The effect of open and closed structures of titanocenes on the control of diastereoselectivity of radical reactions

Article abstract of DOI:10.1021/om0342208

A comparison of the X-ray structures of dichlorobis{(1-methylcyclohexyl)-η⁵-cyclopentadienyl}-titanium (1) and dichlorobis{(1-butyl-1-methylbutyl)-η⁵-cyclopentadienyl}titanium (2), containing tertiary alkyl substituents, and the known complex dichlorobis(cyclohexyl-η⁵-cyclopentadienyl)titanium (3), with two cyclohexyl hexyl substituents, has been carried out. The structural features are of relevance for the understanding of activity and selectivity of these complexes in diastereoselective additions of acrylates to β-titanoxy radicals and for the design of novel catalysts.

Full text of DOI:10.1021/om0342208

1168
Organometallics 2004, 23, 1168-1171
Th e Effect of Op en a n d Closed Str u ctu r es of Tita n ocen es
on th e Con tr ol of Dia ster eoselectivity of Ra d ica l
Rea ction s
Andreas Gansa¨uer,* Bjo¨rn Rinker, Andriy Barchuk, and Martin Nieger
Kekule´-Institut fu¨r Organische Chemie und Biochemie der Rheinischen
Friedrich-Wilhelms-Universita¨t Bonn, Gerhard-Domagk-Strasse 1, D-53121 Bonn, Germany
Received October 8, 2003
Summary: A comparison of the X-ray structures of
dichlorobis{(1-methylcyclohexyl)-η5-cyclopentadienyl}-
titanium (1) and dichlorobis{(1-butyl-1-methylbutyl)-η5-
cyclopentadienyl}titanium (2), containing tertiary alkyl
substituents, and the known complex dichlorobis(cyclo-
hexyl-η5-cyclopentadienyl)titanium (3), with two cyclo-
hexyl substituents, has been carried out. The structural
features are of relevance for the understanding of activity
and selectivity of these complexes in diastereoselective
additions of acrylates to â-titanoxy radicals and for the
design of novel catalysts.
In tr od u ction
The control of selectivity in both free radical reactions
and metal-mediated or -catalyzed radical reactions by
the action of external reagents has attracted consider-
able interest recently.1 Our catalytic approach uses
titanocene complexes as electron transfer reagents and
collidine hydrochloride (Coll*HCl) as mediator for ca-
talysis in reductive epoxide openings to yield â-titanoxy
radicals.2 In this manner the first enantioselective
radical generation was realized3 by the use of menthol-
derived titanocene complexes.4 The method is based on
a reaction stoichiometric in Cp2TiCl2 that was intro-
duced by Nugent and RajanBabu.5
F igu r e 1. Synthesis of the novel titanocenes 1 and 2 and
an improved synthesis of 3.
substituents and with complex 36 containing the sec-
ondary cyclohexyl group in diastereoselective radical
additions and correlate the results to the structures
obtained by X-ray crystallography. These structures can
be used as models for catalyst conformation in solution,
as we have demonstrated earlier for other titanocenes.7
In this note we describe our results employing the
titanocenes 1 and 2 with extremely bulky tertiary alkyl
Resu lts a n d Discu ssion
* Corresponding author. Tel: +49/228-732800. Fax: +49/228-
735683. E-mail: andreas.gansaeuer@uni-bonn.de.
Syn th esis a n d Str u ctu r es of th e Tita n ocen es.
Complexes 1 and 2 were synthesized by addition of MeLi
to known fulvenes8 and ensuing in situ metalation of
the generated cyclopentadienyl anion with TiCl4. The
yield of 2 was lower because of its distinctly higher
solubility in ethereal solvents caused by the long alkyl
groups. For the preparation of 3,6 it proved to be
advantageous to isolate the cyclopentadiene obtained by
LiAlH4 reduction of the fulvene. After deprotonation
with nBuLi metalation with TiCl4 proceeded eventlessly.
The in situ method yielded 3 in only 23% yield. The
preparations are summarized in Figure 1.
(1) (a) Renaud, P.; Gerster, M. Angew. Chem. 1998, 110, 2704;
Angew. Chem., Int. Ed. 1998, 37, 2562. (b) Roberts, B. P. Chem. Soc.
Rev. 1999, 28, 35. (c) Sibi, M. P.; Porter, N. A. Acc. Chem. Res. 1999,
32, 163. (d) Gansa¨uer, A.; Bluhm, H. Chem. Rev. 2000, 100, 2771. (e)
Sibi, M. P.; Manyem, S.; Zimmerman, J . Chem. Rev. 2003, 103, 3263.
(2) (a) Gansa¨uer, A.; Pierobon, M.; Bluhm, H. Angew. Chem. 1998,
110, 107; Angew. Chem., Int. Ed. 1998, 37, 101. (b) Gansa¨uer, A.;
Bluhm, H. Chem. Commun. 1998, 2143. (c) Gansa¨uer, A.; Bluhm, H.;
Pierobon, M. J . Am. Chem. Soc. 1998, 120, 12849. (d) Gansa¨uer, A.;
Pierobon, M. Synlett 2000, 1357. (e) Gansa¨uer, A.; Pierobon, M.; Bluhm,
H. Synthesis 2001, 2500. (f) Gansa¨uer, A.; Pierobon, M.; Bluhm, H.
Angew. Chem. 2002, 114, 3341; Angew. Chem., Int. Ed. 2002, 41, 3206.
(3) (a) Gansa¨uer, A.; Lauterbach, T.; Bluhm, H.; Noltemeyer, M.
Angew. Chem. 1999, 111, 3112; Angew. Chem., Int. Ed. 1999, 38, 2909.
(b) Gansa¨uer, A.; Bluhm, H.; Lauterbach, T. Adv. Synth. Catal. 2001,
343, 785. (c) Gansa¨uer, A.; Bluhm, H.; Rinker, B.; Narayan, S.; Schick,
M.; Lauterbach, T.; Pierobon, M. Chem. Eur. J . 2003, 9, 531. For a
review see: Gansa¨uer, A.; Narayan, S. Adv. Synth. Catal. 2002, 344,
465.
(4) (a) Cesarotti, E.; Kagan, H. B.; Goddard, R.; Kru¨ger, C. J .
Organomet. Chem. 1978, 162, 297. (b) Halterman, R. L.; Vollhardt, K.
P. C. Organometallics 1988, 7, 883.
(5) (a) Nugent, W. A.; RajanBabu, T. V. J . Am. Chem. Soc. 1988,
110, 8561. (b) RajanBabu, T. V.; Nugent, W. A. J . Am. Chem. Soc. 1989,
111, 4525. (c) RajanBabu, T. V.; Nugent, W. A.; Beattie, M. S. J . Am.
Chem. Soc. 1990, 112, 6408. (d) RajanBabu, T. V.; Nugent, W. A. J .
Am. Chem. Soc. 1994, 116, 986.
Crystals of 1 and 2 suitable for X-ray crystallography
were obtained by slow evaporation of concentrated
solutions in CH2Cl2. These structures and that of 3 are
(6) (a) Huang, Q.; Qian, Y.; Tang, Y.; Chen, S. J . Organomet. Chem.
1988, 340, 179. (b) Kapon, M.; Reisner, G. M. J . Organomet. Chem.
1989, 367, 77.
(7) Gansa¨uer, A.; Bluhm, H.; Pierobon, M.; Keller, M. Organome-
tallics 2001, 20, 914.
(8) Stone, K. J .; Little, R. D. J . Org. Chem. 1984, 49, 1849.
10.1021/om0342208 CCC: $27.50 © 2004 American Chemical Society
Publication on Web 01/20/2004

Notes
Organometallics, Vol. 23, No. 5, 2004 1169
Ta ble 2. Cr ysta llogr a p h ic Da ta a n d Su m m a r y of
Da ta Collection a n d Refin em en t for 1 a n d 2
1
2
formula
dimens,mm
cryst syst
space group
unit cell dimens
C
24H34Cl2Ti
C30H50Cl2Ti
0.50 × 0.30 × 0.15
monoclinic
C2/c (No.15)
a ) 30.8243(7) Å,
R ) 90°
0.60 × 0.30 × 0.10
monoclinic
C2/c (No.15)
a ) 23.7298(4) Å,
R ) 90°
b ) 6.5506(1) Å,
â ) 93.429(2)°
c ) 13.7888(2) Å,
γ ) 90°
b ) 6.7389(1) Å,
â ) 93.001(1)°
c ) 13.9847(3) Å,
γ ) 90°
V, Å3
2139.55(6)
4
2900.94(10)
4
Z
F
cacl, g cm-3
1.370
0.657
936
1.212
0.495
1144
µ (Mo KR), mm-1
F(000)
diffractometer
radiation
λ, Å
T, K
Nonius KappaCCD
Mo KR
0.71073
123(2)
50
max 2θ, deg
no. of data
no. of unique data
no. of unique data
[I > 2σ(I)]
no. of variables
R(F)a
50
20 419
1891
1797
7587
2539
2238
123
0.0229
0.0653
150
0.0254
0.0720
wR2(F2)
a
For I > 2σ(I).
tant for an unhindered binding of substrates. For the
following discussion we introduce the expression “open
structure” for this arrangement.
In complex 3 the cyclohexyl groups are positioned
between the chlorine ligands. Compared to 1 and 2
approach of substrates to the titanium center is thus
hindered. We therefore call this alignment “closed
structure”.
Ap p lica tion s in Ra d ica l Ad d ition Rea ction s. We
investigated the use of the three titanocenes in the
opening and addition reactions of cyclopentene, cyclo-
hexene, cycloheptene, and norbornene oxide 4, 6, 8, and
10. Although the values for the diastereoselectivity
given in the figures refer to the isolated material after
chromatography, these values did not deviate signifi-
cantly ((3%) from the ones obtained by analysis of the
crude mixtures in several cases examined. The workup
procedures also do not affect the isolated yields of the
products as has already been established before.2,3
Usually, in these reactions starting material is com-
pletely consumed within less than 6 h (TLC, GC
analysis). The extended reaction times shown here have
been used for the sake of convenience (overnight reac-
tions) and do not affect the outcome of the transforma-
tions (yields, selectivities). The byproducts formed con-
sist mainly of the chlorohydrins generated by Lewis
acid- or acid-mediated ring opening via SN reactions.
This pathway is becoming more relevant when catalyst
activity is reduced by bulky ligands.
F igu r e 2. X-ray structures of 1, 2, and 3.
Ta ble 1. Selected Bon d Len gth s [Å] a n d An gles
[d eg] for 1 a n d 2 (esd ’s in p a r en th eses)
dist. or angle
1
2
Ti-Cl
2.3724(4)
2.3742(4)
Cl-Ti-Cl
C5-Ti
91.700(18)
2.4843(12)
140.34(6)
1.5247(17)
125.44(11)
-11.69(17)
92.092(19)
2.4867(12)
137.78(6)
1.5251(18)
124.20(12)
-136.36(14)
#1
C5-Ti-C5
C5-C6
C4-C5-C6
C1-C5-C6-CH3
shown in Figure 2. Selected data for 1 and 2 are
presented in Table 1. Crystal data are presented in
Table 2.
In both complexes 1 and 2 the bulky alkyl groups are
positioned laterally behind the chlorine ligands and
point away from each other. The backside of the complex
is completely blocked by the cyclopentadienyl ligand and
the alkyl groups in both cases. This is indicated by a
close contact between the only hydrogens (HA and HB)
in 1 and 2 shown in Figure 2 (235 and 236 pm,
respectively). It should be noted that due to steric
interactions the tertiary carbons are not, as one would
expect, placed in the plane of the cyclopentadienes. The
overall orientation of the ligands results in a fairly open
front space around the chlorine ligands that is impor-
In the opening of 4 all complexes exhibited similar
degrees of diastereoselectivity. The yields with 1 and 2
were essentially the same (61% and 63%), whereas 3
resulted in a better result (72%), as depicted in Figure
3.
It seems that complexes with an “open” and a “closed”
structure are suitable for binding of the sterically
undemanding 4 containing the essentially flat cyclo-

1170 Organometallics, Vol. 23, No. 5, 2004
Notes
F igu r e 3. Titanocene-catalyzed opening of 4.
F igu r e 6. Titanocene-catalyzed opening of 10.
The complexes with the “open structure” give much
better yields of 11 than 3. This constitutes a clear
indication of a wider binding pocket resulting from the
lateral positioning of the bulky substituents. Moreover,
3 is distinctly less able to override the high inherent
cis-selectivity of the norbornene system than 1 and 2.
Thus, as in the case of 8, the reagent control exercised
by 3 is rather limited and essentially the same as for
Cp2TiCl2.2c
Su m m a r y
F igu r e 4. Titanocene-catalyzed opening of 6.
In summary, we have demonstrated that an “open
structure” of potential titanocene catalysts for epoxide
opening via electron transfer and ensuing addition to
an acrylate is to be preferred over a “closed structure”
concerning both yield and diastereoselectivity of the
overall process except for sterically unhindered cases.
For complexes possessing the “open structure” care
has to be taken in avoiding substituents on the cyclo-
pentadienyl ligand that are too bulky. In these cases
substrate binding is retarded and accordingly low yields
of the desired product will be obtained. However, control
of diastereoselectivity in these cases is usually good to
excellent.
F igu r e 5. Titanocene-catalyzed opening of 8.
Exp er im en ta l Section
pentane unit. Also, control of diastereoselectivity in the
resulting â-titanoxy cyclopentyl radical addition to yield
5 is efficient in all cases.
As shown in Figure 4 the situation changes slightly
for the opening of 6. Here, only 1 gives an acceptable
yield of the desired product 7. Both the extremely bulky
ligand in 2 and the “closed structure” in 3 prevent
efficient binding of the substrate and result in decreased
yields. Diastereoselectivity is in the usual range for
â-substituted cyclohexyl radicals.9
This trend is becoming even more pronounced in the
opening of 8 (Figure 5). In this case both 2 and 3 result
in poor yields. Additionally control of diastereoselectivity
is low for 3. It seems that in the “closed structure” an
efficient chirality transfer to more distant regions of the
metal-bound radical necessary for high selectivity as
observed for 1 and 2 is impossible.
In the case of norbornene oxide (10) the epoxide is in
close proximity to one of the hydrogens of the methylene
bridge. Thus, binding of catalysts to 10 must be con-
sidered as sterically difficult. This notion is borne out
experimentally by the very low yield in the reaction with
3 as shown in Figure 6.
X-r a y Cr ysta llogr a p h ic Stu d ies of 1 a n d 2. The struc-
tures were solved by Patterson methods (SHELXS97).10 The
non hydrogen atoms were refined anisotropically on F2
(SHELXL-97).11 H atoms were refined using a riding model.
An absorption correction was applied to 2. Further details are
given in Table 2.
Gen er a l P r oced u r es. The fulvenes were prepared accord-
ing to literature procedures.8 All reactions were carried out
using dry THF (freshly distilled from K) under an argon
atmosphere. Collidine hydrochloride was dried by gentle
heating under vacuum prior to use. Flash chromatography was
carried out according to the procedure of Still.12
P r ep a r a tion of 1. Cyclopenta-2,4-dienylidenecyclohexane
(5.84 g, 40 mmol) was dissolved in diethyl ether (40 mL) at 0
°C, and a solution of MeLi (1.5 M, 25.3 mL, 38 mmol) was
added over 30 min. After stirring for 2 h the mixture was added
to a solution of TiCl4 (3.22 g, 17 mmol) in diethyl ether (40
mL). The suspension was stirred at 0 °C for 2 and 16 h at
room temperature. The reaction was quenched by addition of
HCl (1 M, 40 mL, containing NaCl, 4.00 g) and stirring for 30
min. After filtration through Celite the residue was washed
with HCl (1 M, 100 mL). The red precipitate on top of the
(10) Sheldrick, G. M. SHELXL-97. Acta Crystallogr. 1990, A46, 467.
(11) Sheldrick, G. M. SHELXL-97; Universita¨t Go¨ttingen: FRG,
1997.
(9) Kirsch, A.; Lu¨ning, U.; Kru¨ger, O. J . Prakt. Chem. 1999, 341,
649.
(12) Still, W. C.; Kahn, A.; Mitra, A. J . Org. Chem. 1978, 43, 2923.

Notes
Organometallics, Vol. 23, No. 5, 2004 1171
Celite was dissolved in CH2Cl2 (800 mL), the solution was dried
(MgSO4), and the volatiles were removed in a vacuum to yield
1 (5.36 g, 12 mmol) as a red solid (71%). Crystals suitable for
crystallography were grown by slow evaporation of a concen-
trated solution in CH2Cl2. Mp: 223-225 °C. 1H NMR (400
MHz, CDCl3): δ 6.57 (t, 3J ) 4J ) 2.7 Hz, 2H), 6.44 (t, 3J ) 4J
) 2.7 Hz, 2H), 1.18-1.76 (m, 10H), 1.34 (s, 3H). 13C NMR (100
MHz, CDCl3): δ 150.5, 120.2, 117.2, 38.7, 37.6, 26.1, 23.8, 22.3.
Ca ta lyst 2. According to the GP, 2 (106 mg) and 4 (168 mg,
2.00 mmol) for 48 h gave 5 (270 mg, 1.26 mmol) as a 92:8
mixture of diastereoisomers (63%) by SiO2 chromatography
(CH/EE ) 84:16).
Ca ta lyst 3. According to the GP, 3 (82.7 mg) and 4 (168
mg, 2.00 mmol) for 48 h gave 5 (308 mg, 1.44 mmol) as a 96:4
mixture of diastereoisomers (72%) by SiO2 chromatography
(CH/EE ) 84:16).
MS (70 eV, EI): m/z (%) 440 (16), 405 (38), 279 (100), 243 (5),
P r ep a r a tion of 7 (F igu r e 4):3c Ca ta lyst 1. According to
the GP, 1 (88.3 mg), 6 (196 mg, 2.00 mmol), and tert-butyl
acrylate (257 mg, 2.00 mmol) for 48 h gave 7 (308 mg, 1.34
mmol) as a 70:30 mixture of diastereoisomers (67%) by SiO2
chromatography (CH/EE ) 97:3-95:5).
227 (18). HRMS (70 eV, EI): calcd for (C24H34TiCl2 ) M+•
)
+•
438.1564, found 438.1560. IR (KBr): ν 3085, 2925, 1475, 1390,
1050, 910, 830 cm-1. Anal. Calcd for C24H34TiCl2 (441.31): C,
65.32; H, 7.77. Found: C, 65.06; H, 7.66.
Ca ta lyst 2. According to the GP, 2 (106 mg), 6 (196 mg,
2.00 mmol), and tert-butyl acrylate (257 mg, 2.00 mmol) for
48 h gave 7 (238 mg, 1.04 mmol) as a 74:26 mixture of
diastereoisomers (52%) by SiO2 chromatography (CH/EE ) 97:
3-95:5).
P r ep a r a tion of 2. The preparation was carried out as for
1 with 5-(1-butylpentylidene)cyclopenta-1,3-diene13 (13.5 g, 71
mmol), MeLi (1.6 M, 50 mL, 80 mmol), and TiCl4 (6.45 g, 34
mmol) to yield 3 (4.40 g, 8.3 mmol) as a red solid (24%). Mp:
1
3
157-159 °C. H NMR (300 MHz, CDCl3): δ 6.53 (t, J ) 4J )
3
2.6 Hz, 2H), 6.45 (t, J ) 4J ) 2.6 Hz, 2H), AB signal (δ 1.68,
Ca ta lyst 3. According to the GP, 3 (82.7 mg) and 6 (168
mg, 2.00 mmol) for 48 h gave 7 (234 mg, 1.02 mmol) as a 70:
30 mixture of diastereoisomers (51%) by SiO2 chromatography
(CH/EE ) 97:3-95:5).
2
3
δ 1.54, J ) 13.4 Hz, additionally split by J ) 11.8, 11.5, 5.2,
3
5.1 Hz, 4H), 1.31 (s, 3H), 0.90-1.29 (m, 8H), 0.85 (t, J ) 7.3
Hz, 6H). 13C NMR (75 MHz, CDCl3): δ 149.5, 120.3, 117.5,
40.4, 40.2, 26.2, 23.9, 23.5, 14.2. MS (70 eV, EI): m/z (%) 493
(100), 456 (4), 400 (15), 323 (19). IR (KBr): ν 3080, 2925, 1465,
1375, 1045, 905, 830 cm-1. Anal. Calcd for C30H50TiCl2
(529.50): C, 68.05; H, 9.52. Found: C, 68.00; H, 9.56.
P r ep a r a tion of 9 (F igu r e 5):3c Ca ta lyst 1. According to
the GP, 1 (44.1 mg, 0.10 mmol) and 8 (112 mg, 1.00 mmol) for
20 h gave 9 (140 mg, 0.58 mmol) as a 93:7 mixture of
diastereoisomers (58%) by SiO2 chromatography (CH/EE ) 94:
6-90:10).
P r ep a r a tion of 3. Cyclohexylcyclopentadiene (13.5 g, 91
mmol) was dissolved in diethyl ether (500 mL), cooled to 0 °C,
and deprotonated with nBuLi (2.45 M, 37.1 mL, 91 mmol) over
a period of 30 min. After stirring for 2 h the mixture was added
to TiCl4 (4.9 mL, 44 mmol) dissolved in diethyl ether (200 mL)
at 0 °C. After stirring for 16 h at reflux and cooling to 0 °C
the reaction was quenched by the addition of aqueous HCl (1
M, 100 mL). The solid was filtered off and washed with
pentane and combined with the solid obtained by reextracting
the aqueous layer with CH2Cl2 (100 mL) to give 36a (10.5 g, 25
mmol) as a red solid (58%).
Ca ta lyst 2. According to the GP, 2 (53 mg, 0.10 mmol) and
8 (112 mg, 1.00 mmol) for 20 h gave 9 (117 mg, 0.48 mmol) as
a 93:7 mixture of diastereoisomers (48%) by SiO2 chromatog-
raphy (CH/EE ) 94:6-90:10).
Ca ta lyst 3. According to the GP, 3 (88.3 mmol) and 8 (112
mg, 1.00 mmol) for 16 h gave 9 (85.4 mg, 0.35 mmol) as a 82:
18 mixture of diastereoisomers (35%) by SiO2 chromatography
(CH/EE ) 94:6-90:10).
P r ep a r a tion of 11 (F igu r e 6):3c Ca ta lyst 1. According to
the GP, 1 (44.1 mg, 0.10 mmol) and 10 (110 mg, 1.00 mmol)
for 60 h gave 11 (136 mg, 0.56 mmol) as a 49:51 mixture of
diastereoisomers (56%) by SiO2 chromatography (petrol ether
(PE)/tert-butyl methyl ether (MTBE) ) 80:20-75:25).
Ca ta lyst 2. According to the GP, 2 (53 mg, 0.10 mmol) and
10 (110 mg, 1.00 mmol) for 48 h gave 11 (131 mg, 0.54 mmol)
as a 51:49 mixture of diastereoisomers (54%) by SiO2 chro-
matography (PE/MTBE ) 80:20-75:25).
Ca ta lyst 3. According to the GP, 3 (41.3 mg, 0.10 mmol)
and 10 (110 mg, 1.00 mmol) for 60 h gave 11 (46 mg, 0.19
mmol) as a 65:35 mixture of diastereoisomers (20%) by SiO2
chromatography (PE/MTBE ) 80:20-75:25).
Gen er a l P r oced u r e (GP ) for Tita n ocen e-Ca ta lyzed
Ep oxid e Op en in g a n d Ad d ition to Acr yla tes. To a suspen-
sion of collidine hydrochloride (394 mg, 2.50 mmol) in THF
(10 mL) was added the titanocene (0.10 mmol, 10 mol %), zinc
(131 mg, 2.00 mmol), acrylic acid tert-butyl ester (385 mg, 3.00
mmol), and the epoxide (1.00 mmol). After stirring at room
temperature for 16 h tert-butyl methyl ether (30 mL) was
added, and the mixture was filtered and washed with water
(30 mL). The aqueous phase was extracted with CH2Cl2 (2 ×
20 mL). The combined organic layers were washed with HCl
(2 M, 20 mL), saturated NaHCO3 (20 mL), brine (20 mL), and
water (20 mL). After drying (MgSO4) the volatiles were
removed in a vacuum and the crude product was purified by
chromatography over SiO2.
Ack n ow led gm en t. A.G. is indebted to the Fonds der
Chemischen Industrie for a Dozentenstipendium. We
thank the Deutsche Forschungsgemeinschaft (Ga 619/
1-2, Gerhard-Hess-Programm) for financial support of
our work.
P r ep a r a tion of 5 (F igu r e 3):3c Ca ta lyst 1. According to
the GP, 1 (88.3 mg) and 4 (168 mg, 2.00 mmol) for 48 h gave
5 (262 mg, 1.22 mmol) as a 95:5 mixture of diastereoisomers
(61%) by SiO2 chromatography (cyclohexane (CH)/ethyl acetate
(EE) ) 84:16).
Su p p or tin g In for m a tion Ava ila ble: Crystallographic
tables for 1 and 2. This material is available free of charge
via the Internet at http://pubs.acs.org.
(13) Kurata, H.; Ekinaka, T.; Kawase, T.; Oda, M. Tetrahedron Lett.
1993, 34, 3445.
OM0342208