Stereoselective Synthesis of γ-Butyrolactones Subunit of Polycavernoside A

Article abstract of DOI:10.1002/jhet.3240

An efficient and versatile linear synthesis of γ-butyrolactone subunit of polycavernolide A has been reported in 14.2% overall yield with 10 linear steps by employing Sharpless asymmetric epoxidation, ring-closing metathesis, and diastereoface selective h

Full text of DOI:10.1002/jhet.3240

Month 2018
Stereoselective Synthesis of γ-Butyrolactones Subunit of
Polycavernoside A
*
*
Sudhakar Kadari,
Hemasri Yerrabelly, Thirupathi Gogula, Yadaiah Goud Erukala, Jayaprakash Rao Yerrabelly, and
Prem Kumar Begari
Department of Chemistry, Osmania University, Hyderabad, Telangana 500007, India
*
E-mail: sudhakarou@osmania.ac.in; hemay2@yahoo.com
Received November 10, 2017
DOI 10.1002/jhet.3240
Published online 00 Month 2018 in Wiley Online Library (wileyonlinelibrary.com).
An efficient and versatile linear synthesis of γ-butyrolactone subunit of polycavernolide A has been re-
ported in 14.2% overall yield with 10 linear steps by employing Sharpless asymmetric epoxidation, ring-
closing metathesis, and diastereoface selective hydrogenation.
J. Heterocyclic Chem., 00, 00 (2018).
INTRODUCTION
methodology and reduction of exo-methylene C–C
double bond producing the syn-disubstituted lactone as
The red alga Polycavernosa tsudai is a delicacy widely
consumed in the Far East. When several Guam natives
died suddenly following its ingestion in April 1991,
immediate attempts were launched to identify the
responsible toxin. Shortly thereafter, Yasumoto and co-
workers succeeded in characterizing polycavernoside A
[1] (Fig. 1) as one of the principal risk factors.
The proposed macrolactone disaccharide structure
was confirmed via the first total synthesis by Murai
et al., which furthermore established the absolute
stereochemistry [2]. Despite the molecular complexity of
polycavernoside, the lactone moiety has proved to be
essential for the biological activity.
Lactone rings constitute structural features of a wide
range of natural products that display important
biological activities such as anti-HIV, antifungal,
antibacterial, and antitumor properties. Among them,
six-membered lactone, five-membered lactone, and
macrolide lactone moieties are relatively common in
various types of natural products [3]. In recent years,
synthesis of novel heterocycles has gained considerable
attention due to their potential role in organic synthesis
for the formation of bioactive natural products and their
semisynthetic derivatives [4–8]. γ-Butyrolactones have
a
single diastereoisomer [9]. Our strategy relies
on Sharpless asymmetric epoxidation, reductive
elimination, RCM, and stereoselective hydrogenation to
synthesize γ-butyrolactones subunit of polycavernoside
A (Scheme 1).
RESULTS AND DISCUSSION
γ-Butyrolactones subunit can be synthesized from
readily available 2,2-dimethyl-1,3-propane diol, 1. The
selective monobenzylation of the diol was achieved by
treatment with one equivalent of NaH and BnBr to
afford 2 in 80% yield. The free primary hydroxy group
was oxidized under Swern condition to afford crude
aldehyde which without further purification immediately
subjected to Wittig-Horner homologation to obtain α, β-
unsaturated ester 3 along with minor amount of cis
diastereomer in a 9:1 ratio as a separable mixture.
Compound 3 was treated with DIBAL-H in CH2Cl2 at
0°C to provide the desired trans known [10] allyl
alcohol 4. Sharpless asymmetric epoxidation of 4 with
D-(+)-DET, Ti (OiPr)4, molecular sieves in dry CH2Cl2,
TBHP, at À20°C for 6 h afforded the epoxy alcohol 5
in 80% of yield [11]. Compound 5 was confirmed from
1H NMR spectrum wherein the protons attached to the
epoxide ring resonated as multiplet at δ 3.03 for one
proton and at δ 2.87 (d, J = 2.2 Hz, 1H) for one proton
while the remaining protons resonated at their respective
chemical shifts. In ESI-MS, [M + H]+ peaks observed
at m/z 237 further supported the formation of the
product 5.
been
utilized
as
intermediates
for
synthetic
transformations, and much attention has been paid to
their synthesis. To the best of our knowledge, this is the
first report for the preparation of γ-butyrolactones using
ring-closing metathesis (RCM).
Previously, synthesis of γ-butyrolactones subunit of
polycavernoside A was achieved by novel ene reaction
followed by intramolecular oxidative cyclization
© 2018 Wiley Periodicals, Inc.

S. Kadari, H. Yerrabelly, T. Gogula, Y. G. Erukala, J. R. Yerrabelly, and P. K. Begari
Vol 000
NMR, and mass spectroscopy clearly indicates the
1
presence of bis olefin. In H NMR, the resonance due
to terminal double bond proton appeared at δ 6.08 ppm
as a doublet for one proton, at δ 5.83 as a multiplet for
two protons, at δ 5.53 as a multiplet for two protons,
and with the other required protons confirmed the
product. Further, the formation of the product was
supported by ESI-MS, which showed [M + Na]+ peaks
at m/z 311.
Figure 1. Polycavernoside A.
In order to construct the lactone ring, compound 8 was
subjected to RCM using Grubbs’ 2nd generation catalyst
in refluxing toluene for 8 h under nitrogen atmosphere to
afford α, β-unsaturated γ-lactone 9 [15]. In 1H NMR,
terminal olefinic proton peaks disappeared and resonance
due to α, β-unsaturated double bond proton appeared at δ
7.06 ppm as a doublet for one proton indicating the
cyclization and rest of the protons appeared at their
respective chemical shifts. Further, the formation of the
product was supported by ESI-MS, which showed
[M + Na]+ peaks at m/z 283.
Diastereoface selective reduction of olefin 9 with
simaltaneous debenzylation with Pd\C under hydrogen
atmosphere afforded 10. The alcohol was protected as
its TBDPS-ether with TBDPSCl and imidazole in dry
CH2Cl2 to yield 11. In 1H NMR, the presence of
compound 11, δ 7.60–7.66 multiplet for four protons
and δ 7.46–7.35 multiplet for six protons, indicates
the presence of TBDPS protection and the resonance
of methyl peak at δ 1.26 ppm (d, J = 7.0 Hz, 3H),
while the remaining protons at their respective
chemical shifts further support the structure. In
addition, the product formation was supported by ESI-
MS, which showed [M + Na]+ peaks at m/z 433.
Compound 5 was converted to its iodo derivative 6
by treating with triphenylphosphine, imidazole, and
iodine in ether and acetonitrile (8:2) [12]. Product
structure was established from its 1H NMR spectrum
where the chemical shift of epoxy protons was
resonated at δ 2.93 ppm (dd, J = 9.4, 2.3 Hz, 1H)
and δ 2.74 ppm (d, J = 2.1 Hz, 1H) and other
protons were resonated at expected regions and further
supported by its ESI-MS, which showed [M + Na]+
peaks at m/z 369. Then the compound
6 was
converted into allylic alcohol 7 with zinc dust and
sodium iodide in methanol at reflux for 4 h to
furnish 7 in 80% yield [13].
Compound 7 structure was confirmed from its 1H
NMR spectrum in which terminal double bond protons
were resonated at δ 5.79–6.02 (m, 1H) and δ 5.08–5.37
(m, 2H), while the remaining protons were observed at
their respective chemical shifts. Esterification of
compound 7 with methacryloyl chloride and triethyl
amine in CH2Cl2 at 0°C to room temperature afforded
corresponding bis-olefine 8 [14]. The 1H NMR, 13C
Scheme 1. Synthesis of g-butyrolactone 10.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Sharpless Epoxidation, Reductive Elimination, Ring-Closing Metathesis (RCM),
Steroselective Asymmetric Hydrogenation, γ-Butyrolactones
Compound 11 was identical with reported compound in
literature [16].
combined organic layers were washed with brine, dried
(Na2SO4), and concentrated, and the residue was purified
over silica gel (EtOAc: n-Hexane, 1:9) to give the
product 3 (0.79 g, 88%, for two steps) as colorless syrup.
1H NMR (300 MHz, CDCl3) δ 7.23–7.44 (m, 5 H); 7.02
(d, J = 16.0, 1H); 5.81 (d, J = 16.0, 1H); 4.51 (s, 2H);
4.18 (q, J = 7.0, 2H); 3.26 (s, 2H); 1.29 (t, J = 7.0, 3H);
1.09 (s, 6H). 13C NMR (75 MHz, CDCl3): 166.5; 160.0;
137.6; 128.2; 127.4; 125.0; 115.1; 83.1; 72.3; 61.1; 35.8;
23.0; 14.5. IR (neat) νmax: 3425, 2966, 2929, 2860, 1719,
1650, 1456, 1307, 1181, 1101, 738 cmÀ1. Mass (ESI-
MS) m/z: 285 (M + Na)+.
CONCLUSION
In summary, we have reported a stereoselective synthesis
(10 steps, 14.2% overall yields) of γ-butyrolactones subunit
of polycavernoside A, relying on Sharpless assymitric
epoxidation, reductive elimination, RCM, and diastereo
face selective hydrogenation. Current efforts are now
directed towards the synthesis of polycavernoside A.
(E)-5-(benzyloxy)-4,4-dimethylpent-2-en-1-ol (4).
To a
stirred solution of 3 (1.60 g, 3.20 mmol) in anhydrous
ether (10 mL) DIBAL-H (5.68 mL, 8.00 mmol, 20%
solution in toluene) was added dropwise at 0°C and
stirred at rt for 6 h and diluted with methanol, sodium
potassium tartrate, and extracted with EtOAc. The
combined organic layers were washed with brine, dried
(Na2SO4), and concentrated, and the residue was purified
over silica gel (EtOAc: n-Hexane, 1:3) to afford 4
(1.42 g, 92%) as thick syrup. 1H NMR (300 MHz,
CDCl3) δ 7.22–7.40 (m, 5H); 6.08 (d, J = 12.5, 1H);
5.72–5.92 (m, 1H); 5.53–5.58 (m, 1H); 5.18–5.39 (m,
2H); 4.51 (s, 1H); 4.46 (d, J = 3.2, 1H); 4.01 (s, 1H);
3.14–3.32 (m, 2H); 1.92–1.82 (m, 3H); 0.94–1.18 (m,
6H). 13C NMR (75 MHz, CDCl3): 166.2; 138.5; 136.6;
133.2; 128.2; 127.3; 125.0; 118.1; 78.5; 76.3; 73.2; 38.5;
21.1; 20.7; 18.2. IR (neat) νmax: 3420, 2906, 2860, 1650,
1456, 1254, 1181, 1080, 720 cmÀ1. Mass (ESI-MS) m/z:
243 (M + Na)+.
EXPERIMENTAL PART
Silica gel (60–120 mesh) for column chromatography
was purchased from M/s Acme Synthetic Chemicals
(Mumbai, India), and pre-coated TLC plates (Silica gel
60F254) were purchased from Merck (Darmstadt,
Germany). All the chemicals, reagents, and solvents
were purchased from M/s SD Fine Chemicals (Mumbai,
1
India) and were of the highest grade of purity. The H-
NMR and 13C-NMR spectra were recorded on Bruker
400 and 100 MHz, respectively, and TMS was used as
an internal standard. Chemical shifts relative to TMS
as internal standards were given as δ values in ppm.
Mass spectra were recorded using electron spray ioniza-
tion on Waters e2695 Seperators module (Waters, Mil-
ford, MA, USA) mass spectrometer. IR spectra were
recorded on a Fourier transform infrared, USA (Perkin-
Elmer model 337) instrument. The melting points were
(2S, 3S)-3-(1-(benzyloxy)-2-methylpropan-2-yl) oxiran-2-yl)
methanol (5). To a stirred solution of D-(+)-DET (2.78 g,
determined on
Instrument.
a Barnstead Electro Thermal 9200
13.50 mmol) and molecular sieves (4 A°, 5 g) in CH2Cl2 at
À23°C was added Ti(OiPr)4 (3.08 g, 10.80 mmol). The
resulting mixture was stirred at À23°C for 10 min, and a
solution of compound 9 (12.0 g, 54.0 mmol) in CH2Cl2
was added. After 30 min, TBHP (27.0 mL, 4 M solution in
toluene, 108.1 mmol) was added and stirred for 4 h at the
same temperature. The reaction mixture was quenched by
the addition of brine solution (68 mL, containing 10%
NaOH), saturated Na2SO4 and Et2O. The resulting mixture
was stirred at room temperature for 2 h. Filtration through
a pad of celite followed by concentration and purification
by column chromatography using silica gel (100–200
mesh, 30% EtOAc/hexane) afforded the pure compound 5
(E)-Ethyl 5-(benzyloxy)-4,4-dimethylpent-2-enoate (3).
To a stirred solution of oxalyl chloride (0.37 mL,
3.36 mmol) in CH2Cl2 (5 mL) at À78°C, DMSO
(0.32 mL, 4.93 mmol) was added followed by compound
2 (1.0 g, 2.24 mmol) in CH2Cl2 (5 mL), and the contents
were stirred for 1 h at À78°C. Later, the reaction mixture
was quenched with Et3N (0.94 mL, 6.72 mmol) and
diluted with CH2Cl2 (25 mL). The combined organic
layers were washed with brine (1 × 15 mL), dried
(Na2SO4), and concentrated, and the residue was passed
through a pad of silica gel to give the corresponding
aldehyde (0.89 g, 90%), which was used as such for
further reaction. To a stirred solution of (Ph)3PCH2CO2Et
(0.56 mL, 2.66 mmol), 18-crown-6 (2.47 g, 9.36 mmol)
in anhydrous THF (10 mL) at À0°C followed by
KHMDS (0.67 g, 2.93 mmol) was added and stirred the
reaction mixture for 30 min. To the reaction mixture, the
aldehyde (0.80 g, 1.80 mmol) dissolved in THF (5 mL)
was added. The reaction mixture was stirred for 4 h and
quenching with aq NH4Cl and extracted into EtOAc. The
(10.2 g, 80%) as colorless oil. Rf
= 0.3 (30%
1
EtOAc/hexane). H NMR (300 MHz, CDCl3): 7.19–7.42
(m, 6H); 4.48 (s, 2H); 3.83 (d, J = 12.3, 1H); 3.55 (m,
1H); 3.21 (dd, J = 8.9, 15.9, 2H); 3.03 (m, 1H); 2.87 (d,
J = 2.3, 1H); 0.91 (d, J = 3.0, 6H). 13C NMR (75 MHz,
CDCl3): 138.4; 128.2; 127.4; 127.4; 127.3; 73.2; 62.1;
60.9; 55.3; 35.0; 20.9; 20.6. IR (neat) νmax: 3437, 2965,
2865, 1739, 1456, 1096, 740 cmÀ1. Mass (ESI-MS) m/z:
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DOI 10.1002/jhet

S. Kadari, H. Yerrabelly, T. Gogula, Y. G. Erukala, J. R. Yerrabelly, and P. K. Begari
Vol 000
1
259 (M + Na)+. Optical rotation: [α]2D5 À 14 [c 0.5,
thick yellow syrup. H NMR (300 MHz, CDCl3): 7.23–
7.44 (m, 6H); 6.08 (d, J = 12.0, 1H); 5.83 (m, 1H); 5.53–
5.58 (m, 2H); 4.51 (s, 2H); 3.96 (d, J = 6.4); 3.41 (d,
J = 8.7, 1H); 3.29 (d, J = 8.7, 1H); 0.96 (s, 3H); 0.92 (s,
3H). 13C NMR (75 MHz, CDCl3): 166.8; 138.3; 137.8;
133.4; 128.9; 128.6; 124.9; 118.2; 79.9; 78.4; 73.4; 38.2;
21.3; 21.6; 18.4. IR (neat) νmax: 3511, 2968, 2932, 2859,
1720, 1453, 1292, 1165, 1102, 933, 741 cmÀ1. Mass
(CHCl3)].
(2S,
3R)-2-(1-(benzyloxy)-2-methylpropan-2-yl)-3-
(iodomethyl) oxirane (6).
To a solution of alcohol 5
(210 mg, 0.96 mmol) in THF (6 mL) were added
imidazole (138 mg, 2.0 mmol), triphenylphosphine
(530 mg, 2.0 mmol), and iodine (490 mg, 1.92 mmol) at
0°C, allowed the reaction mixture to warm to room
temperature, and stirred for 2 h. The reaction was
quenched with saturated aqueous Na2SO3. The resultant
mixture was diluted with EtOAc, washed with water and
brine, dried (Na2SO4), and concentrated under reduced
pressure. Purification of the residue by column
chromatography gave iodide 6 (290 mg, 95%) as a
colorless clear oil. Rf = 0.4 (SiO2, 10% EtOAc in
hexane). 1H NMR (300 MHz, CDCl3): 7.16–7.40 (m,
6H); 4.48 (s, 2H); 3.09–3.29 (m, 4H); 2.93 (dd, J = 2.3,
9.4, 2H); 2.74 (d, J = 2.1, 1H); 0.94 (s, 3H); 0.90 (s, 3H).
13C NMR (75 MHz, CDCl3): 138.6; 128.4; 127.6; 127.5;
76.9; 73.4; 67.7; 55.5; 35.6; 21.1; 20.8; 5.6. IR (neat)
(ESI-MS) m/z: 311 (M
+
Na)+. Optical rotation:
[α]2D5 + 35 [c 0.02, (CHCl3)].
(S)-5-(1-(benzyloxy)-2-methylpropan-2-yl)-3-methylfuran-
2(5H)-one (9). A solution of 8 (0.41 g, 0.72 mmol) and
Grubbs’ 2nd generation catalyst (0.06 g, 0.07 mmol) in
anhydrous CH2Cl2 (15 mL) was stirred at reflux for 8 h.
After completion of the reaction, solvent was removed
under reduced pressure, and the residue purified by
column chromatography (silica gel, EtOAc: n-Hexane,
1
1:4) to afford 9 (0.33 g, 50%) as thick syrup. H NMR
(300 MHz, CDCl3): 7.2–7.44 (m, 6H); 7.06 (t, J = 1.5,
1H); 4.92 (t, J = 1.9, 1H); 4.51 (dd, J = 12.3, 25.9, 2H);
3.41 (d, J = 9.1, 1H); 3.23 (d, J = 9.1, 1H); 1.91 (t,
J = 1.9, 3H); 0.95 (s, 3H); 0.92 (s, 3H). 13C NMR
(75 MHz, CDCl3): 179.3; 137; 130.8; 128.7; 128.2;
127.3; 82.1; 76.3; 73.3; 38.6; 20; 19.6; 18.4. IR (neat)
ν
max: 3447, 2963, 2861, 1599, 1454, 1102, 739 cmÀ1
.
Mass (ESI-MS) m/z: 369 (M + Na)+. Optical rotation:
[α]2D5 + 44 [c 1, (CHCl3)].
(R)-5-(benzyloxy)-4,4-dimethylpent-1-en-3-ol
(7).
ν
max: 3427, 2933, 2880, 1720.9, 1454, 1452, 1383, 1247,
Compound 6 was dissolved in methanol (40 mL) and
stirred with zinc dust (6.77 g) for 4 h. The methanol was
then removed under reduced pressure, and water (30 mL)
was added. The mixture was extracted with
dichloromethane (3 × 30 mL), and the combined organic
layers were dried (Na2SO4), and the solvent was removed
under reduced pressure. Purification of the crude by
column chromatography on silica yielded 7 (2.65 g, 80%)
as a white solid. Rf = 0.2 (SiO2, 80% EtOAc in hexane).
1H NMR (300 MHz, CDCl3): 7.23–7.44 (m, 6H); 5.79–
6.02 (m, 1H); 5.08–5.37 (m, 2H); 4.51 (s, 2 H); 3.96 (d,
J = 6.4, 1H); 3.41 (d, J = 8.7, 1H); 3.29 (d, J = 8.7, 1H);
0.96 (s, 3H); 0.9 (s, 3H). 13C NMR (75 MHz, CDCl3):
137.8; 137.4; 128.4; 127.7; 127.5; 116.3; 79.9; 79.2;
73.5; 38.2; 22.6; 19.9. IR (neat) νmax: 3292, 2862, 2361,
1612, 1513, 1461, 1361, 1247, 1176, 1097, 1034, 822,
642 cmÀ1. Mass (ESI-MS) m/z: 243 (M + Na)+. Optical
rotation: [α]2D5 + 14 [c 0.5, (CHCl3)].
1220, 1097, 1032, 991, 738, 698 cmÀ1. Mass (ESI-MS)
m/z: 283 (M + Na)+. Optical rotation: [α]2D5 + 11.6 [c 0.5,
(CHCl3)].
(3R,
5S)-5-(1-hydroxy-2-methylpropan-2-yl)-3-methyl-
dihydrofuran-2(3H)-one (10). To a solution of 9 (0.1 g,
0.28 mmol) in methanol (1 mL) was added 10% Pd–C
under the hydrogen atmosphere, and the mixture was
stirred for 6 h. Then the reaction mixture was filtered
through a celite pad, and filtrate was evaporated under
reduced pressure to obtain a residue, which was subjected
to chromatography (silica gel 60–120 mesh, EtOAc: n-
1
Hexane, 3.6:6.4) to afford 10 (0.06 g) yield of 90%. H
NMR (300 MHz, CDCl3): 4.38 (dd, J = 5.5, 11.1, 1H);
3.55 (d, J = 10.9, 1H); 3.43 (d, J = 10.9, 1H); 2.50–2.88
(m, 2H); 2.32–2.36 (m, 1H); 1.27 (d, J = 7.2, 3H); 0.92
(d, J = 4.9, 6H). 13C NMR (75 MHz, CDCl3): 177.2;
84.4; 73.2; 38.3; 34.7; 32.2; 21.6; 16.6. IR (neat) νmax
:
3501, 2960, 2942, 2850, 1725, 1353, 1292, 1165, 1102,
933, 741 cmÀ1. Mass (ESI-MS) m/z: 433 (M + Na)+.
Optical rotation: [α]2D5 + 30 [(c 0.8, CHCl3)].
(3R, 5S)-5-(1-(tert-butyldiphenylsilyloxy)-2-methylpropan-
2-yl)-3-methyl-dihydrofuran-2(3H)-one (11). To a stirred
solution of 10 (3.5 g, 22.15 mmol) in anhydrous CH2Cl2
(30 mL) was added imidazole (4.19 g, 66.45 mmol)
followed by tert-butyldiphenylsilyl chloride (24.36 mL,
13.6 mmol) at 0°C, and the reaction mixture allowed to
stir at room temperature for 5 h. Then, reaction mixture
was quenched with saturated aq. NH4Cl solution (25 mL)
and extracted with CH2Cl2 washed with water (40 mL),
(R)-5-(benzyloxy)-4,4-dimethylpent-1-en-3-yl methacrylate
(8).
To a stirred solution of compound 7 (0.40 g,
0.78 mmol), triethyl amine (0.14 mL, 1.56 mmol) in
CH2Cl2 (5 mL) was added catalytic amount of sssDMAP
and methacryloyl chloride (0.07 mL, 1.09 mmol)
dropwise at 0°C and stirred at room temperature for 10 h.
The reaction mixture was treated with water (1 × 15 mL)
and extracted into CH2Cl2 (2 × 15 mL). The combined
organic layers were washed with brine, dried (Na2SO4),
and evaporated under reduced pressure. The residue was
purified by column chromatography (silica gel, EtOAc: n-
Hexane, 1:9) to afford the acrylate 8 (0.39 g, 90%) as
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

Month 2018
Sharpless Epoxidation, Reductive Elimination, Ring-Closing Metathesis (RCM),
Steroselective Asymmetric Hydrogenation, γ-Butyrolactones
[5] Zareai, Z.; Khoob, M.; Ramazani, A.; Foroumadi, A.;
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brine (40 mL), the combined organic layers were dried
over (Na2SO4), concentrated, and the crude residue was
purified by column chromatography (60–120 silica gel
1:20 (EtOAc: n-Hexane, 1:9) to afford product 11
1
(7.54 g) in 86% yield as a pale-yellow liquid. H NMR
(300 MHz, CDCl3): δ 7.66–7.60 (m, 4H), 7.46–7.35 (m,
6H), 4.45 (dd, J = 5.6, 11.1 Hz, 1H), 3.55 (d, J = 9.9 Hz,
1H), 3.40 (d, J = 9.9 Hz, 1H), 2.67 (ddq, J = 7.0, 8.6,
12.5 Hz, 1H), 2.23 (ddd, J = 5.6, 8.6, 12.5 Hz, 1H), 1.68
(q, J = 12.2 Hz, 1H), 1.26 (d, J = 7.0 Hz, 3H), 1.07 (s,
9H), 0.93 (s, 3H), 0.89 (s, 3H). 13C NMR (75 MHz,
CDCl3): δ 179.5, 135.6 (4C), 133.4, 133.3, 129.7 (2C),
127.7 (4C), 81.6, 69.7, 38.5, 36.0, 31.9, 26.9 (3C), 19.6,
19.4, 19.2, 14.9; IR (neat) νmax: 3395, 3029, 2960, 2911,
2859, 1725, 1453, 1396, 1259, 1086, 1028, 761,
705 cmÀ1. Mass (ESI-MS) m/z: 433 (M + Na)+. Optical
rotation: [α]2D5 + 42.11 [c 0.5, (CHCl3)].
[9] Dumeunier, R.; Marko, I. E. Tetrahedron Lett 2000, 41,
10219.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of the
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Journal of Heterocyclic Chemistry
DOI 10.1002/jhet