Stereodivergent total synthesis of Br-nannocystins underpinning the polyketide (10R,11S) configuration as a key determinant of potency

Article abstract of DOI:10.1016/j.molstruc.2018.12.107

Continuing our investigation into the structure-activity relationship of antiproliferative macrocyclic nannocystins, we describe herein total synthesis of all four stereoisomers of Br-nannocystins as well as a simplified analogue varying at the polyketide C10 and C11 positions. Biological evaluation of these compounds against PANC1 cancer cell lines showed that both the (10R,11S) configuration and its associated two substituents are crucial for high potency.

Full text of DOI:10.1016/j.molstruc.2018.12.107

Journal of Molecular Structure 1181 (2019) 568e578
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Stereodivergent total synthesis of Br-nannocystins underpinning the
polyketide (10R,11S) configuration as a key determinant of potency
Yunfeng Tian, Jiyan Wang, Wenjie Liu, Xiaoya Yuan, Yang Tang, Jing Li, Yue Chen**,
Weicheng Zhang*
The State Key Laboratory of Medicinal Chemical Biology, College of Pharmacy, and Tianjin Key Laboratory of Molecular Drug Research, Nankai University,
Tianjin, 300071, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Continuing our investigation into the structure-activity relationship of antiproliferative macrocyclic
nannocystins, we describe herein total synthesis of all four stereoisomers of Br-nannocystins as well as a
simplified analogue varying at the polyketide C10 and C11 positions. Biological evaluation of these
compounds against PANC1 cancer cell lines showed that both the (10R,11S) configuration and its asso-
ciated two substituents are crucial for high potency.
Received 20 September 2018
Received in revised form
25 December 2018
Accepted 27 December 2018
Available online 27 December 2018
© 2019 Elsevier B.V. All rights reserved.
Keywords:
Nannocystin
Macrocycle
Stereostructure-activity relationship
Stereodivergent synthesis
1. Introduction
cyclodepsipeptide featuring a bipartite polyketide/tripeptide
composition. It was originally isolated, together with other minor
homologues, from the cultivation broth of the myxobacterial strains
Nannocyctis sp. ST201196 and Nannocyctis sp. MB1016, respectively
[6,7]. Its remarkable cytotoxicity against a panel of cancer cell lines
and interesting mode of action targeting eukaryotic translation
elongation factor 1 A (eEF1A) [7,8] warrant concurrent efforts
hitherto from seven research groups to accomplish total synthesis
of 1 or its 2,3-desoxy congener via different routes [9e15]. Taking
advantage of a motif-oriented approach, Fürstner et al. further
prepared 10 additional non-natural analogues and conducted sys-
tematic SAR study at the polyketide trisubstituted 6,7-alkene re-
gion [15]. Interestingly, it was found that the neighboring 5-methyl
ether seems to play a more influential role in target binding than
the 6,7-alkene moiety, albeit a somewhat inconsistent trend may
be deduced from the previous docking analysis. Meanwhile, we
also launched independent SAR study on nannocystins based on
our total synthesis of 1 [11]. The first foray was to clarify the po-
tential binding role of the (2R,3S)-epoxide in 1. Total synthesis and
biological evaluation of two compound series, each bearing (2R,3S)-
epoxide, (2S,3R)-epoxide, or desoxy 2,3-alkene subunit, indicated
that the epoxide moiety is mainly a conformation controlling
element without making direct contact with the target surface [16].
Inspired by our docking solution of Br-nannocystin (2a, Fig. 1 right)
Macrocycles are a distinct class of small-molecule compounds
with their functional groups distributed along a constrained ring
scaffold comprising at least 12 atoms [1]. This trait is thought to
favor modulation of challenging biomacromolecular processes such
as protein-protein interactions [2]. In the context of macrocycle-
based drug discovery, chemical synthesis has become an indis-
pensable tool for resolving structural ambiguity, securing synthetic
access to the compound of low natural abundance, and perhaps
more importantly, probing structure-activity relationship (SAR) in a
systematic fashion. Moreover, for structurally complex sp3 rich
macrocyclic compounds, it is worthwhile to investigate the influ-
ence of stereochemical configuration on bioactivity [3]. Such efforts
on stereostructure-activity relationship (SSAR) [4] represent a
strategic sampling of biologically relevant chemical space con-
cerning the target-binding hot spot that would be difficult to cover
with non-macrocyclic small molecules [5].
Nannocystin
A
(1, Fig.
1
left) is
a
21-membered
* Corresponding author.
** Corresponding author.,
E-mail addresses: yuechen@nankai.edu.cn (Y. Chen), zhangweicheng@nankai.
edu.cn (W. Zhang).
https://doi.org/10.1016/j.molstruc.2018.12.107
0022-2860/© 2019 Elsevier B.V. All rights reserved.

Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
569
Fig. 1. Structures of naturally occurring nannocystin A (1) and bromo-substituted congener 2a.
[16], we next sought to modulate the assumed hydrophobic in-
2.2. Synthetic procedures
teractions between nannocystins and the target eEF1A by system-
atic variation of the polyketide C11-phenyl substituent. The second
SAR study showed that small to medium sized groups are tolerated
at the para or meta position of the C11-phenyl ring, whereas a
bulkier substituent led to considerable drop in activity [17].
Although these data align well with our docking calculation, we
cautioned that to date without known X-ray crystallographic
structure of human eEF1A [18] it is no more than a provisional
model that requires continuing validation when more experi-
mental results are available. Given the pronounced interactions
found between the C11-phenyl substituent and the target binding
site [17], we wonder whether the naturally occurring (10R, 11S)
configuration of 2a is optimal compared with other possible ste-
reochemistry combination. Also it remained to be ascertained
whether or not removing the C10-methyl and C11-phenyl groups
toward a simplified structure would bring about any substantial
impact on activity. To address these issues, it would be necessary to
embark on total synthesis of all four stereoisomers (10R, 11S)-2a,
(10S, 11S)-2b, (10S, 11R)-2c, and (10R, 11R)-2d, as well as the trun-
cated version 3 (Fig. 2). Biological assessment in conjunction with
molecular modeling would return valuable information about the
nannocystin pharmacophore and enrich our knowledge for devel-
oping potent nannocystin-based anticancer agents.
2.2.1. (2S,3R)-1-((3aR,6R)-8,8-dimethyl-2,2-dioxidotetrahydro-3H-
3a,6-methanoben-zo[c]isothiazol-1(4H)-yl)-3-hydroxy-2-methyl-3-
phenylpropan-1-one (5a)
To a solution of 4a (15.0 g, 54.5 mmol) in dry CH2Cl2 (60 mL) was
added Et3N (9.3 mL, 67.0 mmol) and TBSOTf (14.0 mL, 61.1 mmol).
The reaction mixture was stirred overnight. All volatiles were
evaporated under reduced pressure and then dry hexane (60 mL)
was added. The suspension was stirred for 10 min and hexane was
decanted under argon to afford the silyl enol ether as a solid, which
was re-dissolved in dry CH2Cl2 (10 mL).
To a solution of benzaldehyde (12.9 g, 121.6 mmol) in dry CH2Cl2
(60 mL) was added dropwise TiCl4 (7.3 mL, 66.4 mmol) at ꢀ78 ꢁC.
After being stirred for 10 min at ꢀ78 ꢁC, a solution of the above-
mentioned silyl enol ether in CH2Cl2 (10 mL) was added slowly. The
reaction mixture was stirred for 3 h at this temperature. The reac-
tion was quenched by saturated aqueous NH4Cl (30 mL) and
extracted with EtOAc (60 mL ꢂ 3). The combined organic layers
were dried over Na2SO4, and concentrated to get the residue. The
residue was purified by column chromatography (petroleum ether/
EtOAc, 8:1) to give the aldol product 5a (15.7 g, 76%) as a white
solid. mp ¼ 176e177 ꢁC. 1H NMR (400 MHz, CDCl3)
d 7.32e7.16 (m,
5H), 4.66 (t, J ¼ 7.6 Hz, 1H), 3.77 (dd, J ¼ 7.4, 5.2 Hz, 1H), 3.49 (p,
J ¼ 6.8 Hz, 1H), 3.37 (q, J ¼ 13.8 Hz, 2H), 3.11 (d, J ¼ 8.4 Hz, 1H),
1.96e1.69 (m, 5H), 1.34e1.18 (m, 2H), 1.12 (d, J ¼ 6.8 Hz, 3H), 0.84 (s,
3H), 0.78 (s, 3H); 13C NMR (100 MHz, CDCl3)
d 175.32, 142.12,
2. Experimental section
128.55, 128.00, 126.41, 77.90, 65.35, 53.26, 48.36, 47.76, 46.47, 44.77,
38.37, 33.01, 26.54, 20.67, 20.00, 14.68. HRMSeESI (m/z): [MþNa]þ
calcd for C20H27NO4SNaþ, 400.1558; found:400.1553.
2.1. General methods
1H NMR and 13C NMR spectra were recorded on a Bruker
Avance-400 digital spectrometer at ambient temperature. High-
resolution mass spectra (HRMS) were detected by using a Varian
7.0 T FTMS instrument. Infrared spectra (IR) were recorded by using
Bruker Tensor37 FTIR spectrometer. Optical rotations were deter-
mined with Insmark IP120 automatic polarimeter. Melting points
were measured on a digital melting-point apparatus without
correction of the thermometer. Unless otherwise noted, all re-
actions were carried out under argon atmosphere with dry solvents
under anhydrous conditions. Tetrahydrofuran (THF) was distilled
immediately before use from sodium-benzophenone ketyl; meth-
ylene chloride (DCM) and triethylamine (Et3N) were distilled from
calcium hydride. All reagents were purchased at the highest com-
mercial quality and used without purification. Intermediates 13 and
14 were prepared according to the literature methods [16]. Total
syntheses of 2a-2d and 3 are shown in Scheme 1.
2.2.2. (2R,3R)-1-((3aR,6R)-8,8-dimethyl-2,2-dioxidotetrahydro-
3H-3a,6-methanobenz-o[c]isothiazol-1(4H)-yl)-3-hydroxy-2-
methyl-3-phenylpropan-1-one (5b)
To a solution of 4a (15.0 g, 54.5 mmol) in dry CH2Cl2 (60 mL)
stirred at ꢀ10 ꢁC under Ar was added nBu2BOTf (45.3 mL,
45.3 mmol, 1 M in CH2Cl2) and DIPEA (9.1 mL, 65.4 mmol). The
resulting mixture was stirred at ꢀ10 ꢁC for 30 min before being
cooled to ꢀ78 ꢁC. Then a solution of benzaldehyde in CH2Cl2 was
added. The reaction mixture was stirred for 3 h at this temperature.
The reaction was quenched by saturated aqueous NH4Cl (30 mL)
and extracted with EtOAc (60 mL ꢂ 3). The combined organic layers
were dried over Na2SO4, concentrated to get the residue. The res-
idue was purified by column chromatography (petroleum ether/
EtOAc, 8:1) to give the aldol product 5b (16.5 g, 80%) as a white
solid. mp ¼ 176e178 ꢁC. 1H NMR (400 MHz, CDCl3)
d 7.33 (d,

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Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
Fig. 2. Structures of stereoisomeric nannocystins 2a-2d and the truncated analogue 3.
J ¼ 7.4 Hz, 2H), 7.27 (t, J ¼ 7.6 Hz, 2H), 7.19 (d, J ¼ 3.1 Hz, 1H), 5.12 (s,
1H), 3.86 (t, J ¼ 6.3 Hz, 1H), 3.59 (s, 1H), 3.50e3.38 (m, 2H), 3.30 (qd,
J ¼ 7.0, 2.4 Hz, 1H), 2.01 (d, J ¼ 6.3 Hz, 2H), 1.86 (dd, J ¼ 16.0, 7.3 Hz,
3H), 1.36e1.26 (m, 2H), 1.09e1.06 (m, 6H), 0.91 (s, 3H); 13C NMR
The reaction mixture was stirred overnight. After all volatiles were
evaporated under reduced pressure, dry hexane (60 mL) was
added. The suspension was stirred for 10 min and hexane was
decanted under argon to afford the desired silyl enol ether as a
solid, which was re-dissolved in dry CH2Cl2 (10 mL).
(100 MHz, CDCl3)
d 177.29, 141.00, 128.31, 127.40, 126.21, 72.19,
65.11, 53.19, 48.60, 47.92, 46.05, 44.75, 38.44, 32.98, 26.55, 20.98,
19.99, 10.93. HRMSeESI (m/z): [MþNa]þ calcd for C20H27NO4SNaþ,
400.1558; found:400.1553.
To a solution of benzaldehyde (12.9 g, 121.6 mmol) in dry CH2Cl2
(60 mL) was added dropwise TiCl4 (7.3 mL, 66.4 mmol) at ꢀ78 ꢁC.
After being stirred for 10 min at ꢀ78 ꢁC, a solution of the above-
mentioned silyl enol ether in CH2Cl2 (10 mL) was added slowly. The
reaction mixture was stirred for 3 h at this temperature. The reac-
tion was quenched by saturated aqueous NH4Cl (30 mL) and
extracted with EtOAc (60 mL ꢂ 3). The combined organic layers
were dried over Na2SO4, concentrated to get the residue. The res-
idue was purified by column chromatography (petroleum ether/
2.2.3. (2R,3S)-1-((3aR,6S)-8,8-dimethyl-2,2-dioxidotetrahydro-3H-
3a,6-methanobenz-o[c]isothiazol-1(4H)-yl)-3-hydroxy-2-methyl-3-
phenylpropan-1-one (5c)
To a solution of 4b (15.0 g, 54.5 mmol) in dry CH2Cl2 (60 mL) was
added Et3N (9.3 mL, 67.0 mmol) and TBSOTf (14.0 mL, 61.1 mmol).

Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
571
Scheme 1. Synthesis of nannocystin analogues 2a-2d and 3. Reagents and conditions: (a) TBSOTf, NEt3, CH2Cl2, then PhCHO, TiCl4, DCM, ꢀ78 ꢁC; (b) nBu2BOTf, iPrNEt2, CH2Cl2, -5 ꢁC,
then PhCHO, ꢀ78 ꢁC; (c) TBSOTf, 2,6-lutidine, CH2Cl2, -78 ꢁC; (d) DIBAL-H, CH2Cl2, -78 ꢁC; (e) MePPh3Br, tBuOK, 0 ꢁC; (f) TBAF, THF; (g) 13, PPh3, DEAD, THF; (h) TFA, THF; (i) 14, EDC,
HOBT, NaHCO3, THF; (j) Pd(OAc)2, Cs2CO3, NEt3, DMF; (k) CH2]CHCH2CH2OH (18), DMAP, EDC, NEt3, THF.
EtOAc, 8:1) to give the aldol product 5c (15.5 g, 75%) as a white
45.3 mmol, 1 M in CH2Cl2) and DIPEA (9.1 mL, 65.4 mmol). The
resulting mixture was stirred at ꢀ10 ꢁC for 30 min before cooled
to ꢀ78 ꢁC. Then a solution of Benzaldehyde in CH2Cl2 was added.
The reaction mixture was stirred for 3 h at this temperature. The
reaction was quenched by saturated aqueous NH4Cl (30 mL) and
extracted with EtOAc (60 mL ꢂ 3). The combined organic layers
were dried over Na2SO4, concentrated to get the residue. The res-
idue was purified by column chromatography (petroleum ether/
EtOAc, 8:1) to give the aldol product 5d (15.7 g, 76%) as a white
solid. mp ¼ 170e172 ꢁC. 1H NMR (400 MHz, CDCl3)
d 7.36e7.29 (m,
4H), 7.27e7.22 (m, 1H), 4.72 (t, J ¼ 7.6 Hz, 1H), 3.83 (dd, J ¼ 7.5,
5.1 Hz, 1H), 3.55 (p, J ¼ 6.8 Hz, 1H), 3.43 (q, J ¼ 13.8 Hz, 2H), 3.23 (d,
J ¼ 8.3 Hz, 1H), 2.00e1.74 (m, 5H), 1.37e1.26 (m, 2H), 1.17 (d,
J ¼ 6.8 Hz, 3H), 0.91 (s, 3H), 0.85 (s, 3H); 13C NMR (100 MHz, CDCl3)
d
175.24, 142.09, 128.46, 127.91, 126.36, 77.79, 65.25, 53.16, 48.28,
47.67, 46.39, 44.69, 38.29, 32.91, 26.46, 20.61, 19.93, 14.61;
HRMSeESI (m/z): [MþNa]þ calcd for C20H27NO4SNaþ, 400.1558;
found:400.1553.
solid. mp ¼ 169e171 ꢁC. 1H NMR (400 MHz, CDCl3)
d 7.41 (d,
J ¼ 7.2 Hz, 2H), 7.34 (t, J ¼ 7.1 Hz, 2H), 7.25 (t, J ¼ 6.0 Hz, 1H), 5.19 (s,
1H), 3.93 (t, J ¼ 5.6 Hz,1H), 3.66 (s,1H), 3.50 (q, J ¼ 13.8 Hz, 2H), 3.37
(d, J ¼ 6.8 Hz, 1H), 2.08 (d, J ¼ 5.3 Hz, 2H), 1.98e1.85 (m, 3H), 1.39
(dt, J ¼ 18.4, 10.8 Hz, 2H), 1.14 (s, 6H), 0.98 (s, 3H).; 13C NMR
2.2.4. (2S,3S)-1-((3aR,6S)-8,8-dimethyl-2,2-dioxidotetrahydro-3H-
3a,6-methanobenz-o[c]isothiazol-1(4H)-yl)-3-hydroxy-2-methyl-3-
phenylpropan-1-one (5d)
(100 MHz, CDCl3)
d 177.24, 141.02, 128.28, 127.37, 126.20, 72.17,
To a solution of 4d (15.0 g, 54.5 mmol) in dry CH2Cl2 (60 mL)
65.08, 53.16, 48.57, 47.90, 46.03, 44.74, 38.42, 32.95, 26.53, 20.96,
stirred at ꢀ10 ꢁC under Ar was added nBu2BOTf (45.3 mL,

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Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
19.96, 10.92. HRMSeESI (m/z): [MþNa]þ calcd for C20H27NO4SNaþ,
88% yield as a white solid. mp ¼ 109e110 ꢁC. 1H NMR (400 MHz,
400.1558; found:400.1553.
CDCl3)
d
7.36e7.30 (m, 2H), 7.27e7.18 (m, 3H), 4.88 (d, J ¼ 8.4 Hz,
1H), 3.63 (t, J ¼ 6.3 Hz, 1H), 3.36 (dt, J ¼ 39.4, 12.0 Hz, 3H), 1.96 (d,
J ¼ 6.0 Hz, 2H),1.80 (d, J ¼ 7.4 Hz, 3H),1.37 (d, J ¼ 6.9 Hz, 3H), 1.26 (d,
J ¼ 11.4 Hz, 2H), 1.09 (s, 3H), 0.89 (d, J ¼ 10.1 Hz, 3H), 0.82 (s, 9H),
0.00 (s, 3H), ꢀ0.31 (d, J ¼ 4.6 Hz, 3H); 13C NMR (100 MHz, CDCl3)
2.2.5. (2S,3R)-3-((tert-butyldimethylsilyl)oxy)-1-((3aR,6R)-8,8-
dimethyl-2,2-dioxidohex-ahydro-1H-3a,6-methanobenzo[c]
isothiazol-1-yl)-2-methyl-3-phenylpropan-1-one (6a)
To a solution of the aldol product 5a (1.8 g, 4.8 mmol) in dry
CH2Cl2 (30 mL) was added 2.6-lutidine (1.7 mL, 14.2 mmol) and
then TBSOTf (2.2 mL, 9.6 mmol) at ꢀ78 ꢁC. The temperature was
warmed to room temperature slowly. Then the reaction was
quenched by CH3OH (1 mL) and saturated aqueous NH4Cl (10 mL),
extracted with EtOAc (100 mL ꢂ 3). The combined organic layers
were dried over Na2SO4, filtered, concentrated to give the residue.
The residue was purified by column chromatography (petroleum
ether/EtOAc, 20:1) to give the TBS protected product 6a (2.1 g, 92%)
as a white solid. mp ¼ 200e201 ꢁC. 1H NMR (400 MHz, CDCl3)
d 173.96, 143.21, 127.84, 127.80, 127.48, 75.48, 64.84, 53.00, 49.50,
48.33, 47.82, 44.66, 38.38, 32.71, 26.46, 25.90, 20.91, 19.95, 18.26,
16.24, ꢀ4.53, ꢀ4.92. HRMSeESI (m/z): [MþNa]þ calcd for
C
26H41NO4SSiNaþ, 514.2418; found: 514.2414.
2.2.9. (2S,3R)-3-((tert-butyldimethylsilyl)oxy)-2-methyl-3-
phenylpropanal (7a)
To a solution of the amide 6a (0.84 g, 1.7 mmol) in dry CH2Cl2
(10 mL) was added DIBAl-H (2.2 mL, 2.2 mmol, 1 M in toluene)
at ꢀ78 ꢁC. After being stirred for 3 h at this temperature, saturated
aqueous potassium sodium tartrate (5 mL) was added, and the
solution was extracted with EtOAc (60 mL ꢂ 3). The combined
organic layers were dried over Na2SO4, filtered, and concentrated to
give the residue. The residue was purified by column chromatog-
raphy (petroleum ether/EtOAc, 50:1) to give the aldehyde 7a (0.3 g,
d
7.33e7.13 (m, 5H), 4.75 (d, J ¼ 8.9 Hz, 1H), 3.84 (dd, J ¼ 7.7, 5.0 Hz,
1H), 3.49e3.27 (m, 3H), 2.20e2.10 (m, 1H), 2.04 (dd, J ¼ 13.8, 7.9 Hz,
1H), 1.91e1.77 (m, 3H), 1.38e1.24 (m, 2H), 1.15 (s, 3H), 0.90 (s, 3H),
0.80 (d, J ¼ 6.8 Hz, 3H), 0.69 (s, 9H), ꢀ0.15 (s, 3H), ꢀ0.36 (s, 3H); 13
C
NMR (100 MHz, CDCl3)
d 175.02, 142.49, 128.18, 127.93, 127.75,
78.61, 65.82, 53.31, 48.95, 48.28, 47.85, 44.99, 38.99, 33.16, 26.65,
26.01, 21.49, 20.05, 18.17, 14.75, ꢀ4.53, ꢀ4.57. HRMSeESI (m/z):
[MþNa]þ calcd for C26H41NO4SSiNaþ, 514.2418; found: 514.2414.
70% yield) as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 10.06 (d,
J ¼ 2.6 Hz, 1H), 7.56e7.49 (m, 5H), 5.03 (d, J ¼ 7.7 Hz, 1H), 2.94 (pd,
J ¼ 7.2, 2.6 Hz, 1H), 1.10 (s, 12H), 0.27 (s, 3H), ꢀ0.00 (s, 3H). 13C NMR
(100 MHz, CDCl3)
d 204.07, 142.26, 128.30, 127.83, 126.68, 76.81,
2.2.6. (2R,3R)-3-((tert-butyldimethylsilyl)oxy)-1-((6R,7aR)-8,8-
dimethyl-2,2-dioxidotet-rahydro-3H-3a,6-methanobenzo[c]
isothiazol-1(4H)-yl)-2-methyl-3-phenylpro-pan-1-one (6b)
The title compound was synthesized from 5b following the
general procedure described for 6a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 20:1) afforded the product in
95% yield as a white solid. mp ¼ 105e107 ꢁC. 1H NMR (400 MHz,
54.55, 25.72, 18.06, 11.04, ꢀ4.52, ꢀ5.23. HRMSeESI (m/z): [MþNa]þ
calcd for C16H26O2SiNaþ, 301.1600; found: 301.1603.
2.2.10. (2R,3R)-3-((tert-butyldimethylsilyl)oxy)-2-methyl-3-
phenylpropanal (7b)
The title compound was synthesized from 6b following the
general procedure described for 7a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 60:1) afforded the product in
CDCl3)
d
7.34 (d, J ¼ 6.9 Hz, 2H), 7.21 (dd, J ¼ 14.5, 6.9 Hz, 3H), 4.88
(d, J ¼ 8.2 Hz,1H), 3.63 (s,1H), 3.33 (dd, J ¼ 40.2,14.3 Hz, 3H),1.97 (s,
2H), 1.80 (d, J ¼ 7.9 Hz, 3H), 1.37 (d, J ¼ 6.5 Hz, 2H), 1.25 (d,
J ¼ 11.1 Hz, 3H), 1.09 (s, 3H), 0.89 (s, 3H), 0.82 (s, 9H), 0.00 (s,
75% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 9.69 (s, 1H),
7.29e7.15 (m, 5H), 5.08 (s, 1H), 2.51 (s, 1H), 0.97 (d, J ¼ 6.7 Hz, 3H),
0.82 (s, 9H), ꢀ0.04 (s, 3H), ꢀ0.25 (s, 3H); 13C NMR (100 MHz, CDCl3)
3H), ꢀ0.30 (s, 3H); 13C NMR (100 MHz, CDCl3)
d
173.94, 143.20,
d 204.45, 142.45, 128.30, 127.60, 126.41, 74.40, 54.93, 25.89, 18.28,
127.82, 127.79, 127.47, 75.46, 64.83, 52.99, 49.48, 48.32, 47.81, 44.66,
38.37, 32.71, 26.46, 25.90, 20.90, 19.95, 18.25, 16.21, ꢀ4.53, ꢀ4.92.
HRMSeESI (m/z): [MþNa]þ calcd for C26H41NO4SSiNaþ, 514.2418;
found: 514.2414.
8.16, ꢀ4.38, ꢀ5.10. HRMSeESI (m/z): [MþNa]þ calcd for
C
16H26O2SiNaþ, 301.1600; found: 301.1603.
2.2.11. (2R,3S)-3-((tert-butyldimethylsilyl)oxy)-2-methyl-3-
phenylpropanal (7c)
2.2.7. (2R,3S)-3-((tert-butyldimethylsilyl)oxy)-1-((6R,7aR)-8,8-
dimethyl-2,2-dioxidotetrahydro-3H-3a,6-methanobenzo[c]
isothiazol-1(4H)-yl)-2-methyl-3-phenylpropan-1-one (6c)
The title compound was synthesized from 5c following the
general procedure described for 6a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 20:1) afforded the product in
92% yield as a white solid. mp ¼ 180e181 ꢁC. 1H NMR (400 MHz,
The title compound was synthesized from 6c following the
general procedure described for 7a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 60:1) afforded the product in
72% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 9.80 (d,
J ¼ 2.3 Hz, 1H), 7.35e7.24 (m, 5H), 4.75 (d, J ¼ 7.6 Hz, 1H), 2.72e2.64
(m, 1H), 0.87 (d, J ¼ 7.0 Hz, 3H), 0.84 (s, 9H), ꢀ0.00 (s, 3H), ꢀ0.27 (s,
3H);13C NMR (100 MHz, CDCl3)
d 204.66, 142.40, 128.45, 127.97,
CDCl3)
d
7.36e7.26 (m, 5H), 4.84 (d, J ¼ 8.9 Hz, 1H), 3.93 (dd, J ¼ 7.7,
126.84, 76.96, 54.72, 25.83, 18.21, 11.22, ꢀ4.39, ꢀ5.09. HRMSeESI
(m/z): [MþNa]þ calcd for C16H26O2SiNaþ, 301.1600; found:
301.1603.
5.0 Hz, 1H), 3.56e3.34 (m, 3H), 2.24 (d, J ¼ 11.9 Hz, 1H), 2.12 (dd,
J ¼ 13.8, 7.9 Hz, 1H), 1.90 (dd, J ¼ 14.4, 5.4 Hz, 3H), 1.47e1.35 (m, 2H),
1.23 (s, 3H), 0.99 (s, 3H), 0.88 (d, J ¼ 6.8 Hz, 3H), 0.78 (s, 9H), ꢀ0.06
(s, 3H), ꢀ0.27 (s, 3H); 13C NMR (100 MHz, CDCl3)
d
174.99, 142.47,
2.2.12. (2S,3S)-3-((tert-butyldimethylsilyl)oxy)-2-methyl-3-
phenylpropanal (7d)
128.16, 127.91, 127.73, 78.59, 65.79, 53.28, 48.93, 48.26, 47.83, 44.97,
38.97, 33.14, 26.63, 25.99, 21.47, 20.03, 18.15, 14.73, ꢀ4.54, ꢀ4.59.
HRMSeESI (m/z): [MþNa]þ calcd for C26H41NO4SSiNaþ, 514.2418;
found: 514.2414.
The title compound was synthesized from 6d following the
general procedure described for 7a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 60:1) afforded the product in
78% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 9.74 (d,
2.2.8. (2S,3S)-3-((tert-butyldimethylsilyl)oxy)-1-((6R,7aR)-8,8-
dimethyl-2,2-dioxid-otetrahydro-3H-3a,6-methanobenzo[c]
isothiazol-1(4H)-yl)-2-methyl-3-phe-nylpropan-1-one (6d)
The title compound was synthesized from 5d following the
general procedure described for 6a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 20:1) afforded the product in
J ¼ 1.2 Hz, 1H), 7.31e7.19 (m, 5H), 5.13 (d, J ¼ 4.2 Hz, 1H), 2.55 (ddd,
J ¼ 6.9, 4.3, 1.2 Hz, 1H), 1.01 (d, J ¼ 7.0 Hz, 3H), 0.86 (s, 9H), ꢀ0.00 (s,
3H), ꢀ0.21 (s, 3H); 13C NMR (100 MHz, CDCl3)
d 204.31, 142.30,
128.15, 127.45, 126.26, 74.25, 54.78, 25.75, 18.13, 8.01, ꢀ4.53, ꢀ5.24.
HRMSeESI (m/z): [MþNa]þ calcd for C16H26O2SiNaþ, 301.1600;
found: 301.1603.

Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
573
2.2.13. tert-Butyldimethyl(((1R,2R)-2-methyl-1-phenylbut-3-en-1-
yl)oxy)silane (8a)
on silica gel (petroleum ether:EtOAc ¼ 5:1) to give the product 10a
(513 mg, 78%) as a colorless oil. 1H NMR (400 MHz, CDCl3)
To a solution of MePPh3Br (1283.2 mg, 3.6 mmol) in dry THF
(7.2 mL) was added potassium tert-butoxide (402.83 mg, 3.6 mmol)
at 0 ꢁC. After being stirred for 1 h at this temperature, the solution
of 7a (500 mg 1.8 mmol) in THF (5 mL) was added at 0 ꢁC, and then
the temperature was warmed to room temperature slowly. After
being stirred for 2 h, the reaction was quenched by saturated
aqueous NH4Cl (30 mL), and extracted with EtOAc (60 mL ꢂ 3). The
combined organic layers were dried over Na2SO4, concentrated to
get a residue. The residue was used directly for the next step
without further purification.
d
7.26e7.15 (m, 5H), 5.62 (d, J ¼ 7.0 Hz, 1H), 5.55 (dd, J ¼ 17.4, 8.8 Hz,
1H), 5.33 (d, J ¼ 6.6 Hz, 1H), 4.91 (d, J ¼ 14.4 Hz, 2H), 4.16 (d,
J ¼ 8.3 Hz, 1H), 2.69 (dd, J ¼ 13.3, 6.6 Hz, 1H), 2.57 (br s, 1H), 1.37 (s,
9H), 1.12 (s, 3H), 1.02 (s, 6H); 13C NMR (100 MHz, CDCl3)
d 171.64,
155.83, 138.93, 138.15, 128.33, 128.26, 127.43, 116.22, 80.57, 71.97,
61.28, 42.83, 28.41, 27.01, 26.24, 15.39; HRMSeESI (m/z): [MþNa]þ
calcd for C21H31NO5Naþ, 400.2100; found:400.2100.
2.2.19. (1S,2S)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((tert-
butoxycarbonyl)amino)-3-hydroxy-3-methylbutanoate (10b)
The title compound was synthesized from 9b and 13 following
the general procedure described for 10a. Flash column chroma-
tography (eluent: petroleum ether:EtOAc ¼ 5:1) afforded the
product in 72% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
Compounds 8b-8d were prepared by the same procedure.
2.2.14. (1R,2R)-2-methyl-1-phenylbut-3-en-1-ol (9a)
To a solution of 8a (3.8 g, 13.7 mmol) in THF (380 mL) was added
TBAF (20.1 mL, 20.1 mmol) at room temperature. After being stirred
for 2 h at this temperature, saturated aqueous NH4Cl (30 mL) was
added to quench the reaction, and then extracted with EtOAc
(50 mL ꢂ 3). The combined organic layers were dried over Na2SO4,
concentrated to get a residue. The residue was purified by column
chromatography (petroleum ether/EtOAc, 50:1) to give the pure
product 9a (1.9 g, 85%) as a colorless oil. 1H NMR (400 MHz, CDCl3)
d
7.34e7.26 (m, 5H), 5.82e5.71 (m, 1H), 5.61 (d, J ¼ 8.4 Hz, 1H), 5.37
(d, J ¼ 8.9 Hz, 1H), 5.08 (t, J ¼ 13.3 Hz, 2H), 4.19 (d, J ¼ 9.2 Hz, 1H),
2.75e2.63 (m, 2H),1.48e1.33 (m, 9H),1.17 (s, 3H),1.13e1.05 (m, 3H),
0.84 (dd, J ¼ 17.5, 5.5 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 171.10,
155.69, 139.86, 138.16, 128.43, 128.39, 127.45, 116.31, 80.61, 80.08,
71.93, 61.35, 43.54, 28.35, 26.92, 26.22, 16.41. HRMSeESI (m/z):
[MþNa]þ calcd for C21H31NO5Naþ, 400.2100; found:400.2100.
d
7.33e7.09 (m, 5H), 5.82e5.63 (m, 1H), 5.13 (m, 2H), 4.29 (dd,
J ¼ 7.9, 1.8 Hz, 1H), 2.49e2.35 (m, 1H), 2.09 (d, J ¼ 2.3 Hz, 1H), 0.80
2.2.20. (1R,2S)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((tert-
butoxycarbonyl)amino)-3-hydroxy-3-methylbutanoate (10c)
The title compound was synthesized from 9c and 13 following
the general procedure described for 10a. Flash column chroma-
tography (eluent: petroleum ether:EtOAc ¼ 5:1) afforded the
product in 72% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
(d, J ¼ 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 143.56, 141.79,
129.39, 128.81, 127.99, 118.01, 79.01, 47.46, 17.69.
2.2.15. (1R,2S)-2-methyl-1-phenylbut-3-en-1-ol (9b)
The title compound was synthesized from 8b following the
general procedure described for 9a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 60:1) afforded the product in
d
7.29 (s, 5H), 5.69 (s, 2H), 5.31 (s, 1H), 5.00 (d, J ¼ 13.0 Hz, 2H), 4.26
(s, 1H), 2.70 (s, 2H), 1.43 (s, 9H), 1.24 (s, 6H), 1.07 (s, 3H); 13C NMR
(100 MHz, CDCl3) 171.43, 155.74, 139.03, 138.38, 128.33, 128.06,
84% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d
7.27e7.11
d
(m, 5H), 5.73e5.57 (m, 1H), 5.04e4.86 (m, 2H), 4.49 (d, J ¼ 5.3 Hz,
127.00, 116.15, 80.68, 80.21, 71.83, 61.34, 42.87, 28.39, 27.15, 26.18,
15.04. HRMSeESI (m/z): [MþNa]þ calcd for C21H31NO5Naþ,
400.2100; found:400.2100.
1H), 2.56e2.41 (m, 1H), 2.13 (s, 1H), 0.93 (d, J ¼ 6.8 Hz, 3H).13C NMR
(100 MHz, CDCl3)
d 142.67, 140.42, 128.18, 127.47, 126.64, 115.65,
77.42, 44.75, 29.83, 25.76, 14.17.
2.2.21. (1R,2R)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((tert-
butoxycarbonyl)amino)-3-hydroxy-3-methylbutanoate (10d)
The title compound was synthesized from 9d and 13 following
the general procedure described for 10a. Flash column chroma-
tography (eluent: petroleum ether:EtOAc ¼ 5:1) afforded the
product in 70% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
2.2.16. (1S,2S)-2-methyl-1-phenylbut-3-en-1-ol (9c)
The title compound was synthesized from 8c following the
general procedure described for 9a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 60:1) afforded the product in
80% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 7.23 (d,
J ¼ 18.8 Hz, 5H), 5.73 (d, J ¼ 7.1 Hz, 1H), 5.11 (d, J ¼ 17.3 Hz, 2H), 4.27
d
7.31 (s, 5H), 5.82e5.67 (m, 1H), 5.58 (d, J ¼ 7.8 Hz, 1H), 5.29 (d,
(s, 1H), 2.41 (d, J ¼ 5.7 Hz, 1H), 2.17 (s, 1H), 0.79 (s, 3H); 13C NMR
J ¼ 8.1 Hz,1H), 5.07 (dd, J ¼ 23.3,13.7 Hz, 2H),4.23 (d, J ¼ 8.9 Hz,1H),
(100 MHz, CDCl3)
d 142.60, 140.75, 128.31, 127.71, 126.93, 116.77,
2.80e2.65 (m, 2H), 1.38 (d, J ¼ 13.2 Hz, 9H), 1.22 (d, J ¼ 7.5 Hz, 6H),
77.96, 46.30, 16.59.
0.88 (t, J ¼ 8.6 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 171.23, 155.68,
139.75, 138.38, 128.47, 128.29, 127.22, 116.43, 81.06, 80.14, 72.05,
61.35, 43.52, 28.36, 27.06, 26.07, 16.65. HRMSeESI (m/z): [MþNa]þ
calcd for C21H31NO5Naþ, 400.2100; found:400.2100.
2.2.17. (1S,2R)-2-methyl-1-phenylbut-3-en-1-ol (9d)
The title compound was synthesized from 8d following the
general procedure described for 9a. Flash column chromatography
(eluent: petroleum ether:EtOAc ¼ 60:1) afforded the product in
2.2.22. But-3-en-1-yl (S)-2-((tert-butoxycarbonyl)amino)-3-
hydroxy-3-methylbutano-ate (15)
82% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 7.27e7.11
(m, 5H), 5.73e5.57 (m, 1H), 5.04e4.86 (m, 2H), 4.49 (d, J ¼ 5.3 Hz,
To a solution of 18 (138 mg, 1.93 mmol) and 13 (300 mg,
1.29 mmol) in THF (4 mL) were added EDCI (492 mg, 2.57 mmol),
DMAP (15.6 mg, 0.13 mmol), and TEA (0.35 mL, 2.57 mmol) at room
temperature. After being stirred overnight, all volatiles were
evaporated under reduced pressure. The mixture was diluted with
EtOAc (50 mL). The EtOAc solution was washed with brine, dried
over Na2SO4 and the solvent were evaporated under reduced
pressure. The residue was purified through column chromatog-
raphy (petroleum ether/EtOAc, 20:1 to 10:1) to give the product 15
1H), 2.56e2.41 (m,1H), 2.13 (s,1H), 0.93 (d, J ¼ 6.8 Hz, 3H); 13C NMR
(100 MHz, CDCl3)
d 142.67, 140.42, 128.18, 127.47, 126.64, 115.65,
77.42, 44.75, 29.83, 25.76, 14.17.
2.2.18. (1S,2R)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((tert-
butoxycarbonyl)amino)-3-hydroxy-3-methylbutanoate (10a)
To a solution of 9a (278 mg, 1.7 mmol) and PPh3 (900 mg,
3.4 mmol) in THF (1.8 mL) were added 13 (480 mg, 2.1 mmol) and
DIAD (612 mg, 3.0 mmol) at 0 ꢁC. After stirring at room temperature
overnight, the reaction mixture was evaporated under reduced
pressure, and the residue was purified by column chromatography
(60 mg, 20%) as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 5.78 (dq,
J ¼ 11.6, 6.7 Hz, 1H), 5.37 (s, 1H), 5.12 (t, J ¼ 13.3 Hz, 2H), 4.22 (dd,
J ¼ 16.2, 9.5 Hz, 3H), 2.60 (s, 1H), 2.43 (q, J ¼ 6.6 Hz, 2H), 1.45 (s, 9H),

574
Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
1.26 (s, 6H); 13C NMR (100 MHz, CDCl3)
d
171.79, 155.77, 133.65,
133.00, 130.03, 129.70, 128.33, 127.55, 116.52, 116.35, 110.15, 80.86,
117.72, 80.16, 71.85, 64.43, 61.33, 32.90, 29.67, 28.28, 26.88, 26.27.
HRMSeESI (m/z): [MþNa]þ calcd for C14H25NO5Naþ, 301.1625;
found: 301.1630.
80.36, 71.67, 62.06, 60.50, 60.27, 59.35, 56.49, 54.54, 43.24, 36.56,
32.83, 31.84, 31.12, 29.75, 27.02, 26.95, 24.78, 18.93, 16.15, 15.37,
15.05, 14.20, 10.66. HRMSeMALDI (m/z): [MþNa]þ calcd for
C
42H57BrIN3O9Naþ, 976.2215; found: 976.2218.
2.2.23. (S)-3-Hydroxy-3-methyl-1-(((1S,2R)-2-methyl-1-
phenylbut-3-en-1-yl)oxy)-1-oxobutan-2-aminium 2,2,2-
trifluoroacetate (11a)
To a solution of 10a (735 mg, 1.9 mmol) in CH2Cl2 (4 mL) was
added TFA (1.3 mL, 19.0 mmol) at room temperature. After being
stirred at the same temperature for 3 h, all volatiles were evapo-
rated under reduced pressure and toluene (5 mL ꢂ 3) was added to
remove the TFA to get the residue 11a. The residue was used
directly for the next step without further purification. Compounds
11b-11d and 16 were prepared by the same procedure.
2.2.26. (1R,2S)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((R)-3-(3-
bromo-4-hydroxypheny-l)-2-((2S,3S)-2-((2R,3S)-3-((R,E)-4-iodo-2-
methoxy-3-methylbut-3-en-1-yl)-N,2-dimethyloxirane-2-
carboxamido)-3-methylpentanamido)propanamido)-3-hydroxy-3-
methylbutanoate (12c)
The title compound was synthesized from 14 and 11c following
the general procedure described for 12a. Flash column chroma-
tography (petroleum ether/EtOAc, 4:1 to 3:1) afforded the product
in 70% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 7.33e7.25
(m, 5H), 6.92 (t, J ¼ 7.0 Hz, 2H), 6.77 (d, J ¼ 8.2 Hz, 1H), 6.56 (d,
J ¼ 7.9 Hz, 1H), 6.33 (s, 1H), 5.72e5.65 (m, 2H), 5.02 (s, 1H), 4.99 (d,
J ¼ 7.5 Hz, 1H), 4.65 (dd, J ¼ 14.0, 8.0 Hz, 1H), 4.55 (d, J ¼ 8.8 Hz, 1H),
4.23 (d, J ¼ 10.8 Hz, 1H), 3.86 (t, J ¼ 6.5 Hz,1H), 3.21 (s, 3H), 3.07 (dd,
J ¼ 12.3, 5.8 Hz, 2H), 3.00 (s, 3H), 2.84 (dd, J ¼ 14.2, 8.4 Hz, 1H), 2.70
(dd, J ¼ 13.2, 6.6 Hz, 1H), 2.04 (d, J ¼ 6.3 Hz, 1H), 1.78 (d, J ¼ 5.6 Hz,
5H), 1.33 (s, 3H), 1.27 (s, 4H), 1.15 (s, 3H), 1.04 (d, J ¼ 6.8 Hz, 3H), 0.86
(dt, J ¼ 19.2, 6.9 Hz, 5H), 0.69 (d, J ¼ 6.4 Hz, 3H); 13C NMR (100 MHz,
2.2.24. (1S,2R)-2-methyl-1-phenylbut-3-en-1-yl(S)-2-((R)-3-(3-
bromo-4-hydroxyphenyl)-2-((2S,3S)-2-((2R,3S)-3-((R,E)-4-iodo-2-
methoxy-3-methylbut-3-en-1-yl)-N,2-dimethyloxirane-2-
carboxamido)-3-methylpentanamido)propanamido)-3-hydroxy-3-
methylbutanoate (12a)
To a solution of 14 (200 mg, 0.288 mmol) and 11a (129.20 mg,
0.345 mmol) in THF (4 mL) were added HOBT (77.77 mg,
0.576 mmol), EDCI (66.97 mg, 0.432 mmol), and NaHCO3 (36.24 mg,
0.432 mmol) at room temperature. After being stirred overnight, all
volatiles were evaporated under reduced pressure. The mixture
was diluted with EtOAc (50 mL). The EtOAc solution was washed
with brine, dried over Na2SO4 and the solvent were evaporated
under reduced pressure. The residue was purified through column
chromatography (petroleum ether/EtOAc, 4:1 to 3:1) to give the
product 12a (225 mg, 82%) as a colorless oil. 1H NMR (400 MHz,
CDCl3) d 171.92, 170.54, 170.35, 170.10, 151.67, 146.92, 139.15, 138.33,
132.69, 130.13, 129.91, 128.34, 128.06, 126.98, 116.58, 116.14, 110.28,
83.65, 80.87, 80.35, 71.69, 62.84, 60.47, 59.35, 56.59, 54.41, 42.93,
36.48, 32.93, 31.89, 31.61, 27.31, 26.99, 25.00, 22.77, 18.99, 15.64,
15.05, 14.95, 14.24, 10.75. HRMSeMALDI (m/z): [MþNa]þ calcd for
C
42H57BrIN3O9Naþ, 976.2215; found: 976.2218.
2.2.27. (1R,2R)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((R)-3-(3-
bromo-4-hydroxypheny-l)-2-((2S,3S)-2-((2R,3S)-3-((R,E)-4-iodo-2-
methoxy-3-methylbut-3-en-1-yl)-N,2-dimethyloxirane-2-
carboxamido)-3-methylpentanamido)propanamido)-3-hydroxy-3-
methylbutanoate (12d)
CDCl3)
d
7.35e7.26 (m, 6H), 7.06 (d, J ¼ 7.1 Hz, 1H), 6.91 (t, J ¼ 7.8 Hz,
2H), 6.52 (d, J ¼ 7.7 Hz, 1H), 6.33 (s, 1H), 5.70 (s, 1H), 5.62 (d,
J ¼ 7.7 Hz, 1H), 5.57 (dd, J ¼ 10.0, 7.5 Hz, 1H), 4.98 (d, J ¼ 3.7 Hz, 1H),
4.95 (s, 1H), 4.68 (dd, J ¼ 14.2, 8.4 Hz, 1H), 4.41 (d, J ¼ 8.3 Hz, 1H),
4.29 (d, J ¼ 10.9 Hz, 1H), 3.85 (t, J ¼ 6.6 Hz, 1H), 3.14 (s, 4H), 3.02 (s,
3H), 2.89 (dd, J ¼ 14.3, 8.7 Hz, 1H), 2.82 (s, 1H), 2.77 (dd, J ¼ 13.9,
7.1 Hz, 1H), 2.06 (s, 1H), 1.82e1.76 (m, 5H), 1.42 (s, 3H), 1.28 (s, 1H),
1.11 (d, J ¼ 6.7 Hz, 3H), 1.07 (s, 3H), 1.02 (s, 3H), 0.89 (s, 1H),
0.87e0.82 (m, 3H), 0.70 (d, J ¼ 6.4 Hz, 3H); 13C NMR (100 MHz,
The title compound was synthesized from 14 and 11d following
the general procedure described for 12a. Flash column chroma-
tography (petroleum ether/EtOAc, 4:1 to 3:1) afforded the product
in 65% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 7.30 (dd,
J ¼ 10.9, 6.8 Hz, 4H), 7.23 (d, J ¼ 1.7 Hz, 1H), 6.91 (dd, J ¼ 11.5, 5.0 Hz,
2H), 6.75 (d, J ¼ 8.3 Hz, 1H), 6.71e6.66 (m, 1H), 6.32 (s, 1H),
5.79e5.69 (m, 1H), 5.58 (d, J ¼ 7.5 Hz, 1H), 5.09e5.01 (m, 2H), 4.64
(dd, J ¼ 14.1, 8.1 Hz, 1H), 4.52 (d, J ¼ 8.9 Hz, 1H), 4.24 (d, J ¼ 10.8 Hz,
1H), 3.86 (t, J ¼ 6.6 Hz, 1H), 3.20 (s, 3H), 3.07 (t, J ¼ 5.8 Hz, 1H), 3.02
(d, J ¼ 6.1 Hz, 1H), 2.97 (d, J ¼ 12.0 Hz, 3H), 2.79 (dd, J ¼ 14.5, 8.6 Hz,
1H), 2.68 (dt, J ¼ 14.0, 7.0 Hz, 1H), 2.19 (d, J ¼ 12.8 Hz, 1H), 2.00 (d,
J ¼ 5.2 Hz, 1H), 1.80e1.75 (m, 4H), 1.34 (s, 3H), 1.24 (s, 3H), 1.15e1.09
(m, 3H), 0.91e0.80 (m, 8H), 0.65 (d, J ¼ 6.5 Hz, 3H); 13C NMR
CDCl3) d 171.72,170.66,170.61,170.02,151.52,146.90,138.64,138.09,
132.60, 130.31, 129.98, 128.29, 127.73, 116.44, 116.37, 110.39, 83.70,
80.91, 80.41, 71.58, 62.36, 60.61, 60.26, 59.45, 56.58, 54.35, 42.85,
36.34, 32.98, 31.73, 31.26, 27.01, 24.84, 22.80, 18.97, 15.96, 15.55,
15.11, 14.28, 10.74; HRMSeMALDI (m/z): [MþNa]þ calcd for
C
42H57BrIN3O9Naþ, 976.2215; found: 976.2218.
2.2.25. (1S,2S)-2-methyl-1-phenylbut-3-en-1-yl (S)-2-((R)-3-(3-
bromo-4-hydroxyphenyl)-2-((2S,3S)-2-((2R,3S)-3-((R,E)-4-iodo-2-
methoxy-3-methylbut-3-en-1-yl)-N,2-dimethyloxirane-2-
carboxamido)-3-methylpentanamido)propanamido)-3-hydroxy-3-
methylbutanoate (12b)
(100 MHz, CDCl3)
d 171.85, 170.50, 170.26, 170.02, 151.70, 146.85,
139.79, 138.34, 132.78, 130.06, 129.83, 128.42, 128.24, 127.15, 116.55,
116.35, 110.17, 83.63, 81.22, 80.41, 71.82, 62.55, 60.47, 59.31, 56.58,
54.38, 43.50, 36.50, 32.87, 31.85, 31.42, 27.22, 26.89, 24.94, 18.96,
16.57, 15.53, 14.95, 10.74. HRMSeMALDI (m/z): [MþNa]þ calcd for
The title compound was synthesized from 14 and 11b following
the general procedure described for 12a. Flash column chroma-
tography (petroleum ether/EtOAc, 4:1 to 3:1) afforded the product
C
42H57BrIN3O9Naþ, 976.2215; found: 976.2218.
in 76% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d
7.31e7.25
2.2.28. But-3-en-1-yl (S)-2-((R)-3-(3-bromo-4-hydroxyphenyl)-2-
((2S,3S)-2-((2R,3S)-3-((R,E)-4-iodo-2-methoxy-3-methylbut-3-en-
1-yl)-N,2-dimethyloxirane-2-carboxamido)-3-methylpentanamido)
propanamido)-3-hydroxy-3-methylbutan-oate (17)
(m, 6H), 7.03 (t, J ¼ 7.9 Hz, 2H), 6.94 (d, J ¼ 8.0 Hz, 1H), 6.85 (d,
J ¼ 8.3 Hz, 1H), 6.30 (s, 1H), 5.81e5.68 (m, 1H), 5.61 (d, J ¼ 7.9 Hz,
1H), 5.09e5.01 (m, 2H), 4.70 (dd, J ¼ 14.2, 8.2 Hz, 1H), 4.44 (d,
J ¼ 8.6 Hz, 1H), 4.34 (d, J ¼ 11.0 Hz, 1H), 3.83 (t, J ¼ 6.5 Hz, 1H),
3.16e3.08 (m, 4H), 3.02 (d, J ¼ 13.8 Hz, 3H), 2.83 (dd, J ¼ 13.5, 8.6 Hz,
1H), 2.73e2.65 (m, 1H), 2.00 (s, 1H), 1.77e1.71 (m, 4H), 1.40 (d,
J ¼ 7.1 Hz, 3H), 1.26 (d, J ¼ 2.7 Hz, 1H), 1.05 (d, J ¼ 3.4 Hz, 5H),
0.88e0.79 (m, 10H), 0.65 (d, J ¼ 6.4 Hz, 3H); 13C NMR (100 MHz,
The title compound was synthesized from 14 and 16 following
the general procedure described for 12a. Flash column chroma-
tography (petroleum ether/EtOAc, 4:1 to 3:1) afforded the product
in 76% yield as a colorless oil. 1H NMR (400 MHz, CDCl3)
d 7.30 (s,
1H), 7.09e7.01 (m, 2H), 6.92e6.82 (m, 2H), 6.35 (d, J ¼ 17.4 Hz, 2H),
CDCl3) d 171.76,170.73,170.40, 169.96,151.88,146.76,139.42,137.99,
5.76 (td, J ¼ 16.7, 7.4 Hz, 1H), 5.09 (t, J ¼ 13.6 Hz, 2H), 4.76e4.66 (m,

Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
575
1H), 4.37 (dd, J ¼ 24.7, 9.6 Hz, 2H), 4.25e4.13 (m, 2H), 3.87 (t,
NMR (100 MHz, DMSO)
d
171.14, 170.40, 169.90, 168.90, 152.75,
J ¼ 6.3 Hz, 1H), 3.19 (d, J ¼ 12.3 Hz, 3H), 3.12 (dd, J ¼ 17.4, 11.3 Hz,
2H), 3.03 (s, 3H), 2.88 (dd, J ¼ 13.8, 8.3 Hz, 1H), 2.45e2.36 (m, 2H),
2.29 (s, 1H), 2.03 (s, 1H), 1.82e1.73 (m, 4H), 1.43 (s, 3H), 1.24 (s, 3H),
1.11 (d, J ¼ 12.8 Hz, 3H), 0.96e0.90 (m, 1H), 0.84 (dd, J ¼ 12.2, 5.7 Hz,
139.16, 138.55, 134.15, 133.78, 130.45, 130.37, 129.84, 128.54, 128.43,
126.82, 116.02, 109.11, 84.65, 80.62, 72.10, 61.85, 59.69, 58.54, 55.56,
53.29, 42.68, 37.87, 31.37, 31.09, 30.14, 27.31, 26.90, 24.61, 18.61,
15.39, 15.19, 10.75, 10.55. HRMSeMALDI (m/z): [MþNa]þ calcd for
3H), 0.68 (d, J ¼ 6.0 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d
171.85,
C
42H56BrN3O9Naþ, 848.3092; found:848.3097.
171.10, 170.86, 170.03, 151.77, 146.82, 133.80, 132.94, 130.21, 129.88,
117.81, 116.48, 110.23, 83.67, 80.40, 71.58, 64.71, 62.26, 60.60, 60.49,
59.36, 56.58, 54.38, 36.50, 32.96, 31.84, 31.20, 27.13, 24.93, 22.75,
18.98, 15.48, 15.11, 14.22, 10.71. HRMSeMALDI (m/z): [MþNa]þ
calcd for C35H51BrIN3O9Naþ, 886.1746; found: 886.1750.
2.2.31. (1R,4S,7R,10S,13R,14S,15E,17E,19R,21S)-7-(3-bromo-4-
hydroxybenzyl)-4-((S)-sec-butyl)-10-(2-hydroxypropan-2-yl)-19-
methoxy-1,3,14,18-tetramethyl-13-phenyl-12,22-dioxa-3,6,9-
triazabicyclo[19.1.0]docosa-15,17-diene-2,5,8,11-tetraone (2c)
The title compound was synthesized from 12c following the
general procedure described for 2a. Flash column chromatography
2.2.29. (1R,4S,7R,10S,13S,14R,15E,17E,19R,21S)-7-(3-bromo-4-
hydroxybenzyl)-4-((S)-sec-butyl)-10-(2-hydroxypropan-2-yl)-19-
methoxy-1,3,14,18-tetramethyl-13-phenyl-12,22-dioxa-3,6,9-
((hexanes/acetone, 4:1) afforded the product in 60% yield as a white
21
solid. mp ¼ 143e145 ꢁC. [
a
]
¼ ꢀ 17.00 (c 0.20, CHCl3); IR (KBr)
D
triazabicyclo[19.1.0]docosa-15,17-diene-2,5,8,11-tetraone (2a)
To a solution of 12a (20.05 mg, 0.021 mmol) in 5 mL anhydrous
DMF (degassed via freeze-pump-thaw) was added a solid mixture
of Pd(OAc)2 (8.6 mg, 0.038 mmol) and Cs2CO3 (13.6 mg,
0.042 mmol), followed by a solution of Et3N (3.2 mg, 0.031 mmol) in
0.4 mL of DMF. The resulting solution was stirred at room tem-
perature for 2 days. The reaction was quenched with H2O (10 mL),
extracted with EtOAc (20 mL ꢂ 3). The combined organic layers
were washed with brine, dried over Na2SO4, and concentrated
under reduced pressure. The crude product was purified by flash
chromatography on silica gel (hexanes/acetone, 4:1) to afford the
nmax: 3419, 2969, 2932, 2877, 1734, 1669, 1627, 1511, 1453, 1380,
1211, 1093, 972, 702 cmꢀ1; 1H NMR (400 MHz, DMSO)
d 9.93 (s, 1H),
8.49 (s, 1H), 7.43 (d, J ¼ 9.2 Hz, 1H), 7.34 (dd, J ¼ 28.2, 6.3 Hz, 6H),
7.07 (d, J ¼ 5.8 Hz,1H), 6.80 (dd, J ¼ 8.1, 5.1 Hz,1H), 6.48 (dd, J ¼ 18.3,
7.5 Hz, 1H), 6.06e5.87 (m, 2H), 5.65 (s, 1H), 4.94 (d, J ¼ 4.6 Hz, 1H),
4.67e4.58 (m, 1H), 4.56e4.47 (m, 1H), 4.36 (s, 1H), 3.65 (d,
J ¼ 9.8 Hz, 1H), 3.10 (d, J ¼ 5.0 Hz, 3H), 3.03 (d, J ¼ 4.5 Hz, 3H), 2.90
(d, J ¼ 13.6 Hz, 1H), 2.79 (s, 2H), 2.59 (d, J ¼ 12.2 Hz, 1H), 2.10 (d,
J ¼ 5.2 Hz, 3H), 1.83 (s, 1H), 1.68 (s, 3H), 1.43 (d, J ¼ 4.8 Hz, 3H), 1.24
(s, 1H), 1.09 (s, 3H), 1.03 (s, 3H), 0.84 (d, J ¼ 6.0 Hz, 3H), 0.78 (d,
J ¼ 5.3 Hz, 3H), 0.37 (d, J ¼ 5.4 Hz, 3H); 13C NMR (100 MHz, DMSO)
desired product 2a (10 mg, 58%) as a white solid. mp ¼ 238e239 ꢁC.
d 170.20, 170.09, 169.80, 169.17, 152.38, 138.26, 137.32, 133.48,
21
[
a
]
¼ ꢀ 46.28 (c 0.33, CHCl3); IR (KBr) nmax: 3290, 2963, 1748,
133.19, 130.31, 129.37, 129.28, 128.09, 127.25, 126.11, 125.90, 115.85,
108.74, 83.78, 78.57, 71.20, 60.96, 59.30, 58.80, 57.98, 55.16, 54.61,
41.20, 34.71, 31.69, 30.69, 29.72, 27.80, 24.13, 23.89, 14.94, 14.65,
13.85, 10.45, 10.09. HRMSeMALDI (m/z): [MþNa]þ calcd for
D
1684, 1649, 1261, 1097, 1020, 801 cmꢀ1; 1H NMR (400 MHz, DMSO)
9.90 (s, 1H), 8.68 (d, J ¼ 10.0 Hz, 1H), 8.09 (d, J ¼ 9.6 Hz, 1H), 7.57
d
(d, J ¼ 7.5 Hz, 2H), 7.49 (s, 1H), 7.32 (t, J ¼ 7.4 Hz, 2H), 7.25 (t,
J ¼ 7.2 Hz, 1H), 7.13 (d, J ¼ 8.4 Hz, 1H), 6.73 (d, J ¼ 8.3 Hz, 1H), 6.38
(m, 1H), 6.20 (dd, J ¼ 15.2, 4.2 Hz, 1H), 6.12 (d, J ¼ 10.7 Hz, 1H), 5.89
(s, 1H), 5.16 (s, 1H), 4.69 (d, J ¼ 10.0 Hz, 2H), 4.46 (d, J ¼ 11.3 Hz, 1H),
3.61 (dd, J ¼ 10.5, 2.9 Hz, 1H), 3.08 (s, 3H), 2.98 (s, 3H), 2.80 (d,
J ¼ 11.1 Hz, 1H), 2.65 (s, 1H), 2.61 (d, J ¼ 7.5 Hz, 1H), 2.46 (d, 1H), 2.11
(t, J ¼ 11.9 Hz, 1H), 1.69 (s, 4H), 1.50e1.44 (m, 1H), 1.42 (s, 3H), 1.24-
1-17 (m, 1H), 1.13 (s, 3H), 1.02 (s, 3H), 0.96 (d, J ¼ 6.7 Hz, 3H),
0.89e0.82 (m, 1H), 0.76 (t, J ¼ 7.2 Hz, 3H), 0.30 (d, J ¼ 6.5 Hz, 3H);
C
42H56BrN3O9Naþ, 848.3092; found:848.3098.
2.2.32. (1R,4S,7R,10S,13R,14R,15E,17E,19R,21S)-7-(3-bromo-4-
hydroxybenzyl)-4-((S)-sec-butyl)-10-(2-hydroxypropan-2-yl)-19-
methoxy-1,3,14,18-tetramethyl-13-phenyl-12,22-dioxa-3,6,9-
triazabicyclo[19.1.0]docosa-15,17-diene-2,5,8,11-tetraone (2d)
The title compound was synthesized from 12d following the
general procedure described for 2a. Flash column chromatography
13C NMR (100 MHz, DMSO)
d
170.88, 170.72, 169.32, 168.80, 152.34,
((hexanes/acetone, 4:1) afforded the product in 55% yield as a white
21
140.08, 138.62, 133.73, 133.38, 130.01, 129.96, 129.09, 127.84, 126.99,
126.02, 124.64, 115.54, 108.68, 84.10, 78.90, 72.04, 61.46, 59.20,
59.13, 58.15, 55.18, 52.84, 41.99, 37.18, 31.17, 29.76, 28.29, 24.34,
24.34, 14.91, 14.69, 10.66, 10.12, 9.51; HRMSeMALDI (m/z):
[MþNa]þ calcd for C42H56BrN3O9Naþ, 848.3092; found:848.3098.
solid. mp ¼ 142e144 ꢁC. [
a
]
¼ ꢀ 12.75 (c 0.19, CHCl3); IR (KBr)
D
nmax: 3421, 2968, 1730, 1627, 1511, 1380, 1211, 1092 cmꢀ1 1H NMR
;
(400 MHz, DMSO)
d
9.89 (s, 1H), 8.30 (d, J ¼ 6.6 Hz, 1H), 7.63 (s, 1H),
7.43 (s,1H), 7.35 (t, J ¼ 6.8 Hz, 2H), 7.24 (dd, J ¼ 14.9, 7.3 Hz, 3H), 7.06
(d, J ¼ 8.2 Hz, 1H), 6.77 (d, J ¼ 8.1 Hz, 1H), 6.23e6.12 (m, 1H), 5.95 (d,
J ¼ 10.8 Hz, 1H), 5.90e5.82 (m, 1H), 5.61 (d, J ¼ 6.1 Hz, 1H), 4.75 (s,
1H), 4.59 (d, J ¼ 11.0 Hz, 1H), 4.44 (dd, J ¼ 20.3, 9.4 Hz, 2H), 3.66 (d,
J ¼ 8.5 Hz, 1H), 3.11 (s, 3H), 2.99 (s, 3H), 2.88e2.78 (m, 2H), 2.66 (d,
J ¼ 6.2 Hz, 1H), 2.57 (t, J ¼ 12.8 Hz, 1H), 2.08e2.00 (m, 1H), 1.83 (s,
1H), 1.59 (s, 3H), 1.52 (s, 1H), 1.40 (s, 3H), 1.23 (s, 1H), 1.04 (d,
J ¼ 9.4 Hz, 9H), 0.87 (dd, J ¼ 13.8, 6.9 Hz, 1H), 0.77 (t, J ¼ 6.5 Hz, 3H),
2.2.30. (1R,4S,7R,10S,13S,14S,15E,17E,19R,21S)-7-(3-bromo-4-
hydroxybenzyl)-4-((S)-sec-butyl)-10-(2-hydroxypropan-2-yl)-19-
methoxy-1,3,14,18-tetramethyl-13-phenyl-12,22-dioxa-3,6,9-
triazabicyclo[19.1.0]docosa-15,17-diene-2,5,8,11-tetraone (2b)
The title compound was synthesized from 12b following the
general procedure described for 2a. Flash column chromatography
0.39 (d, J ¼ 5.9 Hz, 3H); 13C NMR (100 MHz, DMSO)
d 170.37, 169.77,
(hexanes/acetone, 4:1) afforded the product in 60% yield as a white
169.60, 169.36, 152.34, 139.42, 135.14, 133.44, 133.09, 130.18, 129.32,
128.21, 128.02, 127.51, 126.50, 125.38, 115.78, 108.70, 82.86, 80.34,
71.04, 60.58, 59.01, 58.88, 57.78, 55.34, 54.30, 40.59, 35.34, 31.52,
31.46, 29.69, 27.45, 24.42, 23.83, 16.01, 14.85, 14.67, 11.37, 10.12.
HRMSeMALDI (m/z): [MþNa]þ calcd for C42H56BrN3O9Naþ,
848.3092; found:848.3097.
21
solid. mp ¼ 230e231 ꢁC. [
a]
¼ ꢀ 96.52 (c 0.17, CHCl3); IR (KBr)
D
nmax: 3419, 3289, 2966, 2928, 1747, 1649, 1598, 1545, 1517, 1454,
1421, 1384, 1374, 1184, 1148, 1099, 1075, 697 cmꢀ1
;
1H NMR
(400 MHz, DMSO)
d
9.62 (s, 1H), 8.23 (d, J ¼ 9.9 Hz, 1H), 7.56 (d,
J ¼ 9.4 Hz, 1H), 7.23 (s, 1H), 7.17e7.05 (m, 5H), 6.87 (d, J ¼ 8.2 Hz,
1H),6.50 (d, J ¼ 6.7 Hz, 1H), 6.22 (dd, J ¼ 14.8, 11.1 Hz, 1H), 5.82 (d,
J ¼ 10.9 Hz, 1H), 5.60e5.49 (m, 1H), 5.20 (d, J ¼ 10.2 Hz, 1H), 4.45 (t,
J ¼ 8.8 Hz, 1H), 4.24 (dd, J ¼ 20.6, 10.6 Hz, 3H), 3.35 (dd, J ¼ 10.9,
2.8 Hz, 1H),2.82 (s, 3H), 2.71 (s, 3H), 2.67e2.54 (m, 3H), 2.38 (d,
J ¼ 9.1 Hz, 1H), 1.84 (t, J ¼ 11.7 Hz, 1H), 1.44 (s, 4H), 1.27e1.14 (m,
4H),0.98 (d, J ¼ 9.2 Hz, 1H), 0.65 (s, 3H), 0.58 (s, 3H), 0.53 (t,
J ¼ 7.1 Hz, 4H), 0.41 (d, J ¼ 6.8 Hz, 3H), 0.11 (d, J ¼ 6.4 Hz, 3H); 13C
2.2.33. (1R,4S,7R,10S,15E,17E,19R,21S)-7-(3-bromo-4-
hydroxybenzyl)-4-((S)-sec-butyl)-10-(2-hydroxypropan-2-yl)-19-
methoxy-1,3,18-trimethyl-12,22-dioxa-3,6,9-triazabicyclo[19.1.0]
docosa-15,17-diene-2,5,8,11-tetraone (3)
The title compound was synthesized from 17 following the
general procedure described for 2a. Flash column chromatography

576
Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
((hexanes/acetone, 4:1) afforded the product 3 in 58% yield as a
With the key chiral intermediates 9a-9 d at hand, further elabora-
tion proceeded smoothly through stereospecific Mitsunobu ester-
ification to attach the b-OH-L-valine unit 13, deprotection of the N-
Boc group, and coupling with another major fragment 14 via ami-
dation to afford the Heck precursors 12a-12d. Additionally, 17 was
prepared en route starting with esterification of 13 with the
unsubstituted achiral homoallylic alcohol 18.
21
white solid. mp ¼ 133e135 ꢁC. [
a
]
¼ ꢀ1.71 (c 0.16, CHCl3); IR (KBr)
D
nmax: 3417, 2967, 2930, 1737, 1658, 1626, 1512, 1384, 1262, 1210,
1093, 1024,802 cmꢀ1; 1H NMR (400 MHz, DMSO)
d
9.86 (s, 1H), 8.55
(d, J ¼ 9.9 Hz, 1H), 7.99 (d, J ¼ 9.4 Hz, 1H), 7.47 (s, 1H), 7.11 (d,
J ¼ 8.3 Hz, 1H), 6.74 (d, J ¼ 8.2 Hz, 1H), 6.42e6.32 (m, 1H), 6.00 (d,
J ¼ 11.1 Hz, 1H), 5.80e5.71 (m, 1H), 4.76 (s, 1H), 4.67 (t, J ¼ 9.2 Hz,
1H), 4.50e4.37 (m, 3H), 3.90 (d, J ¼ 11.0 Hz, 1H), 3.58 (dd, J ¼ 11.1,
3.2 Hz, 1H), 3.09e3.01 (m, 3H), 2.97 (s, 3H), 2.83 (d, J ¼ 10.8 Hz, 1H),
2.56 (d, J ¼ 10.2 Hz, 1H), 2.51 (d, J ¼ 4.9 Hz, 1H), 2.41 (s, 1H),
2.08e2.00 (m, 1H), 1.65 (s, 4H), 1.46 (t, J ¼ 9.8 Hz, 1H), 1.40 (s, 3H),
1.23e1.15 (m, 2H), 1.09 (d, J ¼ 18.9 Hz, 6H), 0.88e0.80 (m, 1H), 0.75
(t, J ¼ 7.2 Hz, 3H), 0.32 (d, J ¼ 6.5 Hz, 3H); 13C NMR (100 MHz,
Single-point chiral inversion of an open-chain precursor pos-
sessing multiple stereogenic centers gives rise to its diastereomer,
which repopulates conformational distribution and as such may
profoundly influence the macrocycle-forming reaction. In their
total synthesis of chondramide C stereoisomers [3c,3f], Arndt and
Waldmann exploited ring relay closing metathesis (RRCM) to forge
the 18-membered ring by the creation of a new alkene joining the
polyketide C4 and C5 atoms. Notably, switching the reactant
configuration at C2, C6, or C7 exerted a strong impact on the
metathesis outcome in terms of both reactivity and stereo-
selectivity. Under the same RRCM condition, for example, a (2R, 6S,
7S)-configured diene yielded the product bearing exclusively 6E-
alkene in 47% yield, whilst its (2R, 6R, 7R) diastereomer was
unreactive, and yet another (2S, 6S, 7S) homologue differing from
the former compound merely in the silyl protecting group (TIPS vs.
TBS) besides the C2-chirality led to the cyclized product in 44%
yield but surprisingly with the opposite 6Z-alkene geometry.
Though mechanistically different and substrate dependent, alter-
ation of the precursor chirality could arguably compromise ring-
closing Heck reaction entailed in this study, especially in view of
the fact that (1) the configurational change occurs in close prox-
imity to the ring closure site and (2) existence of the isomeric 8Z-
nannocystin, which is likely to emerge as an unwanted side product
from our intended Heck reaction, was confirmed by Wang et al. [10]
during the endgame of their total synthesis of 1 employing ring
closing metathesis (RCM). Therefore, at the outset of this study it
was unclear whether Heck macrocyclization of stereoisomeric (20R,
30R)-12b, (20R, 30S)-12c, and (20S, 30S)-12d would render the desired
products with acceptable reactivity and selectivity. To our delight,
under previously established cyclization condition [11] 12b-12d
were successfully converted to the corresponding products 2b-2d
in 55e60% yields (unoptimized) comparable to that of 2a (58%). In
each case, only 8E-configured product was found in the crude
product within the detectable limits of NMR. Finally, the truncated
intermediate 17 was also converted to 3 in 58% yield and excellent
E-selectivity (Z/E > 20:1). Altogether, our result converges with
earlier studies from other groups [24] to highlight the utility of
stereoselective intramolecular Heck reaction in the total synthesis
of natural macrocycles.
The antiproliferative activity of 2a-2d and 3 was determined
against PANC1 cancer cell lines. Despite aforementioned advantage
of engaging difficult in vivo targets, drug discovery based on natural
macrocycles has been receiving less attention from pharmaceutical
industry compared with conventional small-molecule drugs [1].
This situation conceivably originates from structural complexity
inherent in most natural macrocycles, which not only in several
aspects constitutes an apparent deviation from Lipinski's Rule of
Five [25,26], a set of empirical guidelines widely adopted for
assessing a compound's druggability, but also demands often
complicated synthesis at a potentially high manufacturing cost [27].
Hence in consonance with drug discovery from other types of nat-
ural products, there is a pressing need to simplify the lead structure
by eliminating redundant portions while at the same time keeping
essential pharmacophoric elements. In this regard, we attempted a
simpler analogue 3 that is devoid of C10 and C11 substitution. As
shown in Table 1, however, the deletion abolished activity dramat-
ically (entry 5), thus suggesting an indispensable role of the
DMSO)
d 170.99, 170.79, 169.29, 168.80, 152.30, 133.66, 133.34,
132.90, 129.92, 129.85, 129.37, 127.34, 115.57, 115.19, 108.66, 84.18,
71.42, 63.56, 61.37, 59.06, 58.17, 55.02, 52.88, 37.15, 31.76, 31.21,
30.53, 29.66, 27.49, 25.14, 24.24, 14.86, 14.62, 10.17, 10.00.
HRMSeMALDI (m/z): [MþNa]þ calcd for C35H50BrN3O9Naþ,
758.2623; found: 758.2627.
2.3. MTT assay
Human pancreatic cancer cell PANC1 was cultured in DMEM
medium supplemented with 10% fetal bovine serum, penicillin (100
U/mL), and streptomycin (100 m
g/mL) at 37 ꢁC with 5% CO2. The
candidate compounds were dissolved in dimethyl sulfoxide
(DMSO) and diluted with cell culture medium to a final working
concentration. To measure cell cytotoxicity, PANC1 cells were
seeded into 96 well plates at a density of 2 ꢂ 103 cells per well,
followed by incubation of candidate compounds with serial con-
centrations for 72 h. After treatment, 20
mL) was added into each well for an additional 4 h at 37 ꢁC. The
supernatant was removed and 150 L of DMSO was added to
mL of MTT reagent (5 mg/
m
dissolve the formazan crystal. The absorbance was recorded at an
optical density of 570 nm. The IC50 value was calculated with
GraphPad Prism 5.
3. Results and discussion
Since our previous total synthesis of 1 called for late-stage
incorporation of the C10eC11 segment into the target molecule
[11], it is amenable to structural diversification at this particular
region. Using this route, we recently built a focused library of 10
nannocystin analogues modified at the C11-position for SAR study
[17]. Likewise, it can be extended to current research by the use of
stereodivergent building blocks (20R, 30R)-9a, (20S, 30R)-9b, (20S,
30S)-9c, and (20R, 30S)-9d (Scheme 1). Although so far a number of
methods [19] exist for direct preparation of 9a-9d with varying
degrees of stereoselectivity, we opted for an alternative strategy
that hinges upon asymmetric aldol condensation due to our accu-
mulating experience [20] with Oppolzer's bornanesultam chemis-
try [21]. Thus optically pure crystalline anti aldol (20S, 30R)-5a was
synthesized via condensation of a Z-configured O-silyl-N,O-ketene
acetal derived from 4a and TiCl4-complexed benzaldehyde [22],
whereas changing the Lewis acid to diethylboryl triflate enabled a
chelated transition state [23] to deliver the syn aldol product (20R,
30R)-5b (Table in Scheme 1). Preparation of enantiomeric anti (20R,
30S)-5c and syn (20S, 30S)-5d followed the same procedures as for 5a
and 5b with the exception of starting with 4b instead to induce the
opposite facial selectivity. 5a-5d were subsequently transformed
into 9a-9d respectively through a sequence of standard operations
including (1) protection of the resultant 30eOH with a TBS group,
(2) reductive cleavage of the bornanesultam moiety, (3) Wittig
olefination, and (4) removal of the O-TBS group to unmask 30eOH.

Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
577
Table 1
To rationalize these results, we calculated energy minimized
structures of 2a-2d and 3 (Fig. 3). According to our proposed
docking model [16,17], the C11-phenyl group is at the forefront of
interacting with the target surface, specifically within a hydro-
phobic pocket. Its configurational switch would inevitably disrupt
the normal binding process, which suffices to lose activity regard-
less of the C10-chirality. This is indeed the case as found in the
Inhibitory activities of nannocystin analogues 2a-2d and 3 against human pancreatic
carcinoma (PANC1) cell lines.
Entry
Compound
IC50 (nM)
1
2
3
4
5
(10R, 11S)-2a
(10S, 11S)-2b
(10S, 11R)-2c
(10R, 11R)-2d
3
1.52 0.21
10.12 0.19
>200
>200
>200
diastereomers (10S, 11R)-2c and (10R, 11R)-2d (entries 3e4,
Table 1). In addition, the vicinal 10R-methyl substituent may also
contribute favorably to binding interactions at this location, but
probably to a lesser extent than the 11S-phenyl group due to its
smaller size. Thus inappropriate positioning of this group as in the
case of (10S, 11S)-2b would deteriorate but not completely cancel
activity (entry 2, Table 1). Taken together, it can be concluded that
the (10R,11S) segment and its affiliated substituents have to be
conserved as a crucial motif for high activity.
truncated groups in acting upon the target. We also explored
configurational permutation at this subdomain by preparing all four
stereoisomers 2a-2d. Again, the native (10R,11S) configuration
turned out to be superior to other chirality pairs in attaining optimal
potency (entry 1 vs. entries 2e4). Chiral inversion of the 10R-methyl
group alone resulted in roughly 6.7-fold reduction in activity (entry
2). In marked contrast, such transformation at C11 position basically
deactivated the resulting 2d (entry 4). Therefore, the 11S-phenyl
group seems to make a predominant contribution to target binding
relative to the neighboring 10R-methyl group.
4. Conclusion
Aiming at elucidation of structure-activity correlations in
Fig. 3. Energy minimized structures of 2a-2d and 3 (the polyketide 11R-phenyl groups of 2c and 2d are highlighted in cyan).

578
Y. Tian et al. / Journal of Molecular Structure 1181 (2019) 568e578
111e129.
nannocystins, a small collection of highly antiproliferative natural
macrocycles, we synthesized all four stereoisomers of Br-
nannocystins differing in the polyketide C10 and C11 configura-
tions, along with an analogue simplified therein. Biological
assessment of these compounds against PANC1 cancer cell lines
showed that the natural (10R,11S) configuration with which the
methyl and phenyl groups are positioned is essential to achieve
high activity.
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This work was generously supported by the National Natural
Science Foundation of China (Grant No. 21772101) and the Natural
Science Foundation of Tianjin City (Grant No. 17JCYBJC28400) to W.
Zhang, and the National Natural Science Foundation of China (Grant
No. 81573282) and the National Science Fund for Distinguished
Young Scholars (Grant No. 81625021) to Y. Chen.
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