Absolute configurations of melanoxadin, MR-93A, melanoxazal, and MR-93B

Article abstract of DOI:10.1002/ejoc.201403287

Fungal metabolites melanoxadin, MR-93A, melanoxazal, and MR-93B were synthesized with the key stereogenic centers derived from commercially available chiral building blocks. The optically active synthetic products with well-defined absolute configurations provided authentic samples for the stereoisomers of these oxazole-containing natural products and thus allowed for unambiguous assignments of their relative and absolute configurations. The large discrepancies in the optical rotations between the natural and the pure synthetic samples are discussed. Some errors in the previously reported NMR signal assignments are also corrected.

Full text of DOI:10.1002/ejoc.201403287

FULL PAPER
DOI: 10.1002/ejoc.201403287
Absolute Configurations of Melanoxadin, MR-93A, Melanoxazal,
and MR-93B
Tian-Jun He,[a,b] Shijun Zhu,[b] Xiao-Wei Lu,[b] Yikang Wu,*[b] and Yan Li*[a]
Keywords: Natural products / Chiral pool / Configuration determination / Oxazoles / Diols
Fungal metabolites melanoxadin, MR-93A, melanoxazal, and
MR-93B were synthesized with the key stereogenic centers
derived from commercially available chiral building blocks.
The optically active synthetic products with well-defined ab-
solute configurations provided authentic samples for the
stereoisomers of these oxazole-containing natural products
and thus allowed for unambiguous assignments of their rela-
tive and absolute configurations. The large discrepancies in
the optical rotations between the natural and the pure syn-
thetic samples are discussed. Some errors in the previously
reported NMR signal assignments are also corrected.
Introduction
Melanoxadin (1; Figure 1), a melanin biosynthesis inhib-
itor, was isolated from the fungus Trichoderma sp. ATF-606
by Nakagawa.[1] Its planar structure was assigned on the
basis of extensive spectroscopic analyses. Interestingly, the
same planar structure was also assigned to MR-93A[2] (2;
isolated from Trichoderma harzianum KCTC 0114BP) by
Kho in 1995. However, neither relative nor absolute config-
urations were determined in either case. The same status is
also fund with melanoxazal[3] (also a melanin biosynthesis
inhibitor) and MR-93B,[4] another two closely related natu-
ral oxazoles. The identities for these compounds are there-
fore not completely established.
Given that nearly twenty years has elapsed by now, it
seems unlikely that the incomplete structural assignments
for these natural oxazoles made in the mid-1990s will be
followed up by the original investigators. It thus appears
warranted to clarify the unavoidable stereochemical ambi-
guity and to establish the full identities for these natural
products. To this end, chemical synthesis of optically active
authentic samples of the enantiomers would serve as stereo-
chemical references for the configuration determination.
Figure 1. The planar structures for melanoxadin, melanoxazal,
MR-93A, and MR-93B along with the numbering as reported in
the literature.
Results and Discussion
Melanoxadin and MR-93A showed nearly identical
NMR spectra but substantially different optical rotations;
therefore, they seemed likely to be diastereomers. It was
therefore desirable to adopt a synthetic route that would
allow for access to both the syn- and anti-diols in the end.
With both isomers in hand, it would be possible to unam-
biguously assign the relative configurations for both struc-
tures by NMR spectroscopy. Comparison of the signs for
the optical rotations would reveal the absolute configura-
tions of the natural oxazoles.
[a] Hubei Collaborative Innovation Center for Advanced Organic
Chemical Materials and Ministry-of-Education Key
Laboratory for Synthesis and Application of Organic
Functional Molecules, Hubei University,
Wuhan 430062, P. R. China
Our synthesis of the diols started with protection[5] of the
commercially available ethyl (S)-2-hydroxy-propionate to
give the known[5] enantiopure ester 5, which, on treatment
with the carbanion derived from MeP(O)(OMe)2, afforded
the desired phosphate 6[6] (Scheme 1). Deprotonation of 6
with nBuLi followed by condensation with aldehyde 8[7]
[b] State Key Laboratory of Bioorganic and Natural Products
Chemistry, Shanghai Institute of Organic Chemistry, Chinese
Academy of Sciences,
345 Lingling Road, Shanghai 200032, P. R. China
E-mail: yikangwu@sioc.ac.cn
http://www.sioc.ac.cn
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201403287.
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Y. Wu, Y. Li et al.
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(prepared from commercially available ester 7) afforded relative configuration for 1 was thus secured. Finally, the
ketone 9.
optical rotation for 12 ([α]D = –20.1) measured under com-
parable conditions showed that the absolute configuration
for the natural melanoxadin ([α]D = –20.0) must be the same
as that for 12.
It is interesting to note that when reduction of 9 was
carried out under conditions developed by Luche,[9] the 12Ј/
13Ј ratio increased to 13:1, providing a much improved ster-
eoselective access to natural melanoxadin (1).
The assignment of the configurations for MR-93A (2)
was less straightforward. Contrary to previously reported
data, which indicated that 1 and 2 have different relative
configurations, neither the optical rotation ([α]D = +17.3 for
1
13 vs. –6.0 for 2) nor the H (δ = 4.29 and 3.97 ppm for 13
vs. 4.04 and 3.78 ppm for 2) and 13C NMR (δ = 129.9, 75.7
and 17.7 ppm for 13 vs. 131.2, 76.5 and 19.0 ppm for 2)
spectra for the anti diol 13 were compatible with those for
MR-93A. Compared with the undeniable discrepancies be-
tween the NMR data for 13 and 2, the discrepancies be-
tween the NMR data for 1 and 2 are almost negligible (cf.
Tables S1 and S2 in the Supporting Information). There-
fore, the possibility for MR-93A being an anti diol can thus
be excluded with confidence; MR-93A must have exactly
the same configurations as melanoxadin,[10] although a sat-
isfactory explanation for the substantially smaller absolute
value of the optical rotation for 2 than that for 1 remains
to be found.
The synthesis of aldehyde 3 was first attempted as shown
in Scheme 2. Thus, Knoevenagel condensation of 8 with
acetoacetate 14 gave expected 15 and its (Z)-isomer 16 in
58 and 38% yield, respectively.[11] The isomers (slowly inter-
changeable in CDCl3 over time) were separated on silica
gel. The isolated (E)-isomer 15 was subjected to the CBS[12]
Scheme 1. Reagents and conditions: (a) i. TBSCl, imidazole,
CH2Cl2; ii. MeP(O)(OMe)2, nBuLi, THF, –78 °C; (b) DIBAL-H,
CH2Cl2, –78 °C, 71%; (c) tBuOK, THF, 0 °C, 55 min, 75%; (d) DI-
BAL-H, CH2Cl2, –78 °C, 100% (crude, 10/11 = 2.6:1), or NaBH4,
CeCl3, MeOH, 0 °C, 30 min, 93% (10/11 = 13:1); (e) nBu4NF,
THF; (f) Me2C(OMe)2, pTsOH, 63% for 12Ј, 18% for 13Ј overall
from 9, respectively (when using DIBAL-H for the reduction);
(g) 1 n HCl, THF, 84% for 12, 83% for 13.
Subsequent reduction of ketone 9 with diisobutylalumi- (Corey–Bakshi–Shibata) conditions in the presence of (R)-
num hydride (DIBAL-H) at –78 °C led to the correspond- 2-methyloxazaborolidine, which was expected to give the
ing alcohol as a 2.6:1 (10/11) inseparable mixture of two (S)-isomer 18 as the major product. Unfortunately, in this
diastereomers. At the beginning of this study the relative case, the stereoselectivity was far less than anticipated. The
configurations of the target compounds 1 and 2 were un-
known; therefore, it was desirable to obtain both the syn
and anti diols. Thus, the otherwise not so impressive dia-
stereoselectivity, in this case, became an advantage because
it allowed access to both 12 and 13 in a single endeavor. To
elaborate the inseparable mixture into the desired cleanly
separated diastereomers, the tert-butyldimethylsilyl (TBS)
protecting group in 10 and 11 was removed with nBu4NF
and the resulting inseparable mixture of diols 12 and 13
was treated with Me2C(OMe)2 to give the corresponding
acetonides 12Ј and 13Ј, which were separated by column
chromatography on silica gel. The relative configurations of
the separated diastereomers 12Ј and 13Ј were then unam-
biguously established based on NOE experiments.[8]
The chromatographically separated acetonides 12Ј and
13Ј were then hydrolyzed with 1 n HCl to provide the syn
and anti diols 12 and 13, respectively. The 1H and 13C
Scheme 2. Reagents and conditions: (a) piperidine, AcOH, CH2Cl2,
room temp., overnight, 58% for 15, 38% for 16; (b) (R)-2-methyl-
NMR spectra of both diols were taken. The results unam-
biguously showed that the data for the syn diol 12 were fully
consistent with those reported for the natural melanoxadin
–78 °C, 30 min; iii. Dess–Martin periodinane, NaHCO3, CH2Cl2,
(1), whereas those for the anti-isomer 13 were not; the syn 15 min; iv. nBu4NF, THF, 30 min, 55% overall from 17 and 18.
CBS-oxazaborolidine, Me2S·BH3, THF, –78 °C to –40 °C, 10 h,
72%; (c) i. TBSCl, imidazole, CH2Cl2, 4 h; ii. DIBAL-H, CH2Cl2,
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Absolute Configurations of Fungal Metabolites
isolated product turned out to be a 2:1 mixture of the two
Reduction of aldehyde 19 (i.e., the natural melanoxazal)
isomers (20/19, with the absolute configurations also con- with NaBH4 in EtOH afforded the corresponding (R)-diol
firmed later by the results given below) as measured by chi- 27 (Scheme 4). Interestingly, this compound did not share
ral HPLC analysis of the end products.
the same sign for the optical rotation as the natural MR-
Separation of such enantiomers was clearly not a readily 93B ([α]2D7 = +62.6 for 27 vs. –4.0 for 4), although the H
executable task; therefore, we turned to another approach and 13C NMR were fully consistent.
relying on separation of diastereomers to gain relative facile
access to a single enantiomer of a predefined absolute con-
figuration. To this end, (R)-3-hydroxybutanoic acid[13] (de-
rived from commercially available methyl ester) was con-
verted into 22 by condensation with 2,2-dimethylpropanal
in the presence of pyridinium p-tosylate (PPTS) under azeo-
tropic conditions.[14] The resulting lactone-acetal was iso-
lated as a 9:1 mixture of the diastereomers (based on HPLC
analysis as previously reported,[14] but only “5:1” if based
on 1H NMR, presumably because of insufficient relax-
ation).
1
The 9:1 diastereomeric mixture of 22 isolated by column
chromatography was then further purified by recrystalli-
zation as described by Seebach[14] to give pure 22. Conver-
sion of 22 into the corresponding carbanion by treatment
with LiN(SiMe3)2 (LiHMDS) followed by reaction with al-
dehyde 8 furnished aldols 23 and 24 (along with only traces
of other diastereomers), the absolute configurations of
which were later established by single-crystal X-ray crystal-
lography.[15]
Subjecting either 23 or 24 to the activation-elimination
conditions led to a mixture of 25 (88%) and 26 (in compar-
able ratios and yields). Reduction of the isolated major
product 25 with DIBAL-H provided the end product 19,
1
which showed H and 13C NMR signals as well as an op-
tical rotation value that were fully consistent with those re-
ported for the natural melanoxazal 2 (Scheme 3).
Scheme 4. Reagents and conditions: (a) NaBH4, EtOH, 0 °C, 1 h,
97% for 27, 98% for ent-27; (b) PPTS, benzene, 110 °C, 85% (9:1
mixture), 34% after recrystallization (pure ent-22); (c) LiN(SiMe3)
2, THF, –78 °C, 1 h, 89% (ent-23/ent-24 = 3:1); (d) MsCl, Et3N,
CH2Cl2, 0 °C to room temp. overnight, then Et3N, 60 °C, 6 h, 78%
(from ent-23) for ent-25, along with 15% of mixture (ent-25 and
ent-26); (e) DIBAL-H, CH2Cl2, –78 °C, 15 min, 83%; (f) Ac2O,
Et3N, DMAP, CH2Cl2, 95%; (g) K2CO3, MeOH, room temp.,
15 min, 97%.
We then utilized the antipode of 22 (i.e., ent-22) as the
starting material and went through the same route to obtain
ent-19. Subsequent treatment with NaBH4 in EtOH finally
afforded the (S)-diol ent-27, which gave an optical rotation
of the same absolute value but opposite sign compared with
27. Compound ent-27 shared the same sign for the optical
rotation with the natural MR-93B (4).
To explain the large discrepancy between the absolute
values for the optical rotations for 4 and ent-27, and the
aforementioned discrepancy between corresponding values
for 1 (12) and 2, we also measured the purity of our samples
Scheme 3. Reagents and conditions: (a) LiN(SiMe3)2, THF, –78 °C,
by HPLC, which showed that the purity for 12 and ent-27
1 h, 89% (23/24 = 3:1); (b) MsCl, Et3N, DMAP, CH2Cl2, 0 °C to
were Ͼ 98.9 and Ͼ 99.9%, respectively. These values, to-
room temp. overnight, then Et3N, 60 °C, 6 h, 88% for 25, 4% for
1
gether with their H and 13C NMR spectra, excluded the
26; (c) DIBAL-H, CH2Cl2, –78 °C, 15 min, 83%; MsCl = meth-
anesulfonyl chloride, DMAP = 4-(dimethylamino)pyridine.
contamination of 12 and ent-27 by impurities with very
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Y. Wu, Y. Li et al.
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nBuLi (2.5 m, in hexanes, 4.0 mL, 10 mmol) was added to a solu-
tion of MeP(O)(OMe)2 (1.24 g, 10.00 mmol) in anhydrous THF
(5 mL) stirred under argon (balloon) in a –78 °C dry ice-acetone
bath. After completion of the addition, stirring was continued at
the same temperature for 1 h before a solution of crude 5 (1.16 g,
5.00 mmol) in anhydrous THF (5 mL) was introduced. The re-
sulting mixture was stirred at –78 °C for 1 h, when TLC showed
completion of the reaction. Aqueous saturated NH4Cl was added
and the mixture was extracted with EtOAc (3ϫ 25 mL). The com-
bined organic layers were washed with water and brine before being
dried with anhydrous Na2SO4. Removal of the solvent by rotary
evaporation gave crude 6 (2.02 g, ca. 100%) as a colorless oil, which
was used in the subsequent reaction without further purification.
large optical rotations. The optical rotation data acquired
in this work thus reliably support the conclusion that com-
pounds 12, 27, and ent-27, have the structures depicted.[16]
Conclusions
In efforts to confirm the structures and establish the ab-
solute configurations of the natural products melanoxadin,
MR-93A, melanoxazal, and MR-93B, the proposed struc-
tures were synthesized in enantiopure forms through chiral
pool based routes. The absolute configurations for the natu-
ral melanoxadin (1) and melanoxazal (3) were unambigu-
ously shown to be identical to 12 [with a syn (S,S) configu-
ration] and 19 [with an (R) configuration], respectively.
With the aid of the optically active synthetic syn and anti
diols, the possibility for MR-93A being an anti dia-
stereomer was definitely excluded, leaving the only possibil-
ity of being the same compound as melanoxadin. Similarly,
Given that no data for 6 could be found in the literature, a small
sample of crude 6 was purified by chromatography on silica gel
(PE/EtOAc, 1:1) to give the following: [α]2D3 = +8.9 (c = 1.0,
CHCl3). 1H NMR (500 MHz, CDCl3): δ = 4.27–4.23 (q, J =
6.8 Hz, 1 H), 3.80 (d, JP,H = 11.3 Hz, 3 H), 3.79 (d, JP,H = 11.4 Hz,
3 H), 3.35 (dd, JP,H = 21.1, JH,H = 14.7 Hz, 1 H), 3.26 (dd, JP,H
=
21.1, JH,H = 14.7 Hz, 1 H), 1.32 (d, J = 6.7 Hz, 3 H), 0.92 (s, 9 H),
the planar structure and absolute configuration for MR- 0.11 (d, J = 4.3 Hz, 6 H) ppm. 13C NMR (125 Hz, CDCl3): δ =
205.2 (d, JP,C = 6.8 Hz), 74.8 (d, JP,C = 3.2 Hz), 52.96 (d, JP,C
6.3 Hz), 52.89 (d, JP,C = 6.3 Hz), 35.22 (d, JP,C = 134 Hz), 25.7,
=
93B were also secured. Although it is still not clear whether
the natural MR-93A and MR-93B were partially racemized
in the first place or during the subsequent purification (or
some unknown factors led to the erroneous optical rota-
tions), the previously reported optical rotations should be
disconnected with any pure enantiomers of these two natu-
ral products. The previous assignments for the C-1Ј/H-1Ј
and C-2Ј/H-2Ј made for MR-93A should also be inverted.
20.2, 18.0, –4.6, –5.0 ppm. FTIR (film): ν = 3486, 2955, 2931, 2896,
˜
2857, 1723, 1472, 1463, 1390, 1258, 1033, 938, 837, 810, 799, 669,
500 cm–1. MS (ESI): m/z = 311.2 [M + H]+, 333.2 [M + Na]+.
HRMS (ESI): m/z calcd. for C12H28O5PSi [M + H]+ 311.1438;
found 311.1441, C12H28NaO5PSi [M + Na]+ 333.1261; found
333.1258.
Condensation of Phosphonate 6 with Aldehyde 8 to Afford Enone 9:
A solution of crude 6 (414.4 mg, 1.34 mmol) in anhydrous THF
(2 mL) was added to a solution of tBuOK (149.8 mg, 1.34 mmol)
in anhydrous THF (4 mL) stirred in an ice-water bath. Stirring was
continued for 15 min at the same temperature before a solution of
aldehyde 8 (86.6 mg, 0.89 mmol) in anhydrous THF (1 mL) was
added. The mixture was stirred in an ice-water bath for 40 min,
when TLC showed completion of the reaction. Aqueous saturated
NH4Cl was added and the mixture was extracted with EtOAc (2ϫ
15 mL). The combined organic layers were washed with water and
brine before being dried with anhydrous Na2SO4. Removal of the
solvent by rotary evaporation and column chromatography (PE/
EtOAc, 4:1) on silica gel gave enone 9 as a colorless oil (187.1 mg,
0.66 mmol, 75%). [α]2D5 = –33.9 (c = 1.0, CHCl3). 1H NMR
(500 MHz, CDCl3): δ = 7.91 (s, 1 H), 7.86 (s, 1 H), 7.55 (d, J =
15.6 Hz, 1 H), 7.41 (d, J = 15.6 Hz, 1 H), 4.36–4.32 (q, J = 6.8 Hz,
1 H), 1.36 (d, J = 6.9 Hz, 3 H), 0.93 (s, 9 H), 0.09 (s, 6 H) ppm.
13C NMR (125 Hz, CDCl3): δ = 201.8, 151.6, 139.9, 137.4, 130.7,
Experimental Section
General Methods: NMR spectra were recorded with either a Bruker
Avance 400 NMR operating at 400 MHz, for 1H or an Agilent 500/
54 NMR spectrometer operating at 500 MHz for 1H. IR spectra
were measured with a Nicolet 380 Infrared spectrophotometer. UV/
Vis spectra were recorded with a UV-4802 UV/Vis spectrophotome-
ter [UNICO (Shanghai) Instruments Co., Ltd.]. EI-MS data were
acquired with an Agilent Technologies 5973N spectrometer.
HRMS (EI) data were recorded with a Waters Micromass GCT
Premier instrument. ESI-MS data were acquired with a Shimadzu
LCMS-2010EV mass spectrometer. HRMS (ESI) data were ob-
tained with a Bruker APEXIII 7.0 Tesla FT-MS spectrometer. Op-
tical rotations were measured with a Jasco P-1030 polarimeter.
Melting points are uncorrected (measured on a hot-stage melting-
point apparatus equipped with a microscope). Anhydrous THF was
obtained by distillation over Na/Ph2CO under argon prior to use.
Anhydrous toluene and CH2Cl2 were acquired by drying over acti-
vated 4 Å MS (molecular sieves). All reagents were of reagent grade
and used as purchased. Column chromatography was performed on
silica gel (300–400 mesh) under slightly positive pressure. Petroleum
ether (PE; b.p. 60–90 °C) was used as chromatography solvent.
122.7, 74.7, 25.8, 21.0, 18.2, –4.8, –4.9 ppm. FTIR (film): ν = 3135,
˜
2956, 2930, 2887, 2858, 1692, 1623, 1259, 1120, 1094, 1063, 940,
889, 833, 778, 610 cm–1. MS (ESI): m/z = 282.3 [M + H]+, 304.2
[M + Na]+. HRMS (ESI): m/z calcd. for C14H24NO3Si [M + H]+
282.1520; found 282.1520, C14H23NNaO3Si [M + Na]+ 304.1339;
found 304.1341.
Conversion of Ethyl (S)-Propionate into Phosphonate 6: A solution
of ethyl (S)-propionate (1.60 g, 13.55 mmol), imidazole (1.38 g,
20.33 mmol) and TBSCl (2.65 g, 17.62 mmol) in anhydrous CH2Cl2
(27 mL) was stirred at ambient temperature for 6 h, when TLC
showed completion of the reaction. Water was added and the mix-
ture was extracted with EtOAc (2ϫ 30 mL). The combined organic
layers were washed with water and brine before being dried with
anhydrous Na2SO4. Removal of the solvent by rotary evaporation
left a colorless oil (crude TBS protected 5, 3.61 g, ca. 100%).
Conversion of Enone 9 into Acetonide 12Ј and 13Ј via 10/11 and 12/
13: DIBAL-H (1.0 m in cyclohexane, 0.5 mL, 0.5 mmol) was added
to a solution of enone 9 (95.7 mg, 0.34 mmol) in anhydrous CH2Cl2
(3.4 mL) stirred at –78 °C under argon (balloon). The mixture was
stirred at the same temperature for 30 min, when TLC showed com-
pletion of the reduction. The excess hydride was destroyed by care-
ful addition of MeOH before aqueous saturated sodium potassium
tartrate was introduced (ca. 1 mL). The mixture was stirred at am-
bient temperature for 1.5 h and then extracted with EtOAc (3ϫ
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Absolute Configurations of Fungal Metabolites
5 mL). The combined organic layers were washed with water and
brine, and dried with anhydrous Na2SO4. Removal of the solvent
by rotary evaporation left the crude mixture of 10 and 11 as a
colorless oil (97.2 mg, ca. 100%), which was directly dissolved in
6.3 Hz, 3 H) ppm. 13C NMR (100 Hz, CDCl3): δ = 151.4, 137.8,
135.5, 131.5, 119.7, 76.9, 70.8, 19.0 ppm. FTIR (film of a solution
in CH Cl ): ν = 3376, 2974, 2927, 2360, 2342, 1671, 1518, 1400,
˜
2
2
1374, 1278, 1106, 1065, 972, 913, 613 cm–1. UV λmax (ε) = 228
THF (3.4 mL). nBu4NF (1.0 m in THF, 0.52 mL, 0.52 mmol) was (29992) nm. MS (ESI): m/z = 170.0 [M + H]+, 192.1 [M + Na]+.
then added and the mixture was stirred at ambient temperature for
HRMS (ESI): m/z calcd. for C8H12NO3 [M + H]+ 170.0812; found
10 min. Water was added and the mixture was extracted with Et2O 170.0811; and calcd. for C8H11NNaO3 [M + Na]+ 192.0631; found
(3ϫ 15 mL). The combined organic layers were washed with water
and brine, and dried with anhydrous Na2SO4. Removal of the sol-
vent by rotary evaporation gave a crude mixture of 12 and 13 as a
yellowish oil (59.4 mg, ca. 100%), which was directly dissolved in
Me2C(OMe)2 (1.7 mL). pTsOH (monohydrate, 11.7 mg,
0.034 mmol) was then added and the mixture was stirred at ambi-
ent temperature for 1 h (TLC showed completion of the reaction).
Aqueous saturated NaHCO3 was added and the mixture was ex-
tracted with Et2O (3ϫ 10 mL). The combined organic layers were
washed with water and brine, and dried with anhydrous Na2SO4.
Removal of the solvent by rotary evaporation and column
chromatography on silica gel (PE/Et2O, 9:4) gave 12Ј (the less polar
component, 45.5 mg, 0.22 mmol, 63% overall from 9) and 13Ј (the
more polar component, 12.6 mg, 0.06 mmol, 18% overall from 9).
192.0630.
Hydrolysis of Acetonide 13Ј to Afford anti-Diol 13: Using the same
procedure given above for the conversion of 12Ј into 12, compound
13Ј (10.8 mg, 0.052 mmol) gave anti-diol 13 as a colorless oil
(7.3 mg, 0.04 mmol, 83%): [α]2D5 = +17.3 (c = 0.17, MeOH). 1H
NMR (400 MHz, CDCl3): δ = 7.85 (s, 1 H), 7.60 (s, 1 H), 6.59 (d,
J = 15.8 Hz, 1 H), 6.48 (dd, J = 5.5, 15.7 Hz, 1 H), 4.30–4.27 (dd,
J = 4.0, 5.0 Hz, 1 H), 3.99–3.93 (dq, J = 3.7, 6.5 Hz, 1 H), 2.01
(br. s, 2 H), 1.20 (d, J = 6.5 Hz, 3 H) ppm. 13C NMR (100 Hz,
CDCl3): δ = 151.2, 137.9, 135.4, 129.9, 120.2, 75.7, 70.3, 17.7 ppm.
FTIR (film): ν = 3385, 2972, 2924, 2854, 1666, 1518, 1454, 1376,
˜
1260, 1104, 1065, 971, 912, 613 cm–1. MS (ESI): m/z = 192.2 [M +
Na]+, 170.2 [M + H]+. HRMS (ESI): m/z calcd. for C8H11NNaO3
[M + Na]+ 192.0631; found 192.0635.
Compound 12Ј: Obtained as a colorless oil; [α]2D5 = –5.2 (c = 0.75,
1
Reduction of Enone 9 under Luche Conditions to Give 10 and 11:
NaBH4 (9.7 mg, 0.26 mmol) was added to a solution of 9 (35.6 mg,
0.13 mmol) and CeCl3·7H2O (47.7 mg, 0.13 mmol) in MeOH
(3 mL) stirred in an ice-water bath. The stirring was continued at
the same temperature for 30 min. The bath was removed and the
mixture was diluted with water, extracted with EtOAc (3ϫ 5 mL).
The combined organic layers were washed with water and brine,
and dried with anhydrous Na2SO4. Removal of the solvent by ro-
tary evaporation and column chromatography (PE/EtOAc, 3:1) on
silica gel gave a 13:1 inseparable mixture of 10 and 11 (93%, the
NMR signals for the minor isomer were essentially negligible and
CHCl3). H NMR (500 MHz, CDCl3): δ = 7.84 (s, 1 H), 7.60 (s, 1
H), 6.58 (d, J = 15.6 Hz, 1 H), 6.41 (dd, J = 6.9, 15.5 Hz, 1 H),
4.09–4.06 (m, 1 H), 3.91–3.86 (dq, J = 6.0, 8.4 Hz, 1 H), 1.45 (d, J
= 8.5 Hz, 6 H), 1.30 (d, J = 6.0 Hz, 3 H) ppm. 13C NMR (125 Hz,
CDCl3): δ = 151.2, 137.7, 135.5, 128.3, 120.9, 108.6, 83.2, 76.9,
27.4, 26.9, 16.5 ppm. FTIR (film): ν = 3128, 2984, 2932, 2873,
˜
1520, 1456, 1379, 1370, 1241, 1173, 1090, 1063, 1029, 1006, 967,
930, 910, 864, 753, 613 cm–1. MS (EI): m/z (%) = 209 (1.5) [M+],
194 (28), 165 (85), 152 (66), 136 (34), 122 (21), 107 (100), 79 (38),
43 (45) [M + H]+. HRMS (EI): m/z calcd. for C11H15NO3 [M+]
209.1052; found 209.1055.
1
thus not given in the list) as a colorless oil. H NMR (500 MHz,
Compound 13Ј: Obtained as a colorless oil; [α]2D5 = +5.2 (c = 0.45,
CDCl3): δ = 7.83 (s, 1 H), 7.57 (s, 1 H), 6.57 (d, J = 15.6 Hz, 1 H),
6.44 (dd, J = 5.4, 15.6 Hz, 1 H), 4.02–4.00 (dd, J = 5.4 Hz, 1 H),
3.77–3.72 (dq, J = 6.1, 6.1 Hz, 1 H), 2.74 (br. s, 1 H), 1.21 (d, J =
6.2 Hz, 3 H), 0.90 (s, 9 H), 0.08 (d, J = 8.3 Hz, 6 H) ppm. 13C
NMR (125 Hz, CDCl3): δ = 151.1, 138.2, 135.1, 131.6, 119.1, 76.3,
1
CHCl3). H NMR (500 MHz, CDCl3): δ = 7.84 (s, 1 H), 7.59 (s, 1
H), 6.51 (d, J = 15.6 Hz, 1 H), 6.41 (dd, J = 7.0, 15.5 Hz, 1 H),
4.67–4.66 (dd, J = 6.4, 6.4 Hz, 1 H), 4.43–4.38 (dq, J = 6.4, 6.4 Hz,
1 H), 1.54 (s, 3 H), 1.40 (s, 3 H), 1.20 (d, J = 6.4 Hz, 3 H) ppm.
13C NMR (125 Hz, CDCl3): δ = 151.1, 137.9, 135.3, 128.3, 120.4,
72.0, 25.8, 20.2, 18.0, –4.3, –4.8 ppm. FTIR (film): ν = 3416, 3154,
˜
108.2, 79.0, 74.3, 28.2, 25.6, 16.3 ppm. FTIR (film): ν = 3129, 2983,
˜
2955, 2930, 2885, 2857, 1692, 1519, 1472, 1463, 1376, 1361, 1255,
1136, 1095, 1065, 1004, 969, 911, 837, 777, 747, 611 cm–1. MS
(ESI): m/z = 306.2 [M + Na]+. HRMS (ESI): m/z calcd. for
C14H25NNaO3Si [M + Na]+ 306.1496; found 306.1495.
2927, 2855, 1519, 1455, 1379, 1372, 1246, 1216, 1169, 1093, 1063,
1027, 933, 860, 753, 612 cm–1. MS (EI): m/z (%) = 209 (1.6) [M+],
194 (31), 165 (96), 152 (67), 136 (34), 107 (100), 79 (40), 43 (55).
HRMS (EI): m/z calcd. for C11H15NO3 [M+] 209.1052; found
209.1050.
Condensation of Aldehyde 8 with Ethyl Acetoacetate to Give 15 and
16: A solution of aldehyde 8 (155.1 mg, 1.6 mmol), ethyl acetoacet-
ate (0.2 mL, 1.9 mmol), piperidine (4.7 μL, 0.05 mmol) and glacial
acetic acid (3.8 μL, 0.06 mmol) was stirred at ambient temperature
overnight (the colorless solution gradually turned yellow). Water
was added and the mixture was extracted with EtOAc (2ϫ 15 mL).
The combined organic layers were washed with water and brine,
and dried with anhydrous Na2SO4. Removal of the solvent by ro-
tary evaporation and column chromatography on silica gel (PE/
Et2O, 2:1) afforded 15 as a colorless oil (the less polar component,
192.8 mg, 0.92 mmol, 58% from 8) and 16 as a white solid (the
more polar component, 127.7 mg, 0.61 mmol, 38% from 8).
Hydrolysis of Acetonide 12Ј to Afford syn-Diol 12, which was Iden-
tical to Natural Melanoxadin 1: A solution of 12Ј (38.2 mg,
0.18 mmol) in THF (1.8 mL) containing 1 n HCl (0.36 mL) was
stirred at ambient temperature for 4 h, when TLC showed comple-
tion of the reaction. Water was added and the mixture was ex-
tracted with Et2O (3ϫ 15 mL). The combined organic layers were
washed with water and brine, and dried with anhydrous Na2SO4.
Removal of the solvent by rotary evaporation and column
chromatography on silica gel (MeOH/CH2Cl2, 1:10) gave diol 12
(25.9 mg, 0.15 mmol, 84%) as a white solid. M.p. 72–74 °C; [α]2D3
=
–20.1 (c = 0.16, MeOH); Ͼ 98.9% pure as shown by HPLC analysis
on a Agilent Eclipse XDB C18 column (4.6ϫ150 mm, pore size
Compound 15: 1H NMR (500 MHz, CDCl3): δ = 7.99 (s, 1 H), 7.89
5 μm) eluting with 10:90 to 100:0 MeCN/H2O at a flow rate of (s, 1 H), 7.47 (s, 1 H), 4.30 (q, J = 7.0, 14.5 Hz, 2 H), 2.51 (s, 3 H),
1.0 mL/min with the UV detector set to 220 nm. 1H NMR
(400 MHz, CDCl3): δ = 7.86 (s, 1 H), 7.59 (s, 1 H), 6.57 (d, J =
15.7 Hz, 1 H), 6.42 (dd, J = 6.0, 15.7 Hz, 1 H), 4.03 (dd, J = 6.3,
1.33 (t, J = 7.0 Hz, 3 H) ppm. 13C NMR (125 Hz, CDCl3): δ =
202.0, 164.3, 151.4, 141.0, 135.0, 134.3, 127.8, 61.6, 31.2, 14.1 ppm.
FTIR (film): ν = 3136, 2983, 2929, 1730, 1698, 1666, 1639, 1521,
˜
6.3 Hz, 1 H), 3.76–3.70 (dq, J = 6.4, 6.4 Hz, 1 H), 1.22 (d, J = 1397, 1251, 1203, 1098, 1040, 910, 613 cm–1. MS (ESI): m/z = 232.1
Eur. J. Org. Chem. 2015, 647–654
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
651

Y. Wu, Y. Li et al.
FULL PAPER
[M + Na]+. HRMS (ESI): m/z calcd. for C10H11NNaO4 [M +
(s, 1 H), 7.90 (s, 1 H), 7.36 (d, J = 2.0 Hz, 1 H), 5.63 (dq, J = 2.0,
6.0 Hz, 1 H), 4.85 (s, 1 H), 1.49 (d, J = 6.5 Hz, 3 H), 1.01 (s, 9
H) ppm. 13C NMR (125 Hz, CDCl3): δ = 167.5, 151.4, 141.5, 136.0,
131.7, 125.0, 105.5, 72.9, 34.6, 24.0, 20.9 ppm. FTIR (film of a
Na]+ 232.0580; found 232.0583.
Compound 16: M.p. 79–81 °C. 1H NMR (500 MHz, CDCl3): δ =
8.01 (s, 1 H), 7.89 (s, 1 H), 7.40 (s, 1 H), 4.14 (q, J = 7.0, 14.0 Hz,
2 H), 2.40 (s, 3 H), 1.36 (t, J = 7.0 Hz, 3 H) ppm. 13C NMR
(125 Hz, CDCl3): δ = 194.1, 167.3, 151.4, 141.5, 135.0, 134.5, 128.2,
solution in CH Cl ): ν = 3136, 2978, 2935, 2873, 1724, 1642, 1519,
˜
2
2
1323, 1251, 1141, 1101, 995, 965, 911, 616 cm–1. MS (ESI): m/z =
274.4 [M + Na]+. HRMS (ESI): m/z calcd. for C13H17NNaO4 [M
+ Na]+ 274.1050; found 274.1048.
61.8, 26.7, 14.0 ppm. FTIR (film of a solution in CH Cl ): ν =
˜
2
2
3136, 3065, 2982, 2927, 1728, 1665, 1638, 1521, 1397, 1382, 1250,
1205, 1099, 1041, 976, 910, 613 cm–1. MS (ESI): m/z = 232.1 [M + Compound 26: Obtained as a white solid; m.p. 83–85 °C; [α]2D5
=
1
Na]+. HRMS (ESI): m/z calcd. for C10H11NNaO4 [M + Na]+
+36.0 (c = 0.4, CHCl3). H NMR (500 MHz, CDCl3): δ = 8.89 (s,
1 H), 7.89 (s, 1 H), 6.81 (d, J = 2.0 Hz, 1 H), 4.88 (s, 1 H), 4.81
(dq, J = 2.0, 6.5 Hz, 1 H), 1.51 (d, J = 6.0 Hz, 3 H), 1.01 (s, 9
H) ppm. 13C NMR (125 Hz, CDCl3): δ = 164.0, 150.1, 142.7, 134.3,
129.8, 128.6, 106.4, 75.4, 34.7, 24.0, 22.2 ppm. FTIR (film of a
232.0580; found 232.0582.
Condensation of Adehyde 8 with Lactone 22 to Afford 23 and 24:
A solution of lactone 22 (330.0 mg, 3.4 mmol) in anhydrous THF
(1.7 mL) was added to a solution of LiHMDS (1.0 m in THF,
3.4 mL, 3.4 mmol) in anhydrous THF (3 mL) stirred under argon
(balloon) at –78 °C. The resulting solution was stirred at the same
temperature for 30 min (TLC showed completion of the reaction).
Aqueous saturated NH4Cl was added to quench the reaction, and
the mixture was extracted with Et2O (3ϫ 15 mL). The combined
organic layers were washed with water and brine, and dried with
anhydrous Na2SO4. Removal of the solvent by rotary evaporation
and column chromatography on silica gel (PE/Et2O, 1:1) afforded
pure 23 (the less polar component, 307.6 mg, 1.1 mmol, 39%), a
mixture (281.7 mg, 1.0 mmol, 36%) of 23 and 24, and pure 24 (the
more polar component, 102.5 mg, 0.4 mmol, 14%).
solution in CH Cl ): ν = 2960, 2925, 2871, 1727, 1633, 1519, 1393,
˜
2
2
1318, 1242, 1199, 1101, 984, 913, 748, 618 cm–1. MS (ESI): m/z =
274.4 [M + Na]+. HRMS (ESI): m/z calcd. for C13H17NO4Na [M
+ Na]+ 274.1050; found 274.1053.
Reduction of 25 with DIBAL-H to Afford 19; The Antipode of Natu-
ral Melanazal: DIBAL-H (1.0 m in cyclohexane, 0.9 mL, 0.9 mmol)
was added to a solution of enone 25 (121.0 mg, 0.48 mmol) in an-
hydrous CH2Cl2 (3.5 mL) stirred at –78 °C under argon (balloon).
The mixture was then stirred at the same temperature for 15 min
(TLC showed completion of the reduction). The excess hydride was
destroyed by careful addition of MeOH. The mixture was acidified
to pH 1–2 with 1 n HCl before being extracted with Et2O (3ϫ
10 mL). The combined organic layers were washed with water and
brine, and dried with anhydrous Na2SO4. Removal of the solvent
by rotary evaporation and column chromatography (PE/EtOAc,
1:1) on silica gel gave aldehyde 19 as a white solid (66.4 mg,
0.4 mmol, 83%). M.p. 69–72 °C; [α]2D5 = +38.3 (c = 0.15, MeOH).
1H NMR (500 MHz, CDCl3): δ = 9.49 (s, 1 H), 8.06 (s, 1 H), 8.05
(s, 1 H), 6.99 (s, 1 H), 5.62 (br. s, 1 H, OH), 5.16 (q, J = 6.5,
13.5 Hz, 1 H), 1.44 (d, J = 7.0 Hz, 3 H) ppm. 13C NMR (125 Hz,
CDCl3): δ = 193.9, 151.7, 146.6, 141.8, 135.8, 134.5, 63.7, 22.9 ppm.
Compound 23: Obtained as a white solid; m.p. 89–91 °C; [α]2D5
=
1
+21.2 (c = 1.0, CHCl3). H NMR (500 MHz, CDCl3): δ = 7.86 (s,
1 H), 7.72 (s, 1 H), 5.08 (d, J = 3.5 Hz, 1 H), 5.00 (s, 1 H), 4.21
(dq, J = 6.0, 10.0 Hz, 1 H), 2.94 (dd, J = 3.5, 10.5 Hz, 1 H), 1.24
(d, J = 6.0 Hz, 3 H), 0.96 (s, 9 H) ppm. 13C NMR (125 Hz, CDCl3):
δ = 171.6, 151.0, 141.2, 136.8, 108.2, 71.4, 66.2, 53.1, 35.0, 23.8,
20.2 ppm. FTIR (film of a solution in CH Cl ): ν = 3388, 3138,
˜
2
2
2977, 2962, 2912, 2874, 1731, 1511, 1485, 1379, 1366, 1251, 1225,
1153, 1057, 1030, 996, 917, 619 cm–1. MS (ESI): m/z = 292.3 [M +
Na]+, 270.4 [M + H]+. HRMS (ESI): m/z calcd. for C13H19NO5Na
[M + Na]+ 292.1155; found 292.1161.
FTIR (film): ν = 3374, 3129, 2974, 2926, 1681, 1639, 1525, 1455,
˜
1367, 1147, 1101, 1072, 1036, 984, 915, 804, 763, 689, 615 cm–1.
MS (ESI): m/z = 190.1 [M + Na]+. HRMS (ESI): m/z calcd. for
C8H9NNaO3 [M + Na]+ 190.0475; found 190.0468.
Compound 24: Obtained as a white solid; m.p. 103–105 °C; [α]2D5
=
1
–0.34 (c = 1.0, CHCl3). H NMR (500 MHz, CDCl3): δ = 7.88 (s,
1 H), 7.72 (s, 1 H), 5.22 (d, J = 3.5 Hz, 1 H), 4.88 (s, 1 H), 4.04
(dq, J = 6.0, 9.5 Hz, 1 H), 2.96 (dd, J = 3.5, 9.5 Hz, 1 H), 2.96 (br.
s, 1 H), 1.38 (d, J = 5.5 Hz, 3 H), 0.96 (s, 9 H) ppm. 13C NMR
(125 Hz, CDCl3): δ = 169.2, 151.1, 140.7, 136.1, 108.0, 72.7, 66.7,
NaBH4-Mediated Reduction of Aldehyde 19 to Afford Diol 27:
NaBH4 (5.4 mg, 0.14 mmol) was added to a solution of aldehyde
19 (20 mg, 0.12 mmol) in anhydrous EtOH (0.25 mL) stirred in an
ice-water bath. The mixture was stirred at the same temperature for
15 min (when TLC showed completion of the reduction). Aqueous
saturated NH4Cl was added, followed by water, and the mixture
was stirred at ambient temperature for 1 h before being extracted
with EtOAc (3ϫ 5 mL). The combined organic layers were washed
with water and brine, and dried with anhydrous Na2SO4. Removal
of the solvent by rotary evaporation and column chromatography
on silica gel (PE/EtOAc, 1:3) gave diol 27 as a colorless oil
(19.6 mg, 0.12 mmol, 97%). [α]2D7 = +62.6 (c = 0.8, MeOH). Other
spectroscopic data were identical to those given below for ent-27
(the natural isomer).
53.6, 35.0, 23.8, 21.1 ppm. FTIR (film of a solution in CH Cl ): ν
˜
2
2
= 3420, 2978, 2909, 1731, 1510, 1366, 1247, 1225, 997, 617 cm–1.
MS (ESI): m/z = 292.2 [M + Na]+. HRMS (ESI): m/z calcd. for
C13H19NNaO5 [M + Na]+ 292.1155; found 292.1158.
Conversion of Alcohol 23 into Alkene 25: A solution of 23
(183.4 mg, 0.72 mmol), Et3N (0.14 mL, 1.05 mmol) and MsCl
(90 μL, 1.05 mmol) in anhydrous CH2Cl2 (1.8 mL) was stirred at
ambient temperature overnight (TLC showed completion of the re-
action). Water was added and the mixture was extracted with Et2O
(2ϫ 15 mL). The combined organic layers were washed with water
and brine, and dried with anhydrous Na2SO4. Removal of the sol-
vent by rotary evaporation and column chromatography on silica
gel (PE/EtOAc, 4:1) afforded pure 25 (the less polar component,
157.5 mg, 0.63 mmol, 88%), a mixture of 25 and 26 (8.1 mg,
0.03 mmol, 4%), and pure 26 (the more polar component, 8.7 mg,
0.03 mmol, 4%). Use of 24 in place of 23 gave comparable results
(yield and 25/26 ratio) under the same conditions.
Synthesis of ent-22 from Methyl (S)-3-Hydroxybutanoate: A mix-
ture of 1 n KOH (18.4 mL) and methyl (S)-3-hydroxybutanoate
(1.90 g, 16.8 mmol) was stirred at ambient temperature for 3 d
(TLC showed completion of the reaction). The mixture was acidi-
fied with 1 n HCl to pH 1–2. Most water was removed on a rotary
evaporator under diaphragm pump vacuum. The residue was di-
luted with CH2Cl2 and dried with anhydrous Na2SO4. Removal of
the solvent by rotary evaporation gave the crude acid as a colorless
oil (1.7 g, ca. 100%), which was directly dissolved in benzene
Compound 25: Obtained as a white solid; m.p. 122–124 °C; [α]2D5
+112.9 (c = 0.7, CHCl3). 1H NMR (500 MHz, CDCl3): δ = 7.93
=
652
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© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2015, 647–654

Absolute Configurations of Fungal Metabolites
(67 mL). To this solution were added pivalaldehyde (2,2-dimeth-
ylpropanal, 14.6 mL, 134.4 mmol) and PPTS (422.1 mg, 1.6 mmol).
The mixture was heated in a 110 °C bath and the water formed was
removed azeotropically by using a Dean–Stark trap. After ca. 8 h,
TLC showed completion of the reaction. Water was added and the
biphasic mixture was extracted with Et2O (3ϫ 75 mL). The com-
bined organic layers were washed with water and brine, and dried
with anhydrous Na2SO4. Removal of the solvent by rotary evapora-
tion and column chromatography on silica gel (PE/EtOAc, 2:1)
gave aldehyde ent-22 and its diastereomer as a white solid (2.4 g,
13.9 mmol, 85% over two steps), which was recrystallization in n-
petane/Et2O to give pure ent-22 (976.8 mg, 5.7 mmol, 40% for
recrystallization) as a white solid. M.p. 83–85 °C; [α]2D5 = +57.9 (c
= 1.0, CHCl3). 1H NMR (500 MHz, CDCl3): δ = 4.91 (s, 1 H),
3.98 (m, 1 H), 2.65 (dd, J = 4.5, 17.5 Hz, 1 H), 2.36 (dd, J = 10.5,
17.5 Hz, 1 H), 1.33 (d, J = 6.0 Hz, 3 H), 0.98 (s, 9 H) ppm. 13C
NMR (125 Hz, CDCl3): δ = 168.3, 108.5, 70.4, 37.8, 35.2, 23.9,
0.07 mmol, 19%) of ent-25 and ent-26, and pure ent-26 (the more
polar component, a white solid, 2.7 mg, 0.01 mmol, 3%).
Reduction of 25 with DIBAL-H to Afford 19; The Antipode of Natu-
ral Melanazal: By using the same procedure given above for re-
duction of 25 with DIBAL-H to afford 19, but with ent-25
(16.1 mg, 0.06 mmol) to replace 25 as the substrate, the reaction
gave ent-19 (9.1 mg, 0.05 mmol, 85%): M.p. 69–72 °C; [α]2D5 = –39.1
(c = 0.13, MeOH). Other data were the same as those given above
for its antipode 19.
NaBH4-Mediated Reduction of Aldehyde ent-19 to Afford Diol ent-
27: By using the same procedure given above for NaBH4-mediated
reduction of aldehyde 19 to afford diol 27 but with ent-19 to replace
19 as the substrate, reaction of ent-19 (55.4 mg, 0.33 mmol) gave
diol ent-27 as a colorless oil (54.7 mg, 0.32 mmol, 98%).
Compound ent-27: [α]2D3 = –61.9 (c = 1.2, MeOH); Ͼ 99.9% pure as
shown by HPLC analysis with a Waters Xselect CSH C18 column
(4.6ϫ250 mm, pore size 5 μm) eluting with 10:90 to 100:0 MeCN/
H2O at a flow rate of 1.0 mL/min with the UV detector set to
220 nm; 97%ee as determined by chiral HPLC with a Lux Celli-
lose-4 column (4.6ϫ250 mm, pore size 5 μm) eluting with 70:30 n-
hexane/iPrOH at a flow rate of 0.7 mL/min with the UV detector
set to 214 nm. 1H NMR (500 MHz, CDCl3): δ = 7.90 (s, 1 H), 7.64
(s, 1 H), 6.32 (s, 1 H), 4.80 (q, J = 7.0 Hz, 1 H), 4.33 (part of an
AB system, d, J = 14.0 Hz, 1 H), 4.23 (part of an AB system, d, J
= 14.0 Hz, 1 H), 1.42 (d, J = 6.5 Hz, 3 H) ppm. 13C NMR (125 Hz,
CDCl3): δ = 150.7, 146.8, 137.2, 136.6, 112.8, 66.7, 65.4, 21.8 ppm.
21.1 ppm. FTIR (film of a solution in CH Cl ): ν = 2982, 2965,
˜
2
2
2936, 2871, 1724, 1384, 1358, 1288, 1256, 1107, 979, 752 cm–1. MS
(ESI): m/z = 195.2 [M + Na]+. HRMS (ESI): m/z calcd. for
C9H16NaO3 [M + Na]+ 195.0992; found 195.0990.
Condensation of Aldehyde 8 with Lactone ent-22 to Afford ent-23
and ent-24: By using the same procedure given above for the con-
densation of aldehyde 8 with lactone 22 to afford 23 and 24, reac-
tion of 8 (158.5 mg, 1.6 mmol) with ent-22 (235.0 mg, 1.4 mmol)
afforded ent-23 (the less polar component, a white solid, 164.3 mg,
0.61 mmol, 44% from ent-22), a mixture of ent-23 and ent-24
(83 mg, 0.31 mmol, 22%) and ent-24 (the more polar component,
a white solid, 77.8 mg, 0.29 mmol, 21% from ent-22).
FTIR (film): ν = 3365, 2974, 2921, 2850, 1646, 1102, 1068, 912,
˜
615 cm–1. MS (ESI): m/z 192.2 [M + Na]+. HRMS (ESI): m/z calcd.
for C8H11NNaO3 [M + Na]+ 192.0631; found 192.0634.
Compound ent-23: M.p. 88–90 °C; [α]2D5 = –21.0 (c = 1.0, CHCl3).
1H NMR (500 MHz, CDCl3): δ = 7.85 (s, 1 H), 7.71 (s, 1 H), 5.02
(s, 1 H), 5.00 (s, 1 H), 4.46 (br. s, 1 H), 4.23 (dq, J = 6.0, 10.0 Hz,
1 H), 2.94 (dd, J = 3.5, 10.5 Hz, 1 H), 1.29 (d, J = 5.5 Hz, 3 H),
0.96 (s, 9 H) ppm. 13C NMR (125 Hz, CDCl3): δ = 171.7, 150.9,
141.2, 136.9, 108.3, 71.5, 66.0, 53.1, 35.0, 23.8, 20.0 ppm. FTIR
and MS (ESI) were identical to those for its antipode 23.
Acetylation of ent-27 to Afford Diacetate 28: A solution of ent-
27 (36.1 mg, 0.21 mmol), Ac2O (50 μL, 0.53 mmol), Et3N (70 μL,
0.53 mmol), and DMAP (5.1 mg, 0.04 mmol) in anhydrous CH2Cl2
(1 mL) was stirred at ambient temperature for 10 min (TLC showed
completion of the reaction). The mixture was acidified to pH 1–2
with 1 n HCl, followed by addition of water. The mixture was then
extracted with EtOAc (3ϫ 5 mL). The combined organic layers
were washed in turn with aqueous saturated NaHCO3, water and
brine before being dried with anhydrous Na2SO4. Removal of the
solvent by rotary evaporation and column chromatography on sil-
ica gel (PE/EtOAc, 2:1) gave diacetate 28 as a colorless oil (50.7 mg,
0.20 mmol, 95%). [α]2D4 = +4.8 (c = 1.0, CHCl3). 1H NMR
(500 MHz, CDCl3): δ = 7.89 (s, 1 H), 7.72 (s, 1 H), 6.59 (q, J =
7.0 Hz, 1 H), 6.39 (s, 1 H), 4.84 (part of an AB system, d, J =
13.5 Hz, 1 H), 4.68 (part of an AB system, d, J = 13.5 Hz, 1 H),
2.11 (s, 3 H), 2.05 (s, 3 H), 1.47 (d, J = 6.5 Hz, 3 H) ppm. 13C
NMR (125 Hz, CDCl3): δ = 170.5, 170.1, 150.9, 137.6, 136.7, 136.3,
Compound ent-24: M.p. 104–106 °C; [α]2D5 = +2.34 (c = 1.0, CHCl3).
1H NMR (500 MHz, CDCl3): δ = 7.88 (s, 1 H), 7.71 (s, 1 H), 5.23
(d, J = 3.5 Hz, 1 H), 4.88 (s, 1 H), 4.03 (dq, J = 6.0, 9.5 Hz, 1 H),
3.61 (br. s, 1 H), 2.95 (dd, J = 3.0, 9.5 Hz, 1 H), 1.39 (d, J = 5.5 Hz,
3 H), 0.95 (s, 9 H) ppm. 13C NMR (125 Hz, CDCl3): δ = 169.3,
151.1, 140.8, 136.2, 107.9, 72.7, 66.7, 53.5, 35.0, 23.8, 21.1 ppm.
FTIR and MS (ESI) were identical to those for its antipode 24.
Conversion of Alcohol ent-23 into Alkene ent-25: By using the same
procedure given above for conversion of alcohol 23 into alkene 25,
the reaction of ent-23 (109.9 mg, 0.41 mmol) afforded ent-25 (the
less polar component, a white solid, 80.9 mg, 0.32 mmol, 78%) and
a mixture (15.8 mg, 0.06 mmol, 15%) of ent-25 and ent-26 (the
more polar component).
117.7, 69.0, 63.9, 21.1, 20.9, 19.3 ppm. FTIR (film): ν = 3139, 2984,
˜
2937, 1738, 1518, 1455, 1371, 1243, 1184, 1100, 1023, 965, 943,
912, 871, 761, 615 cm–1. MS (ESI): m/z 276.2 [M + Na]+. HRMS
(ESI): m/z calcd. for C12H16NO5 [M + H]+ 254.1023; found
254.1022.
Compound ent-25: M.p. 119–121 °C; [α]2D5 = –113.0 (c = 0.85,
1
CHCl3). H NMR (500 MHz, CDCl3): δ = 7.94 (s, 1 H), 7.92 (s, 1
Hydrolysis of Diacetate 28 to Afford Diol ent-27: A solution of di-
acetate 28 (39.7 mg, 0.16 mmol) and K2CO3 (32.5 mg, 0.24 mmol)
in MeOH (0.8 mL) was stirred at ambient temperature for 15 min
(TLC showed completion of the reaction). The reaction mixture
was diluted with water and extracted with EtOAc (3ϫ 5 mL). The
combined organic layers were washed with water and brine before
H), 7.36 (d, J = 2.0 Hz, 1 H), 5.63 (dq, J = 2.0, 6.0 Hz, 1 H), 4.85
(s, 1 H), 1.49 (d, J = 6.5 Hz, 3 H), 1.01 (s, 9 H) ppm. 13C NMR
(125 Hz, CDCl3): δ = 167.4, 151.5, 141.5, 136.0, 131.7, 125.0, 105.5,
72.9, 34.5, 24.0, 20.9 ppm. FTIR and MS (ESI) were identical to
those for its antipode 25.
Conversion of Alcohol ent-24 into Alkene ent-25: By using the same being dried with anhydrous Na2SO4. Removal of the solvent by
procedure given above for conversion of alcohol 23 into alkene 25,
but with ent-23 (109.9 mg, 0.41 mmol) to replace 23 as the sub-
strate, the reaction gave pure ent-25 (the less polar component, a
white solid, 64.8 mg, 0.26 mmol, 70%) and a mixture (17.1 mg,
rotary evaporation and column chromatography on silica gel (PE/
EtOAc, 1:3) gave ent-27 as a colorless oil (25.9 mg, 0.15 mmol,
98%). [α]2D4 = –60.9 (c = 1.0, MeOH). Other data were the same as
given above.
Eur. J. Org. Chem. 2015, 647–654
© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjoc.org
653

Y. Wu, Y. Li et al.
FULL PAPER
[10] The assignments for H-1Ј and H-2Ј, also C-1Ј and C-2Ј, re-
ported by Lee et al.[2] were inverted by mistake; exclusive re-
liance on HMBC without H–H coupling information from a
COSY spectrum may explain why this happened.
Supporting Information (see footnote on the first page of this arti-
1
cle): Copies of the H and 13C NMR spectra, FTIR spectra for all
new compounds.
[11] The configurations of 15 and 16 were established through a
NOESY experiment with the corresponding diol (i.e., 27/ent-
27) derived from reduction of 15.
[12] a) E. J. Corey, R. K. Bakshi, S. Shibata, J. Am. Chem. Soc.
1987, 109, 5551–5553; b) E. J. Corey, B. E. Roberts, J. Am.
Chem. Soc. 1997, 119, 12426–12431; c) for a review, see: V. K.
Singh, Synthesis 1992, 605–617.
Acknowledgments
This work was supported by the National Natural Science Founda-
tion of China (NSFC) (grant numbers 21372248, 21172247, and
21032002) and the Chinese Academy of Sciences.
[13] D. Seebach, U. Brändli, P. Schnurrenberger, M. Przybylski,
Helv. Chim. Acta 1988, 71, 155–167.
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[15] CCDC-1027063 (for 23), -1027064 (for 24), -1027065 (for ent-
23), and -1027066 (for ent-24) contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
[16] Compound ent-27, which was determined to be of 97%ee by
chiral HPLC analysis, was also converted into its diacetate 28
to gain additional chance to remove any potential impurity.
The compound was purified by chromatography as the diacet-
ate, then re-hydrolyzed back to ent-27, and this compound still
gave the same optical rotation, lending further support to the
reliability of the data acquired in this work.
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Received: October 1, 2014
Published Online: December 11, 2014
654
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Eur. J. Org. Chem. 2015, 647–654