Novel ether-ring transformation via a phenonium ion

Article abstract of DOI:10.1016/S0040-4039(02)01009-2

Upon treatment with CF₃COOH at 70°C, trisubstituted-tetrahydropyrans, which were stereoselectively prepared from δ-lactones, were found to be converted into the corresponding 2,5-disubstituted-tetrahydrofurans stereospecifically via a phenonium

Full text of DOI:10.1016/S0040-4039(02)01009-2

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 5333–5337
Novel ether-ring transformation via a phenonium ion
Shinji Nagumo,* Yusuke Ishii, Yo-ichiro Kakimoto and Norio Kawahara*
Hokkaido College of Pharmacy, Katuraoka 7-1, Otaru 047-0264, Japan
Received 11 April 2002; revised 20 May 2002; accepted 24 May 2002
Abstract—Upon treatment with CF3COOH at 70°C, trisubstituted-tetrahydropyrans, which were stereoselectively prepared from
d-lactones, were found to be converted into the corresponding 2,5-disubstituted-tetrahydrofurans stereospecifically via a phenon-
ium ion. Furthermore, the stereocontrolled formal synthesis of pamamycin 607 was achieved based on the ether-ring transforma-
tion. © 2002 Elsevier Science Ltd. All rights reserved.
Symmetrical s-bridged ethylenebenzenium (phenon-
ium) ions were proposed by Cram to explain the stereo-
chemical results in solvolyses of optically active threo-
and erythro-3-aryl-2-butyltosylates.1 Recently, as an
application of this ion to synthetic organic chemistry,
we reported the solvolytic lactonization of methyl 4-
aryl-5-tosyloxyalkanoates via the formation of a pheno-
nium ion and the sequential attack of the internal ester
group on the ion.2 The chemistry of a phenonium ion
can also be applied to lactone-ring transformation.
Treatment of d-lactone 1 with TsOH·H2O in CH3NO2
at 70°C gives g-lactone 2 in high yield (Scheme 1).3 It is
noteworthy that the benzylic asymmetric center on the
ring can be stereospecifically transferred to a side chain
in this ring transformation. As an extension of the
chemistry, we report here a stereospecific ring transfor-
mation of tetrahydropyrans into tetrahydrofurans via a
phenonium ion, as shown in Scheme 1, which would be
applicable to the syntheses of natural products contain-
ing 2,5-disubstituted tetrahydrofurans as polyether
antibiotics.4
The 1H NMR spectrum of 7 showed that its stable
structure has the aryl group at an axial position.6 The
coupling constants between benzyl proton and its three
vicinal protons are all ca. 4 Hz. Furthermore, two
proton peaks at an ortho position on the benzene ring
of 7 appeared at a lower magnetic field (ca. 0.2 or 0.3
ppm) than those of 6. Molecular mechanics calculation
(MM2) showed that the chair conformer with an axial
aryl group was ca. 0.9 kcal/mol more stable than that
with an equatorial aryl group.7
Di- and tri-substituted pyrans 6, 7, 9, 11, 12 and 15
were selected as substrates of ring transformation and
synthesized from trans-d-lactone 1 or cis-d-lactone 3
(Scheme 2). After DIBALH reduction of 1 and 3,
treatment of the corresponding lactol 4 and 5 with
Et3SiH and BF3·Et2O gave 65 and 75 in excellent yield.
Keywords: phenonium ion; stereospecific ether-ring transformation;
tetrahydrofuran; pamamycin 607.
Scheme 1. Reagents and conditions: (a) TsOH·H2O, CH3NO2,
* Corresponding authors. E-mail: nagumo@hokuyakudai.ac.jp
70°C.
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)01009-2

5334
S. Nagumo et al. / Tetrahedron Letters 43 (2002) 5333–5337
refluxing toluene to give thiolactone 13 in 92%
Scheme 2
Ar
yield,9,10 which was successfully converted into thioac-
etal 14 by allylation followed by methylation in situ.
Reduction of 14 with Et3SiH and BF3·Et2O proceeded
stereoselectively and the resulting syn-allylated com-
pound was hydrogenated to give 15 in high yield. The
1H NMR spectrum of 155 showed that its aryl group
took an axial position similar to that of 7. The total
energetic difference (ca. 4.6 kcal/mol) between the sta-
ble and unstable chair conformations of 15 was found
by MM2 calculation to be considerably larger than
that of 7 (ca. 0.9 kcal/mol).7 The result can be ration-
alized by considering the steric repulsion between
axial methyl and propyl groups in the unstable chair
conformation of 15.
Ar
Me
Me
a
O
O
O
OH
1 (α-Me)
3 (β-Me)
4 (quant. from 1)
5 (94% from 3)
b
b
Ar
Ar
O
O
6 (98% from 4)
Ar
7 (99% from 5)
Ar
d
c
4
O
O
Ether-ring transformation of tetrahydropyrans 6, 7, 9,
11, 12 and 15 was carried out upon treatment with
CF3COOH at 70°C (Scheme 3).11 Tetrahydropyrans 6
and 11, which had no axial substituents, were not
converted into furans under the conditions. This
might be due to the overwhelming stability of the
pyrans relative to the corresponding furans. On the
other hand, substrates 7, 9, 12 and 15, which had an
axial substituent, undertook ether-ring transformation
to give the corresponding tetrahydrofurans 16–195
along with recovered substrates. Furthermore, treat-
ment of isolated 17–19 with CF3COOH at 70°C gave
pyrans 9, 12 and 15 along with recovered furans
(Scheme 4). The ratio between pyrans and furans was
consistent with that in the conversion of pyrans into
furans. The fact that the reversible ring transforma-
tion proceeded stereospecifically can be rationalized
by considering the presence of a phenonium ion as an
intermediate.
84%
93%
9
8
Ar
Ar
b,d
e
O
1
O
HO
11 (78% from 1)
10
Ar
Ar
c, d
68%
3
O
O
OH
f
92%
5
12
Ar
Ar
Ar
g
b, d
O
O
O
MeS
14
Interestingly, the ring transformation of pyrans 7 and
15 having an axial aryl group proceeded more slowly
than that of 9 and 12. This finding also suggests the
presence of a phenonium ion as an intermediate. An
axial aryl group in the stable conformations of 7 and
15 is not antiperiplanar to a C2ꢁO bond. Thus, 7 and
15 should be converted into the corresponding pheno-
nium ion via a less-stable conformation in which the
phenyl group takes an equatorial position. The fact
that the ring transformation of 15 was particularly
slow can be rationalized by considering the remark-
able energetic difference between two chair conforma-
tions of 15 obtained by molecular mechanics
calculation.
S
15 (76% from 13)
13
Scheme 2. Reagents and conditions: (a) DIBALH, toluene,
−78°C; (b) Et3SiH, BF3·Et2O, CH3CN, −30°C; (c)
CH2ꢀCHCH2Si(CH3)3, BF3·Et2O, CH2Cl2; (d) 20%
Pd(OH)2–C, H2, AcOEt; (e) CH2ꢀCHCH2MgBr, THF,
−50°C; (f) Lawesson’s reagent, toluene, reflux; (g) (i)
CH2ꢀCHCH2MgBr, THF, −78°C, (ii) CH3I. Ar=3,4-
dimethoxyphenyl.
When anomeric substitution of lactol
4
with
allyltrimethylsilane was carried out according to
Kishi’s method, anti-alkylated pyran 8 was stereose-
lectively obtained.8 Furthermore, hydrogenation of 8
afforded the desired pyran 9.5 On the other hand,
treatment of 1 with allylmagnesium bromide in THF
at −50°C produced hemiacetal 10, which was sub-
jected to reduction with Et3SiH and the subsequent
hydrogenation to afford syn-alkylated pyran 115
stereoselectively.8
As shown above, anomeric substitution of lactol hav-
ing a six-membered ring developed by Kishi et al.
proceeds with high stereoselectivity to give 2,6-disub-
stituted tetrahydropyran.8 However, when the proce-
dure is applied to lactol having a five-membered ring,
its stereoselectivity is not always satisfactory.12 Conse-
quently, our newly developed ether-ring transforma-
tion of 2,6-disubstituted pyrans into 2,5-disubstituted
furans via a phenonium ion should provide us with a
new stereoselective synthetic route of natural products
containing 2,5-disubstituted tetrahydrofuran. In order
to clarify this expectation, we carried out a synthetic
study of pamamycin 607 (Scheme 5).13,14 Compound
Compound 125 was prepared from 5 by the same
procedure as that used for the conversion of 4 into 9.
Compound 3 was treated with Lawesson’s reagent in

S. Nagumo et al. / Tetrahedron Letters 43 (2002) 5333–5337
5335
Scheme 3. Reagents and conditions: CF3COOH, 70°C. Ar=3,4-dimethoxyphenyl.
8 was subjected to oxidative cleavage of a double bond
with OsO4/NaIO4 to give an aldehyde compound,
which was converted into two separable alcohols 20
and 21 upon treatment with allyl Grignard reagent in
the presence of ZnCl2. These compounds were hydro-
genated to give the corresponding alcohols 22 and 23.
The ether-ring transformation of 22 and 23 followed
by alkaline hydrolysis afforded tetrahydrofurans 24
and 25 along with recovered substrates. Furan 24 was
subjected to conventional acetylation to afford 26,
while another furan 25 was also converted into 26
through mesylation and subsequent substitution with
CH3COOCs. Acetate 26 was subjected to oxidative
decomposition of the aromatic ring to afford carboxy-
late 27.15 Finally, 27 was successfully converted into
285 through a sequence of esterification with CH2N2
and alkaline hydrolysis. Compound 28 corresponds to
the C1%ꢁC11% moiety of pamamycin 607 and has
already been converted into pamamycin 607 by Lee et
al.14a The spectroscopic data of the obtained 28 are in
agreement with reported data.14a
hydrofuran via a phenonium ion. The efficiency of this
method was demonstrated by its application to the
formal synthesis of pamamycin 607. Further applica-
tion of this reaction is in progress.
In conclusion, we have developed a stereospecific ring
transformation of tetrahydropyran into tetra-
Scheme 4. Reagents and conditions: CF3COOH, 70°C. Ar=
3,4-dimethoxyphenyl.

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S. Nagumo et al. / Tetrahedron Letters 43 (2002) 5333–5337
Scheme 5. Reagents and conditions: (a) OsO4, NaIO4, t-BuOH, H2O, Et2O; (b) CH2CHꢀCH2MgBr, ZnCl2, THF, −78°C; (c) 5%
Pd–C, H2, MeOH; (d) CF3COOH, 70°C; (e) K2CO3, MeOH; (f) Ac2O, DMAP, Py; (g) MsCl, DMAP, Et3N; (h) CH3COOCs,
DMF; (i) RuCl3·nH2O (cat.), NaIO4, CCl4, CH3CN, H2O; (j) (1) CH2N2, Et2O, (2) K2CO3, MeOH. Ar=3,4-dimethoxyphenyl.
Tf=COCF3.
References
m/z 236.1396 (calcd for C14H20O3: 236.1411); 1H NMR
(270 MHz, CDCl3) l 7.01 (1H, d, J=2.0 Hz), 6.94
(1H, dd, J=8.0, 2.0 Hz), 6.81 (1H, d, J=8.0 Hz),
4.10–3.98 (1H, m), 3.96–3.86 (1H, qd, J=6.6, 3.6 Hz),
3.88 (3H, s), 3.87 (3H, s), 3.68–3.56 (1H, m), 2.78 (1H,
td, J=4.6, 3.6 Hz), 2.02–1.78 (3H, m), 1.48–1.36 (1H,
m), 1.06 (3H, d, J=6.6 Hz). 13C NMR (68 MHz,
CDCl3) l 148.36 (s), 147.25 (s), 135.32 (s), 121.28 (d),
112.80 (d), 110.79 (d), 75.58 (d), 66.62 (t), 55.85 (q),
55.78 (q), 44.23 (d), 29.17 (t), 22.18 (t), 17.75 (q). 9
(colorless oil): IR (neat) 1593, 1518 cm−1; EI-MS m/z
278 (M+), 164; HR-MS m/z 278.1877 (calcd for
C17H26O3: 278.1881); 1H NMR (270 MHz, C6D6) l
6.76–6.60 (3H, m), 3.98–3.85 (2H, m), 3.47 (3H, s), 3.44
(3H, s), 2.40–2.30 (1H, m), 2.04–1.70 (4H, m), 1.60–
1.20 (4H, m), 1.18 (3H, d, J=5.9 Hz), 0.96 (3H, t,
J=7.3 Hz); 13C NMR (68 MHz, C6D6) l 150.17 (s),
148.85 (s), 137.53 (s), 119.96 (d), 112.73 (d, ×2), 71.69
(d), 70.19 (d), 55.77 (q), 55.73 (q), 49.66 (d), 33.36 (t),
28.97 (t), 27.73 (t), 20.42 (q), 19.69 (t), 14.32 (q). 11
(colorless oil): IR (neat) 1593, 1518 cm−1; EI-MS m/z
278 (M+), 164; HR-MS m/z 278.1857 (calcd for
C17H26O3: 278.1881); 1H NMR (270 MHz, CDCl3) l
6.80 (1H, d, J=8.1 Hz), 6.70 (1H, dd, J=8.1, 1.9 Hz),
6.67 (1H, d, J=1.9 Hz), 3.87 (3H, s), 3.85 (3H, s),
3.57–3.37 (2H, m), 2.28 (1H, ddd, J=11.9, 9.7, 3.8 Hz),
2.00–1.88 (1H, m), 1.82–1.62 (2H, m), 1.62–1.20 (5H,
m), 0.99 (3H, d, J=6.3 Hz), 0.94 (3H, t, J=7.3 Hz);
13C NMR (68 MHz, CDCl3) l 148.71 (s), 147.38 (s),
1. (a) Cram, D. J. J. Am. Chem. Soc. 1949, 71, 3863; (b)
Cram, D. J.; Davis, R. J. Am. Chem. Soc. 1949, 71,
3871; (c) Cram, D. J. J. Am. Chem. Soc. 1949, 71,
3875; (d) Cram, D. J. J. Am. Chem. Soc. 1964, 86,
3767.
2. Nagumo, S.; Furukawa, T.; Ono, M.; Akita, H. Tetra-
hedron Lett. 1997, 38, 2849.
3. Nagumo, S.; Hisano, T.; Kakimoto, Y.; Kawahara, N.;
Ono, M.; Furukawa, T.; Takeda, S.; Akita, H. Tetra-
hedron Lett. 1998, 39, 8109.
4. Westley, J. W. Polyether Antibiotics: Naturally Occur-
ring Acid Ionophores; Marcel Dekker: New York, 1982;
Vols. I and II.
5. All new compounds could be identified by spectroscopic
data. For representative compounds, 6 (colorless oil):
IR (neat) 2936, 1593 cm−1; EI-MS m/z 236 (M+), 164;
HR-MS m/z 236.1419 (calcd for C14H20O3: 236.1411);
1H NMR (270 MHz, CDCl3) l 6.80 (1H, d, J=8.0
Hz), 6.71 (1H, dd, J=2.0, 8.0 Hz), 6.67 (1H, d, J=2.0
Hz), 4.04 (1H, dt, J=3.0, 11.2 Hz), 3.88 (3H, s), 3.86
(3H, s), 3.55 (1H, dt, J=3.0, 11.2 Hz), 3.48 (1H, dq,
J=6.3, 9.9 Hz), 2.33 (1H, ddd, J=3.6, 9.9, 11.2 Hz),
2.00–1.90 (1H, m), 1.83–1.57 (3H, m), 0.98 (3H, d, J=
6.3 Hz); 13C NMR (68 MHz, CDCl3) l 148.70 (s),
147.41 (s), 136.33 (s), 119.38 (d), 119.38 (d), 111.13 (d),
110.79 (d), 78.27 (d), 68.23 (t), 55.71 (q, ×2), 55.01 (d),
32.21 (t), 26.45 (t), 19.96 (q). 7 (colorless oil): IR (neat)
2936, 1589 cm−1; EI-MS m/z 236 (M+), 164; HR-MS

S. Nagumo et al. / Tetrahedron Letters 43 (2002) 5333–5337
5337
136.48 (s), 119.45 (d), 111.14 (d), 110.82 (d), 78.10 (d),
77.48 (d), 55.73 (q, ×2), 49.89 (d), 38.49 (t), 32.51 (t),
31.90 (t), 20.04 (q), 18.79 (t), 14.06 (q). 12 (white crys-
tals): EI-MS m/z 278 (M+), 164, 149; HR-MS m/z
m/z 278 (M+), 165; HR-MS m/z 278.1906 (calcd for
C17H26O3: 278.1881); H NMR (270 MHz, C6D6) l 6.89
1
(1H, d, J=2.0 Hz), 6.83 (1H, dd, J=8.2 Hz), 6.81 (1H,
dd, J=8.2, 2.0 Hz), 3.91 (1H, q, J=5.9 Hz), 3.70 (1H,
quintet, J=6.6 Hz), 3.53 (3H, s), 3.44 (3H, s), 2.81 (1H,
quintet, J=6.9 Hz), 1.68–1.22 (7H, m), 1.34 (3H, d,
J=6.9 Hz), 1.20–1.08 (1H, m), 0.88 (3H, t, J=6.9 Hz);
13C NMR (68 MHz, CDCl3) l 148.40 (s), 147.32 (s),
136.94 (s), 119.85 (d), 111.69 (d), 110.81 (d), 83.51 (d),
79.07 (d), 55.80 (q), 55.69 (q), 44.00 (d), 38.28 (t), 31.05
(t), 28.39 (t), 19.35 (t), 17.34 (q), 14.20 (q). 28 (colorless
oil): EI-MS m/z 226 (M+−H2O), 157; HR-MS m/z
1
278.1884 (calcd for C17H26O3: 278.1881); H NMR (270
MHz, CDCl3) l 6.81 (1H, d, J=8.5 Hz), 6.74–6.69 (2H,
m), 4.23 (1H, quintet, J=6.8 Hz), 3.86 (3H, s), 3.85 (3H,
s), 3.69–3.58 (1H, m), 3.08 (1H, ddd, J=12.4, 6.8, 3.4
Hz), 2.13–1.95 (1H, m), 1.86–1.71 (2H, m), 1.55–1.30
(5H, m), 0.98 (3H, t, J=6.8 Hz), 0.93 (3H, t, J=6.8 Hz);
13C NMR (68 MHz, CDCl3) l 148.69 (s), 147.41 (s),
135.35 (s), 119.30 (d), 111.05 (d, ×2), 73.81 (d), 67.89 (d),
55.80 (q), 55.77 (q), 44.60 (d), 38.39 (t), 32.08 (t), 22.58
(t), 18.71 (t), 14.05 (q), 12.26 (q). 15 (colorless oil): EI-MS
m/z 278 (M+), 164; HR-MS m/z 278.1860 (calcd for
C17H26O3: 278.1881); 1H NMR (270 MHz, CDCl3) l 7.12
(1H, d, J=1.6 Hz), 6.96 (1H, dd, J=8.2, 1.6 Hz), 6.79
(1H, d, J=8.2 Hz), 3.88 (3H, s), 3.86 (3H, s), 3.90–3.78
(1H, m), 3.51–3.40 (1H, m), 2.61 (1H, br), 2.00 (1H, tt,
J=13.2, 4.9 Hz), 1.80 (1H, br d, J=13.2 Hz), 1.73–1.30
(6H, m), 1.09 (3H, d, J=6.6 Hz), 0.95 (3H, t, J=6.9 Hz);
13C NMR (68 MHz, CDCl3) l 148.05 (s), 147.01 (s),
135.38 (s), 121.90 (d), 113.38 (d), 110.61 (d), 78.01 (d),
75.79 (d), 55.67 (q, ×2), 43.62 (d), 38.95 (t), 31.74 (t),
25.68 (t), 19.92 (q), 18.54 (t), 14.18 (q). 16 (colorless oil):
EI-MS m/z 236 (M+), 165; HR-MS m/z 236.1426 (calcd
1
226.1569 (calcd for C13H22O3: 226.1568); H NMR (270
MHz, CDCl3) l 4.20–3.94 (3H, m), 3.70 (3H, s), 2.54
(1H, quintet, J=6.9 Hz), 2.10–1.90 (2H, m), 1.81–1.54
(5H, m), 1.21 (3H, d, J=6.9 Hz), 1.13 (3H, d, J=6.9 Hz);
13C NMR (68 MHz, CDCl3) l 13.52 (q), 81.06 (d), 77.26
(d), 65.16 (d), 51.70 (q), 45.28 (d), 42.61 (t), 30.50 (t),
28.82 (t), 23.16 (q), 13.52 (q).
6. Similar result has been found in cis-2-methyl-1-phenyltet-
rahydropyran: Perrott, A. L.; de Lijser, H. J. P.; Arnold,
D. R. Can. J. Chem. 1997, 75, 384.
7. The structures of tetrahydropyrans 7 and 15 were opti-
mized with the MM2 force field. The initial geometries of
these molecules were obtained by CAChe (ver. 4.5 for
PowerMac).
8. Lewis, M. D.; Cha, J. K.; Kishi, Y. J. Am. Chem. Soc.
1982, 104, 4976.
9. Direct alkylation of 3 with allyl Grignard reagent was not
successful due to the further alkylation of the hemiacetal
product.
10. (a) Nicolaou, K. C.; McGarry, D. G.; Somers, P. K.;
Kim, B. H.; Ogilvie, W. W.; Yiannikouros, G.; Prasad,
C. V. C.; Veale, C. A.; Hark, R. R. J. Am. Chem. Soc.
1990, 112, 6263; (b) Scheibye, S.; Kristensen, J.; Lawes-
son, S.-O. Tetrahedron 1979, 35, 1339.
1
for C14H20O3: 236.1411); H NMR (270 MHz, CDCl3) l
6.80 (3H, s), 4.00–3.64 (3H, m), 3.88 (3H, s), 3.85 (3H, s),
2.74 (1H, quintet, J=7.0 Hz), 2.03–1.72 (3H, m), 1.64–
1.48 (1H, m), 1.26 (3H, d, J=7.0 Hz); 13C NMR (68
MHz, CDCl3) l 148.65 (s), 147.37 (s), 137.28 (s), 119.60
(d), 111.08 (d), 111.01 (d), 83.78 (d), 68.14 (t), 55.80 (q),
55.77 (q), 44.01 (d), 29.54 (t), 25.81 (t), 18.54 (q). 17
(colorless oil): EI-MS m/z 278 (M+), 165; HR-MS m/z
1
278.1869 (calcd for C17H26O3: 278.1881); H NMR (270
MHz, C6D6) l 6.79–6.70 (2H, m), 6.63 (1H, d, J=7.9
Hz), 3.91 (1H, q, J=7.3 Hz), 3.79 (1H, quintet, J=6.9
Hz), 3.48 (3H, s), 3.44 (3H, s), 2.74 (1H, quintet, J=7.3
Hz), 1.70–1.16 (8H, m), 1.51 (3H, d, J=7.3 Hz), 0.94
(3H, t, J=6.9 Hz); 13C NMR (68 MHz, C6D6) l 150.08
(s), 148.84 (s), 137.95 (s), 119.97 (d), 112.66 (d), 112.55
(d), 84.17 (d), 79.46 (d), 55.72 (q), 55.65 (q), 45.91 (d),
38.82 (t), 31.44 (t), 29.99 (t), 19.96 (t), 19.35 (q), 14.47 (q).
18 (colorless oil): EI-MS m/z 278 (M+), 165; HR-MS m/z
11. A solution of pyran (100 mg) in CF3COOH (5 ml) was
stirred at 70°C.
12. For example, anomeric substitution of the lactol com-
pound, which was obtained by DIBALH reduction of 2,
did not proceed stereoselectively to give a ca. 1:1 mixture
of two diastereomeric 2,5-disubstituted tetrahydrofurans.
13. For isolation of pamamycin 607, see: Kondo, S.; Yasui,
K.; Katayama, M.; Marumo, S.; Kondo, T.; Hattori, H.
Tetrahedron Lett. 1987, 28, 5861.
1
278.1877 (calcd for C17H26O3: 278.1881); H NMR (270
14. For synthesis of pamamycin 607, see: (a) Lee, E.; Jeong,
E. J.; Kang, E. J.; Sung, L. T.; Hong, S. K. J. Am. Chem.
Soc. 2001, 123, 10131; (b) Germay, O.; Kumar, N.;
Thomas, E. J. Tetrahedron Lett. 2001, 42, 4969; (c)
Wang, Y.; Bernsmann, H.; Gruner, M.; Metz, P. Tetra-
hedron Lett. 2001, 42, 7801.
MHz, CDCl3) l 6.86–6.74 (3H, m), 4.14–4.03 (1H, m),
3.87 (3H, s), 3.85 (3H, s), 3.87–3.75 (1H, m), 2.79 (1H,
quintet, J=6.9 Hz), 1.95–1.82 (2H, m), 1.72–1.30 (6H,
m), 1.27 (3H, 6.9 Hz), 0.89 (3H, t, J=6.9 Hz); 13C NMR
(68 MHz, CDCl3) l 148.66 (s), 147.43 (s), 137.07 (s),
120.01 (d), 111.76 (d), 111.08 (d), 82.77 (d), 79.11 (d),
55.90 (q), 55.83 (q), 43.96 (d), 38.09 (t), 32.16 (t), 29.37
(t), 19.41 (t), 17.60 (q), 14.13 (q). 19 (colorless oil): EI-MS
15. Carlsen, P. H. J.; Katsuki, T.; Martin, V. S.; Sharpless,
K. B. J. Org. Chem. 1981, 46, 3936.