Total synthesis of narbonolide and biotransformation to pikromycin

Article abstract of DOI:10.1021/jo062047u

An improved total synthesis of narbonolide and its biotransformation to pikromycin is reported. This total synthesis utilized an intramolecular Nozaki-Hiyama-Kishi coupling that significantly improved macrocyclization yields (90-96%) and allowed for diffe

Full text of DOI:10.1021/jo062047u

Nozaki-Hiyama-Kishi (NHK) macrocyclization to 14-mem-
bered macrolides.
Total Synthesis of Narbonolide and
Biotransformation to Pikromycin
The modular pikromycin (Pik) polyketide synthase (PKS)
system of S. Venezuelae ATCC 15439 catalyzes the biosynthesis
of the 12- and 14-membered macrolactones 10-deoxymethyno-
lide and narbonolide (1), respectively (Scheme 1).3,4 Upon
thioesterase (TE)-catalyzed macrolactonization to narbonolide,
it undergoes further modifications by the post-PKS tailoring
enzymes DesVII and PikC.3,4 DesVII catalyzes the C5-glyco-
sylation of narbonolide to narbomycin (2), which is followed
by C12-oxidation to pikromycin (3) by PikC. Because the Pik
PKS system biosynthesizes both 12- and 14-membered mac-
rolides, the inherent substrate tolerance of this system provides
a unique opportunity to study the mechanism and regulation of
the size of macrolactone ring formation and the potential of
Pik PKS and post-PKS tailoring enzymes as components of the
combinatorial biosynthesis toolbox.
Lakshmanan Venkatraman,† Christine E. Salomon,‡
David H. Sherman,§ and Robert A. Fecik*,†
Department of Medicinal Chemistry, 308 HarVard Street
Southeast, 8-101 WDH, UniVersity of Minnesota, Minneapolis,
Minnesota 55455-0353, Department of Microbiology, 420
Delaware Street Southeast, UniVersity of Minnesota,
Minneapolis, Minnesota 55455, and Departments of Medicinal
Chemistry, Microbiology & Immunology, and Chemistry, 210
Washtenaw AVenue, UniVersity of Michigan Life Sciences
Institute, Ann Arbor, Michigan 48109-2216
fecik001@umn.edu
To extend the studies by us and others of Pik PKS modules
and enzymes with unnatural substrates5-7 and mimics of the
natural chain elongation intermediates,8-15 we sought to develop
a convenient total synthesis of narbonolide that was readily
amenable to the synthesis of analogues for the study of DesVII
and PikC. We were intrigued by the possibility of utilizing a
mutant S. Venezuelae strain for the biotransformation of nar-
bonolide analogues to novel macrolides. In previous biotrans-
formation studies, narbonolide has been converted to narbo-
mycin16 and pikromycin17 by Streptomyces narbonensis. Mutant
strains of Streptomyces platensis18 and S. Venezuelae19 have been
used to convert narbonolide into pikromycin analogues with
modified sugars. We anticipated that a S. Venezuelae mutant
with a disrupted PKS system would provide a rapid method
to glycosylate and oxidize narbonolide analogues with its native
enzymes, DesVII and PikC, in ViVo. Total synthesis
ReceiVed October 3, 2006
An improved total synthesis of narbonolide and its biotrans-
formation to pikromycin is reported. This total synthesis
utilized an intramolecular Nozaki-Hiyama-Kishi coupling
that significantly improved macrocyclization yields (90-
96%) and allowed for differentiation of the C3- and C5-
oxidation states. A pikAI deletion mutant of Streptomyces
Venezuelae was used to biotransform synthetic narbonolide
to pikromycin by glycosylation and oxidation in ViVo. This
integration of synthetic chemistry and engineered biotrans-
formations holds great promise for the synthesis of novel
macrolide analogues of biological interest.
(3) Xue, Y.; Zhao, L.; Liu, H.-W.; Sherman, D. H. Proc. Natl. Acad.
Sci. U.S.A. 1998, 95, 12111-12116.
(4) Xue, Y.; Sherman, D. H. Met. Eng. 2001, 3, 15-26.
(5) Beck, B. J.; Aldrich, C. C.; Fecik, R. A.; Reynolds, K. A.; Sherman,
D. H. J. Am. Chem. Soc. 2003, 125, 4682-4683.
(6) Beck, B. J.; Aldrich, C. C.; Fecik, R. A.; Reynolds, K. A.; Sherman,
D. H. J. Am. Chem. Soc. 2003, 125, 12551-12557.
(7) Yin, Y.; Lu, H.; Khosla, C.; Cane, D. E. J. Am. Chem. Soc. 2003,
125, 5671-5676.
(8) Aldrich, C. C.; Beck, B. J.; Fecik, R. A.; Sherman, D. H. J. Am.
Chem. Soc. 2005, 127, 8441-8452.
Macrolide antibiotics are a clinically important class of
polyketide natural products of current interest for the treatment
of drug-resistant bacterial infections, intestinal motility disorders,
and inflammatory airway diseases.1 Recent developments in the
metabolic engineering of polyketide biosynthetic pathways have
opened new avenues for the production of novel macrolides.2
Herein, we report an improved total synthesis of narbonolide
and its biotransformation to the macrolide pikromycin using a
genetically engineered Streptomyces Venezuelae mutant. This
total synthesis significantly improved macrocyclization yields
and revealed the effects that protecting groups have on the
(9) Aldrich, C. C.; Venkatraman, L.; Sherman, D. H.; Fecik, R. A. J.
Am. Chem. Soc. 2005, 127, 8910-8911.
(10) Fecik, R. A.; Nguyen, P. L.; Venkatraman, L. Curr. Opin. Drug
Disc. DeV. 2006, 8, 741-747.
(11) He, W.; Wu, J.; Khosla, C.; Cane, D. E. Bioorg. Med. Chem. Lett.
2006, 16, 391-394.
(12) Wu, J.; He, W.; Khosla, C.; Cane, D. E. Angew. Chem., Int. Ed.
2005, 44, 7557-7560.
(13) Kao, C.-L.; Borisova, S. A.; Kim, H. J.; Liu, H.-w. J. Am. Chem.
Soc. 2006, 128, 5606-5607.
(14) Giraldes, J. W.; Akey, D. L.; Kittendorf, J. D.; Sherman, D. H.;
Smith, J. L.; Fecik, R. A. Nat. Chem. Biol. 2006, 2, 531-536.
(15) Akey, D. L.; Kittendorf, J. D.; Giraldes, J. W.; Fecik, R. A.;
Sherman, D. H.; Smith, J. L. Nat. Chem. Biol. 2006, 2, 537-542.
(16) Suzuki, M.; Hori, T.; Maezawa, I.; Nagahama, N. J. Chem. Soc.,
Chem. Commun. 1971, 304-305.
† Department of Medicinal Chemistry, University of Minnesota.
‡ Department of Microbiology, University of Minnesota.
§ Departments of Medicinal Chemistry, Microbiology & Immunology, and
Chemistry, University of Michigan.
(1) Katz, L.; Ashley, G. W. Chem. ReV. 2005, 105, 499-527.
(2) McDaniel, R.; Welch, M.; Hutchinson, C. R. Chem. ReV. 2005, 105,
543-558.
(17) Maezawa, I.; Hori, T.; Kinumaki, A.; Suzuki, M. J. Antibiot. 1973,
26, 771-775.
(18) Maezawa, I.; Kinumaki, A.; Suzuki, M. J. Antibiot. 1976, 29, 1203-
1208.
(19) Hong, J. S. J.; Park, S. H.; Choi, C. Y.; Sohng, J. K.; Yoon, Y. J.
FEMS Microbiol. Lett. 2004, 238, 391-399.
10.1021/jo062047u CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/22/2006
J. Org. Chem. 2006, 71, 9853-9856
9853

SCHEME 1. Post-PKS Tailoring of Narbonolide in S.
Wenezuelae ATCC 15439
SCHEME 2. Synthesis of the Linear C1-C15 Chain of
Narbonolide
coupled with enzymatic transformations provides a means to
gain access to pikromycin analogues that could not be synthe-
sized by natural product degradation.
Previous syntheses of narbonolide20 and pikronolide,21-23 the
pikromycin aglycone, have one or more complicating factors:
(1) a low yield of the key macrocyclization reaction, (2) the
inability to differentiate the C3- and C5-positions for chemose-
lective reactions, and (3) highly optimized protecting group
strategies that decrease synthetic efficiency. We recently
reported a formal total synthesis of narbonolide to evaluate the
use of an intramolecular NHK coupling for macrocyclization
of 14-membered macrolides.24 Although we were able to obtain
a modest macrocyclization yield (58%), the use of a C3, C5-
ketal protecting group prevented us from addressing the problem
of selective oxidation of the C3-alcohol. Classic studies on
erythromycin by Woodward have demonstrated the effects that
protecting groups at the C3- and C5-positions have on the
macrolactonization of linear intermediates.25 On the basis of
the protecting group effects observed in the pikronolide
syntheses,21-23 we expected that replacement of the C3, C5-
ketal would improve the yield of intramolecular NHK coupling
and allow for selective oxidation of the C3-alcohol.
SCHEME 3. Completion of the Total Synthesis of
Narbonolide
Synthesis of the entire linear C1-C15 chain of narbonolide
began with aldehyde 4 (Scheme 2). The Evans asymmetric aldol
reaction of aldehyde 415 furnished aldol adduct 5, which was
reduced to 1,3-diol 6. Protection of the 1,3-diol as its 4-meth-
oxybenzylidene ketal (7),26 followed by DIBAL-H reduction
gave the secondary alcohol, corresponding to the C5-position
of narbonolide, as PMB ether 8. Oxidation to aldehyde 9, an
Evans asymmetric aldol reaction to alcohol 10, and removal of
the chiral auxiliary afforded acid 11. Complete assembly of the
entire heptaketide chain of narbonolide was accomplished by
esterification with vinyl iodide 1226 to ester 13, followed by
deprotection to primary alcohol 14.
Treatment of alcohol 14 with the Dess-Martin periodinane
and pyridine oxidized both the primary and secondary alcohols
to aldehyde 15; however, the C2-methyl group had epimerized
(Scheme 3). We speculated that pyridine might be responsible
for the epimerization, but Dess-Martin oxidation without
pyridine also furnished aldehyde 15. We were optimistic that
the C2-methyl group could be equilibrated to the desired R
configuration after macrocyclization, so NHK macrocyclization
was performed to give allylic alcohol 16 in high yield.
Gratifyingly, Dess-Martin oxidation with pyridine afforded
ketone 17 with accompanying equilibration of the C2-methyl
group to a single diastereomer. Removal of the C5-protecting
group furnished narbonolide (1), thus confirming the R config-
uration of the C2-stereocenter.
Although the epimerized aldehyde 15 could be used to
complete the total synthesis of narbonolide, we sought to
develop an alternate route that avoided epimerization of the C2-
methyl group. Treatment of alcohol 14 with TEMPO and NaOCl
selectively oxidized the primary alcohol to aldehyde 18 (Scheme
4). Intramolecular NHK coupling afforded allylic alcohol 19,
and Dess-Martin oxidation of the C3- and C9-alcohols to
(20) Kaiho, T.; Masamune, S.; Toyoda, T. J. Org. Chem. 1982, 47,
1612-1614.
(21) Nakajima, N.; Hamada, T.; Tanaka, T.; Oikawa, Y.; Yonemitsu,
O. J. Am. Chem. Soc. 1986, 108, 4645-4647.
(22) Nakajima, N.; Tanaka, T.; Hamada, T.; Oikawa, Y.; Yonemitsu,
O. Chem. Pharm. Bull. 1987, 35, 2228-2237.
(23) Nakajima, N.; Ubukata, M.; Yonemitsu, O. Heterocycles 1997, 46,
105-110.
(24) Venkatraman, L.; Aldrich, C. C.; Sherman, D. H.; Fecik, R. A. J.
Org. Chem. 2005, 70, 7267-7272.
(25) Woodward, R. B.; Logusch, E.; Nambiar, K. P.; Sakan, K.; Ward,
D. E.; Au-Yeung, B.-W.; Balaram, P.; Browne, L. J.; Card, P. J.; Chen, C.
H.; Cheˆnevert, R. B.; Fliri, A.; Frobel, K.; Gais, H.-J.; Garratt, D. G.;
Hayakawa, K.; Heggie, W.; Hesson, D. P.; Hoppe, D.; Hoppe, I.; Hyatt, J.
A.; Ikeda, D.; Jacobi, P. A.; Kim, K. S.; Kobuke, Y.; Kojima, K.; Krowicki,
K.; Lee, V. J.; Leutert, T.; Malchenko, S.; Martens, J.; Matthews, R. S.;
Ong, B. S.; Press, J. B.; Rajan Babu, T. V.; Rousseau, G.; Sauter, H. M.;
Suzuki, M.; Tatsuta, K.; Tolbert, L. M.; Truesdale, E. A.; Uchida, I.; Ueda,
Y.; Uyehara, T.; Vasella, A. T.; Vladuchick, W. C.; Wade, P. A.; Williams,
R. M.; Wong, H. N.-C. J. Am. Chem. Soc. 1981, 103, 3213-3215.
(26) Pilli, R. A.; de Andrade, C. K. Z.; Souto, C. R. O.; de Meijere, A.
J. Org. Chem. 1998, 63, 7811-7819.
9854 J. Org. Chem., Vol. 71, No. 26, 2006

SCHEME 4. Alternate Route To Complete the Total
Synthesis of Narbonolide
FIGURE 1. HPLC trace of the EtOAc extract from the crude
fermentation broth after incubation of narbonolide (1) with the S.
Venezuelae pikAI deletion mutant BB138 showing complete conversion
to pikromycin (3) (blue), overlaid with the HPLC traces of authentic
pikromycin (red), control fermentation (pink), and extraction control
with narbonolide in media only (green).
ketone 17 proceeded with no evidence of C2-epimerization.
Deprotection of PMB ether 17 (Scheme 3) completed the
alternate route to narbonolide (1).
To test our hypothesis that a mutant S. Venezuelae strain
lacking a functional PKS can glycosylate and oxidize narbono-
lide, we elected to use a pikAI deletion mutant of S. Venezuelae
(BB138).27 The BB138 strain does not contain the gene encoding
PikAI (modules L, 1, and 2) in the Pik PKS and therefore lacks
the ability to biosynthesize narbonolide.27 S. Venezuelae BB138
was previously used in genetic complementation studies to
produce novel macrolides,27 but its value in exogenous feeding
experiments was unknown. Incubation of synthetic narbonolide
with BB138 resulted in exclusive and quantitative bioconversion
to pikromycin, as shown by high-performance liquid chroma-
tography (HPLC) (Figure 1). Analysis of the fermentation extract
by liquid chromatography-mass spectrometry (LC-MS) con-
firmed the exclusive metabolite of this bioconversion to be
pikromycin (3). A control fermentation of S. Venezuelae BB138
without narbonolide showed no production of narbonolide,
narbomycin, or pikromycin by HPLC, confirming that exog-
enous narbonolide is the sole source for pikromycin production
(Figure 1).
One of the challenges in the study of modular PKS systems
is access to advanced biosynthetic intermediates for the study
of enzymes that catalyze late-stage extension and processing,
macrolactonization, and post-PKS modifications. We expect this
total synthesis to be valuable in the study of the post-PKS
tailoring enzymes DesVII and PikC. Macrocyclization proceeds
in high yields (90%) and enables the differentiation of oxygen-
ation states at the C3- and C5-positions. Our results also reveal
important structural insights to the NHK macrocyclization of
macrolides. Woodward showed that macrolactonization of linear
erythronolide intermediates occurs only when the C3- and C5-
positions are protected as a cyclic ketal;25 Martin and co-workers
have found one exception to this trend.28 This work establishes
that having the C3- and C5-positions protected as a cyclic ketal
is detrimental to the NHK macrocyclization of linear narbonolide
intermediates.
fully functional enzymes necessary for the biosynthesis of
desosamine and the C5-glycosylation (DesVII) and C12-
oxidation (PikC) of narbonolide.
In conclusion, we have completed a second-generation total
synthesis of narbonolide that markedly improves upon previous
syntheses and demonstrated that a pikAI deletion mutant of S.
Venezuelae can bioconvert narbonolide to pikromycin. This
synthetic route is readily amenable to the synthesis of narbono-
lide analogues and will enable us to examine the substrate
specificity of DesVII and PikC and the utility of S. Venezuelae
BB138 to produce pikromycin analogues. We expect this
integration of synthetic chemistry and engineered biotransfor-
mations to be of great value in the synthesis of novel macrolides
with biological activity.
Experimental Section
(R)-3-((2R,3S,4S,5S,6S,8R)-3-Hydroxy-5-(4-methoxybenzyloxy)-
2,4,6,8-tetramethyl-9-(triisopropylsilyloxy)nonanoyl)-4-benzy-
loxazolidin-2-one (10). To a solution of R-4-benzyl-3-propionyl-
2-oxazolidinone (0.933 g, 4.0 mmol, 1.08 equiv) in CH2Cl2 at -78
°C was added freshly distilled Bu2BOTf (1.22 mL, 4.88 mmol, 1.32
equiv). After 15 min, distilled i-Pr2NEt (0.94 mL, 5.29 mmol, 1.43
equiv) was added, and the temperature was raised to 0 °C; the
mixture was stirred for 1 h. The mixture was cooled back to -78
°C, and a solution of aldehyde 9 (1.72 g, 3.7 mmol, 1.0 equiv) in
CH2Cl2 (5 mL) was added. The reaction mixture was stirred at -78
°C for 2 h, followed by stirring at -20 °C for 2 h. The reaction
was quenched by the addition of pH 7 phosphate buffer (1 M, 5
mL) and MeOH (12 mL). The mixture was treated with a solution
of 30% H2O2 (4.5 mL) and MeOH (8 mL) and extracted with Et2O
(5 × 15 mL). Purification by flash chromatography (25% EtOAc/
hexanes) afforded the title compound (1.96 g, 76% yield) as a
23
colorless oil: Rf ) 0.45 (20% EtOAc/hexanes); [R]D ) -21.44
(c ) 0.49, CH2Cl2); 1H NMR (CDCl3, 300 MHz) δ 7.18-7.35 (m,
7H), 6.84 (d, J ) 8.4 Hz, 2H), 4.59-4.67 (m, 2H), 4.38 (d, J )
10.8 Hz, 1H), 4.11-4.19 (m, 2H), 3.98 (dd, J ) 4.5, 6.6 Hz, 1H),
3.83-3.88 (m, 1H), 3.78 (s, 3H), 3.61 (dd, J ) 4.8, 9.6 Hz, 1H),
3.45 (dd, J ) 6.6, 9.6 Hz, 1H), 3.31 (dd, J ) 2.7, 6.6 Hz, 1H),
3.21 (dd, J ) 2.7, 13.5 Hz, 1H), 2.75 (dd, J ) 9.6, 13.5 Hz, 1H),
1.59-1.82 (m, 3H), 1.17-1.30 (ovlp, 4H), 1.03-1.15 (ovlp, 22H),
0.99 (d, J ) 6.9 Hz, 3H), 0.98 (d, J ) 6.6 Hz, 3H), 0.96-0.94
(ovlp, 4H); 13C NMR (CDCl3, 75 MHz) δ 176.7, 159.1, 152.8,
135.3, 131.0, 129.6, 129.3, 129.1, 127.6, 113.9, 86.8, 75.3, 73.3,
68.4, 66.3, 55.5, 55.3, 40.9, 38.0, 37.5, 37.4, 33.8, 33.5, 19.1, 18.5,
17.0, 13.9, 12.4, 9.1. HRMS calcd for (C40H63NO7Si + Na+):
720.4271, found 720.4288.
Bioconversion of synthetic narbonolide to pikromycin by a
pikAI deletion mutant of S. Venezuelae also has important
implications for the use of engineered bacteria to transform
natural products and their analogues. Despite the lack of a
functional Pik PKS system, S. Venezuelae BB138 retains the
(27) Yoon, Y. J.; Beck, B. J.; Kim, B. S.; Kang, H.-Y.; Reynolds, K.
A.; Sherman, D. H. Chem. Biol. 2002, 9, 203-214.
(28) Hergenrother, P. J.; Hodgson, A.; Judd, A. S.; Lee, W.-C.; Martin,
S. F. Angew. Chem., Int. Ed. 2003, 42, 3278-3281.
J. Org. Chem, Vol. 71, No. 26, 2006 9855

Narbonolide (1). To a solution of the PMB ether 17 (39.0 mg,
0.0826 mmol) in CH2Cl2 (2 mL) and H2O (0.21 mL, 11.57 mmol,
140 equiv) at 0 °C was added DDQ (37.0 mg, 0.1652 mmol, 2.0
equiv). The reaction was stirred for 1.5 h and quenched by the
addition of saturated aqueous NaHCO3 (3 mL). The mixture was
diluted with CH2Cl2 (10 mL), and the layers were separated. The
aqueous layer was extracted with CH2Cl2 (3 × 10 mL), and the
combined organic layers were dried (Na2SO4), filtered off, and
concentrated under reduced pressure. Purification by flash chro-
matography (40% EtOAc/hexanes) followed by HPLC purification
(C18, 40-80% MeCN/0.1% aqueous TFA) afforded the title
compound (24.7 mg, 85% yield) as a solid: Rf ) 0.47 (40% EtOAc/
hexanes); [R]D23 ) +66.15 (c ) 1.24, CH2Cl2); 1H NMR (CDCl3,
300 MHz) δ 6.89 (dd, J ) 5.1, 16.5 Hz, 1H), 6.08 (dd, J ) 16.5,
1.8 Hz, 1H), 5.10-5.15 (m, 1H), 3.83-3.86 (m, 1H), 3.70 (q, J )
6.9 Hz, 1H), 2.98-3.05 (m, 1H), 2.57-2.72 (m, 1H), 2.21 (br s,
1H), 1.38-1.76 (ovlp, 6H), 1.34 (d, J ) 6.9 Hz, 3H), 1.13 (d, J )
6.6 Hz, 3H), 1.12 (d, J ) 6.9 Hz, 3H), 1.08 (d, J ) 6.6 Hz, 3H),
0.92 (d, J ) 6.6 Hz, 3H), 0.91 (t, J ) 7.2 Hz, 3H); 13C NMR
(CDCl3, 75 MHz) δ 207.7, 205.1, 171.1, 148.8, 129.0, 76.9, 73.0,
50.6, 40.1, 39.2, 36.7, 35.4, 24.6, 19.0, 18.7, 14.8, 11.2, 10.8. HRMS
calcd for (C20H32O5 + Na+): 375.2147, found 375.2154.
3H), 1.22 (d, J ) 6.9 Hz, 3H), 1.01-1.04 (ovlp, 6H), 0.96 (d, J )
7.2 Hz, 3H), 0.88 (t, J ) 7.2 Hz, 3H); 13C NMR (CDCl3, 75 MHz)
δ 175.8, 159.5, 135.1, 130.1, 129.9, 129.7, 114.2, 84.3, 78.7, 77.8,
76.7, 70.3, 55.6, 44.0, 38.8, 36.3, 35.9, 32.4, 29.2, 26.7, 17.4, 16.7,
15.8, 11.0, 8.1. HRMS calcd for (C28H44O6 + Na+): 499.3036,
found 499.3039.
Biotransformation of Narbonolide (1) to Pikromycin (3). A
concentrated spore stock (10 µL) of the Streptomyces Venezuelae
pikAI deletion mutant BB13827 was used to inoculate 50 mL of
ATCC172 media (10 g of glucose, 20 g of soluble starch, 5 g of
yeast extract, 5 g of NZ amine type A, and 1 g of CaCO3 per 1 L
of H2O) in a 250 mL baffled flask and incubated at 30 °C for 2
days with shaking. This seed culture (5 mL) was used to inoculate
fresh ATCC172 media (50 mL) containing narbonolide (1, 4 mg
in 400 µL EtOH) or EtOH (400 µL) and incubated at 30 °C for 3
days with shaking. Sterile media (50 mL) containing narbonolide
(1, 0.5 mg) was also incubated under the same conditions as an
extraction control. Samples were centrifuged and the pH of the
decanted supernatant was adjusted to 9.5 with 1 M aqueous NaOH.
The supernatant was extracted with EtOAc (2 × 50 mL), and the
combined organic extracts were dried (Na2SO4) and concentrated
under reduced pressure. The crude extracts were analyzed by HPLC
(C18, 40-80% MeCN/0.1% TFA) and LC-MS (50-100% MeCN/
57 mM ammonium acetate, ESI+). MS (ESI+): 524 (M - H+).
Pikromycin was purified from the crude fermentation extract by
isolation of the corresponding HPLC peak (10.4 min) and was
analyzed by MS. MS (ESI-): 548 (M+ + Na), 526 (M+ + H+).
(3S,9R/S)-3,9-Tetrahydro-5-(4-methoxybenzyloxy)narbono-
lide (19). To a solution of aldehyde 18 (28.0 mg, 0.0466 mmol) in
DMSO (19 mL) at room temperature was added CrCl2 (57.2 mg,
0.4660 mmol, 10 equiv) and NiCl2 (0.6 mg, 0.0047 mmol, 0.1
equiv). The reaction was stirred for 12 h, then quenched by the
addition of H2O (10 mL). The mixture was diluted with EtOAc
(200 mL), and the layers were separated. The organic layer was
washed with H2O (3 × 20 mL) and saturated aqueous NaCl (20
mL), dried (Na2SO4), filtered off, and concentrated under reduced
pressure. Purification by flash chromatography (40% EtOAc/
hexanes) afforded the title compound (20.0 mg, 90% yield) as a
Acknowledgment. This research was supported by grants
FRD 01-13 from the Academic Health Center, University of
Minnesota (to R.A.F. and D.H.S.), NSF 0118926 (to D.H.S.),
and the Minnesota Craniofacial Research Training Program,
NIDCR 5T32-DE007288-10 (fellowship to C.E.S.).
1
moist solid: Rf ) 0.57 (40% EtOAc/hexanes); H NMR (CDCl3,
Supporting Information Available: General experimental
procedures, experimental procedures for compounds 5-9, 11, 13-
18, 1H and 13C NMR spectra for compounds 1, 5-11, and 13-19,
and LC-MS and MS (ESI-) spectra for compound 3. This material
is available free of charge via the Internet at http://pubs.acs.org.
300 MHz) δ 7.25 (d, J ) 8.4 Hz, 2H), 6.87 (d, J ) 8.4 Hz, 2H),
5.74 (dd, J ) 3.0, 15.9 Hz, 1H), 5.52 (dd, J ) 6.6, 15.9 Hz, 1H),
5.06 (dd, J ) 4.5, 9.6 Hz, 1H), 4.43 (AB system, J ) 10.8 Hz, νAB
95.4, 2H), 4.09 (d, J ) 6.6 Hz, 1H), 3.80 (s, 3H), 3.68-3.72 (m
2H), 3.56-3.60 (m, 1H), 2.59 (pent, J ) 6.6 Hz, 1H), 2.16-2.46
(m, 3H), 1.64-1.74 (m, 1H), 1.39-1.58 (ovlp, 5H), 1.25-1.32 (m,
JO062047U
9856 J. Org. Chem., Vol. 71, No. 26, 2006