Total synthesis of gem-difluoromethylenated analogues of pironetin

Article abstract of DOI:10.1055/s-0029-1217099

The straightforward synthesis of four gem-difluoromethylenated analogues of pironetin is described. Our synthesis features the efficient construction of the key intermediates through the indium--mediated gem-difluoropropargylation of aldehydes with the fl

Full text of DOI:10.1055/s-0029-1217099

PAPER
267
Total Synthesis of gem-Difluoromethylenated Analogues of Pironetin
S
ynthesis
iof gem
n
t
hylenated
A
nalogu
L
es of Pironetinin,a Xuyi Yue,b Peng Huang,c Daxiang Cui,c Feng-Ling Qing*a,b
a
College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, 2999 North Renmin Lu, Shanghai 201620, P. R.
of China
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Lu,
Shanghai 200032, P. R. of China
b
Fax +86(21)64166128; E-mail: flq@mail.sioc.ac.cn
Institute of Micro–Nano Science and Technology, Shanghai Jiao Tong University, 800 Dongchuan Lu, Shanghai 200240, P. R. of China
c
Received 17 August 2009; revised 22 September 2009
O
Abstract: The straightforward synthesis of four gem-difluorometh-
ylenated analogues of pironetin is described. Our synthesis features
the efficient construction of the key intermediates through the
indium-mediated gem-difluoropropargylation of aldehydes with the
fluorine-containing building block.
OMe OH
pironetin
O
Key words: pironetin, a,b-unsaturated d-lactones, gem-difluoro-
methylenated analogues
O
O
R
OMe OH
O
F
R
OMe OH
O
F
Pironetin was originally isolated in 1993 by Yoshida and
co-workers1 from the fermentation broths of Streptomyces
prunicolor PA-48153 and then in 1994 by Kobayashi and
co-workers2 from Streptomyces sp. NK-10958 (Figure 1).
It was found to possess plant growth regulatory activity2a
as well as immunosuppressive activity1a on the responses
of both T and B lymphocytes to mitogens. Interestingly,
pironetin binds to the a-subunit of tubulin, differing from
colchicine, vinblastine, rhizoxin, and epothilone B which
bind to b-tubulin.3 Based on all these characteristics,
pironetin is considered to be a very attractive research tar-
get and the total synthesis of pironetin has been reported
by many groups.4 Osada and co-workers reported that the
a,b-unsaturated d-lactone is one of the important portions
for microtubule inhibitory activity of pironetin.4h,5 It has
been revealed by structure–activity relationship studies
that the a,b-unsaturated d-lactone structure plays a key
role in the bioactivity of many natural products, because
such a structural unit is an excellent potential Michael ac-
ceptor for nucleophilic amino acid residues of the natural
receptors interacting with these compounds.6–8 In view of
the strong electron-withdrawing character of a difluoro-
F
F
1a R = H
1b R = H
2a R = Me
2b R = Me
target molecules
Figure 1 Structure of pironetin and the target molecules
We designed a flexible synthetic strategy to obtain com-
pounds 1 and 2, as shown in Scheme 1. gem-Difluoro-
methylenated a,b-unsaturated d-lactones can be con-
structed according to our published procedure;10 that is,
lactones 1 and 2 can be obtained by removal of the tert-
butyldimethylsilyl group of compounds 3 and 4 followed
by oxidation of the resulting 1,5-diols. Compounds 3 and
4 could be prepared by gem-difluoropropargylation of al-
dehydes 5 and 6 with the fluorine-containing building
block 7 followed by selective hydrogenation. The double
bond segment in compounds 5 and 6 could be constructed
by the Wittig reaction of aldehyde 8 with the correspond-
ing ylides. Compound 8 could be prepared from tosylate
9.
methylene group (CF2),9 we were interested in replacing The synthesis of tosylate 9 (Scheme 2) was mainly based
the propyl group of pironetin with a CF2 group. Thus, the on Dias’s strategy with some improvements and the con-
resulting conjugated double bond would be much more struction of the four stereochemical centers was accom-
electron deficient, making it a better candidate to enhance plished by the use of two Evans asymmetric aldol
the reactivity of the conjugated double bond as a Michael reactions.4j The asymmetric aldol reaction of the boron
acceptor. Herein, we describe the total synthesis and bio- enolate derived from (S)-oxazolidinone (+)-10 with alde-
logical evaluation of four gem-difluoromethylenated ana- hyde 11 gave aldol adduct 12 in 91% isolated yield and
logues of pironetin (Figure 1).
with excellent diastereoselectivity (dr >95:5, determined
by 1H NMR spectroscopy). Dias reported that exchange of
the oxazolidinone auxiliary in the syn-aldol (+)-12 with
N,O-dimethylhydroxylamine generated the Weinreb
amide; then, after protection of the second hydroxy group
as its tert-butyldimethylsilyl ether, the resulting amide
was reduced to aldehyde on treatment with diisobutylalu-
minum hydride in toluene at 0 °C. The aldehyde was di-
SYNTHESIS 2010, No. 2, pp 0267–0275
Advanced online publication: 03.11.2009
DOI: 10.1055/s-0029-1217099; Art ID: F16209SS
© Georg Thieme Verlag Stuttgart · New York
x
x
.
x
x
.2
0
0
9

268
J. Lin et al.
PAPER
O
O
O
O
OTBS
OMe OP1 OH
O
OH
a
b
R
OMe OH
O
F
O
N
R
O
N
OPMB
O
OPMB
H
Bn
Bn
11
F
F
F
(+)-10
12
1 R = H
3 R = H
4 R = Me
O
O
N
OTBS
2 R = Me
OH OTBS
c
d
O
OPMB
OMe OP1
OPMB
R
OTBS
CHO
Bn
+
BrF2C
13
14
7
5 R = H
O
O
N
OH OTBS
OTs OMe OTBS
6 R = Me
e
O
OPMB
OPMB
OMe OP1
OMe OP1
Bn
TsO
OP2
OHC
OP2
9
15
9
Scheme 2 Preparation of tosylate 9. Reagents and conditions:
(a) n-Bu2BOTf, Et3N, CH2Cl2, –10 °C, 91%; (b) TBSOTf, 2,6-lutidi-
ne, CH2Cl2, –10 °C to r.t., overnight, 95%; (c) LiBH4, EtOH, THF,
0 °C to r.t., overnight, 89%; (d) i: SO3·py, Et3N, DMSO, CH2Cl2; ii:
(–)-10, n-Bu2BOTf, Et3N, CH2Cl2, 95%; (e) i: LiBH4, EtOH, THF,
0 °C to r.t., overnight; ii: TsCl, DMAP, Et3N, CH2Cl2, r.t., overnight;
iii: Me3OBF4, Proton-Sponge®, CH2Cl2, r.t., overnight, 80% yield
over three steps.
8
Scheme 1 Retrosynthetic analysis of gem-difluoromethylenated
analogues of pironetin
rectly treated with the boron enolate generated from the
(R)-oxazolidinone (–)-10 to give the aldol adduct 15 in
good yield4j and excellent diastereoselectivity.11 In our
procedure, a more common and efficient strategy was em-
ployed to produce compound 15, as outlined in Scheme 2.
Silylation of the hydroxy function of compound 12 afford-
ed compound 13. Removal of the oxazolidinone auxiliary
with lithium borohydride in ethanol and tetrahydrofuran
at 0 °C gave primary alcohol 14 in 89% yield. Oxidation
of the hydroxy group of 14 followed by asymmetric aldol
reaction with the boron enolate derived from (–)-10 gave
aldol adduct 15 in 95% isolated yield (dr >95:5, deter-
mined by 1H NMR spectroscopy). Our strategy avoids the
use of dangerous trimethylaluminum and the procedure is
much easier to handle. According to the literature11 and a
tion of a solution of aldehyde 8 in tetrahydrofuran to this
solution at –78 °C dropwise, stirring for one hour, and
workup.
CN OMe OTBS
OTs OMe OTBS
a
b
OPMB
OPMB
9
16
CHO OMe OTBS
R
OMe OTBS
c
OPMB
OPMB
1
comparison of the H NMR spectrum of adduct 15 with
17 R = H, 90%
8
the authentic sample,4j we could deduce that the resulting
four contiguous stereocenters exhibited syn,anti,syn stereo-
chemistry. The tosylate segment 9 can be prepared from
adduct 15 in three steps according to the published proce-
dure;4j that is, reductive removal of the oxazolidinone
auxiliary from aldol adduct 15, followed by selective to-
sylation of the primary hydroxy group and then methyla-
tion of the second hydroxy group, afforded the desired
product 9 in 80% yield over three steps (Scheme 2).
18 R = Me, 96%
Scheme 3 Synthesis of the key intermediates 17 and 18. Reagents
and conditions: (a) NaCN, DMSO, 80 °C, 2 h, 95%; (b) DIBAL-H,
–78 °C, 1.5 h, quant; (c) RCH3CHPPh3I, base, THF.
With the key fragments 17 and 18 in hand, we embarked
on the synthesis of the target molecules 1a, 1b and 2a, 2b,
as shown in Scheme 4. Removal of the p-methoxybenzyl
group of compound 17 followed by o-iodoxybenzoic acid
(IBX) oxidation of the resulting alcohol 19 gave the de-
sired aldehyde 5 quantitatively. At this point, we focused
our efforts on the coupling of aldehyde 5 and the fluorine-
containing building block 7. Under Hammond reaction
conditions,13 treatment of 5 and 7 in the presence of indi-
um in tetrahydrofuran–water (1:4, v/v) at room tempera-
ture gave the expected product 21 in 78% yield (anti/
syn = 2:1, determined by 19F NMR spectroscopy before
column chromatography). Initial attempts to hydrogenate
the triple bond of compound 21 to the cis double bond in
the presence of Lindlar catalyst were unsuccessful, even
with added quinoline, leading to a complex, inseparable
With compound 9 in hand, we turned our attention to the
lengthening of the carbon chain. Treatment of tosylate 9
with sodium cyanide in dimethyl sulfoxide at 80 °C pro-
vided cyanide 16 in 95% yield (Scheme 3). Reduction of
cyanide 16 with diisobutylaluminum hydride produced al-
dehyde 8 quantitatively. Aldehyde 8 was subjected to
Wittig reaction with ethyltriphenylphosphonium iodide
under Schlosser conditions12 to give the alkene fragment
17 in 90% yield (E/Z = 12:1, determined by GC-MS and
1H NMR spectroscopy). Alternatively, according to clas-
sical Wittig reaction conditions, alkene 18 was afforded in
96% yield by reaction of isopropyltriphenylphosphonium
iodide with n-butyllithium at –10 °C, followed by addi-
Synthesis 2010, No. 2, 267–275 © Thieme Stuttgart · New York

PAPER
Synthesis of gem-Difluoromethylenated Analogues of Pironetin
269
R
OMe OTBS
R
OMe OTBS
a
b
OPMB
OH
19 R = H, 90%
17 R = H
18 R = Me
20 R = Me, 88%
TBS
OMe O OH
R
OMe OTBS
R
OTBS
c
F
F
OTBS
CHO
BrF2C
7
5 R = H
21 R = H, dr = 2:1, 78%
6 R = Me
22 R = Me, dr = 1.5:1, 75%
OH
OTBS
F
TBS
TBS
R
OMe
O
OH
R
OMe
O
OH
d
e
F
F
F
23 R = H, dr = 2:1, 88%
24 R = Me, dr = 2:1, 82%
3 R = H, dr = 2:1, 95%
4 R = Me, dr = 2:1, 90%
O
O
TBS
TBS
R
OMe
O
O
R
OMe
O
O
f
+
F
F
F
F
25a R = H, 44%
26a R = Me, 45%
25b R = H, 30%
26b R = Me, 33%
O
O
R
OMe OH
O
F
R
OMe OH
O
F
g
+
F
F
1a R = H, 95%
2a R = Me, 93%
1b R = H, 92%
2b R = Me, 93%
Scheme 4 Lactone construction and target molecule synthesis. Reagents and conditions: (a) DDQ, CH2Cl2, H2O, 3 h; (b) IBX, EtOAc, reflux,
3 h, quant; (c) In(0), THF–H2O (1:4), r.t., overnight; (d) H2, Pd/BaSO4, quinoline, MeOH, DMF, 3.5 h; (e) CSA, MeOH, 1 h; (f) BAIB, TEMPO,
CH2Cl2, 3 h; (g) 6 M HCl–THF (1:1), 1 h.
mixture. Fortunately, the selective hydrogenation pro- ing the Cell Counting Kit-8 (CCK-8) assay (Figure 2 and
ceeded smoothly in a palladium on barium sulfate–quino- Figure 3). IC50 values were approximately 1500 nM for
line system to provide the expected product 3 in 95% the A375 cells and 600 nM for the MGC803 cells. The
yield.14 Selective deprotection of the primary tert-bu- dose of the gem-difluoromethylenated analogues of
tyldimethylsilyl group of compound 3 with D-camphor- pironetin is over ten times higher than that of pironetin as
10-sulfonic acid (CSA) gave cyclization precursor 23 in reported by Osada and co-workers.5,15 The results indicate
88% yield. Delightfully, treatment of compound 23 with that the activities of the gem-difluoromethylenated ana-
0.2 equivalents of 2,2,6,6-tetramethyl-1-piperidinyloxy logues of pironetin are less effective than pironetin.
(TEMPO) and 3.0 equivalents of [bis(acetoxy)iodo]ben-
zene (BAIB) in dichloromethane at room temperature af-
forded two separable cyclization compounds 25a and 25b
in 44% and 30% isolated yield, respectively. Removal of
the tert-butyldimethylsilyl group of compound 25a af-
forded target molecule 1a in 95% yield. The other three
target gem-difluoromethylenated analogues of pironetin,
1b, 2a, and 2b, were synthesized from 25b, 26a, and 26b,
In conclusion, we have accomplished the total synthesis
of four gem-difluoromethylenated analogues of pironetin,
1a, 1b and 2a, 2b. Our approach features an efficient and
general strategy to construct gem-difluoromethylenated
a,b-unsaturated d-lactones in high yield from aldehydes
under mild conditions. All of the synthesized compounds
were examined in growth inhibition assays against a panel
of human tumor cell lines comprised of human melanoma
respectively, in the same manner.
A375 cells and human MGC803 cells. The inhibition con-
The tumor cell viability of both human melanoma A375 centrations indicated that the resulting fluorinated ana-
cells and human MGC803 cells with the gem-difluoro- logues are approximately ten times less effective than the
methylenated analogues of pironetin was determined us- natural pironetin.
Synthesis 2010, No. 2, 267–275 © Thieme Stuttgart · New York

270
J. Lin et al.
PAPER
(3.6 mL, 15.4 mmol) dropwise at –10 °C for 0.5 h, then the mixture
was warmed to r.t. overnight. The reaction mixture was quenched
with sat. NaHCO3 soln (30 mL) and was extracted with CH2Cl2 (3
× 20 mL). The combined organic extracts were washed with 1 M
HCl (50 mL) and brine (50 mL), dried (anhyd Na2SO4), filtered, and
concentrated. The residue was purified by flash chromatography on
silica gel (PE–EtOAc, 20:1) to afford 13 as a clear oil; yield: 7.2 g
(95%).
[a]D23 +51.4 (c 1.00, CHCl3).
IR (thin film): 3029, 2930, 1782, 1697, 1612, 1513, 1383, 1248,
1106 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.28 (m, 7 H), 6.84 (d, J = 8.4 Hz,
2 H), 4.51 (m, 1 H), 4.40 (d, J = 11.4 Hz, 1 H), 4.35 (d, J = 11.4 Hz,
1 H), 4.10 (m, 2 H), 3.85 (m, 2 H), 3.79 (s, 3 H), 3.57 (m, 1 H), 3.46
(m, 1 H), 3.22 (dd, J = 12.9, 2.4 Hz, 1 H), 2.74 (dd, J = 13.2, 9.6 Hz,
1 H), 1.88 (m, 1 H), 1.24 (d, J = 6.9 Hz, 3 H), 0.87 (s, 9 H), 0.04 (s,
3 H), 0.03 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 175.5, 159.0, 152.9, 135.4, 130.6,
129.5, 129.2, 128.9, 127.3, 113.7, 72.6, 71.0, 65.9, 65.8, 55.5, 55.2,
43.1, 37.7, 35.0, 25.8, 18.0, 13.7, –4.4, –4.8.
Figure 2 Cell viability of A375 cells with pironetin analogues
(3.125–3125 nM), incubated at 37 °C for 24 h. Data represent mean
SD (n = 3).
MS (EI): m/z (%) = 478 (3), 290 (9), 251 (5), 121 (100).
HRMS (EI): m/z calcd for C26H34NO6Si: 484.2155; found:
484.2154.
(2R,3R)-3-(tert-Butyldimethylsilyloxy)-5-(4-methoxybenzyl-
oxy)-2-methylpentan-1-ol (14)
To a soln of compound 13 (6.8 g, 12.6 mmol) in THF (100 mL) was
added EtOH (1.5 mL, 25.1 mmol) followed by 2 M LiBH4 in THF
(12.6 mL) at 0 °C. After the mixture was stirred for 1 h, it was al-
lowed to warm to r.t. overnight. The reaction mixture was quenched
with cold H2O (50 mL) and was extracted with Et2O (3 × 30 mL).
The combined organic extracts were washed with brine (70 mL),
dried (anhyd Na2SO4), filtered, and concentrated. The residue was
purified by flash chromatography on silica gel (PE–EtOAc, 6:1) to
afford 14 as a clear oil; yield: 4.1 g (89%).
[a]D23 +4.6 (c 1.00, CHCl3).
IR (thin film): 3448, 2930, 1612, 1513, 1249, 1038 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.25 (d, J = 8.4 Hz, 2 H), 6.89 (d,
J = 8.7 Hz, 2 H), 4.44 (d, J = 11.4 Hz, 1 H), 4.38 (d, J = 12 Hz, 1
H), 3.94 (m, 1 H), 3.80 (s, 3 H), 3.69 (td, J = 10.8, 3.0 Hz, 1 H), 3.51
(m, 3 H), 2.72 (dd, J = 6.9, 3.0 Hz, 1 H), 1.98 (m, 1 H), 1.79 (m, 2
H), 0.89 (s, 9 H), 0.80 (d, J = 9.3 Hz, 3 H), 0.09 (s, 3 H), 0.05 (s, 3
H).
Figure 3 Cell viability of MGC803 cells with pironetin analogues
(3.125–3125 nM), incubated at 37 °C for 24 h. Data represent mean
SD (n = 3).
MS (ESI): m/z = 391 [M + Na]+.
Anal. Calcd for C20H36O4Si: C, 65.17; H, 9.84. Found: C, 64.93; H,
9.87.
Unless otherwise indicated, all chemicals and solvents were used as
received from commercial sources or purified by standard proce-
dures. Optical rotations were recorded on a Jasco P-1030 polarime-
(4R)-4-Benzyl-3-[(2R,3S,4R,5R)-5-(tert-butyldimethylsilyloxy)-
3-hydroxy-7-(4-methoxybenzyloxy)-2,4-dimethylheptanoyl]ox-
azolidin-2-one (15)
ter. IR spectra were scanned with
a Bio-Rad FTS185
To a soln of compound 14 (2.6 g, 7.0 mmol) and DMSO (9 mL) in
CH2Cl2 (50 mL) at 0 °C was added Et3N (4.9 mL, 35.0 mmol) for
10 min; then, SO3·py (2.2 g, 14.0 mmol) was added. The mixture
was warmed to r.t. and stirred for 3 h. H2O (20 mL) was added, the
layers were separated, and the aqueous layer was extracted with
CH2Cl2 (3 × 20 mL). The combined organic extracts were washed
with 1 M HCl (80 mL) and brine (50 mL), dried (anhyd Na2SO4),
filtered, and concentrated to give a residue, which was used without
further purification.
1
spectrophotometer. H NMR and 13C NMR spectra were obtained
using Bruker AM300 and AM400 spectrometers, respectively, and
19F NMR spectra were recorded on a Bruker AM300 spectrometer
(CFCl3 as external standard, and low field is positive). LRMS and
GC-MS were measured on an Agilent mass spectrometer system
and HRMS on an APEXIII (7.0 Tesla) FTMS or Waters mass spec-
trometer, respectively. Elemental analyses were taken on a Vario
EL III elementary analysis instrument.
To a soln of (4R)-4-benzyl-3-propionyloxazolidin-2-one [(–)-10;
1.63 g, 7.0 mmol] in CH2Cl2 (100 mL) was added dropwise 1 M n-
Bu2BOTf in CH2Cl2 (7.7 mL) followed by Et3N (1.2 mL, 9.1 mmol)
at –10 °C and the mixture was allowed to warm to 0 °C. After being
(4S)-4-Benzyl-3-[(2S,3R)-3-(tert-butyldimethylsilyloxy)-5-(4-
methoxybenzyloxy)-2-methylpentanoyl]oxazolidin-2-one (13)
To a soln of compound 12 (6.0 g, 14.0 mmol) in CH2Cl2 (80 mL)
was added 2,6-lutidine (3.0 mL, 25.2 mmol) followed by TBSOTf
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Synthesis of gem-Difluoromethylenated Analogues of Pironetin
271
stirred for 0.5 h at this temperature, the resultant soln was cooled to
–78 °C and a soln of the above residue in CH2Cl2 (5 mL) was added
dropwise via cannula. The resulting mixture was stirred for 1 h at
–78 °C, then was warmed to 0 °C and stirred for 1 h. The reaction
mixture was treated with pH 7 phosphate buffer (10 mL) followed
by MeOH–H2O2 (2:1, v/v, 25 mL) and the temperature was main-
tained below 8 °C. The layers were separated, and the aqueous layer
was extracted with EtOAc (60 mL). The organic layer was washed
with brine (30 mL), dried (anhyd Na2SO4), filtered, and concentrat-
ed. The residue was purified by flash chromatography on silica gel
(PE–EtOAc, 6:1) to afford 15 as a foamy liquid; yield: 3.98 g
(95%).
1H NMR (300 MHz, CDCl3): d = 7.78 (m, 7 H), 6.86 (d, J = 8.7 Hz,
2 H), 4.67 (m, 1 H), 4.44 (d, J = 12.0 Hz, 1 H), 4.37 (d, J = 11.4 Hz,
1 H), 4.17 (m, 2 H), 4.05 (m, 2 H), 3.86 (m, 1 H), 3.80 (s, 3 H), 3.49
(m, 2 H), 3.35 (dd, J = 13.2, 3.0 Hz, 1 H), 2.73 (dd, J = 13.2, 9.3 Hz,
1 H), 1.85 (m, 3 H), 1.20 (d, J = 6.9 Hz, 3 H), 0.88 (s, 9 H), 0.86 (d,
J = 7.5 Hz, 3 H), 0.12 (s, 3 H), 0.06 (s, 3 H).
tert-Butyl[(3R,4R,5R,6S,E)-5-methoxy-1-(4-methoxybenzyl-
oxy)-4,6-dimethyldec-8-en-3-yloxy]dimethylsilane (17)
To a soln of EtPPh3I (1.38 g, 3.37 mmol) in THF (30 mL) was added
2.0 M NaHMDS in THF (2.0 mL) dropwise at –10 °C. The mixture
was stirred for 15 min before being cooled to –78 °C. Then, a soln
of the above residue 8 in THF (5 mL) was added dropwise. After the
mixture was stirred for 10 min, 2.0 M NaHMDS in THF (1.1 mL)
and excess MeOH were added. The mixture was stirred for 1 h, then
warmed to r.t. for 1 h. The reaction mixture was quenched with sat.
NH4Cl soln (10 mL) and was extracted with Et2O (30 mL). The or-
ganic layer was washed with brine (20 mL), dried (Na2SO4), fil-
tered, and concentrated. The residue was chromatographed on silica
gel (PE–EtOAc, 20:1) to give 17 as a colorless oil; yield: 0.92 g
(90%); E/Z = 12:1 (determined by GC-MS).
1H NMR (300 MHz, CDCl3): d = 7.26 (d, J = 9.0 Hz, 2 H), 6.87 (d,
J = 8.7 Hz, 2 H), 5.45 (m, 2 H), 4.64 (d, J = 12.0 Hz, 1 H), 4.39 (d,
J = 12.0 Hz, 1 H), 4.11 (m, 1 H), 3.80 (s, 3 H), 3.46 (s, 3 H), 3.43
(m, 2 H), 3.16 (d, J = 9.3 Hz, 1 H), 2.15 (m, 2 H), 1.85 (m, 2 H),
1.68 (m, 1 H), 1.63 (d, J = 6.3 Hz, 3 H), 1.52 (m, 1 H), 0.92 (s, 9 H),
0.82 (d, J = 6.9 Hz, 3 H), 0.74 (d, J = 7.2 Hz, 3 H), 0.08 (s, 6 H).
(3S,4R,5R,6R)-6-(tert-Butyldimethylsilyloxy)-4-methoxy-8-(4-
methoxybenzyloxy)-3,5-dimethyloctanenitrile (16)
To a soln of tosylate 9 (1.9 g, 3.2 mmol) in anhyd DMSO (20 mL)
was added NaCN (0.45 g, 9.6 mmol), and the mixture was stirred at
80 °C for 2 h. The reaction mixture was quenched with H2O (30
mL) at 0 °C and was extracted with EtOAc (3 × 20 mL). The organ-
ic layer was washed with brine (30 mL), dried (Na2SO4), filtered,
and concentrated. The residue was chromatographed on silica gel
(PE–EtOAc, 10:1) to give cyanide 16 as a colorless oil; yield: 1.44
g (95%).
tert-Butyl[(3R,4R,5R,6S)-5-methoxy-1-(4-methoxybenzyloxy)-
4,6,9-trimethyldec-8-en-3-yloxy]dimethylsilane (18)
To a soln of i-PrPPh3I (1.89 g, 4.36 mmol) in THF (50 mL) was add-
ed 1.6 M n-BuLi in hexane (2.7 mL) dropwise at –10 °C. The mix-
ture was stirred for 15 min before being cooled to –78 °C. Then, a
soln of the above residue 8 in THF (5 mL) was added dropwise. Af-
ter being stirred for 1 h, the reaction mixture was quenched with sat.
NH4Cl soln (20 mL) and was extracted with Et2O (30 mL). The or-
ganic layer was washed with brine (50 mL), dried (Na2SO4), fil-
tered, and concentrated. The residue was chromatographed on silica
gel (PE–EtOAc, 20:1) to give 18 as a colorless oil; yield: 1.0 g
(96%).
[a]D25 –1.4 (c 1.10, CHCl3).
IR (thin film): 2932, 2249, 1614, 1514, 1250, 1090 cm–1.
1H NMR (300 MHz, CDCl3): d = 7.26 (d, J = 9.0 Hz, 2 H), 6.87 (d,
J = 8.7 Hz, 2 H), 4.44 (d, J = 12 Hz, 1 H), 4.38 (d, J = 11.7 Hz, 1
H), 4.10 (m, 1 H), 3.80 (s, 3 H), 3.49 (s, 3 H), 3.42 (m, 2 H), 3.24
(dd, J = 9.6, 2.7 Hz, 1 H), 2.44 (dd, J = 16.5, 7.8 Hz, 1 H), 2.34 (dd,
J = 16.5, 7.8 Hz, 1 H), 2.10 (m, 1 H), 1.84 (m, 2 H), 1.69 (m, 1 H),
0.92 (d, J = 6.9 Hz, 3 H), 0.88 (s, 9 H), 0.75 (d, J = 6.9 Hz, 3 H),
0.09 (s, 3 H), 0.08 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 159.0, 130.5, 129.1, 119.3, 113.7,
83.4, 72.6, 69.3, 66.9, 61.3, 55.2, 40.4, 35.6, 33.3, 25.9, 22.7, 18.2,
12.6, 9.3, –3.3, –4.2.
[a]D25 –8.4 (c 1.00, CHCl3).
IR (thin film): 2931, 1614, 1514, 1465, 1383, 1249, 1039, 836, 773
cm–1.
1H NMR (300 MHz, CDCl3): d = 7.26 (d, J = 8.7 Hz, 2 H), 6.87 (d,
J = 8.7 Hz, 2 H), 5.13 (m, 1 H), 4.44 (d, J = 12.0 Hz, 1 H), 4.39 (d,
J = 11.7 Hz, 1 H), 4.10 (m, 1 H), 3.80 (s, 3 H), 3.45 (s, 3 H), 3.42
(m, 2 H), 3.17 (dd, J = 9.6, 1.8 Hz, 1 H), 2.06 (m, 2 H), 1.84 (m, 2
H), 1.69 (s, 3 H), 1.66 (m, 1 H), 1.61 (s, 3 H), 1.50 (m, 1 H), 0.88 (s,
9 H), 0.81 (d, J = 6.9 Hz, 3 H), 0.73 (d, J = 7.2 Hz, 3 H), 0.07 (s, 3
H), 0.06 (s, 3 H).
MS (ESI): m/z = 467 [M + NH4]+.
13C NMR (100 MHz, CDCl3): d = 159.0, 132.0, 130.6, 129.1, 124.0,
113.7, 84.0, 72.6, 69.4, 67.2, 60.8, 55.2, 40.9, 35.9, 35.8, 33.6, 26.0,
25.8, 18.3, 17.9, 12.8, 9.5, –3.3, –4.3.
HRMS (ESI): m/z calcd for C25H43NO4SiNa: 472.2853; found:
472.2853.
(3S,4R,5R,6R)-6-(tert-Butyldimethylsilyloxy)-4-methoxy-8-(4-
methoxybenzyloxy)-3,5-dimethyloctanal (8)
MS (EI): m/z (%) = 478 (<1) [M+], 121 (100).
To a soln of cyanide 16 (0.98 g, 2.18 mmol) in CH2Cl2 (30 mL) at
–78 °C was added 1.0 M DIBAL-H in hexane (3.3 mL) dropwise.
After being stirred for 1.5 h, the reaction mixture was quenched
with sat. Rochelle salt soln (20 mL) at –78 °C. The mixture was
warmed to r.t. and stirred for 1 h. The aqueous phase was extracted
with CH2Cl2 (60 mL). The organic phase was dried (anhyd
Na2SO4), filtered, and concentrated to give a residue 8, which was
used without further purification.
1H NMR (300 MHz, CDCl3): d = 9.78 (t, J = 1.8 Hz, 1 H), 7.25 (d,
J = 9.0 Hz, 2 H), 6.86 (d, J = 8.7 Hz, 2 H), 4.44 (d, J = 11.7 Hz, 1
H), 4.38 (d, J = 11.7 Hz, 1 H), 4.13 (m, 1 H), 3.80 (s, 3 H), 3.43 (s,
3 H), 3.41 (m, 2 H), 3.09 (dd, J = 9.3, 1.8 Hz, 1 H), 2.55 (ddd,
J = 15.9, 7.2, 2.1 Hz, 2 H), 2.34 (m, 1 H), 1.84 (m, 2 H), 1.55 (m, 1
H), 0.91 (d, J = 6.9 Hz, 3 H), 0.87 (s, 9 H), 0.78 (d, J = 6.6 Hz, 3 H),
0.07 (s, 3 H), 0.06 (s, 3 H).
HRMS (EI): m/z calcd for C28H50O4Si: 478.3478; found: 478.3482.
(3R,4R,5R,6S)-3-(tert-Butyldimethylsilyloxy)-5-methoxy-4,6,9-
trimethyldec-8-en-1-ol (20)
To a soln of 18 (0.5 g, 1.0 mmol) in CH2Cl2–H2O (19:1, v/v, 20 mL)
at 0 °C was added DDQ (0.27 g, 1.2 mmol). After being stirred for
3 h, the reaction mixture was quenched with 1 M NaOH (10 mL).
The aqueous phase was extracted with CH2Cl2 (10 mL). The organ-
ic phase was washed with H2O (20 mL) and brine (20 mL), dried
(anhyd Na2SO4), filtered, and concentrated. The residue was puri-
fied by flash chromatography on silica gel (PE–EtOAc, 6:1) to af-
ford 20 as a clear oil; yield: 0.33 g (88%).
[a]D25 –8.7 (c 1.00, CHCl3).
IR (thin film): 3400, 2930, 1469, 1382, 1255, 1077, 836 cm–1.
1H NMR (300 MHz, CDCl3): d = 5.13 (m, 1 H), 4.11 (m, 1 H), 3.67
(m, 2 H), 3.47 (s, 3 H), 3.16 (d, J = 9.0 Hz, 1 H), 2.05 (m, 2 H), 1.81
Synthesis 2010, No. 2, 267–275 © Thieme Stuttgart · New York

272
J. Lin et al.
PAPER
(m, 2 H), 1.70 (s, 3 H), 1.67–1.59 (m, 2 H), 1.61 (s, 3 H), 0.89 (s, 9
H), 0.82 (d, J = 6.9 Hz, 3 H), 0.76 (d, J = 7.2 Hz, 3 H), 0.103 (s, 3
H), 0.100 (s, 3 H).
13C NMR (100 MHz, CDCl3): d = 132.4, 124.1, 84.7, 69.7, 61.0,
60.0, 40.6, 38.7, 36.0, 33.9, 26.1, 26.0, 18.5, 18.1, 13.1, 10.1, –3.1,
–4.1.
pound 21 (100 mg, 0.18 mmol) in DMF (5 mL) was added. The
mixture was stirred for another 3.5 h under H2 atmosphere, where-
upon 19F NMR spectroscopy indicated the absence of starting mate-
rial 21. The reaction mixture was filtered and concentrated. The
residue was purified by flash chromatography on silica gel (PE–
EtOAc, 20:1) to afford 3 as a yellow oil; yield: 96 mg (95%).
IR (thin film): 3450, 2931, 1473, 1388, 1260, 1093 cm–1.
MS (ESI): m/z = 359 [M + H]+.
1H NMR (300 MHz, CDCl3): d = 5.95 (m, 1 H), 5.45 (m, 3 H), 4.44
(s, 2 H), 4.14 (m, 1 H), 3.97 (m, 0.34 H), 3.80 (m, 0.66 H), 3.47 (s,
2 H), 3.41 (s, 1 H), 3.15 (m, 0.66 H), 3.09 (m, 0.34 H), 2.14 (m, 2
H), 1.78 (m, 2 H), 1.70 (m, 2 H), 1.61 (d, J = 6.6 Hz, 3 H), 0.92 (s,
18 H), 0.84 (d, J = 6.9 Hz, 4 H), 0.75 (d, J = 7.2 Hz, 2 H), 0.09 (s, 4
H), 0.05 (s, 8 H).
19F NMR (282 MHz, CDCl3): d (major product) = –102.4 (d, J =
254.6 Hz, 1 F), –106.4 (dt, J = 257.2, 12.1 Hz, 1 F); d (minor prod-
uct) = –102.8 (d, J = 257.5 Hz, 1 F), –107.0 (dt, J = 256.9, 11.8 Hz,
1 F).
HRMS (ESI): m/z calcd for C20H42O3SiNa: 381.2795; found:
381.2802.
(7R,8R,9R,10S,E)-1,7-Bis(tert-butyldimethylsilyloxy)-4,4-di-
fluoro-9-methoxy-8,10-dimethyltetradec-12-en-2-yn-5-ol (21)
To a soln of 19 (0.18 g, 0.52 mmol) in EtOAc (10 mL) was added
IBX (0.44 g, 1.57 mmol). After being stirred at reflux for 3 h, the
reaction mixture was filtered through Celite® and the filter cake was
washed with EtOAc. The organic phase was dried (anhyd Na2SO4),
filtered, and concentrated to give a residue 5, which was used with-
out further purification.
MS (ESI): m/z = 565 [M + H]+.
To a stirred suspension of the above residue 5 and compound 7
(0.19 g, 0.62 mmol) in THF–H2O (1:4, v/v, 10 mL) was added indi-
um power (78 mg, 0.68 mmol) at r.t. After being stirred for 22 h, the
reaction mixture was quenched with 1 M HCl (5 mL). The aqueous
phase was extracted with EtOAc (10 mL), and the organic phase
was washed with H2O (5 mL) and brine (3 mL), dried (anhyd
Na2SO4), filtered, and concentrated. The residue was purified by
flash chromatography on silica gel (PE–EtOAc, 20:1) to afford 21
as a yellow oil; yield: 0.23 g (78%).
Anal. Calcd for C29H58F2O4Si2: C, 61.65; H, 10.35. Found: C,
61.83; H, 10.59.
(2Z,7R,8R,9R,10S)-1,7-Bis(tert-butyldimethylsilyloxy)-4,4-di-
fluoro-9-methoxy-8,10,13-trimethyltetradeca-2,12-dien-5-ol (4)
Compound 4 was prepared from compound 22 (150 mg, 0.26
mmol) using the same conditions as described for compound 3;
yield: 135 mg (90%); yellow oil.
IR (thin film): 3450, 2931, 1473, 1260, 1093, 837 cm–1.
IR (thin film): 3450, 2931, 2256, 1473, 1255, 1105 cm–1.
1H NMR (300 MHz, CDCl3): d = 5.97 (m, 1 H), 5.50 (m, 1 H), 5.13
(m, 1 H), 4.45 (m, 2 H), 4.16 (m, 1 H), 3.97 (m, 0.3 H), 3.85 (m, 0.7
H), 3.47 (s, 2 H), 3.45 (s, 1 H), 3.16 (dd, J = 9.3, 1.5 Hz, 0.7 H), 3.10
(dd, J = 8.4, 1.8 Hz, 0.3 H), 2.07 (m, 2 H), 1.79 (m, 1 H), 1.70 (s, 3
H), 1.65 (m, 3 H), 1.61 (s, 3 H), 0.90 (s, 18 H), 0.83 (d, J = 6.6 Hz,
4 H), 0.75 (d, J = 6.6 Hz, 2 H), 0.09 (s, 4 H), 0.07 (s, 8 H).
19F NMR (282 MHz, CDCl3): d (major product) = –102.5 (dt, J =
249.9, 13.8 Hz, 1 F), –106.7 (dt, J = 257.7, 12.1 Hz, 1 F); d (minor
product) = –102.8 (dt, J = 257.5, 17.5 Hz, 1 F), –106.9 (dt, J = 256.9,
11.8 Hz, 1 F).
1H NMR (300 MHz, CDCl3): d = 5.52 (m, 1 H), 5.39 (m, 1 H), 4.40
(m, 2 H), 4.18 (m, 1 H), 4.02 (m, 0.33 H), 3.87 (m, 0.67 H), 3.49 (s,
2 H), 3.46 (s, 1 H), 3.10 (m, 2 H), 2.13 (m, 2 H), 1.85 (m, 2 H), 1.70
(m, 2 H), 1.62 (d, J = 6.6 Hz, 3 H), 0.90 (s, 18 H), 0.85 (m, 4 H),
0.77 (d, J = 6.6 Hz, 2 H), 0.11 (s, 8 H), 0.08 (s, 4 H).
19F NMR (282 MHz, CDCl3): d = –96.2 (s, 0.66 F), –96.6 (s, 1.34
F).
MS (ESI): m/z = 563 [M + H]+.
Anal. Calcd for C29H56F2O4Si2: C, 61.88; H, 10.03. Found: C,
61.80; H, 9.70.
MS (ESI): m/z = 601 [M + Na]+, 579 [M + H]+.
Anal. Calcd for C30H60F2O4Si2: C, 62.03; H, 9.98. Found: C, 62.06;
H, 10.31.
(7R,8R,9R,10S)-1,7-Bis(tert-butyldimethylsilyloxy)-4,4-difluo-
ro-9-methoxy-8,10,13-trimethyltetradec-12-en-2-yn-5-ol (22)
Compound 22 was prepared from compound 20 (0.20 g, 0.56
mmol), using the same conditions as described for compound 21;
yield: 0.32 g (75%); yellow oil.
(2Z,7R,8R,9R,10S,12E)-7-(tert-Butyldimethylsilyloxy)-4,4-di-
fluoro-9-methoxy-8,10-dimethyltetradeca-2,12-diene-1,5-diol
(23)
To a soln of 3 (70 mg, 0.12 mmol) in MeOH (5 mL) was added CSA
(1 mg, 0.005 mmol). After being stirred for 1 h, the reaction mixture
was quenched with sat. NaHCO3 soln (5 mL). The aqueous phase
was extracted with EtOAc (10 mL), and the organic phase was
washed with brine (8 mL), dried (anhyd Na2SO4), filtered, concen-
trated, and purified by flash chromatography on silica gel (PE–
EtOAc, 4:1) to give 23 as a clear oil; yield: 49 mg (88%).
IR (thin film): 3440, 2931, 2260, 1473, 1257, 1107 cm–1.
1H NMR (300 MHz, CDCl3): d = 5.13 (m, 1 H), 4.40 (t, J = 4.5 Hz,
2 H), 4.18 (m, 1 H), 4.00 (m, 0.4 H), 3.85 (m, 0.6 H), 3.47 (s, 1.8 H),
3.45 (s, 1.2 H), 3.17 (m, 0.6 H), 3.10 (m, 0.4 H), 2.95 (br s, 1 H),
2.10 (m, 2 H), 1.90 (m, 2 H), 1.75 (m, 2 H), 1.70 (s, 3 H), 1.61 (s,
3 H), 0.91 (s, 10.8 H), 0.90 (s, 7.2 H), 0.82 (d, J = 7.2 Hz, 3.6 H),
0.76 (d, J = 6.6 Hz, 2.4 H), 0.11 (s, 7.2 H), 0.10 (s, 4.8 H).
19F NMR (282 MHz, CDCl3): d = –96.4 (d, J = 12.7 Hz, 0.75 F),
–96.6 (s, 1.25 F).
MS (ESI): m/z = 599 [M + Na]+, 577 [M + H]+.
IR (thin film): 3450, 2931, 1465, 1254, 1081 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.07 (m, 1 H), 5.55 (m, 2 H), 5.38
(m, 1 H), 4.35 (m, 2 H), 4.14 (m, 1 H), 3.95 (m, 0.33 H), 3.80 (m,
0.67 H), 3.49 (s, 2 H), 3.46 (s, 1 H), 3.18 (d, J = 9.6 Hz, 0.67 H),
3.09 (d, J = 8.4 Hz, 0.33 H), 2.90 (br s, 2 H), 2.15 (m, 2 H), 1.81 (m,
1 H), 1.72 (m, 3 H), 1.62 (d, J = 6.9 Hz, 3 H), 0.9 (s, 3 H), 0.89 (s,
6 H), 0.83 (d, J = 6.9 Hz, 4 H), 0.75 (d, J = 7.2 Hz, 2 H), 0.01 (s, 2
H), 0.009 (s, 4 H).
Anal. Calcd for C30H58F2O4Si2: C, 62.45; H, 10.13. Found: C,
62.64; H, 9.97.
(2Z,7R,8R,9R,10S,12E)-1,7-Bis(tert-butyldimethylsilyloxy)-4,4-
difluoro-9-methoxy-8,10-dimethyltetradeca-2,12-dien-5-ol (3)
To a soln of Pd/BaSO4 (8 mg, 42 mg/mmol) in MeOH (5 mL) was
added quinoline (8 mg, 42 mg/mmol) in MeOH (1 mL) at 0 °C. The
suspension was warmed to r.t. and stirred for 15 min before com-
19F NMR (282 MHz, CDCl3): d (major product) = –100.7 (d, J =
249.3 Hz, 1 F), –106.0 (dt, J = 256.3, 13.8 Hz, 1 F); d (minor prod-
Synthesis 2010, No. 2, 267–275 © Thieme Stuttgart · New York

PAPER
Synthesis of gem-Difluoromethylenated Analogues of Pironetin
273
uct) = –102.1 (d, J = 250.7 Hz, 1 F), –106.4 (dt, J = 251.5, 15.5 Hz,
1 F).
MS (ESI): m/z = 473 [M + Na]+, 451 [M + H]+.
19F NMR (282 MHz, CDCl3): d = –108.6 (dd, J = 288.5, 20.6 Hz, 1
F), –111.9 (dd, J = 288.8, 5.4 Hz, 1 F).
MS (ESI): m/z = 447 [M + H]+.
Anal. Calcd for C23H44F2O4Si: C, 61.30; H, 9.87. Found: C, 61.69;
H, 10.02.
Anal. Calcd for C23H40F2O4Si: C, 61.85; H, 9.03. Found: C, 62.06;
H, 8.63.
(2Z,7R,8R,9R,10S)-7-(tert-Butyldimethylsilyloxy)-4,4-difluoro-
9-methoxy-8,10,13-trimethyltetradeca-2,12-diene-1,5-diol (24)
Compound 24 was prepared from compound 4 (90 mg, 0.16 mmol)
using the same conditions as described for compound 23; yield: 59
mg (82%); clear oil.
6-[(2R,3R,4R,5S)-2-(tert-Butyldimethylsilyloxy)-4-methoxy-
3,5,8-trimethylnon-7-enyl]-5,5-difluoro-5,6-dihydro-2H-pyran-
2-one (26)
Compound 26 was prepared from compound 24 (70 mg, 0.15
mmol) using the same conditions as described for compound 25.
IR (thin film): 3350, 2931, 1473, 1386, 1257, 1037 cm–1.
Compound 26a
Yield: 31 mg (45%); clear oil.
[a]D24 –11.8 (c 1.00, CHCl3).
IR (thin film): 2958, 1752, 1465, 1379, 1255, 1075, 836 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.81 (m, 1 H), 6.28 (d, J = 10.2
Hz, 1 H), 5.13 (m, 1 H), 4.61 (m, 1 H), 4.27 (m, 1 H), 3.47 (s, 3 H),
3.17 (dd, J = 9.0, 1.5 Hz, 1 H), 2.08 (m, 2 H), 1.95 (m, 2 H), 1.70 (s,
3 H), 1.65 (m, 2 H), 1.62 (s, 3 H), 0.89 (s, 9 H), 0.84 (d, J = 6.9 Hz,
3 H), 0.81 (d, J = 7.2 Hz, 3 H), 0.13 (s, 3 H), 0.08 (s, 3 H).
19F NMR (282 MHz, CDCl3): d = –108.5 (dd, J = 284.5, 18.6 Hz, 1
F), –111.6 (dd, J = 281.7, 10.4 Hz, 1 F).
1H NMR (300 MHz, CDCl3): d = 6.06 (m, 1 H), 5.59 (m, 1 H), 5.12
(m, 1 H), 4.34 (m, 2 H), 4.14 (m, 1 H), 3.99 (m, 0.3 H), 3.80 (m, 0.7
H), 3.48 (s, 2 H), 3.45 (s, 1 H), 3.19 (d, J = 9.6 Hz, 0.7 H), 3.11 (d,
J = 8.4 Hz, 0.3 H), 2.62 (br s, 2 H), 2.06 (m, 2 H), 1.79 (m, 1 H),
1.71 (s, 3 H), 1.70 (m, 3 H), 1.61 (s, 3 H), 0.91 (s, 3 H), 0.89 (s, 6
H), 0.83 (d, J = 7.2 Hz, 4 H), 0.75 (d, J = 6.9 Hz, 2 H), 0.12 (s, 2 H),
0.10 (s, 4 H).
19F NMR (282 MHz, CDCl3): d (major product) = –100.7 (dt, J =
268.2, 10.2 Hz, 1 F), –105.8 (dt, J = 252.9, 10.4 Hz, 1 F); d (minor
product) = –101.9 (d, J = 254.4 Hz, 1 F), –106.3 (dt, J = 239.7, 14.7
Hz, 1 F).
MS (ESI): m/z = 487 [M + Na]+, 465 [M + H]+.
MS (ESI): m/z = 483 [M + Na]+, 461 [M + H]+.
Anal. Calcd for C24H46F2O4Si: C, 62.24; H, 10.45. Found: C, 62.35;
H, 10.45.
Anal. Calcd for C24H42F2O4Si: C, 62.57; H, 9.19. Found: C, 62.06;
H, 8.68.
6-[(2R,3R,4R,5S,E)-2-(tert-Butyldimethylsilyloxy)-4-methoxy-
3,5-dimethylnon-7-enyl]-5,5-difluoro-5,6-dihydro-2H-pyran-2-
one (25)
Compound 26b
Yield: 23 mg (33%); clear oil.
[a]D24 +19.7 (c 1.00, CHCl3).
IR (thin film): 2931, 1752, 1465, 1378, 1252, 1070 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.80 (m, 1 H), 6.29 (d, J = 10.2
Hz, 1 H), 5.13 (m, 1 H), 4.52 (m, 1 H), 4.26 (m, 1 H), 3.48 (s, 3 H),
3.18 (dd, J = 8.7, 1.2 Hz, 1 H), 2.10 (m, 2 H), 2.00 (m, 2 H), 1.70 (s,
3 H), 1.65 (m, 2 H), 1.62 (s, 3 H), 0.90 (s, 9 H), 0.83 (d, J = 6.6 Hz,
3 H), 0.76 (d, J = 6.6 Hz, 3 H), 0.13 (s, 3 H), 0.12 (s, 3 H).
19F NMR (282 MHz, CDCl3): d = –108.6 (dd, J = 286.6, 20.0 Hz, 1
F), –112.0 (dd, J = 281.4, 10.4 Hz, 1 F).
To a soln of 23 (50 mg, 0.11 mmol) in CH2Cl2 (5 mL) were added
BAIB (106 mg, 0.33 mmol) and TEMPO (3 mg, 20 mol%) at r.t. Af-
ter being stirred for 3 h, the reaction mixture was quenched with sat.
Na2S2O3 soln (3 mL) and was extracted with CH2Cl2 (3 × 2 mL).
The combined organic extracts were washed with sat. NaHCO3 soln
(5 mL) and brine (5 mL), dried (anhyd Na2SO4), filtered, and con-
centrated. The residue was purified by flash chromatography on sil-
ica gel (PE–EtOAc, 10:1) to afford 25a [yield: 22 mg (44%)] and
25b [yield: 15 mg (30%)].
Compound 25a
MS (ESI): m/z = 499 [M + K]+, 483 [M + Na]+, 461 [M + H]+.
Clear oil; [a]D24 –10.3 (c 1.15, CHCl3).
Anal. Calcd for C24H42F2O4Si: C, 62.57; H, 9.19. Found: C, 62.51;
H, 8.77.
IR (thin film): 2933, 1753, 1642, 1456, 1255, 1073 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.82 (m, 1 H), 6.29 (d, J = 9.9 Hz,
1 H), 5.48 (m, 2 H), 4.61 (m, 1 H), 4.28 (m, 1 H), 3.49 (s, 3 H), 3.16
(d, J = 9.0 Hz, 1 H), 2.16 (m, 3 H), 1.98 (m, 1 H), 1.70 (m, 2 H),
1.61 (d, J = 6.9 Hz, 3 H), 0.89 (s, 9 H), 0.85 (d, J = 5.4 Hz, 3 H),
0.82 (d, J = 7.2 Hz, 3 H), 0.14 (s, 3 H), 0.08 (s, 3 H).
19F NMR (282 MHz, CDCl3): d = –108.7 (dd, J = 283.1, 16.4 Hz, 1
F), –111.5 (dd, J = 288.2, 11.8 Hz, 1 F).
(6R)-5,5-Difluoro-6-[(2R,3S,4R,5S,E)-2-hydroxy-4-methoxy-
3,5-dimethylnon-7-enyl]-5,6-dihydro-2H-pyran-2-one (1a)
To a soln of 25a (20 mg, 0.045 mmol) in THF (5 mL) was added 6
M HCl (5 mL) at r.t. After the starting material had been consumed,
the mixture was diluted with H2O (5 mL). The aqueous phase was
extracted with EtOAc (8 mL), and the organic phase was washed
with sat. NaHCO3 soln (5 mL) and brine (5 mL), dried (anhyd
Na2SO4), filtered, concentrated, and purified by flash chromatogra-
phy on silica gel (PE–EtOAc, 4:1) to afford 1a as a clear oil; yield:
14.2 mg (95%).
[a]D24 –27.8 (c 0.60, CHCl3).
IR (thin film): 3474, 2931, 1751, 1461, 1241, 1082 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.83 (m, 1 H), 6.30 (d, J = 9.9 Hz,
1 H), 5.48 (m, 2 H), 4.86 (m, 1 H), 4.23 (m, 1 H), 3.50 (s, 3 H), 3.05
(m, 1 H), 2.11 (m, 1 H), 1.82 (m, 5 H), 1.64 (d, J = 7.2 Hz, 3 H),
1.02 (d, J = 6.6 Hz, 3 H), 0.98 (d, J = 7.2 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 156.5, 133.6 (dd, J = 34.2, 18.8
Hz), 123.9, 122.4 (dd, J = 10.3, 7.9 Hz), 121.2, 108.6 (t, J = 244.0
MS (EI): m/z (%) = 389 (3) [M – C4H9]+, 291 (87), 265 (100).
Anal. Calcd for C23H40F2O4Si: C, 61.85; H, 9.03. Found: C, 61.71;
H, 8.80.
Compound 25b
Clear oil; [a]D24 +19.0 (c 0.63, CHCl3).
IR (thin film): 2931, 2859, 1751, 1473, 1260, 1073 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.86 (m, 1 H), 6.29 (d, J = 10.2
Hz, 1 H), 5.45 (m, 2 H), 4.51 (m, 1 H), 4.25 (m, 1 H), 3.50 (s, 3 H),
3.17 (d, J = 9.3 Hz, 1 H), 2.17 (m, 3 H), 1.98 (m, 1 H), 1.70 (m, 2
H), 1.63 (d, J = 6.9 Hz, 3 H), 0.90 (s, 9 H), 0.84 (d, J = 7.2 Hz, 3 H),
0.79 (d, J = 6.9 Hz, 3 H), 0.13 (s, 3 H), 0.12 (s, 3 H).
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Hz), 86.8, 71.8 (dd, J = 32.3, 27.2 Hz), 62.0, 57.3, 34.6, 33.1, 32.1,
27.1, 13.8, 11.0, 8.3.
19F NMR (282 MHz, CDCl3): d = –108.6 (dd, J = 290.2, 21.4 Hz, 1
F), –113.0 (d, J = 284.0 Hz, 1 F).
MS (EI): m/z (%) = 300 (8) [M – OCH3 – H]+, 73 (100).
13C NMR (100 MHz, CDCl3): d = 160.4 (t, J = 2.1 Hz), 137.9 (dd,
J = 32.8, 25.8 Hz), 132.7, 126.3 (dd, J = 10.3, 8.2 Hz), 122.5, 112.8
(dd, J = 243.0, 235.4 Hz), 89.5, 77.7 (dd, J = 31.5, 28.3 Hz), 67.9,
61.5, 39.0, 36.4, 33.0, 32.6, 25.8, 17.9, 14.6, 11.4.
19F NMR (282 MHz, CDCl3): d = –108.5 (dd, J = 287.9, 20.3 Hz, 1
F), –112.9 (dd, J = 288.7, 6.4 Hz, 1 F).
HRMS (EI): m/z calcd for C17H26F2O4: 332.1799; found: 332.1801.
MS (ESI): m/z = 385 [M + K]+, 369 [M + Na]+, 347 [M + H]+.
(6S)-5,5-Difluoro-6-[(2R,3S,4R,5S,E)-2-hydroxy-4-methoxy-
3,5-dimethylnon-7-enyl]-5,6-dihydro-2H-pyran-2-one (1b)
Compound 1b was prepared from compound 25b (18 mg, 0.04
mmol) using the same conditions as described for compound 1a;
yield: 12 mg (92%); clear oil.
[a]D25 +54.8 (c 0.60, CHCl3).
IR (thin film): 3455, 2931, 1746, 1439, 1243, 1122 cm–1.
HRMS (ESI): m/z calcd for C18H28F2O4Na: 369.1848; found:
369.1848.
Cell Culture and Cell Viability Assay
Human melanoma A375 cells and human MGC803 cells were cul-
tured to exponential growth phase in RPMI 1640 medium supple-
mented with 10% (v/v) fetal calf serum. Both cell lines were
cultured in a humidified atmosphere containing 5% CO2.
1H NMR (300 MHz, CDCl3): d = 6.83 (m, 1 H), 6.30 (d, J = 10.3
Hz, 1 H), 5.48 (m, 2 H), 4.79 (m, 1 H), 4.23 (m, 1 H), 3.50 (s, 3 H),
3.08 (m, 1 H), 2.10 (m, 4 H), 1.72 (m, 2 H), 1.62 (d, J = 7.2 Hz, 3
H), 0.98 (d, J = 7.2 Hz, 3 H), 0.95 (d, J = 6.9 Hz, 3 H).
13C NMR (100 MHz, CDCl3): d = 156.2 (t, J = 2.3 Hz), 133.7 (dd,
J = 32.6, 25.6 Hz), 124.3, 122.1 (dd, J = 9.9, 8.2 Hz), 121.1, 108.8
(dd, J = 244.6, 236.4 Hz), 85.1, 73.6 (dd, J = 32.1, 28.5 Hz), 63.6,
57.4, 34.9, 33.2, 31.9, 27.1, 13.8, 10.4, 7.1.
The viability of the tumor cells was evaluated using the Cell Count-
ing Kit-8 (CCK-8) assay (Dojindo Laboratories) following the in-
structions of the kit. Briefly, 2 × 104 cells were seeded in each well
of a 96-well plate. Various concentrations of the gem-difluorometh-
ylenated analogues of pironetin were added, then the plate was in-
cubated for 24 h. The CCK-8 soln was added (10 mL/well) and the
cells were incubated for 1–4 h at 37 °C. Then, the absorbance (OD
492 nm) of each well was measured using a microplate reader.
19F NMR (282 MHz, CDCl3): d = –108.4 (dd, J = 287.6, 18.0 Hz, 1
F), –110.2 (dd, J = 288.2, 9.0 Hz, 1 F).
MS (EI): m/z (%) = 300 (7) [M – OCH3 – H]+, 73 (100).
Acknowledgment
We thank the National Natural Science Foundation of China and the
Shanghai Municipal Scientific Committee for funding this work.
HRMS (EI): m/z calcd for C17H26F2O4: 332.1799; found: 332.1796.
(6R)-5,5-Difluoro-6-[(2R,3S,4R,5S)-2-hydroxy-4-methoxy-
3,5,8-trimethylnon-7-enyl]-5,6-dihydro-2H-pyran-2-one (2a)
Compound 2a was prepared from compound 26a (25 mg, 0.05
mmol) using the same conditions as described for compound 1a;
yield: 17 mg (93%); clear oil.
[a]D25 –33.5 (c 0.50, CHCl3).
IR (thin film): 3480, 2932, 1747, 1462, 1377, 1271, 1080 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.81 (m, 1 H), 6.30 (d, J = 10.2
Hz, 1 H), 5.11 (m, 1 H), 4.87 (m, 1 H), 4.24 (d, J = 10.8 Hz, 1 H),
3.49 (s, 3 H), 3.03 (m, 1 H), 2.04 (m, 2 H), 1.91 (m, 4 H), 1.77 (s, 3
H), 1.61 (s, 3 H), 1.03 (d, J = 6.9 Hz, 3 H), 0.97 (d, J = 6.3 Hz, 3 H).
References
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Hz), 132.9, 126.6 (dd, J = 10.3, 7.4 Hz), 122.2, 112.8 (dd, J = 242.6,
234.0 Hz), 91.3, 76.1 (dd, J = 32.8, 27.5 Hz), 66.2, 61.5, 38.7, 36.6,
32.5, 31.7, 25.8, 17.9, 15.3, 12.5.
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(100).
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(6S)-5,5-Difluoro-6-[(2R,3S,4R,5S)-2-hydroxy-4-methoxy-
3,5,8-trimethylnon-7-enyl]-5,6-dihydro-2H-pyran-2-one (2b)
Compound 2b was prepared from compound 26b (20 mg, 0.04
mmol) using the same conditions as described for compound 1a;
yield: 14 mg (93%); clear oil.
[a]D25 +65.5 (c 0.50, CHCl3).
IR (thin film): 3487, 2931, 1752, 1465, 1294, 1075 cm–1.
1H NMR (300 MHz, CDCl3): d = 6.81 (m, 1 H), 6.30 (d, J = 9.9 Hz,
1 H), 5.11 (m, 1 H), 4.79 (m, 1 H), 4.22 (m, 1 H), 3.50 (s, 3 H), 3.08
(m, 1 H), 2.08 (m, 3 H), 1.91 (m, 1 H), 1.80 (m, 2 H), 1.71 (s, 3 H),
1.60 (s, 3 H), 0.98 (d, J = 7.2 Hz, 3 H), 0.92 (d, J = 6.6 Hz, 3 H).
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