Synthesis of novel 2H,5H-dihydrofuran-3-yl ketones via ISNC reactions

Article abstract of DOI:10.1016/j.tetlet.2009.08.110

Unique 1-[2H,5H-dihydrofur-3-yl]ketones have been synthesized from propargylic nitroethers via intramolecular cycloadditions involving silyl nitronates. Various substituent groups were placed on the 2 and 5 positions of the dihydrofuran rings. We examined the scope of the long-range coupling in proton NMR of the oxo-dihydrofuran products. The identities of the diastereomers resulting from the Michael addition/cycloaddition reactions were tentatively assigned for the first time. CAChe MNDO PM5 and CONFLEX programs were engaged to assist with the identification of these stereoisomers. The reaction times and conditions for these oxo-dihydrofurans were found to be different than that of the published dihydrofuranals, which led us to propose a different mechanism.

Full text of DOI:10.1016/j.tetlet.2009.08.110

Tetrahedron Letters 50 (2009) 6446–6449
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of novel 2H,5H-dihydrofuran-3-yl ketones via ISNC reactions
Matthew L. Grandbois a, , Kelsie J. Betsch b, , William D. Buchanan c,à, Jetty L. Duffy-Matzner d,
*
a Chemistry Department, University of Minnesota, Minneapolis, MN 55455-0431, USA
b Department of Physics, University of Virginia, Charlottesville, VA 22904-4714, USA
c Department of Chemistry, Syracuse University, Syracuse, NY 13244-4100, USA
d Chemistry Department, Augustana College, 2001 S. Summit Ave., Sioux Falls, SD 57197, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 29 July 2009
Revised 26 August 2009
Accepted 27 August 2009
Available online 12 September 2009
Unique 1-[2H,5H-dihydrofur-3-yl]ketones have been synthesized from propargylic nitroethers via intra-
molecular cycloadditions involving silyl nitronates. Various substituent groups were placed on the 2 and
5 positions of the dihydrofuran rings. We examined the scope of the long-range coupling in proton NMR
of the oxo-dihydrofuran products. The identities of the diastereomers resulting from the Michael addi-
tion/cycloaddition reactions were tentatively assigned for the first time. CAChe MNDO PM5 and CONFLEX
programs were engaged to assist with the identification of these stereoisomers. The reaction times and
conditions for these oxo-dihydrofurans were found to be different than that of the published dihydrof-
uranals, which led us to propose a different mechanism.
Keywords:
CAChe
CONFLEX
ISNC
Ó 2009 Elsevier Ltd. All rights reserved.
Propargylic nitroethers
Oxo-dihydrofurans
Dihydrofuranyl ketones
Dihydrofuranones
1. Introduction
proposed polyester series would not rely upon the furanyl oxygen
for coordination to the metal and thus avoids these donor or steric
We are interested in the synthesis of a novel tetraester macro-
cycle (see 1 in Scheme 1) that may possibly exhibit antibiotic prop-
erties such as those of the established ionophoric antibiotic,
nonactin. To our knowledge these 3,4-furan macrocycles have
not been extensively investigated as compared to the 2,3-, 2,4-,
and 2,5-furan derivatives.1 Many of these macromolecules have
exhibited efficient cation transport as in the case of the noncyclic
2,5-dioxytetrahydrofuran ligands studied by Wierenga,2 the cyclic
2,5-derivatived furan polyether macrocycles reported by Brad-
shaw,3 and the macrocyclic furan ligands developed by Kobuke.4
Some investigators, however, found little to no effect when com-
pared to similarly sized macrocycles without furan-based compo-
nents as discovered by Tarroago5 and Jackels.6 The former paper
stated that the lack of extraction capability of the macromolecules
for an alkali atom is due to the lower donor character of the furan
oxygen atom compared to that of an alkyl ether oxygen atom. The
latter reports that the steric interactions due to the twisting of the
macrocycle to accommodate a divalent metal cation were in-
creased when the donor oxygen atom was part of a furan-deriva-
tized system and resulted in little evidence in binding. The
strain problems. Computational analysis has found that nonactin
and the proposed macrocycle have similar binding capabilities to-
ward cations with a lower energy of complexation requirement for
the novel macrolide.7
Scheme 1 shows a brief retrosynthetic analysis of the macrocy-
cle 1. The tetraester could be derived from a difunctional tetrahy-
drofuran monomer such as 2. This in turn could be prepared from
the dihydrofuran carbonyl compounds 3. The synthesis of the
related carbaldehydes has been briefly reported.8 The appropriate
nitroethers 4 can be prepared in good yields from nitroalkenes
and propargylic alcohols.9
2. Results
It was decided to explore the reactions to produce the oxo-
dihydrofurans, since these compounds have not been reported in
the literature. An example of the synthesis of these novel dihydrof-
uranyl ketones is provided in Scheme 2 (compounds 10–16).
2-Methylpropanal 5 was treated under Grignard conditions to pro-
vide 2-methyl-4-hexyn-3-ol 6. This propargylic alcohol was then
converted to the secondary alkoxide and nitrostyrene was slowly
added. Racemic 7 [( )( )-5-methyl-4-[1-nitro-2-phenyleth-2-ylox-
y]hex-2-yne] was isolated after work up and purification by
column chromatography. This nitroether was then treated under
silyl nitronate cycloaddition conditions,10 which we refer to as an
* Corresponding author. Tel.: +1 605 274 4822; fax: +1 605 274 4492.
E-mail address: jetty.duffy@augie.edu (J.L. Duffy-Matzner).
This work was supported by NSF-POWRE CHE-0074797.
à
This work was supported by NIH-2 P20 RR016479.
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.08.110

M. L. Grandbois et al. / Tetrahedron Letters 50 (2009) 6446–6449
6447
R1
R2
O
O
O
O
O
O
R1
R2
R2
R1
O
O
O
R2
O
R2
R3
O
R1
R1
NO2
R3
O
4
O
HO
R2
O
O
O
O
OH
R1
O
3
2
O
O
O
O
R2
R1
1
Scheme 1.
Ph
OH
NO2
1) NaH, THF, r.t.
2) -40ºC
1) BrMg
O
O
THF, 0ºC
H
Ph
NO2
78%
6
2) 1M HCl
5
7
3) 1M HCl
68%
Ph
O
SiMe3
Ph
SiMe3
O
O
N
1) TMSCl, TEA
PhH, r,.t.
O
N
Ph
O
O
O
2) 1M HCl
10
O
9
8
91%
Scheme 2.
ISNC (Intramolecular Silyl Nitronate Cycloaddition) so as not to
limit the unsaturated carbon system to alkenes (vs ISOC which re-
fers to olefins only). The anion of the N-trimethylsilyloxy nitronate
8 is prepared in situ and then undergoes a 1,3-dipolar cycloaddi-
tion to form intermediate 9, which upon quenching with acid
affords target compound 10 [( )( )-1-[2-[1-methylethyl]-5-phe-
nyl-2H,5H-dihydrofuran-3-yl]ethanone].
Table 1 displays the oxo-dihydrofurans that were completed.
Ratios of diastereomers were determined via GC analysis of the
crude products. We found that the time involved for the cycloaddi-
tion was dependent on the carbonyl substituent. The previously re-
ported dihydrofuranals require less than 24 h of reaction time,8
with very little time needed for the acidic workup. In contrast,
3. Discussion
The tentative mechanism put forth by Kurth and Duffy-Matz-
ner8 suggested a b-nitroso-carbaldehyde intermediate with subse-
quent lost of HNO to form the dihydrofuranals. We now suggest
the following mechanism, which entails a second intramolecular
reaction during acidic work up of the 1H-isoxazoline intermediate
17 as depicted in Scheme 3. This negates the need for a base under
acidic conditions and explains the reaction time differences due to
the reduced activity of the enol 18. This enol intermediate also
explains why the propargylic nitroethers give dihydrofuro-car-
bonyl products under ISNC, while the vinylic nitroethers give the
expected isoxazole derivatives under ISOC (Intramolecular Silyl
Nitronate Olefin Cycloaddition) conditions.11 There is currently
work in progress to address the regioselectivity of these two cyclo-
addition reactions.12
NMR analysis of the final oxo and carbaldehydes dihydrofurans
showed interesting coupling constants. We noticed that several of
the final ethanones displayed 4J and 5J coupling constants in the
proton spectra while several did not. It was found that phenyl
and t-butyl substituents would provide the necessary spatial inter-
actions to afford this longer distance coupling while methyl, ethyl
the reaction times for the
a,b-unsaturated ketones was greatly
influenced by the length of the carbonyl’s alkyl chain. The novel
oxo-dihydrofurans need a much longer reaction time (four to eight
days) in comparison to their carbaldehyde counter parts. They also
require an increase in the equivalents of both trimethylamine and
trimethylsilylchloride, with additions of these compounds during
the reaction. The acidic work up demanded a higher concentration
of acid for the ketones (6 M) than that of the published dihydrof-
uranals (1 M).
Table 1
#
R1
R2
R3
% Yield
Ratiosa
Rxn time (days)
10
11
12
13
14
15
16
Ph
t-Butyl
H
i-Propyl
Ph
H
i-Propyl
Me
Ph
H
H
Me
Me
Me
Me
Me
Me
Et
91
46
63
83
64
64
96
10:1
2:1
4.5
3.5
5
3
3.5
4
R1
O
R2
R3
Et
Me
Ph
3:1
7
O
a
Ratios determined by gas chromatography and confirmed by 1H NMR.

6448
M. L. Grandbois et al. / Tetrahedron Letters 50 (2009) 6446–6449
Cl
R1
R1
SiMe3
O
N
O
-HNO
HCl
R1
O
N
O
O
H+
O
R3
O
R2
R2
H
O
R3
R3
R2
17
3
18
Scheme 3.
and iso-propyl groups do not. The diastereomers of the cyclic prod-
ucts had different 5J coupling constants for the major (6 Hz) and
minor (5 Hz) isomers. We calculated the pseudo dihedral bond–
bond angles (apparent angle between H2 and H5) present in the
cis and trans isomers of 10. For the entirety of the calculations,
the CAChe 5.1 software system was used. All the initial structures
magnesium sulfate, and filtered. Removal of the solvent under re-
duced pressure gave dihydrofuranyl-ethanone17 as a clear colorless
oil that turns yellow over time.
Acknowledgements
were equilibrated by executing a conformational search (MM213
)
The authors would like to thank NSF-POWRE CHE-0074797 for
the purchase of the CAChe program and Augustana College for the
purchase of the CONFLEX program. We would like to thank Arlen
Viste for his assistance with the computational work and Sally
Kessler for her initial work on the synthetic methodology. The syn-
thetic work was funded through NSF-POWRE CHE-0074797 and
NIH Grant Number 2 P20 RR016479 from the INBRE Program of
the National Center for Research Resources. The authors would also
like to thank the University of South Dakota for the use of their
200 MHz NMR and North Dakota State University for the use of
their 400 MHz NMR. We especially would like to thank Professor
Mark J. Kurth for his support and guidance through the years.
using the CONFLEX method,14 where only the conformation lowest
in energy was saved. The thermodynamic calculations were carried
out at the MNDO15 PM516 level of theory using MOPAC. The trans
isomer resulted in a pseudo-bond angle of 160° and 5° for the cis
isomer. Applying Karplus logic to this data allows us to assign
the major diastereomer as the trans isomer.
We tried to exploit the carbonyl present in our products to
make solid derivatives that could be analyzed via X-ray diffraction
to provide an exact diastereomer assignment. Two attempts were
made with compound 10 to produce the 2,4-dinitrophenylhydraz-
one and semicarbazone derivatives. Both resulted in the formation
of fine powders that did not lead to crystal formation.
Supplementary data
4. Conclusion
Supplementary data (experimental for compounds 10–16 and
spectral information (FTIR, MS, 1H NMR, 13C NMR) associated with
this article can be found, in the online version, at doi:10.1016/
j.tetlet.2009.08.110.
This paper represents the synthesis of a novel class of ketones.
These oxo-dihydrofurans can be prepared in the hands of under-
graduates via intramolecular silyl nitronate cycloaddition reactions
of propargylic nitroethers in good to excellent yields. We found
that the time involved for the cycloaddition was dependent on
the carbonyl substituent, with drastically longer time needed for
bulkier groups. This led us to propose a new mechanism for this
reaction. We also found that long-range coupling constants (5J)
are observed in compounds with bulky substituents placed in
either the 2 or 5 positions. The stereochemistry of these carbonyl
dihydrofuran derivatives was also explored, as expected the trans-
formation proceeds with a preference for the trans orientation. It is
not known whether the diastereomeric ratios of these compounds
are set completely during the Michael addition step. We are cur-
rently examining the possibility of epimerization/retro Michael
Addition of the N-trimethylsilyloxy nitronate intermediate (8) dur-
ing the ISNC reaction. Our group intends to continue the work on
an exact assignment of the stereoisomers produced in this process.
References and notes
1. (a) Newkome, G. R.; Sauer, J. D.; Roper, J. M.; Hager, D. C. Chem. Rev. 1977, 77,
513; (b) Goki, G. W.; Korzeniowski, S. H. Macrocyclic Polyether Synthesis;
Springer: Berlin, Heidelberg, New York, 1982. pp 19–150.
2. Wierenga, W.; Evans, B. R.; Woltersom, J. A. J. Am. Chem. Soc. 1979, 101, 1334.
3. Bradshaw, J. S.; Baxter, S. L.; Scott, D. C.; Lamb, J. D.; Izatt, R. M.; Christensen, J. J.
Tetrahedron Lett. 1979, 36, 3383.
4. Kobuke, Y.; Hanji, K.; Horiguchi, K.; Asada, M.; Nakayama, Y.; Furukawa, J. J. Am.
Chem. Soc. 1976, 98, 7414.
5. Tarrango, G.; Marzin, C.; Najimi, O.; Pellegrin, V. J. Org. Chem. 1990, 55, 420.
6. Rothermal, G. L.; Miao, L.; Hill, A. L.; Jackels, S. C. Inorg. Chem. 1992, 31, 4854.
7. Grandbois, M. L.; Viste, A. E.; Englund, E. A.; Duffy-Matzner, J. L. J.
Undergraduate Chem. Res. 2006, 4, 159.
8. Duffy, J. L.; Kurth, M. J. J. Org. Chem. 1994, 59, 3783.
9. Duffy, J. L.; Kurth, J.; Kurth, M. J. Tetrahedron Lett. 1992, 34, 1259.
10. (a) Hassner, A.; Dehaen, W. J. Org. Chem. 1990, 55, 5505; (b) Dehaen, W.;
Hassner, A. Tetrahedron Lett. 1990, 31, 743; (c) Yan, C.; Li, C. Phosphorus, Sulfur
Silicon Relat. Elem. 1995, 103, 133; (d) Hassner, A.; Friedman, O.; Dahaen, W.
Liebigs Ann. Recl. 1997, 3, 587; (e) Cheng, Q.; Oritani, T.; Horiguchi, T.; Shi, Q.
Eur. J. Org. Chem. 1999, 10, 2689; (f) Huang, K. S.; Lee, E. H.; Olmstead, M. M.;
Kurth, M. J. J. Org. Chem. 2000, 65, 499; (g) Kudoh, T.; Ishikawa, T.; Shimizu, Y.;
Saito, S. Org. Lett. 2003, 5, 3875.
5. Experimental section—general procedure to make
dihydrofuro-ketones
11. Duffy, J.L. Ph.D. Thesis, 1993, University of California, Davis.
12. Li, S.L.; Hassebroek, K.M.; Grandbois, M.L.; Duffy-Matzner, J.L. in preparation.
13. (a) Goto, H.; Osawa, E. J. Chem. Soc., Perkin Trans. 1993, 2, 187. MM2 force field
for hydrocarbons was described in Ref. (b). Extensions to functionalized
molecules have been described in subsequent papers and summarized in Ref.
(c); (b) Allinger, N. L. J. Am. Chem. Soc. 1977, 99, 8127; (c) Burkert, U.; Allinger,
N. L. Molecular Mechanics; American Chemical Society: Washington, DC, 1982.
14. Stewart, J.P.S. MOPAC 2002, CAChe Group, Portland.
To a stirred solution of nitroether (1 equiv) in dry benzene
(0.20 M) under nitrogen was added triethylamine (3.5 equiv) and
trimethylsilyl chloride (3.6 equiv). After stirring overnight, thin
layer chromatography (TLC) revealed the presence of some starting
material so more triethylamine (0.5 equiv) and trimethylsilylchlo-
ride (0.5 equiv) were added. The reaction was followed every 12 h
until no more starting material was detected (2–5 days). The reac-
tion was quenched with 6 M HCl (18 equiv) and allowed to stir un-
til no silyl intermediate was found by TLC (1–2 days). The aqueous
and organic layers were separated. The aqueous layer was ex-
tracted 3Â with 5 mL of ether. The combined organic layers were
washed with 5% sodium bicarbonate, followed by brine, dried over
15. Stewart, J. P. S. J. Comput. Chem. 1989, 10, 221.
16. Stewart, J. P. S. J. Mol. Model. 2004, 10, 6.
17. Compound characterization: ( )( )-1-[2-[1-methylethyl]-5-phenyl-2H,5H-dihydro-
furan-3-yl]ethanone (10): Rf (1:6 EtOAc/hexane) I = 0.363, II = 0.272; FTIR (neat)
3064, 3031, 2963, 2929, 2872, 1674; 1H NMR (500 MHz, CDCl3) d = 0.85 (I d,
J = 9.2 Hz, 3H); 0.89 (II d, J = 9.6 Hz, 3H), 1.12 (I d, J = 9.2 Hz, 3H), 1.35 (II d,
J = 9.6 Hz, 3H), 2.18 (II s, 3H), 2.37 (I s, 3H), 2.25 (I&II pd, J = 9.2, 2.8 Hz 1H), 5.28
(I&II ddd (apparent dt), J = 7.6, 2.8, 2.8 Hz, 1H), 5.93 (I&II dd, J = 8.4, 2.8 Hz, 1H),
6.71 (I dd (apparent t), 2.4, 2.4 Hz, 1H), 6.85 (II m, (apparent s), 1H), 7.34 (I&II m,

M. L. Grandbois et al. / Tetrahedron Letters 50 (2009) 6446–6449
6449
5H);13C NMR (500 MHz, CDCl3): 14.3 (II), 15.3 (I), 20.3 (I), 20.9 (II), 29.6 (II), 29.9
(I), 31.1 (II), 31.6 (I), 88.1 (I&II), 90.6 (I&II), 126.6, 128,5, 128.9, 142.3 (I&II0, 140.7
(I&II), 142.1 (I&II), 194.9 (I), 195.0 (II); GC–MS (initial temp: 80 °C, final temp:
250 °C, hold 2 min, ramp 12 °C/min) RtI = 7.446, RtII = 7.511, M+ = 230, m/z = 187.
( )( )-1-[2-Methyl-5-[1,1-dimethylethyl]-2H,5H-dihydrofuran-3-yl]-ethanone
(11): Rf (1:6 EtOAC/hexane) I = 0.94, II = 1.0; FTIR (neat) 2961, 2872, 1672; 1H
NMR (60 MHz, CDCl3): d = 0.83 (I s, 9 H), 0.85 (II s, 9 H), 1.20 (I d, J = 6.2 Hz, 3H),
1.26 (II d, J = 7.4 Hz, 3H), 4.36 (II dd, J = 1.7, 4.8 Hz), 4.48 (I dd, J = 1.6, 6.0 Hz,), 4.90
(I&II m, 1H), 6.0 (II m(apparent s), 1H), 6.45 (I m, 1H); 13C NMR (60 MHz, CCl4):
20.8 (I&II), 25.5 (I), 26.0 (II), 28.9 (I&II), 35.8 (I&II), 80.5 (II), 81.3 (I), 92.4 (I), 92.8
(II), 137.8 (I), 138.6 (II), 146.6 (I&II), 183.6 (I&II); GC–MS (initial temp: 75 °C, final
temp: 200 °C, hold 2 min, ramp 10 °C/min) RtI = 7.049, RtII = 7.319, m/z = 183
(M+ÀH), 126, 43.
( )-1-[2-Phenyl-2H,5H-dihydrofuran-3-yl]ethanone (14): Rf = 0.29 (1:4 EtOAc/
hexane); FTIR 3063, 3031, 2958, 2922, 2859, 1672, 1096; 1H NMR (60 MHz,
CDCl3) d = 2.35 (s, 3H, CH3–CO), 4.91 (ddd (apparent td), J = 5.0, 2.0, 2.0 Hz, 2H),
5.89 (td, J = 5.0, 2.0 Hz, 1H), 6.61 (dt (apparent q), J = 2.0, 2.0 Hz, 1H), 7.33 (s, 5H);
13C NMR (60 MHz, CDCl3) d = 27.9, 75.3, 89.5, 126.8, 127.0, 129.1, 129.5, 142.3,
141.1, 195.5; GC–MS (initial temp: 75 °C, final temp: 200 °C, hold 2 min, ramp
10 °C/min) Rt = 9.17, M+ = 188, m/z = 145, 105, 77, 43.
( )-1-(4-Ethyl-2H,5H-dihydrofuran-3-yl)ethanone (15): Rf = 0.12 (1:10 EtOAc/
hexane); FTIR 3091, 2967, 2933, 2877, 1672; 1H NMR (60 MHz, CDCl3) d = 0.87
(t, J = 7.3 Hz, 3H), 1.83 (m, 2H), 2.36 (s, 3H), 4.83 (d, J = 1.7 Hz, 2H), 5.05 (m, 1H),
6.81 (d, J = 2 Hz, 1H); 13C NMR (60 MHz, CDCl3) d = 8.6, 26.7 26.9, 74.3, 85.8, 139.1,
143.0, 194.0; GC–MS M+ = 140 (C8H12O2), m/z = 111, 43.
( )( )-1-[2-Methyl-5-phenyl-2H,5H-dihydrofuran-3-yl]propanone (16): Rf (1:6
EtOAC/hexane) I & II = 0.37; FTIR (neat) 3065, 3031, 2976, 2931, 1673; 1H NMR
(200 MHz, CDCl3) d = 1.08 (I t, J = 7.4 Hz, 3H), 1.16 (II t, J = 7.0 Hz, 3H), 1.44 (I d,
J = 6.2 Hz, 3H), 1.52 (II d, J = 6.2 Hz, 3H), 2.68 (I q, J = 7.1 Hz, 2H), 2.76 (I q,
J = 7.0 Hz, 2H), 5.22 (II qdd, J = 6.2, 3.9, 2.0 Hz, 1H), 5.36 (I qdd (apparent pd),
J = 6.2, 6.2, 1.6 Hz, 1H), 5.83 (II dd, J = 3.9, 2.0 Hz, 1H), 5.95 (Idd, J = 6.2, 1.6 Hz, 1H),
6.62 (I dd (apparent t), J = 1.6, 1.6 Hz, 1 H), 6.85 (II m, 1 H), 7.33 (I&II m); 13C NMR
(60 MHz, CDCl3) d = 7.6 (I&II), 20.8 (I), 21.6 (II), 29.3 (II), 32.59 (I), 81.8 (I&II), 86.2
(I), 86.6 (II), 126.1, 127.6,127.9, 128.5 (I&II), 139.2 (I&II), 140.5 (I&II)143.5 (I&II),
197.3 (I&II); GC–MS (initial temp: 75 °C, final temp: 250 °C, hold 2 min, ramp
12 °C/min) RtI = 10.60, RtII = 10.73 (3.4:1), m/z = 159, 105, 91, 77, 57.
( )-1-[2-Phenyl-2H,5H-dihydrofuran-3-yl]-ethanone (12): Rf (1:12 EtOAc/
hexane) = 0.080; FTIR (neat) 3064, 3032, 2619, 2851, 1671; 1H NMR (60 MHz,
CDCl3) d = 1.83 (s, 3H), 4.84 (ddd, 2, 2, 5 Hz, 2H), 5.84 (ddd, 2, 4, 5 Hz, 1H), 6.70 (dt,
2,2 Hz, 1H), 7.21 (s, 5H); 13C NMR (60 MHz, CDCl3) d = 26.9, 86.4, 96.0, 127.7,
127.4, 126.7, 130.3, 136.3, 182.3; GC–MS M+ = 188, m/z = 145, 77.
( )-1-[2-[1-Methylethyl]-2H,5H-dihydrofuran-3-yl]ethanone (13): Rf = 0.34 (1:6
EtOAc/hexane); FTIR 3080, 2962, 2931, 2873, 1672, 1095; 1H NMR (60 MHz,
CDCl3) d = 0.90 (d, J = 6.5 Hz, 6H), 1.26 (m, 1H), 2.36 (s, 3H), 4.80 (d, J = 2.0 Hz, 1H),
4.82 (s, 2H), 6.65 (d, J = 2.0 Hz, 1H); 13C NMR (60 MHz, CDCl3) d = 17.7, 17.9, 26.8,
33.0, 73.8, 92.0, 139.3, 141.9, 194.1; GC–MS (initial temp: 75 °C, final temp:
200 °C, hold 2 min, ramp 10 °C/min) Rt = 4.50, M+ = 154, m/z = 111, 95, 69.