Total Synthesis of (+)-7-epi-Tarchonanthuslactone via a Chelation-Controlled Mukaiyama Aldol Reaction

Article abstract of DOI:10.1080/00397911.2015.1014498

An asymmetric total synthesis of (+)-7-epi-tarchonanthuslactone was achieved from commercially available methyl (R)-3-hydroxybutyrate. The key step employed a diastereoselective chelation-controlled Mukaiyama aldol reaction to construct the chiral hydroxyl group at the C-5 position.

Full text of DOI:10.1080/00397911.2015.1014498

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Total Synthesis of (+)-7-Epi-
tarchonanthuslactone via a Chelation-
Controlled Mukaiyama Aldol Reaction
Shuangping Huanga, Dongwang Liua, Linjun Tangb, Fei Fei Huanga,
Jianting Zhanga & Xiaoji Wanga
a School of Pharmacy, Jiangxi Science and Technology Normal
University, Nanchang, China
b School of Life Science, Jiangxi Science and Technology Normal
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University, Nanchang, China
Accepted author version posted online: 13 Feb 2015.Published
online: 13 Feb 2015.
To cite this article: Shuangping Huang, Dongwang Liu, Linjun Tang, Fei Fei Huang, Jianting
Zhang & Xiaoji Wang (2015): Total Synthesis of (+)-7-Epi-tarchonanthuslactone via a Chelation-
Controlled Mukaiyama Aldol Reaction, Synthetic Communications: An International Journal for Rapid
Communication of Synthetic Organic Chemistry, DOI: 10.1080/00397911.2015.1014498
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Synthetic Communications1, 0: 1–8, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397911.2015.1014498
TOTAL SYNTHESIS OF (+)-7-EPI-
TARCHONANTHUSLACTONE VIA A CHELATION-
CONTROLLED MUKAIYAMA ALDOL REACTION
Shuangping Huang,1 Dongwang Liu,1 Linjun Tang,2
Fei Fei Huang,1 Jianting Zhang,1 and Xiaoji Wang1
1School of Pharmacy, Jiangxi Science and Technology Normal University,
Nanchang, China
2School of Life Science, Jiangxi Science and Technology Normal University,
Nanchang, China
GRAPHICAL ABSTRACT
Abstract An asymmetric total synthesis of (þ)-7-epi-tarchonanthuslactone was achieved
from commercially available methyl (R)-3-hydroxybutyrate. The key step employed a
diastereoselective chelation-controlled Mukaiyama aldol reaction to construct the chiral
hydroxyl group at the C-5 position.
Keywords (þ)-7-Epi-tarchonanthuslactone; 1,3-diol=5,6-pyrone; asymmetric total
synthesis; chelation-controlled Mukaiyama aldol reaction
INTRODUCTION
The syn=anti-1,3-diol=5,6-pyrone moiety has been found in many natural
products exhibiting a variety of biological activities, including anticancer, antibacter-
ial, and antifungal agents.[1,2] The a,b-unsaturated unit, acting as a Michael acceptor,
is assumed to play an important role in these pharmacological properties. Several
methodologies for 1,3-diol=5,6-pyrone synthesis have been developed, and
approaches to molecules containing this functional group, such as strictifolione,[3]
cryptocarya diacetate,[4] and passifloricin A,[5] have been reported. Tarchonanthus-
lactone (Fig. 1), a related natural product, was isolated from the leaves of
Tarchonanthus trilobus by Bohlmann in 1979[6] and subsequently shown to lower
Received December 11, 2014.
Address correspondence to Shuangping Huang or Xiaoji Wang, School of Pharmacy, Jiangxi
Science and Technology Normal University, Nanchang, China. E-mail: 185544590@qq.com;
13767101659@163.com
1

2
S. HUANG ET AL.
Figure 1. Structures of (þ)-tarchonanthuslactone and (þ)-7-epi-tarchonanthuslactone.
Scheme 1. Retrosynthetic analysis of (þ)-7-epi-tarchonanthuslactone.
plasma glucose levels in diabetic rats.[7] The medicinal potential generated interest in
development of routes to tarchonanthuslactone and its analogs, and several synthetic
approaches have been reported since tarchonanthuslactone was first isolated.[8] The
structure of tarchonanthuslactone is composed of a 3,4-dihydroxyhydrocinnamic
acid and a 1,3-diol unit with one hydroxyl group contained in an a,b-unsaturated
d-lactone. To the best of our knowledge, although several total syntheses of tarcho-
nanthuslactone have been reported, only two total syntheses of the anti-1,3-diol iso-
mer of tarchonanthuslactone have been achieved.[8i,8m]
The novel and interesting bioactivity of these molecules encouraged us to
develop approaches toward tarchonanthuslactones. Herein we describe a sub-
strate-controlled synthesis of (þ)-7-epi-tarchonanthuslactone (Scheme 1) via a 4-
methoxybenzyl (PMB) chelation-controlled Mukaiyama aldol reaction.
RESULTS AND DISCUSSION
To date, two total syntheses of (þ)-7-epi-tarchonanthuslactone have been
achieved by Singh and Lee. Singh employed a Kulinkovich reaction and a ring-clos-
ing metathesis as the key steps to obtain (þ)-7-epi-tarchonanthuslactone, in which

TOTAL SYNTHESIS OF (+)-7-EPI-TARCHONANTHUSLACTONE
3
the trans-diol was prepared via a sodium borohydride reduction giving a trans=cis
ratio of about 5:1. Lee obtained the trans-diol unit using two epoxide openings
and a hydroxyl directed reduction, using enantiomerically pure epichlorohydrin as
the starting material. For formation of the 1,3-syn=anti diol unit, the 1,3-asymmetri-
cally inductive Mukaiyama aldol reaction is a useful protocol.[9] However, to our
knowledge, this methodology has not been applied to the synthesis of tarchonanthus-
lactones. In 1986, Munro demonstrated chelation-controlled nucleophilic additions
of allylstannanes to b-alkoxy aldehydes could give 1,3-trans diols.[10] As suggested
by Munro's work, we devised we would get a high anti-selectivity of addition product
when treated silyl enol ether with b-alkoxy aldehyde in the presence of Lewis acid. As
shown in Fig. 2, the high anti-selectivity would arise from the b-chelation of titanium
cation with the aldehyde and the ether oxygen of PMB to form the complex A, in
which the R group occupies an axial position. Subsequent attack of bis(trimethylsi-
lyl)dienol ether SEE from the less hindered side would result in the formation of the
anti-diol product P-1, which could be applied in the total synthesis of tarchonanthus-
lactones.
We therefore devised our retrosynthetic analysis as outlined in Scheme 1. The
target molecule was first suitably disconnected to tert-butyldimethylsilyl-protected
3,4-dihydroxyhydrocinnamic acid 1 and a,b-unsaturated d-lactone 2 via an esterifi-
cation. We assumed that a,b-unsaturated lactone 2 could be prepared from com-
pound 3 through hydroxyl group elimination and deprotection of the PMB group.
The intermediate 3 was then assumed to be readily built from b-keto ester 4 via a
few steps including reduction and intramolecular lactonization. For the preparation
of compound 4, the challenge was to construct the 1,3-trans diol structure. According
to our retrosynthetic analysis, we envisioned that the stereochemistry of compound 4
could be obtained from bis(trimethylsilyl)dienol ether 5 and aldehyde 6 via a chela-
tion-controlled Mukaiyama aldol reaction.
In consideration of the retrosynthetic strategy to construct the 1,3-anti diol via
the chelation-controlled Mukaiyama aldol reaction, we first protected the hydroxyl
of the starting material, (R)-3-hydroxybutyrate 7, with a PMB group because the
PMB ether has effectively been used in the 1,3-inductive Mukaiyama aldol reaction
and can easily be removed.[11] Treatment of 7 and freshly prepared trichloroacetimi-
date 8[12] with catalytic camphorsulfonic acid (CSA) in anhydrous dichloromethane
(DCM) provided compound 9 in 94% yield. Subsequently, PMB-protected (R)-3-
hydroxybutyrate was reduced with diisobutylaluminiumhydride (DIBAL-H) at
ꢀ78 °C for 1 h to afford the Mukaiyama aldol precursor, aldehyde 6, in about
83% yield.
Figure 2. Proposed mechanism of the chelation-controlled Mukaiyama aldol addition.

4
S. HUANG ET AL.
With aldehyde 6 in hand, our next concern was to establish the 1,3-trans diol unit.
Thus, compound 6 was subjected to aldol reaction with bis(trimethylsilyl)dienol ether 5.
[13]
Evans found that the standard conditions, such as BF3 · OEt2, SnCl4, TiCl4, or
MgBr2 · OEt2, could give 1,3-trans diol in poor to modest yield, but the use of the mixed
titanium species Ti(OiPr)2Cl2 delivered a high-yielding and stereoselective reaction.[14]
Therefore, in the presence of freshly prepared catalytic Ti(OiPr)2Cl2 (prepared with
TiCl4 and Ti(OiPr)4 in a 1:1 ratio in DCM at 0 °C) at ꢀ78°C, 5 and 6 were reacted
to provide the desired 1,3-anti product 4 in about 82% yield. To prepare intermediate
2, we next needed to convert the b-keto ester to an a,b-unsaturated d-lactone. A reduc-
tion=elimination strategy was employed, because the unsaturated d-lactone could easily
be obtained via elimination. Compound 10 was easily prepared by treatment of 4 with
NaBH4. The chirality of the generated hydroxyl of 10 had no relevance to the final pro-
duct, so, without classification, subsequent lactonization of 10 under weakly acidic con-
ditions using pyridinium 4-toluenesulfonate (PPTS) afforded the mixture 3. Compound
3 subsequently underwent elimination smoothly when treated with MsCl and excess
triethylene amine (TEA) to provide the desired lactone 11. Using thin-layer chromato-
graphy (TLC) analysis we found that the by-product of NaBH4 reduction had no effect
on the subsequent lactonization and elimination. So, without purification, the desired
lactone 11 was obtained from compound 4 in about 51% yield. Removal of the PMB
group with DDQ afforded intermediate 2 in 80% yield (Scheme 2).
The endgame is the coupling of intermediates 1[8k] and 2. As shown in Scheme 3,
after applying normal lactonization conditions [N,N0-dicyclohexylcarbodiimide
(DCC), dimethylaminopyridine (DMAP)], we obtained the TBS protected (þ)-7-
epi-tarchonanthuslactone 12 in 81% yield. Deprotection of 12 using tetrabutylammo-
nium fluoride (TBAF) furnished the desired final product.[8i,8m]
Scheme 2. Synthesis of the intermediate 2. Reagents and conditions: (a) CSA, DCM, 0 °C to rt, 94%; (b)
DIBAL-H, DCM, ꢀ78 °C, 83%; (c) Ti(OiPr)2Cl2, PhCH3, ꢀ78 °C, 82%; (d) (i) NaBH4, MeOH, rt; (ii)
PPTS, benzene, 80 °C; (iii) MsCl, TEA, 0 °C, 51% overall yield from 4; (e) DDQ, DCM-H2O, 0 °C, 80%.

TOTAL SYNTHESIS OF (+)-7-EPI-TARCHONANTHUSLACTONE
5
Scheme 3. Synthesis of (þ)-7-epi-tarchonanthuslactone. Reagents and conditions: (f) 1, DMAP, DCC,
DCM, rt, 81%; (g) TBAF, benzoic acid, THF, rt, 88%.
CONCLUSION
In conclusion, we have successfully obtained (þ)-7-epi-tarchonanthuslactone
from commercially available methyl (R)-3-hydroxybutyrate. Synthetically, a
diastereoselective chelation-controlled Mukaiyama aldol reaction was employed to
construct the 1,3-trans diol, which provides an alternative approach to tarcho-
nanthuslactones. The anti-selectivity of the Mukaiyama aldol addition arises from
the b-chelation of titanium cation with the aldehyde and the ether oxygen of the
PMB ether. Further application of this approach toward tarchonanthuslactone
and its analogs is currently in progress.
EXPERIMENTAL
All reactions were carried out under N2 atmosphere with dry solvents unless
otherwise noted and were monitored by thin-layer chromatography (TLC) carried
out on 0.25-mm silica-gel plates (60 F-254). Silica gel (200–300 mesh) supplied by
Qingdao Marine chemical factory in China was used for flash column
chromatography. Anhydrous tetrahydrofuran (THF) was distilled from sodium-
benzophenone. CH2Cl2 and MeCN were distilled from CaH2. Yield refers to chro-
matographic and spectroscopic yields (1H, 13C NMR), unless otherwise stated.
NMR spectra were recorded on a Bruker Advance DPX 400-MHz spectrometers
(1H: 400 MHz, 13C: 100 MHz). High-resolution mass spectra were obtained from a
MALDI-TOF mass spectrometer. Optical rotations were measured on a digital
polarimeter at 25 °C.
(5S,7R)-Methyl 5-Hydroxy-7-((4-methoxybenzyl)oxy)-3-oxooctanoate (4)
Under an N2 atmosphere, 0.43 mL TiCl4 (3.89 mmol) was added to a solution of
1.3 mL Ti(OiPr)4 (4.24 mmol) in 40mL PhCH3 in a dropwise fashion at 0 °C. The sol-
ution was stirred at 0 °C for 10 min, and then cooled to ꢀ78 °C to give a milky white
slurry. This slurry was treated with a solution of the aldehyde 6 (1.46 g, 7 mmol) in
PhCH3 (20.7 mL) via a cannula. The resulting solution was stirred at ꢀ78 °C for
another 10 min before being treated with a solution of the bis-(trimethylsilyl)dienol
ether 5 (3.28 g, 12.6 mmol) in PhCH3 (10 mL).The reaction mixture was stirred at
ꢀ78 °C for 2 h before being quenched with saturated NH4Cl solution. The mixture
was filtered through a plug of celite and extracted with DCM (3 × 80 mL). The com-
bined organic layer was washed with brine, dried over anhydrous Na2SO4, filtered,
and concentrated under reduced pressure. The residue was purified by silica-gel column
chromatography (hexane=EtOAc¼ 10:1) to provide a yellow oil compound 4 (1.86 g,
25
1
82% yield). ½aꢁD ¼ ꢀ30:2 (c ¼ 0.54, CH2Cl2); H NMR (400 MHz, CDCl3): d 7.18

6
S. HUANG ET AL.
(d, J ¼ 8.5 Hz, 2H), 6.79 (d, J ¼ 8.5 Hz, 2H), 4.47 (d, J ¼ 11.0 Hz, 1H), 4.27 (d, J ¼ 11.0
Hz, 2H), 4.22–4.10 (m, 1H), 3.78–3.73 (m, 1H), 3.71 (s, 3H), 3.64 (s, 3H), 3.40 (s, 2H),
3.26 (d, J ¼ 3.6 Hz, 1H), 2.60–2.53 (m, 2H), 1.64–1.46 (m, 2H), 1.15 (d, J ¼ 6.0 Hz, 3H);
13C NMR (100 MHz, CDCl3): d 202.8, 167.3, 159.0, 130.3, 129.2, 113.7, 71.6, 70.1,
64.6, 55.1, 52.2, 49.8, 49.5, 42.6, 19.2. HRMS (ESI): m=z calcd. for C17H24O6Na
[MþNa]þ 347.1465; found 347.1467.
(R)-6-((R)-2-((4-Methoxybenzyl)oxy)propyl)-5,6-dihydro-2H-pyran-
2-one (11)
NaBH4 (105.9 mg, 2.79 mmol) was added to a stirred solution of compound 4
(824.3 mg, 2.54 mmol) in methanol (13 mL) at room temperature. After the addition,
stirring was continued at the same temperature for 1 h. Then the reaction was
quenched with saturated NH4Cl solution (5 mL) when the TLC monitor indicated
that the reaction had finished. Then we removed most of the methanol and added
the saturated NH4Cl solution (20 mL). The mixture was extracted with DCM
(3 × 30 mL). The combined organic layer was washed with brine, dried over anhy-
drous Na2SO4, and concentrated under reduced pressure to give crude compound
10, which was used for next step without further purification.
Pyridinium 4-toluenesulfonate (PPTS) (191.58 mg, 0.75 mmol) was added to
the solution of the crude compound 10 in benzene (50 mL) at room temperature.
The mixture was then warmed up to 80 °C. After stirring at 80 °C for 2 h, it was
cooled to room temperature. Subsequently, the reaction was quenched with a satu-
rated NaHCO3 solution (40 mL), and extracted with DCM (3 × 50 mL). The com-
bined organic layer was washed with brine, dried over anhydrous Na2SO4, and
concentrated under reduced pressure to give crude compound 3, which was used
for the next elimination step without further purification.
Under an N2 atmosphere, Et3N (1.77 mL, 12.70 mmol) was slowly added to a
solution of crude compound 3 in dry DCM (12 mL) at 0 °C, followed by MsCl
(0.60 mL, 7.62 mmol).The solution was stirred at 0 °C for 30 min and quenched with
a saturated NH4Cl solution (20 mL). Then, the mixture was extracted with DCM
(3 × 50 mL) and the combined organic layer was washed with brine, dried over
anhydrous Na2SO4, and concentrated under reduced pressure. Purification by flash
chromatography (hexane=EtOAc ¼ 1:5) gave the desired compound 11 as a colorless
25
oil (381.3 mg, 51% overall yield from compound 4). ½aꢁD ¼ ꢀ79:3 (c ¼ 0.44, CH2Cl2);
1H NMR (400 MHz, CDCl3): d 7.11 (d, J ¼ 8.4 Hz, 2H), 6.73–6.70 (m, 3H), 5.87 (d,
J ¼ 10.0 Hz, 1H), 4.54–4.47 (m, 1H), 4.42 (d, J ¼ 10.8 Hz, 1H), 4.21 (d, J ¼ 10.8 Hz,
1H), 3.83–3.74 (m, 1H), 3.65 (s, 3H), 2.21–2.10 (m, 2H), 1.75–1.69 (m, 1H), 1.61–
1.55 (m, 1H), 1.08 (d, J ¼ 6.0 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 164.3,
159.1, 145.2, 130.6, 129.3, 121.2, 113.7, 74.8, 70.7, 70.4, 55.2, 42.9, 29.8, 19.8. HRMS
(ESI): m=z calcd. for C16H20O4Na [MþNa]þ 299.1254; found 299.1252.
(R)-1-((R)-6-Oxo-3,6-dihydro-2H-pyran-2-yl)propan-2-yl 3-(3,4-Bis
((tert-butyldimethylsilyl)oxy)phenyl)propanoate (12)
At room temperature, acid 1 (159 mg, 0.39 mmol), DCC (80 mg, 0.39 mmol)
and DMAP (21.5 mg, 0.18 mmol) were added to a solution of (R)-6-((R)-2-

TOTAL SYNTHESIS OF (+)-7-EPI-TARCHONANTHUSLACTONE
7
hydroxypropyl)-5,6-dihydro-2H-pyran-2-one 2 (55 mg, 0.35 mmol) in DCM (10 mL).
The mixture was stirred at rt overnight. Filtration of the reaction mixture,
evaporation of the solvent, and subsequent purification by flash chromatography
(hexane=EtOAc ¼ 2:1) offered the desired compound 12 (156 mg, 81% yield) as a col-
25
1
orless oil. ½aꢁD ¼ 0:5 (c ¼ 0.85, CH2Cl2); H NMR (400 MHz, CDCl3): d 6.85–6.81
(m, 1H), 6.70 (d, J ¼ 8.0 Hz, 1H), 6.64–6.60 (m, 2H), 6.00 (d, J ¼ 9.6 Hz, 1H),
5.16–5.08 (m, 1H), 4.42–4.35(m, 1H), 2.80 (t, J ¼ 7.6 Hz, 2H), 2.53 (t, J ¼ 7.6 Hz,
2H), 2.33–2.25 (m, 2H), 1.94–1.92 (m, 1H), 1.86–1.84 (m, 1H), 1.24 (d, J ¼ 6.4 Hz,
3H), 0.97 (s, 9H), 0.96 (s, 9H), 0.17 (s, 6H), 0.16 (s, 6H); 13C NMR (100 MHz,
CDCl3): d 172.2, 163.8, 146.5, 145.1, 144.6, 133.5, 121.4, 121.1, 121.0, 120.8, 74.6,
67.4, 41.4, 36.2, 30.2, 29.6, 25.9, 20.6, 18.4, ꢀ4.1, ꢀ4.2. HRMS (ESI): m=z calcd.
for C29H48O6Si2Na [M þ Na]þ 571.2882; found 571.2880.
(þ)-7-Epi-tarchonanthuslactone
A solution of compound 12 (54 mg, 0.1 mmol) in THF (2 mL) was treated with
benzoic acid (35 mg, 0.28 mmol) and 1.0 M TBAF (0.22 mL, 0.22 mmol). The mixture
was stirred at room temperature for 1 h before being quenched with H2O (10 mL).
The resulting mixture was extracted with DCM (3 × 20 mL). The combined organic
layer was dried over anhydrous Na2SO4, concentrated under reduced pressure, puri-
fication by flash chromatography (hexane=EtOAc ¼ 1:1) to offer the desired (þ)-7-
25
epi-tarchonanthuslactone as a colorless oil (28 mg, 88% yield). ½aꢁD ¼ 26:5 (c ¼ 0.78,
CH2Cl2); 1H NMR (400 MHz, CDCl3): d 6.87–6.84 (m, 1H), 6.73–6.68 (m, 2H), 6.53
(d, J ¼ 8.0 Hz, 1H), 5.99 (d, J ¼ 10.0 Hz, 1H), 5.14–5.11 (m, 1H), 4.14–4.08 (m, 1H),
2.79 (t, J ¼ 6.8 Hz, 2H), 2.57 (t, J ¼ 6.8 Hz, 2H), 2.22–2.20 (m, 2H), 1.85–1.76 (m,
2H), 1.20 (d, J ¼ 6.4 Hz, 3H); 13C NMR (100 MHz, CDCl3): d 173.0, 165.3, 145.9,
143.8, 142.5, 132.4, 120.7, 120.1, 115.2, 74.5, 67.1, 41.1, 36.1, 30.2, 29.4, 20.4. HRMS
(ESI): m=z calcd. for C17H20O6Na [M þ Na]þ 343.1152; found 343.1149.
FUNDING
We thank the National Natural Science Foundation of China (No. 21362012),
Natural Science Foundation of Jiangxi Province (No. 20132BAB205075), Scientific
Research Fund of Jiangxi Provincial Education Department (No. KJLD12036),
and Training Fund for Excellent young scientist of JiangXi Province (No. [2013]
138) for the funding support.
SUPPORTING INFORMATION
Full experimental details and 1H, 13C NMR, and HRMS spectra for this article
can be accessed on the publisher’s website.
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