Total synthesis of carolacton, a highly potent biofilm inhibitor

Article abstract of DOI:10.1002/anie.201106762

Metals are the key players in the synthesis of caralacton, a strong inhibitor of bacterial biofilms. The total synthesis is based on several metal-mediated key transformations such as the Ley and the Duthaler-Hafner aldol reactions, the Marshall reaction and Breit's substitution, as well as the Nozaki-Hiyama-Kishi and Negishi-Fu C-C coupling reactions. Copyright

Full text of DOI:10.1002/anie.201106762

Angewandte
Chemie
DOI: 10.1002/anie.201106762
Total Synthesis
Total Synthesis of Carolacton, a Highly Potent Biofilm Inhibitor**
Thomas Schmidt and Andreas Kirschning*
Recently, we discovered and isolated carolacton (1) from the
extracts of Sorangium cellulosum strain So ce96.[1] Subse-
quent screening revealed unique biological properties of
carolacton (1): it drastically reduces the number of viable cells
in biofilms at nanomolar concentration.[2] In particular,
biofilms containing the caries- and endocarditis-associated
bacterium Streptococcus mutans were sensitive to a solution
of 1 at a concentration of 0.005 mgmLÀ1, at which approx-
imately 35% of the cells within the biofilm die. Importantly,
planktonic cultures were nearly insensitive to carolacton (1).
Bacterial biofilms have become a major concern in clinical
treatment,[3] as they evade host defenses and are inherently
resistant to antimicrobial agents whether they grow on
natural tissues or medical devices and implants. Importantly,
the antibiotic resistance of bacteria in biofilms is about 1000-
fold higher than in planktonic form.[4]
Our first report on carolacton (1) also covered a concise
structure elucidation that included the assignment of all
stereogenic centers. Carolacton (1) features a polyketide-type
backbone which forms a 12-membered macrolactone with a
1,2-diol moiety with trans orientation in the open-chain
conformation. Remarkably, the other terminus is a carboxylic
acid, a terminal group rarely found in polyketides.
Scheme 1. Retrosynthetic analysis of carolacton (1); metals involved in
key transformations are given in parenthesis. Ms=mesyl, PMB=
p-methoxybenzyl , TIPS=triisopropylsilyl, TBDPS=tert-butyldiphenyl-
benzylsilyl.
The exceptional inhibitory effects of carolacton (1) led us
to initiate a synthetic program that would also be conducive to
the study of structure–activity relationships. Our highly
convergent approach is depicted in Scheme 1 and relies on a
pared in the form of seco acid 2, which is macrocyclized.
Advanced intermediate 2 is further dissected into western
fragment 3 and eastern fragment 4. Fragment 3 was planned
to be prepared from building blocks 5–7 by an aldol reaction
according to Ley[5] and an asymmetric Negishi reaction
between racemic allyl chloride 6 and bromide 5 following
Fuꢀs method.[6] The eastern fragment was to originate from
propargylic mesylate 8[7,8] and aldehyde 9,[8] which are merged
by a Marshall reaction[9] followed by the Duthaler–Hafner
aldol reaction.[10]
À
series of metal-mediated or metal-catalyzed C C coupling
reactions. First, the complete polyketide backbone is pre-
[*] T. Schmidt, Prof. Dr. A. Kirschning
Institut fꢀr Organische Chemie und Zntrum fꢀr Biomolekulare
Wirkstoffe (BMWZ), Leibniz Universitꢁt Hannover
Schneiderberg 1B, 30167 Hannover (Germany)
E-mail: andreas.kirschning@oci.uni-hannover.de
The synthesis of the western fragment 3 commenced with
lactic acid 11, which was transformed into bromide 5[11] by
means of Breitꢀs SN2-type zinc-catalyzed alkylation
[**] We are indepted to R. Jansen and R. Mꢀller (Helmholtz-Zentrum fꢀr
Infektionsforschung, HZI, Braunschweig (Germany)) for an
authentic sample of carolacton (1) and thank the referees for helpful
suggestions.
(Scheme 2).[13] The next C C coupling step was a key step
À
in our synthesis and relied on the asymmetric cross-coupling
reaction between racemic allylic chloride 6[6b,8] and aliphatic
bromide 5 according to the method of Fu et al.[6] Ester 12 was
generated in the presence of the (S,S)-pybox ligand 14 under
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106762.
Angew. Chem. Int. Ed. 2012, 51, 1063 –1066
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1063

.
Angewandte
Communications
Scheme 3. Preparation of eastern fragment 4. Reagents and condi-
tions: a) InI, [Pd(dppf)Cl2], THF/HMPA, 08C–RT, 17 h, 87% single
diastereomer (+ 5% of diastereomeric mixture of side products);
b) TBAF, THF, 08C–RT, 13 h, 98%; c) (MeO)2CHPh-OMe, PPTS, MS
3 ꢂ, CH2Cl2, 08C, 1 h; d) DIBAL-H, CH2Cl2, À788C to RT, 3 h, 85% for
2 steps; e) (COCl)2, DMSO, Et3N, CH2Cl2, À788C to À408C, 1 h; f) 20,
Et2O, À1058C added to 22 (formed from 10 at À1058C according to
Ref. [10]) warmed up to À788C over 1.5 h, then À788C for 12 h, 88%
for two steps (d.r.=5.5:1); g) OMe3BF4, proton sponge, CH2Cl2, RT,
12 h, 90%. dppf=1,1’-bis(diphenylphosphino)ferrocene, HMPA=
hexamethylphosphoramide, TBAF=tetra-n-butylammonium fluoride,
MS=molecular sieves, DMSO=dimethyl sulfoxide, Cp=cyclopenta-
dienyl, proton sponge=1,8-bis(dimethylamino)naphthalene.
Scheme 2. Preparation of western fragment 3. Reagents and condi-
tions: a) Zn, I2 (3 mol%), DMA, 708C, 12 h; b) 6, 14, NiCl2·glyme,
NaCl, DMF/DMA, À108C, 24 h, 82% (d.r.>10:1) and 13 (ca. 3%);
c) DIBAL-H, CH2Cl2, À608C, 45 min, 94%; d) MnO2, CH2Cl2, 08C–RT,
2 h; e) 1. 7, LiHMDS, THF, À788C, 10 min, 2. 15, À788C, 6 min, 57%
for two steps (d.r.>20:1); f) CSA, MeOH, RT, 18 h, 74%, g) 2,2-
dimethoxypropane, PPTS, CH2Cl2, RT, 1 h, 87%. CSA= camphorsul-
fonic acid, DMA=dimethylacetamide, glyme=1,2-dimethoxyethane,
DIBAL-H=diisobutylaluminum hydride, LiHMDS=lithium hexa-
methyldisilazide, PPTS=pyridimium p-toluenesulfonic acid.
different Lewis acids preferentially delivered the undesired
diastereomer (3S)-21 (Table 1, entries 1–3). In fact, in the case
of the Mukaiyama-type aldol reaction (entry 2) this asym-
metric induction was excellent (d.r. 3R/3S = 1:20).
Discarding the idea of substrate-controlled asymmetric
induction, we turned to the Duthaler–Hafner aldol reaction,
nickel catalysis.[14] However, this crucial reaction step suc-
ceeded only after optimization,[15] because initially we
obtained regioisomer 13 as a mixture of diastereomers. We
found that the added sodium chloride must be ground first in
order to generate the cross-coupling product 12 in good yield
and high diastereomeric ratio (d.r. < 10:1).
Table 1: Optimization of acetaldol reaction (selected examples).
Next, the trans-1,2-diol unit at C17–C18 was introduced
employing Leyꢀs butane-2,3-diacetal-protected (BDA)
desymmetrized glycolic acid building block 7.[5,16] Formation
of the lithium enolate 16 and reaction with aldehyde 15, which
was obtained from ester 12 after a reduction/oxidation
sequence, furnished aldol adduct 17 in good yield and with
excellent diastereocontrol. The synthesis of the C9–C19
fragment 3 of carolacton (1) was completed after transesteri-
fication of the butane-2,3-diacetal unit, which was accompa-
nied by TIPS ether cleavage, followed by acetonide protec-
tion of the intermediate diol.
Synthesis of the eastern fragment 4 commenced with the
formation of the (R)-Roche ester derived aldehyde 9
(Scheme 3). The subsequent Marshall reaction[9] with mesy-
late (S)-8 yielded adduct 18 with excellent diastereocontrol.
Alkyne 18 was further elaborated to alcohol 19 by a set of
standard protecting-group manipulations. After oxidation,
aldehyde 20 was subjected to different aldol protocols.[17]
Efforts to prepare the desired anti-Felkin aldol product
(3R)-21 by exploiting 1,2-,1,3-substrate control[18] using
Entry
R
Conditions
3R/
3S
Yield[a]
1[a]
2[a]
TMS TiCl4/Ti(OiPr)4 (1:1), CH2Cl2, À788C,
30 min
TMS TiCl4/Ti(OiPr)4 (3:1), CH2Cl2, À788C,
1:20 66%
1:20 73%
1:4.5 79%
30 min
3[a]
4
5
6
7
TMS BF3·OEt2, toluene, À788C, 30 min
Li
LDA, THF, À788C, 20 min
1:1
–
61%
–
“Ti” TiCl4, NEt3, CH2Cl2, À788C, 30 min
Ti[b,c] Ti,[c] Et2O, RT, Et2O, À788C, 12 h
Ti[b,c] Ti,[c] Et2O, À788C, Et2O, À908C, 12 h
Ti[b,c] Ti,[c] Et2O, À1058C, Et2O, À1058C, 12 h
2.1:1 92%
3.4:1 93%
5.5:1 88%
8
[a] Reactions were conducted with 2 equiv of ketene silyl acetal; Lewis
acids were added 30 s prior to addition of ketene silyl acetal for chelation.
[b] Chiral titanium complex given in Ref. [10]. [c] A cooled solution of the
titanium enolate was added to the cooled reaction mixture.
1064 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1063 –1066

Angewandte
Chemie
which relies on titanium enolate 22 generated from tert-butyl
ester 10. Inexpensive diacetoxy-d-glucose served as the chiral
ligand. In our case the mismatched aldol reaction gave the
two separable diastereomers (3R)-21 and (3S)-21 at À788C in
excellent yield (92%), with moderate stereoselectivity in
favor of the desired 3R diastereomer (d.r. = 2:1).[10] The
stereoselectivity could be improved by lowering the reaction
temperature to À1058C (88%; d.r. = 5.5:1) (Table 1,
entries 6–8). Attempts to convert aldol adduct (3S)-21 into
the desired 3R diastereomer by means of the Mitsunobu
reaction failed and resulted mainly in the formation of the
elimination product. Finally, O-methylation of (3R)-21 fur-
nished the eastern fragment 4.
Next, both fragments 3 and 4 had to be appropriately
refunctionalized. Alcohol 3 was oxidized under Swern con-
ditions to give aldehyde 24 (Scheme 4). Alkyne 4 was
transformed into vinyl iodide 23 by hydrozirconation and
electrophilic trapping of the intermediate vinylzirconium
species with iodine.[19]
C9 but proceeded through an SN2’ process to afford the
rearranged macrocycle 28 in 57% yield (confirmed by HSQC
and HMBC NMR measurements).[15] Therefore, we aimed for
improving the yield of diastereomer 25a. To our delight, the
asymmetric Nozaki–Hiyama–Kishi reaction that utilizes
ligand 27[21] provided carbinol 25a with good stereoselectivity
(d.r. = 4.7:1) and in almost quantitative yield.
At this point we note that differentiating the two
carboxylate groups is one key issue for the endgame towards
carolacton (1). The chosen tert-butyl ester is essential because
it can be orthogonally manipulated in the presence of the
methyl ester and the lactone moiety. Additionally, we found
that the methyl ester that corresponds to 4 was reduced in the
presence of [Cp2ClHZr] (see Scheme 4).
Thus, chemoselective hydrolysis of the ester 25a was
followed by ring closure using Shiinaꢀs protocol to yield
macrolactone 29 (Scheme 5).[23] To our surprise epimer 25b
did not cyclize under these conditions. Importantly, hydrolysis
of the tert-butyl ester is conducted before oxidative removal
of the PMB group and oxidation of C5. In fact, oxidation was
conducted as late as possible in the synthesis in order to avoid
epimerization adjacent to the keto group, which could occur
under the acidic conditions required for the deprotection of
the tert-butyl group.
Scheme 4. Preparation of the complete backbone of carolacton.
Reagents and conditions: a) 1. 4, [Cp2ClHZr], benzene, 1 h, 508C, then
CH2Cl2, 2. I2, benzene/CH2Cl2 2:1, À158C, 73%; b) (COCl)2, DMSO,
Et3N, CH2Cl2, À788C to À408C, 1 h; c) 1. (R)-27, CrCl2, proton sponge,
MeCN, 2 h, RT, 2. 23, 24 (1.8 equiv), [NiCl2(dppp)], MeCN, 16 h, RT,
96% for two steps (25a/25b=4.7:1); d) Dess–Martin periodinane,
CH2Cl2, NaHCO3, 08C to RT, 2.5 h, 70%; e) NaBH4, CeCl3·7H2O,
CH2Cl2, À50 to À308C, 2 h, 95% (25a/25b=1:10). dppp=1,3-bis(di-
phenylphosphino)propane.
Scheme 5. Completion of the total synthesis of carolacton (1).
Reagents and conditions: a) LiOH (1m), THF, H2O, RT, 13 h;
b) MNBA, DMAP, toluene, 508C, 12 h, 76% for two steps;
c) 1. TESOTf, lutidine, CH2Cl2, 08C to RT, 12 h; 2. TBAF, 10 min, 72%;
d) DDQ, CH2Cl2, pH 7 phosphate buffer, 08C to RT, 45 min; e) Dess–
Martin periodinane, CH2Cl2, NaHCO3, RT, 1.5 h, 88% for 2 steps;
f) PPTS, iPrOH/H2O 3:1, 6 d, 688C, 87% (96% based on recovered
starting material). MNBA=2-methyl-6-nitrobenzoic anhydride, TBAF=
tetrabutylammonium fluoride, TESOTf=triethylsilyl trifluoromethane-
sulfonate, DDQ=2,3-dichloro-5,6-dicyanobenzoquinone, DIAD=diiso-
propyl azodicarboxylate).
The Nozaki–Hiyama–Kishi reaction between aldehyde 24
and vinyliodide 23 yielded carbinols 25a and 25b (94%, d.r. =
1.5:1) containing the complete carbon backbone of carolacton
(1).[20–22] The diastereomeric mixture 25a,b was preferentially
transformed into 25b by Dess–Martin oxidation followed by
Luche reduction of 26. The absolute configuration of the
newly formed stereogenic center at C9 was assigned by
transforming 25b into the corresponding (R)- and (S)-Mosher
esters.[12] Macrolactonization of 25b under Mitsunobu con-
ditions (P(Ph3)3, DIAD, THF, 08C to RT, 6 h) did not provide
the desired macrolactone with inversion of configuration at
Thus, in the final sequence, removal of the tert-butyl group
was achieved by mild transesterification with TESOTf.[24]
PMB removal yielded acid 30 which had to be worked up
rapidly, and the crude product was directly oxidized at C5.
This protocol was essential because flash chromatography of
30 led to the formation of lactone 31. Finally, cleavage of the
acetonide moiety under mild acidic conditions provided
Angew. Chem. Int. Ed. 2012, 51, 1063 –1066
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
1065

.
Angewandte
Communications
1989, 28, 495 – 497; c) R. O. Duthaler, P. Herold, S. Wyler-Helfer,
M. Riediker, Helv. Chim. Acta 1990, 73, 659 – 673.
[11] Bromide 5 was already prepared by a different route: J. W. Bode,
E. M. Carreira, J. Org. Chem. 2001, 66, 6410 – 6424.
carolacton (1) in good yield when bulky isopropanol in water
was used as the solvent system. Otherwise, acetonide cleavage
was accompanied by methyl ester formation when methanol
1
was used. H and 13C NMR spectra of synthetically prepared
[12] Optical purities of alcohols S7 (precursor of 5), S16 (precursor of
8), and 25b were checked according to Mosher: J. A. Dale, D. L.
Dull, H. S. Mosher, J. Org. Chem. 1969, 34, 2543 – 2549.
[13] a) C. Studte, B. Breit, Angew. Chem. 2008, 120, 5531 – 5535;
Angew. Chem. Int. Ed. 2008, 47, 5451 – 5455; b) G. J. Brand, C.
Studte, B. Breit, Org. Lett. 2009, 11, 4668 – 4670.
[14] It is postulated that the tridentate pybox ligand 14 coordinates to
Ni thus suppressesing b-hydride elimination for which a vacant
coordination site would be required: a) M. R. Netherton, G. C.
Fu, Adv. Synth. Catal. 2004, 346, 1525 – 1532; b) A. Rudolph, M.
Lautens, Angew. Chem. 2009, 121, 2694 – 2708; Angew. Chem.
Int. Ed. 2009, 48, 2656 – 2670; c) V. B. Phapale, D. J. Cꢂrdenas,
Chem. Soc. Rev. 2009, 38, 1598 – 1607.
and natural carolacton (1) were identical; a mixture of both
showed only one set of signals.[15]
In conclusion, we described the total synthesis of the
highly potent biofilm inhibitor carolacton (1) in 4.3% overall
yield (22 steps in the longest linear sequence starting from
commercially available (S)-(À)-2-acetoxypropionic acid
(S1)). The synthesis is based on several metal-mediated key
transformations such as the Ley and Duthaler–Hafner aldol
reactions, the Marshall reaction, and Breitꢀs substitution as
well as the asymmetric Nozaki–Hiyama–Kishi and asymmet-
À
ric Negishi–Fu C C coupling reactions.
[15] Details are provided in the Supporting Information.
[16] Butane-2,3-diacetal (BDA) 7 was prepared from (S)-3-chloro-
propane-1,2-diol: S. V. Ley, E. Diez, D. J. Dixon, R. T. Guy, P.
Michel, G. L. Nattrass, T. D. Sheppard, Org. Biomol. Chem.
2004, 2, 3608 – 3617.
Received: September 23, 2011
Revised: October 22, 2011
Published online: December 9, 2011
[17] Dissertation of T. Schmidt will be published in 2012.
[18] For 1,2-,1,3-stereoinduction in aldol additions, see a) D. E.
Evans, J. L. Duffy, M. J. Dart, Tetrahedron Lett. 1994, 35,
8537 – 8540; b) D. J. Gustin, M. S. VanNieuwenhze, W. R.
Roush, Tetrahedron Lett. 1995, 36, 3443 – 3446; c) D. E. Evans,
M. J. Dart, J. L. Duffy, M. G. Yang, J. Am. Chem. Soc. 1995, 117,
6619 – 6620; d) D. E. Evans, M. J. Dart, J. L. Duffy, M. G. Yang,
J. Am. Chem. Soc. 1996, 118, 4322 – 4343; e) D. E. Evans, B. D.
Allison, M. G. Yang, C. E. Masse, J. Am. Chem. Soc. 2001, 123,
10840 – 10852.
[19] a) J. Schwartz, J. A. Labinger, Angew. Chem. 1976, 88, 402 – 409;
Angew. Chem. Int. Ed. Engl. 1976, 15, 333 – 340; b) D. W. Hart, J.
Schwartz, J. Am. Chem. Soc. 1974, 96, 8115 – 8116; c) T. Hu, J. S.
Panek, J. Am. Chem. Soc. 2002, 124, 11386 – 11378; d) A.
Fꢁrstner, E. Kattnig, O. Lepage, J. Am. Chem. Soc. 2006, 128,
9194 – 9204.
Keywords: biofilms · carolacton · Duthaler–
Hafner aldol reaction · Marshall reaction · Negishi–Fu reaction ·
total synthesis
.
[1] R. Jansen, H. Irschik, V. Huch, D. Schummer, H. Steinmetz, M.
Bock, T. Schmidt, A. Kirschning, R. Mꢁller, Eur. J. Org. Chem.
2010, 1284 – 1289.
[2] a) B. Kunze, I. Wagner-Dçbler, H. Irschik, H. Steinmetz (HZI),
WO 2009/030773A1, 2009; b) B. Kunze, M. Reck, A. Dçtsch, A.
Lemme, D. Schummer, H. Irschik, H. Steinmetz, I. Wagner-
Dçbler, BMC Microbiol. 2010, 10, 199.
[3] a) M. R. Parsek, P. K. Singh, Annu. Rev. Microbiol. 2003, 57,
677 – 701; b) R. M. Donlan, J. W. Costerton, Clin. Microbiol.
Rev. 2002, 15, 167 – 193.
[20] a) Y. Okude, S. Hirano, T. Hiyama, H. Nozaki, J. Am. Chem. Soc.
1977, 99, 3179 – 3181; b) H. Jin, J. Uenishi, W. J. Christ, Y. Kishi,
J. Am. Chem. Soc. 1986, 108, 5644 – 5646.
[4] a) L. Hall-Stoodley, J. W. Costerton, P. Stoodley, Nat. Rev.
Microbiol. 2004, 2, 95 – 107; b) P. S. Stewart, J. W. Costerton,
Lancet 2001, 358, 135 – 138.
[21] a) Z.-K. Wan, H.-W. Choi, F.-A. Kang, K. Nakajima, D. Demeke,
Y. Kishi, Org. Lett. 2002, 4, 4431 – 4433; b) H.-w. Choi, K.
Nakajima, D. Demeke, F.-A. Kang, H.-S. Jun, Z.-K. Wan, Y.
Kishi, Org. Lett. 2011, 13, 900 – 903; ligand 27 was prepared
according to the published procedure.
[22] Direct one-pot vinylation of aldehyde 24 by transmetalation of
the vinylzirconium intermediate of 4 using dimethyl zinc to give
the corresponding vinyl zinc analogue proceeded in only about
20% yield.
[5] D. J. Dixon, S. V. Ley, A. Polara, T. Sheppard, Org. Lett. 2001, 3,
3749 – 3753.
[6] a) J. Zhou, G. C. Fu, J. Am. Chem. Soc. 2003, 125, 14726 – 14727;
b) S. Son, G. C. Fu, J. Am. Chem. Soc. 2008, 130, 2756 – 2757.
[7] a) G. Gribble, H. H. Joyner, F. L. Switzer, Synth. Commun. 1992,
22, 2997 – 3002; b) R. S. Coleman, X. Lu, I. Modolo, J. Am.
Chem. Soc. 2007, 129, 3826 – 3827.
[8] The preparation of both mesylate 8 and aldehyde 9 is described
in the Supporting Information.
[23] I. Shiina, M. Kubota, H. Oshiumi, M. Hashizume, J. Org. Chem.
2004, 69, 1822 – 1830; Review: A. Parenty, X. Moreau, J.-M.
Champagne, Chem. Rev. 2006, 106, 911 – 939.
[24] C. B. Lee, Z. Wu, F. Zhang, M. D. Chappell, S. J. Stachel, T.-C.
Chou, Y. Guan, S. J. Danishefsky, J. Am. Chem. Soc. 2001, 123,
5249 – 5259.
[9] a) J. A. Marshall, C. M. Grant, J. Org. Chem. 1999, 64, 696 – 697;
b) J. A. Marshall, N. D. Adams, J. Org. Chem. 1999, 64, 5201 –
5204; c) J. A. Marshall, J. Org. Chem. 2007, 72, 8153 – 8166.
[10] a) M. Riediker, R. O. Duthaler, Angew. Chem. 1989, 101, 488 –
490; Angew. Chem. Int. Ed. Engl. 1989, 28, 494 – 495; b) R. O.
Duthaler, P. Herold, W. Lottenbach, K. Oertle, M. Riediker,
Angew. Chem. 1989, 101, 490 – 491; Angew. Chem. Int. Ed. Engl.
1066 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1063 –1066