Total synthesis of a novel oxa-bowl natural product paracaseolide A via a 'putative' biomimetic pathway

Article abstract of DOI:10.1016/j.tetlet.2013.04.097

A total synthesis of bioactive tetracyclic natural product paracaseolide A, embodying an architecturally unusual oxa-bowl framework, has been accomplished from commercially available 5-methyl-2-furfural. The key step involving a thermal [4+2]-dimerization of an appropriately crafted 5-methyl-3- alkenylbutenolide is shown to proceed in a stepwise manner.

Full text of DOI:10.1016/j.tetlet.2013.04.097

Accepted Manuscript
Total synthesis of a novel oxa-bowl natural product paracaseolide A via a
‘putative’ biomimetic pathway
Laxmaiah Vasamsetty, Faiz Ahmed Khan, Goverdhan Mehta
PII:
S0040-4039(13)00703-X
DOI:
http://dx.doi.org/10.1016/j.tetlet.2013.04.097
TETL 42865
Reference:
To appear in:
Tetrahedron Letters
Received Date:
Revised Date:
Accepted Date:
31 March 2013
20 April 2013
23 April 2013
Please cite this article as: Vasamsetty, L., Khan, F.A., Mehta, G., Total synthesis of a novel oxa-bowl natural product
paracaseolide A via a ‘putative’ biomimetic pathway, Tetrahedron Letters (2013), doi: http://dx.doi.org/10.1016/
j.tetlet.2013.04.097
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Total synthesis of a novel oxa-bowl natural
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biomimetic pathway
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Laxmaiah Vasamsettya, Faiz Ahmed Khana and
Goverdhan Mehtab

1
Tetrahedron Letters
journal homepage: www.elsevier.com
Total synthesis of a novel oxa-bowl natural product paracaseolide A via a ‘putative’
biomimetic pathway
Laxmaiah Vasamsettya, Faiz Ahmed Khana and Goverdhan Mehtab
aDepartment of chemistry, Indian Institute of Technology, Hyderabad – 502 205, India
bSchool of Chemistry, University of Hyderabad, Hyderabad - 500 046, India
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A total synthesis of bioactive tetracyclic natural product paracaseolide A, embodying an
architecturally unusual oxa-bowl framework, has been accomplished from commercially
available 5-methyl-2-furfural. The key step involving a thermal [4+2]-dimerization of an
appropriately crafted 5-methyl-3-alkenylbutenolide is shown to proceed in a stepwise manner.
Available online
2009 Elsevier Ltd. All rights reserved.
Keywords:
Singlet oxygen
Suzuki coupling
Biomimetic
3-alkenylbutenolide
[4+2]-type dimerization
In 2011, Guo and co-workers reported1 the isolation and
structure determination of a novel natural product, paracaseolide
A 1 from the stem bark of Sonneratia paracaseolaris, an
endemic mangrove species found in China. In structural terms,
tetracyclic 1 is an unusual construct that features oxa-bowl
architecture and embodies a 3-alkenylbutenolide substructure
reminiscent of many bioactive natural products.2 The five oxygen
atoms present in 1 essentially dot the rim of its bowl-like
framework and the additional presence of two long linear
hydrophobic chains on its convex surface impart the natural
product a dipolarofacial character. Indeed, 1 can be formally
groups of Vassilikogiannakis5 and Kraus6 have appeared in the
very recent past. Herein we report a total synthesis of 1,
essentially along the proposed biosynthetic route1,5 involving the
[4+2]-dimerization of
a butenolide precursor 2. Such a
biomimetically patterned approach to 1 involving the [4+2]-
dimerization was successfully implemented in the first synthesis
of the natural product by Vassilikogiannakis et al.5 However, our
access to the key precursor 2 is shorter, well differentiated and
the outcome of the dimerization protocol is somewhat different,
which in turn sheds some light on the nature of the [4+2]-type
dimerization leading to the natural product.
derived through
a
[4+2]-type dimerization of
a
3-
alkenylbutenolide precursor 2.
In our quest for 1, we initially carried out a model study in
which a sibling 3-alkenylbutenolide 3 was deployed to probe the
Paracaseolide 1 has been shown to exhibit impressive
inhibitory activity against dual-specificity phosphatase CDC25B
with an IC50 value of 6.44 µM. This is a potentially significant
bioactivity attribute as CDC25B is a proto-oncogene in humans
and shown to be over expressed in a number of cancers and is
implicated in cell cycle progression in tumors.3
Both, on account of its complex and unusual molecular
structure and its bioactivity profile, natural product 1 presents a
challenging and interesting target for total synthesis. We were
instantly drawn to a synthesis of 1 in view of our long standing
interest in the oxa-bowl like constructs.4 While our own efforts
towards 1 were underway, two total syntheses of 1 from the
———
 Corresponding author. Tel.: +91-40-23016084; E-mail: faiz@iith.ac.in
 Corresponding author. Tel.: +91-40-23134848; fax: +91-40-23010785; e-mail: gmehta43@gmail.com

2
Tetrahedron Letters
key dimerization process. Commercially available furfural 4
was elaborated to butenolide 5 via a known7 protocol involving
dye sensitized photo-oxygenation as the key step, Scheme 1.
Bromine addition8 to 5 and in situ dehydrobromination led to 3-
bromo-butenolide 6. Pd-mediated Suzuki cross-coupling9
between 6 and vinylicboronate 7 delivered 3-alkenylbutenolide 8
and further 5-methoxy deprotection10 led to the requisite 3-
alkenylbutenolide 3.11 Heating (neat, sealed tube, 100 oC) 3
triggered the tandem [4+2]-type dimerization and concomitant
dehydration to furnish a mixture (40:60) of two diastereomeric
dimers 9 and 10 in 52% isolated yield. Stereo-structures of 9 and
10 were derived through incisive analyses of their 2D NMR
(COSY, HMBC and ROESY) studies and the key connectivities
are displayed in Figure 1. In addition, structure of one of the
diastereomers 10 was further verified by a single crystal X-ray
structure determination and an ORTEP12 is shown in Scheme 1.
Interestingly, in 9 both the alkenyl and the alkyl hydrophobic
Figure 1. Key HMBC, COSY, ROESY connectivities of
compound 9, 10.
chains are cis disposed and located on convex surface as present
in the natural product 1, but in the diastereomer 10 the two chains
are trans with the alkyl chain protruding towards the cavity of the
oxa-bowl. In separate experiments, it was shown that under the
conditions of thermal activation employed to effect dimerization,
both 9 and 10 were stable and did not interconvert. This clearly
established that 9 and 10 were independently produced during the
[4+2]-dimerization protocol.
Having demonstrated the viability of the 3-
alkenylbutenolide dimerization in 3, it was decided to extend
the protocol to 5-methyl-3-alkenylbutenolide 2 to target the
natural product 1. Towards this end, 5-methyl-2-furfural 11
was photooxygenated13 to 5-methylbutenolide 12 (Scheme 2).
Single pot iodination-dehydroiodination in 12 led to 5-methyl-
3-iodobutenolide 13. Pd-mediated Suzuki cross-coupling
between 13 and vinylic boronate 7 delivered 14 in which
methoxy deprotection10 was smoothly implemented to furnish
the desired 5-methyl-3-alkenylbutenolide 2. Thermal
activation (neat, sealed tube, 110 oC) of 2 led to the formation
of two diastereomeric dimers 1, 15 and a ring opened
compound 165 in 66% yield and in a ratio of 4.9:1:2.4. While
it was pleasing to establish the identity of the major dimeric
product with the natural product paracaseolide A 1 through
appropriate spectral comparison, the structure of the minor
product 15 had to be deduced from 2D NMR studies (COSY,
HMBC, ROESY) and the connectivities are displayed in
Figure 2. Once again, we observed that subjecting either 1 or
15 to prolonged thermal activation did not provide any
evidence of interconversion among them. Thus, indicating an
independent origin of both the natural product 1 and its
epimer 15 during the dimerization process.
Scheme 1. Reagents and conditions. (a) (i) 1O2, MeOH; (ii)
MeOH, reflux, 3 d; (b) 1.2 equiv Br2, 0.08 equiv PBr3, CCl4, 0
oC-rt, 12 h; py, 0 oC-rt, 4 h 65%; (c) 1.5 equiv 7, 3 mol%
PdCl2dppf, 4 equiv CsF, 1 equiv TBAB, THF/H2O (4:1), µW,
o
o
100 C, 4 min, 41%; (d) TFA/acetone/H2O (1:1:1), 0 C-rt, 2 h,
59%; (e) neat, sealed tube, 100 oC, 14.5 h 52% (9:10 = 2:3).

3
During their synthesis of paracaseolide A 1, involving the
dimerization of 2, Vassilikogiannakis et al.5 hypothesized initial
formation of a Diels-Alder dimer 17 in a stereoselective manner
through
a concerted endo-transition state. Intramolecular
dehydration in 17 could led to a tetracycle 15 (unknown prior to
the present work). An epimerization of the sp3 anchored alkyl
side arm in 15 was proposed to account for the generation of the
requisite cis stereochemistry of the alkyl and alkenyl arms to
eventuate in the natural product (Scheme 3). As it turned out, the
earlier authors5 observed the formation of only one diastereomer,
namely natural product 1, during the thermal activation of 2,
contrary to our observation of formation of 1 and 15 under
essentially the same conditions. While the reasons for this
discrepancy remain unclear at the moment, formation of two non-
interconvertible diastereomers during the dimerization of 3-
alkenylbutenolides 2 and 3 in the present study is indicative of
the involvement of a step-wise process involving [4+2]-
dimerization. We intend to probe this issue further employing
experimental and quantum mechanical approaches to delineate
the mechanistic contours of the dimerization process.
Scheme 2. Synthesis of diastereomeric dimers paracaseolide A
(1), 15, and 16; Reagents and conditions. (a) (i) 1O2, MeOH; (b) 4
o
equiv I2, Py/CCl4(1:1), 0 C-rt, 24 h, 50%; (c) 1.5 equiv 7, 3
mol% PdCl2dppf, 4 equiv CsF, 1 equiv TBAB, THF/H2O (4:1),
µW, 120 oC, 40 min, 60%; (d) TFA/acetone/H2O (1:1:1), 0 oC-rt,
o
13 h, 82%; (e) neat, sealed tube, 110 C, 12 h, 66% (1:15:16 =
4.9:1:2.4).
Scheme 3. Proposed route5 for the dimerization of 2 to deliver
the natural product 1
In short, we have outlined a concise approach to the bioactive
natural product paracaseolide A from commercially available 5-
methyl-2-furfural through a ‘putative’ biomimetic approach that
involves [4+2]-dimerization of a suitably crafted 5-methyl-3-
alkenylbutenolide 2.
Acknowledgments
One of us (LV) thanks CSIR, India for the award of a research
fellowship. F.A.K. acknowledges CSIR for financial support.
G.M. acknowledges the research support from Eli Lilly and
Jubilant-Bhartia Foundations and the facilities extended by the
University of Hyderabad. We thank Dr. Saikat Sen for
determining the X-ray crystal structure.
Figure 2. Key HMBC, COSY, ROESY connectivities of
compound 15.
Supplementary Material

4
Tetrahedron Letters
1
m/z calcd for C36H59O5 (M + H)+ 571.4363, found 571.4333; 14 IR
(Neat) 2923, 2853, 1774, 1720, 1463, 1376, 1362, 1281, 1178,
1125, 1070, 1032, 1006, 947 cm-1; 1H NMR (400 MHz, CDCl3) δ:
6.86 (dt, J = 15.8, 6.9 Hz, 1H), 6.71 (s,1H), 6.09 (d, J = 15.8 Hz,
1H), 3.21 (s, 3H), 2.17 (q, J = 6.9 Hz, 2H), 1.63 (s, 3H), 1.45-1.40
(m, 2H), 1.25 (br s, 18H), 0.87 (t, J = 6.6 Hz, 3H); 13C NMR (100
MHz, CDCl3) δ: 169.4, 142.9, 141.0, 132.4, 117.8, 106.8, 51.2,
33.5, 31.9, 29.67, 29.65, 29.64, 29.56, 29.5, 29.4, 29.3, 28.6, 24.0,
22.7, 14.1; HRMS (+APCI) m/z calcd for C20H35O3 (M + H)+
323.2581, found 323.2586; 2 mp. 40-42 oC; IR (Neat) 3371, 2922,
2853, 1745, 1659, 1460, 1372, 1260, 1171, 1106, 1055, 971, 930
Supplementary data (experimental procedures and copies of H
and 13C NMR, HMBC, HSQC, COSY, ROESY spectra),
associated with this article can be found in the online version, at
References and notes
1. Chen, X.-L.; Liu, H.-L.; Li, J.; Xin, G.-R.; Guo, Y.-W. Org. Lett.
2011, 13, 5032.
2. Reviews on butenolides: (a) Rao, Y. S. Chem. Rev. 1976, 76, 625;
(b) Ugurchieva, T. M.; Veselovsky, V. V. Russ. Chem. Rev. 2009,
78, 337.
cm-1; H NMR (400 MHz, CDCl3) δ: 6.83 (s, 1H), 6.76 (dt, J =
1
15.9, 7.2 Hz, 1H), 6.02 (d, J = 15.9 Hz, 1H), 4.12 (br s, 1-OH),
2.13 (q, J = 7.2 Hz, 2H), 1.68 (s, 3H), 1.43-1.39 (m, 2H), 1.25 (br
s, 18H), 0.87 (t, J = 6.8 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ:
170.2, 144.1, 140.9, 130.7, 117.8, 104.1, 33.5, 31.9, 29.68, 29.66,
29.64, 29.58, 29.5, 29.4, 29.3, 28.7, 24.8, 22.7, 14.1; HRMS
(+APCI) calcd for C19H33O3 (M + H)+ 309.2424, found 309.2414;
1 IR (Neat) 2922, 2853, 1772, 1671, 1460, 1388, 1351, 1310,
1276, 1197, 1155, 1124, 1055, 983, 910 cm-1; 1H NMR (400 MHz,
CDCl3) δ: 7.25 (dd, J = 7.7, 3.2 Hz, 1H), 5.83 (dt, J = 15.8, 6.9
Hz, 1H), 5.48 (d, J = 15.8 Hz, 1H), 3.37 (dd, J = 9.5, 3.2 Hz, 1H),
3.30 (d, J = 9.5 Hz, 1H), 3.07-3.01 (m, 1H), 2.11 (q, J = 6.9 Hz,
2H), 1.75 (s, 3H), 1.69-1.65 (m, 1H), 1.62 (s, 3H), 1.39-1.15 (m,
41H), 0.88 (t, J = 6.8 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ:
175.1, 166.2, 144.7, 135.1, 129.3, 126.3, 115.4, 113.8, 58.3, 50.3,
46.7, 45.1, 32.7, 31.9, 29.68, 29.66, 29.64, 29.55, 29.42, 29.36,
29.1, 29.0, 28.2, 28.0, 26.6, 25.7, 22.7, 14.1; HRMS (+APCI)
calcd for C38H63O5 (M + H)+ 599.4670, found 599.4646; 15 IR
(Neat) 2922, 2853, 1771, 1673, 1460, 1387, 1311, 1273, 1237,
1121, 1057, 973, 911 cm-1; 1H NMR (400 MHz, CDCl3) δ: 6.98 (t,
J= 3.7 Hz, 1H), 5.69 (dt, J = 15.7, 6.9 Hz, 1H), 5.43 (d, J = 15.7
Hz, 1H), 3.27 (d, J = 9.6 Hz, 1H), 3.19 (d, J = 9.6 Hz, 1H), 2.14-
1.96 (m, 3H), 1.74 (s, 3H), 1.59 (s, 3H), 1.38-1.15 (m, 42H), 0.88
(t, J = 6.8 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ:173.2, 165.8,
144.3, 135.3, 130.3, 127.3, 114.7, 113.7, 59.9, 55.3, 47.4, 45.9,
32.6, 31.9, 29.68, 29.66, 29.6, 29.44, 29.40, 29.37, 29.2, 29.0,
28.6, 27.5, 26.7, 25.6, 22.7, 14.1; HRMS (+APCI) calcd for
C38H66NO5 (M + NH4)+ 616.4936, found 616.4917; 16 IR (Neat)
3. Kristjánsdóttir K.; Rudolph, J. Chem. Biol. 2004, 11, 1043.
4. (a) Mehta, G.; Vidya, R. Tetrahedron Lett. 1997, 38, 4173; (b)
Mehta, G.; Vidya, R. Tetrahedron Lett. 1997, 38, 4177; (c) Mehta,
G; Vidya, R. Tetrahedron Lett. 1998, 39, 6403; (d) Mehta, G.;
Vidya, R.; Venkatesan, K. Tetrahedron Lett. 1999, 40, 2417; (e)
Mehta, G.; Vidya, R. J. Org. Chem. 2000, 65, 3497; (f) Mehta, G.;
Vidya, R. J. Org. Chem. 2001, 66, 6905; (g) Mehta, G.; Vidya, R.
J. Org. Chem. 2001, 66, 6913; (h) Khan, F. A.; Dwivedi, V.; Rout,
B. Tetrahedron Lett. 2007, 48, 207; (i) Khan, F. A.; Rao, C. N.
Tetrahedron Lett. 2009, 50, 5751.
5. Noutsias, D.; Vassilikogiannakis, G. Org. Lett. 2012, 14, 3565.
6. Guney, T.; Kraus, G. A. Org. Lett. 2013, 15, 613.
7. (a) Feringa, B. L.; Lange, B. D. Tetrahedron 1988, 44, 7213; (b)
Moradei O. M.; Paquette. L. A. Org. Synth., Coll. Vol. 2009, 11,
128. (c) Review: Gollnick, K.; Griesback, A. Tetrahedron 1985,
41, 2057.
8. Qiang, P.; Junyong, S.; Chunxue, J.; Manli, N.; Kejing, H.
Chinese J. Org. Chem. 2010, 30, 698.
9. Aquino, M.; Bruno, I.; Riccio, R.; Gomez-Paloma, L. Org. Lett.
2006, 8, 4831.
10. Jauch, J. Eur. J. Org. Chem. 2001, 473.
11. All new compounds reported here were racemic and characterized
on the basis of spectroscopic data (IR, 1H and 13C NMR and
mass). Spectral data for some of the key compounds follows: 8 IR
(Neat) 2923, 2853, 1772, 1662, 1465, 1366, 1341, 1210, 1124,
1
1089, 1028, 981, 939 cm-1; H NMR (400 MHz, CDCl3) δ: 6.86
(dt, J = 15.9, 7.5 Hz, 1H), 6.76 (s, 1H), 6.09 (d, J = 15.9 Hz, 1H),
5.75 (s, 1H), 3.55 (s, 3H), 2.16 (q, J = 7.5 Hz, 2H), 1.45-1.39 (m,
2H), 1.25 (br s, 18H), 0.87 (t, J = 6.8 Hz, 3H); 13C NMR (100
MHz, CDCl3) δ: 169.8, 141.3, 138.8, 133.2, 117.9, 102.0, 56.7,
33.5, 31.9, 29.7, 29.64, 29.56, 29.45, 29.36, 29.2, 28.6, 22.7, 14.1;
HRMS (+APCI) m/z calcd for C19H33O3 (M + H)+ 309.2430,
found 309.2417; 3 mp. 40-42 oC; IR (Neat) 3369, 2956, 2922,
2852, 1767, 1662, 1466, 1282, 1206, 1090, 1012, 973, 925, 873
1
2923, 2853, 1757, 1725, 1465, 1357, 1163, 1092 cm-1; H NMR
(400 MHz, CDCl3) δ: 7.38 (d, J = 1.3 Hz, 1H), 5.00-4.97 (m, 1H),
3.45 (s, 2H), 2.25 (s, 3H), 1.74-1.67 (m, 2H), 1.46-1.24 (m, 20H),
0.88 (t, J = 6.8 Hz, 3H); 13CNMR (100 MHz, CDCl3) δ: 203.1,
173.5, 152.2, 126.6, 82.1, 38.9, 33.4, 31.9, 30.3, 29.7, 29.63,
29.59, 29.5, 29.4, 29.34, 29.31, 25.0, 22.7, 14.1; HRMS(+APCI)
m/z calcd for C19H33O3 (M + H)+ 309.2430, found 309.2422.
1
12. Single crystal X-ray diffraction data was collected on a Bruker
AXS SMART APEX CCD diffractometer. The X-ray generator
was operated at 45 KV and 30 mA using MoKα radiation. The data
was collected with a ω scan width of 0.3o. A total of 600 frames
per set were collected using SMART in three different settings of
 (0o, 90o and 180o), keeping the sample to detector distance of
6.023 cm and the 2θ value fixed at -25o. The data were reduced by
SAINTPLUS; an empirical absorption correction was applied
using the package SADABS and XPREP were used to determine
the space group. The crystal structure was solved by direct
methods using SIR92 and refined by full-matrix least-squares
method using SHELXL97. Crystal data for 10 C36H58O5, MW =
570.82, monoclinic, space group P21/n, cell parameters a =
6.0635(13), b = 71.972(15), c = 8.1928(17) Å, β = 99.310(4)°, V =
3528.3(13) Å3, Z = 4, ρcalcd = 1.075 g/cm3, 15564 reflections
measured, 5795 unique (Rint = 0.1131), R1 = 0.1187 and wR2 =
0.2768 for 1967 observed relections. Crystallographic data
(excluding structure factors) for the structure in this letter have
been deposited with the Cambridge Crystallographic Data Centre
as supplementary publication number CCDC 923267. Copies of
the data can be obtained, free of charge, on application to CCDC,
12 Union Road, Cambridge CB2 1EZ, UK (fax: +44 (0)1223
336033 or e-mail: deposit@ccdc.cam.ac.uk).
cm-1; H NMR (400 MHz, CDCl3) δ: 6.85 (s, 1H), 6.86-6.81 (m,
1H), 6.11 (s, 1H), 6.08 (d, J = 16.5 Hz, 1H), 4.10 (br s, 1-OH),
2.16 (q, J = 7.1 Hz, 2H), 1.44-1.40 (m, 2H), 1.25 (br s, 18H), 0.87
(t, J = 6.3 Hz, 3H); 13C NMR (100 MHz, CDCl3) δ: 170.6, 141.3,
140.5, 132.7, 117.7, 96.5, 33.5, 31.9, 29.68, 29.67, 29.66, 29.59,
29.5, 29.4, 29.3, 28.6, 22.7, 14.1; HRMS (+APCI) m/z calcd for
C18H31O3 (M + H)+ 295.2273, found 295.2257; 9 IR (Neat) 2923,
2853, 1779, 1671, 1461, 1352, 1302, 1207, 1167, 1118, 1010, 933,
894 cm-1; 1H NMR (400 MHz, CDCl3) δ: 7.31 (dd, J = 7.6, 3.3 Hz,
1H), 6.21 (d, J = 5.8 Hz, 1H), 6.00 (d, J = 5.5 Hz, 1H), 5.79 (dt, J
= 15.7, 6.8 Hz 1H), 5.47 (d, J = 15.7 Hz, 1H), 3.65-3.60 (m, 1H),
3.52 (dd, J = 9.9, 5.5 Hz, 1H), 3.14-3.10 (m, 1H), 2.10 (q, J = 6.8
Hz, 2H), 1.73-1.65 (m, 1H), 1.39-1.20 (m, 41H), 0.88 (t, J = 6.8
Hz, 6H); 13C NMR (100 MHz, CDCl3) δ: 175.3, 166.5, 146.0,
135.2, 127.2, 126.0, 106.6, 106.3, 57.5, 46.8, 44.6, 40.9, 32.7,
31.9, 29.69, 29.66, 29.63, 29.5, 29.41, 29.36, 29.1, 28.9, 28.2,
28.1, 22.7, 14.1; HRMS (+APCI) m/z calcd for C36H59O5 (M + H)+
571.4363, found 571.4351; 10 IR (Neat) 2921, 2852, 1775, 1670,
1466, 1349, 1208, 1004, 929, 890 cm-1; 1H NMR (400 MHz,
CDCl3) δ: 7.03 (t, J = 3.8, 1H), 6.21 (d, J = 5.7 Hz, 1H), 5.90 (d, J
= 5.5 Hz, 1H), 5.65 (dt, J = 15.7, 6.8 Hz 1H), 5.44 (d, J = 15.7
Hz, 1H), 3.54-3.49 (m, 1H), 3.43 (dd, J = 10.2, 5.5 Hz 1H), 2.26-
2.22 (m, 1H), 2.12-2.02 (m, 2H), 1.79-1.76 (m, 1H), 1.38-1.20 (m,
41H), 0.88 (t, J = 6.8 Hz, 6H); 13C NMR (100 MHz, CDCl3) δ:
173.4, 166.1, 145.6, 135.2, 128.2, 126.3, 106.5, 106.1, 59.1, 52.2,
45.8, 41.7, 32.6, 31.9, 29.69, 29.66, 29.64, 29.60, 29.59, 29.44,
29.40, 29.37, 29.1, 28.9, 28.6, 27.5, 22.7, 14.1; HRMS (+APCI)
13. Feringa, B. L.; Butselaar, R. J. Tetrahedron Lett. 1983, 24, 1193.