanistic rationale that involves regio-controlled silyl-
palladation of a terminal acetylene followed by coupling
of the resulting alkenylpalladium(II) with Et2Zn. Treatment
of vinylsilane 7 with NIS in CH3CN afforded (E)-iodoolefin
8 in 95% yield with retention of the olefin geometry. The
Negishi coupling with Grignard-derived methylzinc chloride
furnished the terminal trisubstituted (E)-olefin 9 in 96%
yield. Removal of the silyl group and subsequent Swern oxi-
dation yielded an aldehyde, which was then converted to
silylacetylene 10 under the Corey–Fuchs conditions.11 The
terminal silylacetylene 10 was then converted by hydrozirc-
onation using Cp2Zr(H)Cl, generated in situ from Cp2ZrCl2
and DIBAL,12 followed by treatment with NIS to afford
iodovinylsilane 11 in one pot as a single isomer in 69% yield.
The ethyl group was installed by Negishi coupling with
Grignard-derived ethylzinc chloride to give (Z)-vinylsilane
12 in nearly quantitative yield. Upon treatment with NIS
in DMF under Tamao’s conditions,13 iodation of (Z)-
vinylsilane 12 proceeded smoothly with complete inversion
of the olefin geometry, presumably through anti-addition
of iodide and DMF and subsequent anti-elimination, to
afford the desired (E)-iodoolefin 5 in 53% yield.
Next, we turned our attentions to the B-alkyl Suzuki–
Miyaura coupling of alkylborane 4 and 5 leading to the
Scheme 3. Reagents and conditions: (a) TBDPSCl, imidazole, DMF,
98%; (b) Et2Zn, TMSI, 5 mol % Pd(PPh3)4, 1,4-dioxane, 45%; (c) NIS
(1.2 equiv), CH3CN, 95%; (d) ZnCl2, MeMgBr, Pd(PPh3)4, THF, 96%; (e)
Bu4NF, THF; (f) (COCl)2, DMSO, Et3N, CH2Cl2, À78 to À45 °C; (g)
CBr4, PPh3, pyridine, CH2Cl2, 53% (three steps); (h) n-BuLi, TMSCl,
THF, À78 °C, 95%; (i) Cp2ZrCl2 (1.5 equiv), DIBALH (1.5 equiv), then
NIS (1.2 equiv), THF, 10 °C, 69%; (j) EtMgBr, ZnCl2, Pd(PPh3)4, THF,
99%; (k) NIS (1.2 equiv), DMF, 53%.
Scheme 2. Synthetic strategy for prelasalocid (1).

A. Migita et al. / Tetrahedron Letters 49 (2008) 1021–1025
1023
Scheme 5. Reagents and conditions: (a) (i) LDA (1.5 equiv), Et2O, 0 °C,
30 min, (ii) ZnCl2 (1.5 equiv), 0 °C, 30 min, (iii) 2 (1.0 equiv), Et2O, 0 °C,
10 min, 17 (12%), 18 (42%), 16 (15% recovery); (b) (c-Hex)2BCl
(1.2 equiv), Et3N (1.2 equiv), then 2 (1 equiv), THF, À78 °C, 17 (27%),
16 (42% recovery). *Mixture of other aldol adducts.
Scheme 4. Reagents and conditions: (a) Bu2BOTf, (i-Pr)2NEt, methacro-
lein, CH2Cl2, À78 °C, 95%; (b)TESCl, imidazole, DMF, rt, 98%; (c) 14
(3.0 equiv), 9-BBN dimer (3.0 equiv), THF, 0 °C, then Cs2CO3 (3.0 equiv),
10 mol % Pd(dppf)Cl2, 5 (1.0 equiv), THF/1,4-dioxane/H2O = 10:8:1, rt,
82%; (d) EtSH, n-BuLi, THF, À78 °C, 91%; (e) DIBALH, CH2Cl2,
À78 °C, 92%; (f) EtMgBr, THF, À78 °C, 94%; (g) Dess–Martin period-
inane, CH2Cl2, 98%.
and Kishi’s fully elaborated substrate 2017 having cyclic
ether rings, the distinct modes of zinc chelation are sur-
mised to play a pivotal role in the double asymmetric
induction for the condensation with chiral aldehyde 2.
The stereochemistry of the anti-aldol adduct 17 was unambi-
guously determined based on NMR analysis after conver-
sion into 1,3-diol acetonides 21 and 22.18
right segment 3 (Scheme 4). Synthesis of precursor 15 of 3
began with syn-selective Evans aldol reaction14 of 13 with
methacrolein and subsequent silyl ether formation to give
14 in 93% yield (two steps). Diastereo-controlled hydrobo-
ration of 14 with 9-BBN generated the corresponding alkyl
borane 4,15 which was in situ treated with 5 under the con-
ditions [aqueous Cs2CO3, Pd(dppf)Cl2, THF/1,4-dioxane/
H2O = 10:8:1, rt]16 to produce 15 in 82% yield. The (E)-
configuration of the resulting trisubstituted olefin of 15
was confirmed on the basis of NOE correlation between
methylene protons at C17 and a vinyl proton at C19.
Removal of the oxazolidinone group to form a thioester
and stepwise installation of the ethylketone functionality
yielded 3 in good yields.
With the elaborated 3 in hand, model studies for the
critical anti-aldol reaction for the assembly of the left and
right segments were examined using a readily accessible
ethylketone 16 (Scheme 5). As an initial attempt, we
applied the Kishi’s procedure employing a zinc-enolate,
which was reported to effect the anti-aldol reaction leading
to the total synthesis of lasalocid A.17 Deprotonation of 16
with LDA (1.5 equiv) in Et2O at 0 °C and subsequent addi-
tion of ZnCl2 (1.5 equiv) generated the zinc enolate, which
was treated with aldehyde 2 in one pot at 0 °C for 10 min to
form a mixture of aldol adducts in 60% yield. Apart from
the minor isomers, the resulting two adducts (17:18 = 2:7)
were anti-aldol adducts. However, desired 17 (C11,C12-anti,
C12,C14-syn) was obtained in 12% yield as a minor product,
and instead, 18 (C11,C12-anti, C12,C14-anti) predominantly
formed in 42% yield. As a rationale for the different stereo-
chemical outcomes between model 16 with C14 silyl ether
To attain the desired stereoselectivity (C11,C12-anti,
C12,C14-syn), we next adopted an alternative approach for
the anti-aldol reaction using sterically demanding di-
alkylboron enolate reported by Paterson et al.19 Treatment
of 16 with (c-Hex)2BCl (1.2 equiv) and Et3N (1.2 equiv)
generated the (E)-boron enolate, which was then condensed
with aldehyde 2 to afford the desired anti-aldol adduct 17 as
a major product in 27% yield along with minor aldol
adducts (17:others = 10:1) and recovered 16 (42%). Since
lasalocid A is labile to the retro-aldol fragmentation at
C11–C12 under basic conditions, equilibrium between sub-
strates and aldol adducts is probably responsible for the
low yield of this reaction. Despite several experimentations,
no significant improvement has been achieved so far. None-
theless, it is worth mentioning that the formation of unde-
sired 18 was completely suppressed under the conditions.
Taking into account for the remarkable diastereoselectivity
as well as the substantial recovery of ethylketone 16 which
was actually recycled, we employed the boron-aldol
approach for the condensation of the elaborated 3.
As shown in Scheme 6, the assembly of the left and right
segments (2, 3) was achieved by the boron enolate mediated
anti-aldol coupling to produce desired aldol adduct 19 in
27% yield (3: 40% recovery) in a highly stereo-controlled
manner (19:other aldol adducts = 10:1).20 Finally, the pro-
tecting group manipulations were conducted in three steps:
(1) silylation of C11 hydroxyl group; (2) palladium-cata-
lyzed cleavage of O-allyl groups; (3) removal of TES

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A. Migita et al. / Tetrahedron Letters 49 (2008) 1021–1025
6. Leadlay, P. F.; Staunton, J.; Oliynyk, M.; Bisang, C.; Cortes, J.; Frost,
E.; Hughes-Thomas, Z. A.; Jones, M. A.; Kendrew, S. G.; Lester, J. B.;
Long, P. F.; McArthur, H. A. I.; McCormick, E. L.; Oliynyk, Z.; Stark,
C. B. W.; Wilkinson, C. J. J. Ind. Microbiol. Biotechnol. 2001, 27, 360.
7. Bhatt, A.; Stark, C. B. W.; Harvey, B. M.; Gallimore, A. R.;
Demydchuk, Y. A.; Spencer, J. B.; Staunton, J.; Leadlay, P. F.
Angew. Chem., Int. Ed. 2005, 44, 7075.
8. The left segment 2 was readily prepared from sodium salt of lasalocid
A: (1) allyl bromide, K2CO3, dioxane (92%); (2) pyrolysis (230 °C),
5 min, 65%, see: Ireland, R. E.; Anderson, R. C.; Badoud, R.;
Fitzsimmons, B. J.; McGarvey, G. J.; Thaisrivongs, S.; Wilcox, C. S.
J. Am. Chem. Soc. 1983, 105, 1988.
9. For excellent reviews, see: (a) Miyaura, N.; Suzuki, A. Chem. Rev.
1995, 95, 2457; (b) Chemler, S. R.; Traunar, D.; Danishefsky, S. J.
Angew. Chem., Int. Ed. 2001, 40, 4544.
10. Chatani, N.; Amishiro, N.; Morii, T.; Yamashita, T.; Murai, S. J.
Org. Chem. 1995, 60, 1834.
11. Corey, E. J.; Fuchs, P. L. Tetrahedron Lett. 1972, 3769.
12. Kiyotsuka, Y.; Igarashi, J.; Kobayashi, Y. Tetrahedron Lett. 2002, 43,
2725.
13. Tamao, K.; Akita, M.; Maeda, K.; Kumada, M. J. Org. Chem. 1987,
52, 1100.
Scheme 6. Reagents and conditions: (a) (c-Hex)2BCl (1.2 equiv), Et3N
(1.2 equiv), then 2 (1.0 equiv), THF, À78 °C, 1 h, 19 (27%), 3 (40%
recovery); (b) TESCl, imidazole, DMF, rt; (c) HCO2H, Et3N, Pd(PPh3)4,
THF, rt; (d) HF, CH3CN, rt, 84% (three steps).
groups at C11 and C15 by treatment with HF, leading to the
targeted prelasalocid 1 in 84% overall yield.21
In summary, the convergent synthesis of prelasalocid 1
has been achieved in a highly stereo-controlled manner.
The trisubstituted olefins (C18–C19 and C22–C23) were
successfully synthesized by transition-metal mediated
construction of vinyl silanes, stereo-divergent iodination
by NIS, and Negishi and Suzuki–Miyaura cross-couplings.
The efficient access to prelasalocid 1 and its analogues
paves the way for extensive investigation for substrate spec-
ificities of epoxidase and hydrolase that effect the cyclic
ether formations on the right side. Recently, we have iden-
tified a gene cluster for lasalocid biosynthesis. The enzy-
matic conversions of 1 and its analogues will be reported
in due course.
14. Evans, D. A.; Bartroli, J.; Shih, T. L. J. Am. Chem. Soc. 1981, 103, 2127.
15. Still, W. C.; Barrish, J. C. J. Am. Chem. Soc. 1983, 105, 2487.
16. Mickel, S.; Sedelmeier, G. H.; Niederee, D.; Schuerch, F.; Seger, M.;
Schreiner, K.; Daeffler, R.; Osmani, A.; Bixel, D.; Loiseleur, O.; Cercus, J.;
Stettler, H.; Schaer, K.; Gamboni, R. Org. Process Res. Dev. 2004, 8, 113.
17. Nakata, T.; Schmid, G.; Vranesic, B.; Okigawa, M.; Smith-Palmer,
T.; Kishi, Y. J. Am. Chem. Soc. 1978, 100, 2933. Structure of right
segment 20 for the anti-aldol reaction for the Kishi’s total synthesis of
lasalocid A is shown below:
.
18. The stereochemistry of the aldol adduct 17 was determined as
C11,C12-anti and C12,C14-syn by extensive NMR analysis (1H, 13C
NMR, and NOE experiments) on the corresponding acetonides
converted from 17. The stereochemistry of 18 was also determined in
a similar manner. Selected 1H NMR data of 21: d 3.78 (br d,
J = 10.5 Hz, H11), 3.47 (br d, J = 9.6 Hz, H13), 1.79 (ddq, J = 10.5,
9.6, 3.3 Hz, H12), 22: d 3.55 (t, J = 6.0 Hz, H13), 3.31 (dd, J = 9.6,
3.3 Hz, H15), 1.40 (ddt, J = 6.0, 3.3, 3.3 Hz, H14).
Acknowledgments
We are grateful to Kaken Pharmaceutical Co. Ltd for
supplying generous amount of natural lasalocid A, and
to High Resolution NMR Laboratory, Faculty of Science,
Hokkaido University, for NMR measurements. This work
was supported by the Mitsubishi Foundation, and the
Grants-in-Aid for Scientific Research [(A)17208010 to H.
Oikawa and 17710175 to H. Oguri] from the Japan Society
for the Promotion of Science (JSPS). A fellowship to A.M.
from JSPS is gratefully acknowledged.
References and notes
1. Polyether Antibiotics: Naturally Occurring Acid Ionophores, Biology;
Westley, J. W., Ed.; Marcel Dekker: New York, 1982.
2. Breger, J.; Rachlin, A. I.; Scott, W. E.; Sternbach, L. H.; Goldberg,
M. W. J. Am. Chem. Soc. 1951, 73, 5295.
3. Westley, J. W.; Blount, J. F.; Evans, R. H., Jr.; Stempel, A.; Berger, J.
J. Antibiot. 1974, 27, 597.
4. (a) Hutchinson, C. R.; Sherman, M. M.; Vederas, J. C.; Nakashima,
T. T. J. Am. Chem. Soc. 1981, 103, 5953; (b) Hutchinson, C. R.;
Sherman, M. M.; McInnes, A. G.; Walter, J. A.; Vederas, J. C. J. Am.
Chem. Soc. 1981, 103, 5956.
5. Cane, D. E.; Celmer, W. D.; Westley, J. W. J. Am. Chem. Soc. 1983,
105, 3594.
.

A. Migita et al. / Tetrahedron Letters 49 (2008) 1021–1025
1025
19. Vulpetti, A.; Bernardi, A.; Gennari, C.; Goodman, J. M.; Paterson, I.
Tetrahedron 1993, 49, 685.
20. The stereochemistry of the aldol adduct 19 was confirmed based on a
comparison of 1H NMR data of 17.
21. Data for prelasalocid 1: 1H NMR (300 MHz, CDCl3): d 7.19 (d,
J = 7.5 Hz, 1H), 6.63 (d, J = 7.5, 1H), 5.14 (q, J = 6.9 Hz, 1H), 5.07
(t, J = 6.6 Hz, 1H), 3.95 (dd, J = 8.2, 2.7 Hz, 1H), 3.71 (dd, J = 9.6,
1.5 Hz, 1H ), 3.16 (dt, J = 10.1, 3.9 Hz, 1H), 2.96 (qn, J = 7.8 Hz,
1H), 2.72 (dt, J = 9.6, 1.5 Hz, 1H), 2.70 (dt, J = 10.1, 3.9 Hz, 1H),
2.21 (s, 3H), 1.83–2.02 (m, 12H), 1.17 (m, 1H), 1.56 (d, J = 6.6 Hz,
3H), 1.30–1.53 (m, 3H), 1.02 (d, J = 8.2 Hz, 3H), 0.95 (t, J = 7.5 Hz,
3H), 0.94 (t, J = 6.6 Hz, 3H), 0.89 (d, J = 9.0 Hz, 3H), 0.86 (t,
J = 7.5 Hz, 3H), 0.79 (d, J = 6.6 Hz, 3H); 13C NMR (75 MHz,
CDCl3): d 217.6, 171.1, 167.8, 161.4, 143.7, 141.6, 139.4, 134.9, 126.3,
124.3, 121.3, 117.6, 77.2, 76.8, 74.7, 73.9, 57.3, 67.6, 41.9, 36.9, 36.8,
34.5, 34.3, 34.0, 26.5, 23.7, 23.0, 22.8, 22.7, 13.4, 13.2, 13.1, 12.9, 12.8;
IR(neat) mmax: 2963, 1700, 1655, 1257, 1093, 844 cmÀ1; HRMS(ESI)
calcd for C34H54O6Na [M+Na]+ 581.7787; found 581.7762.