Pinacol-type rearrangement reactions of 2-phenyl-3-silyloxyoxetanes: The influence of the Lewis acid on the regioselectivity

Full text of DOI:10.1021/jo990802g

J . Org. Chem. 1999, 64, 8041-8044
8041
Ta ble 1. Rea r r a n gem en t Rea ction s of Oxeta n e 4 u p on
Tr ea tm en t w ith Lew is Acid s in Va r iou s Solven ts
P in a col-Typ e Rea r r a n gem en t Rea ction s of
2-P h en yl-3-silyloxyoxeta n es: Th e In flu en ce
of th e Lew is Acid on th e Regioselectivity
Lewis
acid
time,a temp,
product ratio
(5a :5b):(6a :6b)
entry
solvent
h
°C
b
1
2
3
4
5
6
7
8
9
ZnCl2
ZnCl2
ZnCl2
SnCl4
TiCl4
TiCl4
TiCl4
AlCl3
ether
18
18
18c
12
2.5
0.1
0.1
reflux
-
Thorsten Bach* and Frank Eilers
CH2Cl2
toluene
toluene
ether
CH2Cl2
toluene
toluene
ether
reflux (43:0):(57:0)
reflux (11:0):(58:31)
Fachbereich Chemie der Philipps-Universita¨t Marburg,
25
25
-78
-78
25
25
25
25
25
(29:0):(71:0)
(100:0):(0:0)
(74:0):(26:0)
(80:0):(20:0)
(21:0):(79:0)
(11:0):(49:40)
(28:0):(42:30)
(9:0):(17:74)
(50:6):(18:26)
D-35032 Marburg, Germany
Received May 17, 1999
Oxetanols of the general structure 1 (R, R1, R2 ) alkyl)
undergo acid-catalyzed rearrangement reactions in the
course of which a hydroxymethyl group migrates to the
carbon atom C-2 in a pinacol-type 1,2-shift.1,2 2,2-Diphen-
yl-3-[(trimethylsilyl)oxy]oxetanes 2 similarly undergo
hydroxymethyl group migrations to yield the correspond-
ing rearrangement products 3 upon treatment with a
Lewis acid (BF3 or TiCl4) in CH2Cl2.3,4
18
d
d
d
AlMeCl2
AlMeCl2
AlMeCl2
20e
20f
20
10
11
12
CH2Cl2
toluene
CH2Cl2
g
AlMe3
20
a
Time required for complete conversion at the designated
temperature. No reaction occurred. c 90% conversion. A 1 M
b
d
solution in hexane was used. e 35% conversion. f 60% conversion.
g
A 2 M solution in heptane was used.
Cl2. The four possible products as depicted in eq 1 were
detected by 1H NMR spectroscopy, and their relative ratio
was determined by integration. To ascertain their iden-
tity they were purified by chromatography and fully
characterized (see Experimental Section). Silyl ether 5b
proved to be extremely moisture sensitive and decom-
posed even on silica gel. It is therefore possible that this
compound was not detected after workup, although it had
been present in the reaction mixture in some instances.
Generally, these reactions proceeded fastest in CH2Cl2
as the solvent and slowest in ether, as exemplified by
the ZnCl2-promoted reactions (entries 1-3). Whereas
there was no reaction in ether, the rearrangement
occurred in toluene, but it was incomplete after 18 h. It
was complete in CH2Cl2 after the same period of time.
Similar trends were observed with other Lewis acids, e.g.
TiCl4 (entries 5-7) and AlMe3 (entry 12), in the case of
which only the reaction in CH2Cl2 went to completion.
With almost all Lewis acids except TiCl4 and AlMe3
compounds 6 prevailed as products over 5. If compounds
6 were the major products, the highest selectivity was
often observed in toluene as the solvent. This tendency
can be observed for ZnCl2 (entries 1-3) and AlMeCl2
(entries 9-11) from Table 1 and it also held true for SnCl4
(entry 4) and AlCl3 (entry 8), for which only the most
selective reactions were included in the table. In addition,
the solvent toluene often favored preferential formation
of the silylated product 6b (entries 3 and 11). In terms
of regioselectivity, the best results were recorded either
by employing TiCl4 in ether, which resulted in the
exclusive formation of 5a (entry 5), or by employing
AlMeCl2 in toluene (entry 11), which promoted the
Recently, we observed that 2-phenyl-3-[(trimethylsilyl)-
oxy]oxetanes related to 2 exhibit migratory aptitudes
different from 1 and 2. It was found that the 1,2-
migration of the R group at C-3 is preferred as compared
to the hydroxymethyl shift (3:1 ratio).5 In view of the
potential use of the Lewis acid catalyzed rearrangement
of 3-silyloxyoxetanes for kinetic resolution experiments6
with chiral Lewis acids, we have studied the regioselec-
tivity of these rearrangements more closely. To this end,
the oxetane 4, which is readily available by the photo-
cycloaddition of benzaldehyde and 3,3-dimethyl-2-[(tri-
methylsilyl)oxy]-1-butene,7 was treated with various
Lewis acids in three different solvents, i.e., in a polar,
coordinating solvent (diethyl ether), in a polar, noncoor-
dinating solvent (CH2Cl2), and in a nonpolar solvent
(toluene). The structure of the products so obtained are
depicted in eq 1 and their relative ratios are summarized
in Table 1. The table does not include all the experiments
we have conducted. A full list of data is contained in the
Supporting Information.
(4) Related epoxide rearrangements: (a) Maruoka, K.; Hasegawa,
M.; Yamamoto, H.; Suzuki, K.; Shimazaki, M.; Tsuchihashi, G.-i. J .
Am. Chem. Soc. 1986, 108, 3827. (b) Suzuki, K.; Miyazawa, M.;
Tsuchihashi, G.-i. Tetrahedron Lett. 1987, 28, 3515. (c) Maruoka, K.;
Ooi, T. Yamamoto, H. J . Am. Chem. Soc. 1989, 111, 6431. (d) Maruoka,
K.; Ooi, T.; Nagahara, S.; Yamamoto, H. Tetrahedron 1991, 47, 6983.
(e) Raman, J . V.; Lee, H. K.; Vleggaar, R.; Cha, J . K. Tetrahedron Lett.
1995, 36, 3095. (f) J ung, M. E.; Anderson, K. L. Tetrahedron Lett. 1997,
38, 2605.
In all runs the reaction mixture was quenched with
water and the aqueous layer was extracted with CH2-
(1) (a) Kagan, J .; Przybytek, J . T. Tetrahedron 1973, 29, 1163. (b)
Donnelly, J . A.; Hoey, J . G.; O’Donnell, R. J . Chem. Soc., Perkin Trans.
1, 1974, 1218.
(2) General reviews on oxetane ring opening reactions: (a) Searles,
S. In Comprehensive Heterocyclic Chemistry; Katritzky, A. R., Ed.;
Pergamon Press: Oxford, 1984; Vol. 7, p 363. (b) Linderman, R. J . In
Comprehensive Heterocyclic Chemistry, 2nd ed.; Katritzky, A. R., Ed.;
Pergamon Press: Oxford, 1996; Vol. 1, p 721.
(5) Bach, T.; Kather, K. Tetrahedron 1994, 50, 12319.
(6) Previous experiments in this area: (a) Ito, K.; Yoshitake, M.;
Katsuki, T. Chem. Lett. 1995, 1027. (b) Ito, K.; Yoshitake, M.; Katsuki,
T. Tetrahedron 1996, 52, 3905.
(3) Shimizu, N.; Yamaoka, S.; Tsuno, Y. Bull. Chem. Soc. J pn. 1983,
56, 3853.
(7) Bach, T.; J o¨dicke, K. Chem. Ber. 1993, 126, 2457.
10.1021/jo990802g CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/23/1999

8042 J . Org. Chem., Vol. 64, No. 21, 1999
Notes
Sch em e 1
process to occur. In a preparative run in which AlMeCl2
was employed in substoichiometric quantities (0.5 equiv)
relative to oxetane 4, the silylated product 6b was
isolated in 40% yield together with 31% of the desilylated
aldol 6a .
Concerning the regioselectivity of the reaction, the
pathway followed in most cases (entries 2-4, 8-11 in
the table) is the route 7 f 8 f 10. This observation might
be associated with the fact that the nucleophilicity of the
tert-butyl group in 7 is not high enough to account for a
successful competition of step 7 f 9 vs 7 f 8. An
increased nucleophilicity would be expected if the TMS
group in 7 is either activated by an anion or replaced by
a metal. Accordingly, the high tendency of TiCl4 to effect
the desilylation of silyl ethers rapidly even at low
temperature8 might be the clue to its different reaction
mode. If desilylation of the oxetanol proceeds before the
oxetane oxygen is activated for the migration or if TiCl4
is capable of providing a chloride ion intramolecularly,
as depicted in formula 11,9 the tert-butyl migration
should be favored.
formation of products 6a and 6b. In preparative runs on
a 1 mmol scale, the latter reaction yielded 73% of aldol
6a after acidic workup and the former reaction yielded
70% of R-hydroxy ketone 5a .
A proposed mechanism for the reaction is shown in
Scheme 1 although it is certainly not possible to trace
down every single experimental observation to a mecha-
nistic key step. It postulates two competitive pathways
responsible for the formation of compounds 5 and 6
(Scheme 1).
Ether should be the best solvent for this process, as
oxetane activation is retarded in this solvent due to
competing complexation of ether with the Lewis acid.
AlMe3, on the other hand, might lead to a significant
degree of tert-butyl migration because its Lewis acidity
is relatively low10 and the ring opening 7 f 8 may be
slower than the migration 7 f 9.
We briefly looked into another possible pathway for the
formation of compounds 6a and 6b by rearrangement of
oxetane 4. The AlMeCl2-catalyzed process 4 f 6 can
either proceed via the free carbenium ion 8 by an
intramolecular 1,2-shift as discussed above or it could be
due to a complete separation of formaldehyde from
oxetane 4 in a retro-[2 + 2] fashion and a subsequent
intermolecular Mukaiyama aldol reaction (eq 2).
The Lewis acid (depicted by the general structure MXn)
activates the oxetane by complexation to the Lewis-basic
oxetane oxygen atom, yielding onium ion 7. The tert-butyl
migration can occur directly from this intermediate to
yield rearrangement product 9. Alternatively, the oxetane
ring can open to benzylic carbenium ion 8, which rear-
ranges to intermediate 10 via a hydroxymethyl 1,2-shift.
The rearrangement of 8 to 9 is less likely based on
previous experiments2,3 with oxetanes 1 and 2 (vide
supra), in which carbenium ions related to 8 were the
intermediates and in which the hydroxymethyl and not
the alkyl migration dominated. In turn, 7 cannot be
considered a viable precursor for rearrangement product
10, as this hydroxymethyl migration would have to occur
in a disfavored [σ2s + σ2s]-process (vide infra). As the
oxetane competes with other Lewis bases for the Lewis
acid, it is clear why the reaction proceeded with the
lowest reaction rate in ether as solvent. CH2Cl2 is suited
to stabilize ions and zwitterions better than toluene and
is consequently the solvent in which the rearrangement
proceeded most readily. The ratio of silylated to desily-
lated products is determined by the competition of silyl
migration (9 f 5b, 10 f 6b) with silyl cleavage (9 f 5a ,
10 f 6a ). The dissociation of X- is facilitated in CH2Cl2
and it leads to desilylation upon attack at the TMS group.
On the contrary, the intramolecular silyl migration can
proceed via a tight five- or six-membered transition state
which should be favored in toluene. Indeed, the highest
amounts of silylated products were formed in toluene.
The process of silyl migration to afford products 5b or
6b is not only mechanistically interesting but it is also
mandatory for a successful catalytic cycle. Only if the
Lewis acid MXn is regenerated can it activate another
oxetane molecule for the rearrangement. So far, AlMeCl2
was the only Lewis acid which allowed for a catalytic
In agreement with previous crossover experiments,3 we
found no indication to support the latter notion. In
competition reactions with acetaldehyde there was no
hint for the incorporation of this substrate into the
product, i.e., for the formation of a 1-hydroxyethyl-
substituted ketone. Consequently, it appears reasonable
to assume a carbenium ion as an intermediate in the
AlMeCl2-catalyzed reaction which is intramolecularly
attacked in a pinacol-type rearrangement. This picture
(8) Some recent examples: (a) J enal, A.; Zahra, J .-P.; Santelli, M.
Tetrahedron Lett. 1983, 24, 1395. (b) Castedo, L.; Granja, J .; Maestro,
M. A.; Mourino, A. Tetrahedron Lett. 1987, 28, 4589. (c) Crossley, M.
J .; Hambley, T. W.; Stamford, A. W. Aust. J . Chem. 1990, 43, 1827.
(d) Yamamoto, Y.; Abe, H.; Nishi, S.; Yamada, J .-i. J . Chem. Soc.,
Perkin Trans. 1 1991, 3253.
(9) We very much thank a referee for pointing out this possibility
to us.
(10) Childs, R. F.; Mulholland, D. L.; Nixon, A. Can. J . Chem. 1982,
60, 801.

Notes
J . Org. Chem., Vol. 64, No. 21, 1999 8043
preparative runs, 10 mL of 2 N aqueous HCl was used to quench
the reaction mixture. The purification of the individual sub-
stances was conducted by chromatography, as indicated below.
(RS)-1-Hydr oxy-4,4-dim eth yl-3-ph en yl-2-pen tan on e (5a).5
Following the general procedure, 1.0 mmol of oxetane 4 (278
mg) and 3.0 mmol of TiCl4 (570 mg, 330 µL) were allowed to
react in 5 mL of ether at -78 °C for 10 min. Workup and
subsequent flash chromatography (P/TBME/NEt3 ) 95/4/1) gave
145 mg (70%) of ketone 5a as a colorless solid. Rf ) 0.20 (P/
is further supported by the fact that the rearrangement
reaction of oxetane 1211 (eq 3) proceeded nonstereospe-
1
TBME ) 9/1); H NMR (CDCl3) δ 1.00 (s, 9 H), 3.11 (t, J ) 4.7
Hz, 1 H), 3.49 (s, 1 H), 4.11 (dd, J ) 19.0 Hz, J ) 4.7 Hz, 1 H),
4.20 (dd, J ) 19.0 Hz, J ) 4.7 Hz, 1 H), 7.26-7.30 (m, 5 H). All
other analytical data were in agreement with the literature
values.5
cifically in the presence of AlMeCl2. As expected, a
1-hydroxyethyl shift occurred predominantly and the
reaction products 14a and 14b were isolated (66% yield).
Their ratio was slightly dependent on the reaction
temperature and varied between 70/30 and 80/20 in favor
of 14b. The assignment of the relative configuration was
(RS)-4,4-Dim eth yl-3-ph en yl-1-[(tr im eth ylsilyl)oxy]-2-pen -
ta n on e (5b). For comparison, this sensitive silyl ether was
prepared from 0.66 mmol of compound 5a (137 mg) by silylation
with 1.3 mmol of TMSCl (140 mg, 165 µL) and 1.3 mmol of NEt3
(130 mg, 180 µL) in 5 mL of CH2Cl2 at ambient temperature.
The workup was conducted with pH 7 buffer. Purification with
neutral alumina (P/TBME ) 95/5) gave 172 mg (94%) of silyl
3
based on the J HH coupling constants of the protons
CHOH and CHPh and on the 13C NMR data. In general,
intramolecularly hydrogen-bonded syn-aldols are known
1
ether 5b as a colorless oil. Rf ) 0.49 (P/TBME ) 9/1); H NMR
3
to exhibit smaller J HH values than the anti-aldols.12 Also,
(CDCl3) δ 0.04 (s, 9 H), 0.99 (s, 9 H), 3.65 (s, 1 H), 4.08 (d, J )
18.0 Hz, 1 H), 4.17 (d, J ) 18.0 Hz, 1 H), 7.25-7.30 (m, 5 H);
13C NMR (CDCl3) δ -0.7 (q), 28.0 (q), 34.5 (s), 62.2 (d), 69.4 (t),
127.1 (d), 127.9 (d), 130.4 (d), 135.5 (s), 209.1 (s). Anal. Calcd
for C16H26O2Si (278.5): C, 69.01; H, 9.41. Found: C, 68.80; H,
9.48.
the 13C NMR signals of carbinol and methine carbon
atoms are shifted upfield in syn-aldols relative to anti-
aldols.13
Contrary to the result obtained with AlMe2Cl and in
agreement with our previous arguments, aldol-type
products were observed in the TiCl4-induced rearrange-
ment of compound 12 only to a small extent. If the
reaction was conducted at -78 °C, the aldols 14a and
14b were isolated in a ratio of 95/5 (17% yield). The major
product was an R-hydroxyketone (47% yield), which was
tentatively assigned the depicted configuration 13 based
on the assumption that an inversion at the former C-2
carbon atom of oxetane 12 has occurred. The structure
of compound 12 in the crystal14 shows an anticlinal
arrangement of the C-2/O and the t-Bu/C-3 bond (dihe-
dral angle ) 122.9°) which should facilitate a backside
attack of the migrating group, as was postulated for the
analogous migration 7 f 9.
(RS)-1-Hydr oxy-4,4-dim eth yl-2-ph en yl-3-pen tan on e (6a).5
Following the general procedure, 1.0 mmol of oxetane 4 (278
mg) and 3.0 mmol of AlMeCl2 (3.0 mL of a 1 M solution in
hexane) were allowed to react in 5 mL of toluene at room
temperature for 2 h. Workup and subsequent flash chromatog-
raphy (P/TBME/NEt3 ) 95/4/1 f 80/19/1) gave 150 mg (73%) of
ketone 6a as a colorless solid. In addition, 35 mg (17%) of ketone
5a was obtained. Rf ) 0.05 (P/TBME ) 9/1); 1H NMR (CDCl3) δ
1.08 (s, 9 H), 1.94 (s, 1 H), 3.70 (dd, J ) 11.0 Hz, J ) 5.3 Hz, 1
H), 4.08 (dd, J ) 11.0 Hz, J ) 8.6 Hz, 1 H), 4.38 (dd, J ) 8.6 Hz,
J ) 5.3 Hz, 1 H), 7.22-7.40 (m, 5 H). All other analytical data
were in agreement with the literature values.5
(RS)-4,4-Dim eth yl-2-ph en yl-1-[(tr im eth ylsilyl)oxy]-3-pen -
ta n on e (6b). Following the general procedure, 1.0 mmol of
oxetane 4 (278 mg) and 0.5 mmol of AlMeCl2 (0.5 mL of a 1 M
solution in hexane) were allowed to react in 5 mL of toluene at
room temperature for 18 h. Workup and subsequent flash
chromatography (P/TBME/NEt3 ) 98/1/1 f 85/14/1) gave 112
mg (40%) of ketone 6b as a colorless oil. In addition, 64 mg (31%)
of ketone 6a and 42 mg (20%) of ketone 5a were obtained. Rf )
Further studies regarding the course of 1,2-rearrange-
ments of 3-oxetanols and their derivatives are currently
underway in our laboratories.
1
0.65 (P/TBME ) 9/1); H NMR (CDCl3) δ 0.04 (s, 9 H), 1.09 (s,
9 H), 3.58 (dd, J ) 9.4 Hz, J ) 5.0 Hz, 1 H), 4.14 (t, J ) 9.4 Hz,
1 H), 4.38 (dd, J ) 9.4 Hz, J ) 5.0 Hz, 1 H), 7.23-7.27 (m, 5 H);
13C NMR (CDCl3) δ -0.7 (q), 26.2 (q), 45.1 (s), 55.1 (d), 66.1 (t),
127.2 (d), 128.4 (d), 128.6 (d), 136.3 (s), 214.1 (s). Anal. Calcd
for C16H26O2Si (278.5): C, 69.01; H, 9.41. Found: C, 69.04; H,
9.65.
Exp er im en ta l Section
Gen er a l. For general remarks, see ref 15. Solvents (P )
pentane, TBME ) tert-butyl methyl ether) used for chromatog-
raphy were distilled prior to use. The oxetanes 4 and 13 were
prepared as described previously.7,11 13C NMR multiplicities were
obtained by DEPT experiments.
Gen er a l P r oced u r e for Lew is Acid Ca ta lyzed Rea r -
r a n gem en ts. A solution of 1.0 mmol of oxetane was dissolved
in 5 mL of the corresponding solvent and the mixture was cooled
to -78 °C. Lewis acid (3.0 mmol) was slowly added to the
vigorously stirred solution. After the addition was complete the
mixture was warmed to the temperature indicated in Table 1.
The reaction was quenched with 5 mL of water and the aqueous
layer was extracted with CH2Cl2 (3 × 20 mL). The combined
organic layers were washed with a saturated aqueous NaHCO3
solution (5 mL) and with brine (10 mL). After drying over MgSO4
and filtration, the solvent was removed and the residue was
analyzed by GLC and 1H NMR spectroscopy (see Table 1). In
(2R S ,4R S )-2-H yd r oxy-5,5-d im e t h yl-4-p h e n yl-3-h e xa -
n on e (13). Following the general procedure, 1.0 mmol of oxetane
12 (293 mg) and 3.0 mmol of TiCl4 (570 mg, 330 µL) were allowed
to react in 5 mL of ether at -78 °C for 10 min. Workup and
subsequent flash chromatography (P/TBME/NEt3 ) 95/4/1 f 85/
14/1) gave 104 mg (47%) of ketone 13 as a colorless oil. Rf )
1
0.23 (P/TBME ) 9/1); H NMR (CDCl3) δ 1.01 (s, 9 H), 1.38 (d,
J ) 7.1 Hz, 3 H), 3.42 (d, J ) 5.5 Hz, 1 H), 3.74 (s, 1 H), 4.04
(dq, J ) 7.1 Hz, J ) 5.5 Hz, 1 H), 7.15-7.18 (m, 2 H), 7.26-7.31
(m, 3 H); 13C NMR (CDCl3) δ 20.1 (q), 28.1 (q), 34.4 (s), 63.0 (d),
71.7 (d), 127.5 (d), 128.3 (d), 130.3 (d), 134.7 (s), 212.5 (s). Anal.
Calcd for C14H20O2 (220.3): C, 76.33; H, 9.15. Found: C, 76.00;
H, 9.15.
5-Hyd r oxy-2,2-d im eth yl-4-p h en yl-3-h exa n on e (14). Fol-
lowing the general procedure, 1.0 mmol of oxetane 12 (293 mg)
and 3.0 mmol of AlMeCl2 (3.0 mL of a 1 M solution in hexane)
were allowed to react in 5 mL of toluene at -78 °C for 18 h.
Workup and subsequent flash chromatography (P/TBME/NEt3
) 95/4/1 f 85/14/1) gave 145 mg (66%) of the ketones 14a and
14b (14a /14b ) 35/65) as a yellowish solid. The diastereoisomers
were not separable. Rf ) 0.12 (P/TBME ) 9/1). Anal. Calcd for
(11) Bach, T. Liebigs Ann. Chem. 1995, 855.
(12) Heathcock, C. H. In Asymmetric Synthesis; Morrison, J . D., Ed.;
Academic Press: Orlando, FL, 1984; Vol. 3 B, p 115.
(13) (a) Heathcock, C. H.; Pirrung, M. C.; Sohn, J . E. J . Org. Chem.
1979, 44, 4294. (b) Hoffmann, R. W.; Weidmann, U. Chem. Ber. 1985,
118, 3980.
(14) Kotila, S.; J a¨ntti, A.; Penttinen, S.; Bach, T. Acta Crystallogr.
Sect. C 1996, C52, 1722.
(15) Bach, T.; Schro¨der, J . J . Org. Chem. 1999, 64, 1265.
C14H20O2 (220.3): C, 76.33; H, 9.15. Found: C, 75.93; H, 9.53.

8044 J . Org. Chem., Vol. 64, No. 21, 1999
Notes
(4RS,5RS)-Diastereoisomer (14a ): 1H NMR (CDCl3) δ 0.94
(d, J ) 6.3 Hz, 3 H), 0.99 (s, 9 H), 2.50 (s, 1 H), 3.96 (d, J ) 8.7
Hz, 1 H), 4.19-4.30 (m, 1 H), 7.15-7.26 (m, 5 H, arom. H); 13C
NMR (CDCl3) δ 20.2 (q), 26.7 (q), 45.4 (s), 61.7 (d), 70.6 (d), 127.3
(d), 128.7 (d), 129.4 (d), 136.3 (s), 216.4 (s).
Ack n ow led gm en t. This work was generously sup-
ported by the Deutsche Forschungsgemeinschaft (Ba
1372/1-2), and by the Fonds der Chemischen Industrie.
Su p p or tin g In for m a tion Ava ila ble: A table of all data
recorded and complete spectral data for compounds 5b, 6b,
13 and 14 (IR, NMR assignments, MS). This material is
available free of charge via the Internet at http://pubs.acs.org.
(4RS,5SR)-Diastereoisomer (14b): 1H NMR (CDCl3) δ 0.99
(s, 9 H), 1.05 (d, J ) 6.2 Hz, 3 H), 2.44 (s, 1 H), 3.94 (d, J ) 6.7
Hz, 1 H), 4.19 (virt. quintet, J ) 6.4 Hz, 1 H), 7.15-7.26 (m, 5
H); 13C NMR (CDCl3) δ 20.7 (q), 26.5 (q), 45.4 (s), 59.5 (d), 69.7
(d), 127.5 (d), 128.7 (d), 129.4 (d), 135.1 (s), 216.4 (s).
J O990802G