Two-step synthesis of trans-2-arylcyclopropane carboxylates with 98-100% ee by the use of a phosphazene base

Article abstract of DOI:10.1002/(SICI)1521-3773(19980703)37:12<1689::AID-ANIE1689>3.0.CO;2-9

Only the axial diastereomer of sulfonium salts 2 are formed upon alkylation of 1. Deprotonation of 2 results in ylides, which allow the highly enantioselective cyclopropanation of Michael systems. The chirat auxiliary 1 is recovered and can be reused.

Full text of DOI:10.1002/(SICI)1521-3773(19980703)37:12<1689::AID-ANIE1689>3.0.CO;2-9

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[7] J. D. Corbett, Struct. Bonding (Berlin) 1997, 87, 157, and references
therein.
[8] T. A. Albright, J. K. Burdett, M. H.Whangbo, Orbital Interaction in
Chemistry, Wiley, New York, 1985.
[9] The spatial drawing of MO obitals was ploted with Molden v3.3
program. The Molden package was written by G. Schaftenaar, CAOS/
CAMM Center Nijmegen, Toernooiveld, Nijmegen, The Netherlands
1997.
[10] a) Z. Lin, I. D. Williams, Polyhedron 1996, 15, 3277; b) Z. Lin, M.-
F.Fan, Struct. Bonding (Berlin) 1997, 87, 35; c) Z. Xu, Z. Lin, Chem.
Eur. J. 1998, 4, 28.
[11] a) G. Igel-Mann, H. Stoll, H. Preuss, Mol. Phys. 1988, 65, 1321; b) D.
Andrae, U. Haeussermann, M. Dolg, H. Stoll, H. Preuss, Theor. Chim.
Acta 1990, 77, 123; c) W. Kuechle, M. Dolg, H. Stoll, H. Preuss, Mol.
Phys. 1991, 74, 1245.
[12] M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson,
M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A.
Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski,
J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayak-
kara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong,
J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S.
Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C.
Gonzalez, J. A. Pople, Gaussian, Inc., Pittsburgh PA, 1995.
propane with 99% ee by adding only 1 mol% of a chiral
copper catalyst,[5] an exotic and very hindered ester had to be
used to obtain a trans/cis ratio of 94/6. Starting from the
inexpensive and commercially available ethyl ester, low trans/
cis diastereoselectivities (trans/cis ꢀ 70/30) are obtained. Fur-
ther difficulties arise from the fact that diazo reagents have to
be used and that they have to be added slowly (over about
16 h for 0.02 mol) to avoid the formation of side products.[6, 7]
Therefore, for large-scale preparations the sulfur ylide
method might be more promising, as already stated by
Corey.[8] However, until now the chirality had only been
located on the olefinic moiety, and either yields[9] or diaster-
eomeric excesses[10] were low. In the method described here
the chirality is located at the sulfur center of the reagent,
which is recoverable and can thus be reused (Scheme 1).
Scheme 1. Synthesis of cyclopropanes 4a ± d. 2a: R ˆ Ph, Yˆ TfO; 2b:
R ˆ p-NCC6H4, Yˆ TfO; 2c: R ˆ p-tBuC6H4, Yˆ BF4; 2c': R ˆ p-
tBuC6H4, Yˆ TfO; 2d: R ˆ 2-naphthyl, Yˆ BF4; 4a: R ˆ Ph; 4b: R ˆ p-
NCC6H4; 4c: R ˆ p-tBuC6H4; 4d: R ˆ 2-naphthyl. B ˆ NaH, Et P2.[13]
Two-Step Synthesis of trans-2-Arylcyclo-
propane Carboxylates with 98 ± 100% ee
by the Use of a Phosphazene Base
Â
Arlette Solladie-Cavallo,* Ahn Diep-Vohuule, and
Thomas Isarno
(Arylmethyl)sulfonium salts 2a ± d were prepared in about
80 ± 85% yield from RCH2OH, Tf2O, and pyridine in
CH2Cl2 or from RCH2Br AgBF4. Only one diastereomer
was detected by 1H and 13C NMR spectroscopy, and the axial
position was assigned for the arylmethyl group in 2b ± d based
on a comparison with spectra of the known 2a.[12] The
corresponding ylides were generated in situ with either NaH
[11]
We found recently that oxathiane 1 (see Scheme 1) is a very
efficient chiral auxiliary that allows the preparation of various
pure trans-diarylepoxides in high yields ( ꢀ 85%) and with
high enantiomeric purities (98.5 ± 99.9%).[1] We report here
the extension of this method to the synthesis of disubstituted
cyclopropanes.
Since the first Simmons ± Smith reaction in 1958,[2] exten-
sive work has been devoted to the synthesis of enantiomeri-
cally enriched polysubstituted cyclopropanes. Pure trans- (or
pure cis-) disubstituted cyclopropanes with levels of enantio-
selectivity up to 93% have been obtained from trans- (or cis-)
olefins by the use of stoichiometric quantities of an external
chiral promoter and an excess of the preformed Zn(CH2I)2 ´
DME complex.[3] However, with catalytic amounts of a chiral
promoter only 90% ee could be achieved.[4] Although styrene
was converted into a trans-alkoxycarbonyl-substituted cyclo-
or EtN P(NMe2)2 N P(NMe2)3 (ªEt P2º)[13] as base (Ta-
bles 1 and 2). With NaH in THF at 308C full conversions
were achieved, as seen from the 1H NMR spectra of the crude
product, but 24 to 76 hours were required. Whereas cyclo-
propane 4a was isolated in high yield (83%) and with
complete enantioselectivity (100% ee), 4b and 4d were
obtained with slightly lower enantioselectivities ( ꢀ 96% for
4b and ꢀ 95% for 4d, Table 1). Only traces of the cis isomer
of crude 4d were detected, but about 15% of the cis isomer
was observed for 4b. Surprisingly, in the case of 4c, although
ˆ
ˆ
Table 1. Synthesis of cyclopropanes 4a ± d from 2a ± d in THFat 308C with NaH
as base.
Â
[*] Dr. A. Solladie-Cavallo, T. Isarno
 Â
Â
Â
Laboratoire de Stereochimie Organometallique associe au CNRS
Â
ECPM/Universite L. Pasteur
4
R
cylide [m] t[h] Crude product
trans/cis[a]
trans-4
Reisolat-
yield[%][b] ee[%][c] ed 1[%]
1 rue B. Pascal, F-67008 Strasbourg (France)
Fax: (‡33)3-88-61-65-31
a
b
c
C6H5
p-NCC6H4
p-tBuC6H4 0.23
2-naphthyl 0.19
0.22
0.23
24
48
95/5
80/20
83
62
50
78
100
95.8
60.8
94.8
89
89
80
80
E-mail: ascava@chimie.u-strasbg.fr
Dr. A. Diep-Vohuule
Stereochimie et Interactions (Bat. LR2-LR3)
ENS Lyon (France)
72 ꢁ 99/1
 Â
d
6
95/5
1
Supporting information for this article is available on the WWW [a] Determined by H NMR spectroscopy (200 MHz). [b] Not optimized. [c] De-
under http://www.wiley-vch.de/home/angewandte/ or from the au- termined by chromatography on a chiral phase (see ref. [16] and the supporting
thor.
information).
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the cis isomer was not detected, the enantiomeric purity was
only 61%.
cyclopropanes 4b ± d and 6 on the basis of the minus sign of
their optical rotations in ethanol; the 1R,2R isomer of the
known 4a is levorotatory in ethanol according to Walbor-
sky[17] and Evans.[18] The R,R absolute configurations (ob-
tained in all cases) are reasonably explained by the model
already proposed for the syntheses of mono- and diary-
lepoxides:[1, 12] The approach of the Michael acceptor occurs
opposite to the gem-dimethyl group (Figure 1). The very high
ee values observed (98 ± 100%) could well be due to the
In an attempt to improve these results and to shorten the
reaction time, we decided to generate more reactive ªnakedº
carbanions by using a phosphazene base. Et P2 (pKa ꢀ 30),[14]
is potentially able to provide a high concentration of ylide and
could be used in CH2Cl2. The results are summarized in
Table 2. As expected, full conversions were achieved in only
‡
participation of the salt present (Et P2H TfO or BF4 ) in a
Table 2. Synthesis of cyclopropanes 4a ± d from 2a,b,c',d in CH2Cl2 with Et P2 as
coordinative and/or electrostatic interaction in the transition
state leading to the cyclization.
base.
4
R
t[min] T[8C] cylide [m] trans/cis
trans-4
Config.
ee[%][a] [a]2D4 [b]
a
C6H5
15
15
30
50
30
50
30
30
50
0.05
0.25
0.05
0.25
0.05
0.05
0.25
95/5
96.7
97.8
99.2
98.8
98.9
84.4
94.8
232
339
R,R
R,R
94/6[a]
85/15
94/6[a]
96/4[a]
93/7
b
p-NCC6H4 30
30
p-tBuC6H4 30
2-naphthyl 30
30
c
d
245
215
R,R
R,R
95/5[a]
[a] Determined by gas chromatography on CP-Chirasil-DEX CB and by HPLC on
Chiralcel OJ for 4a, and by HPLC on Chiralcel OJ for 4b ± d (see ref. [16] and the
supporting information). [b] In ethanol.
Figure 1. Possible participation of the salt (symbolized by a charge within a
circle) in the mechanism of the cyclopropanation.
The chiral sulfur ylide derived from oxathiane 1 provides, in
15 to 30 minutes and with about 90% conversion, 2-
arylcyclopropane carboxylates (4a ± d, 6) with high enantio-
selectivities (98 ± 100% ee, 95% for 4d). It is worth noting
that these ylides also react smoothly with an enolizable ketone
to provide in high yield enantiomerically pure ketocyclopro-
panes of type 6, which can then be further derivatized.
Although used in stoichiometric amount, the chiral auxiliary
is recovered in high yield; we are now searching for a way to
avoid the presently necessary chromatographic separation. It
is also worth noting that the phosphazene base can be
15 to 30 minutes. The percentage of the cis isomer for 4b was
reduced to 6%. It is worth noting that in all cases, after
chromatographic separation from the starting oxathiane, the
cis isomers were not detected by 1H NMR spectroscopy
(200 MHz). The enantiomeric purities of 4a ± c ranged from
97.8 to 98.9%, and reached 94.8% for 4d.
The enolizable ketone 5 also reacted smoothly under the
same conditions to provide the corresponding cyclopropane 6
in high yield and with high enantioselectivity (98.5% ee,
Scheme 2). In an analogous manner acrolein led mainly to the
‡
recovered through precipitation of Et P2H TfO .
Experimental Section
The base (1 equiv) and the Michael acceptor (2 equiv) were successively
added (at the desired temperature) to a stirred solution of the sulfonium
salt (5 mmol) in anhydrous CH2Cl2 (20 mL). The smooth reaction was
monitored by thin-layer chromatography (TLC). After the reation was
complete, the mixture was poured into cold water, the aqueous phase was
extracted with CH2Cl2 (5 Â 10 mL), and the organic phases were combined
and dried over NaSO4. The cyclopropane derivative and the chiral auxiliary
were then separated by chromatography (silica gel, hexane/ether 95/5), and
the chiral auxiliary was reused. All compounds gave correct elemental
analyses.
2a: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.48 (m, 2H), 7.38 (m, 3H), 5.60
(d, 1H, 2J ˆ 12 Hz), 4.80 (d, 1H, 2J ˆ 12 Hz), 4.66 (s, 2H), 3.80 (td, 1H, 3J ˆ
10.5, 10.5, 4.5 Hz), 2.01 (m, 1H), 1.80 (m, 2H), 1.74 (s, 3H), 1.68 (s, 3H),
1.50 (m, 2H), 1.30 (q, 1H, 3J ˆ 11 Hz), 1.10 (m, 2H), 0.95 (d, 3H, 3J ˆ
6.5 Hz); 13C NMR (50 MHz, CDCl3, TMS): d ˆ 130.7, 71.0 (C), 130.1, 129.8,
127.0, 78.0, 43.0, 31.0 (CH), 58.0, 40.0, 36.0, 33.5, 24.0 (CH2), 23.5, 22.0, 21.0
(CH3), 120.7 (q, JCF ˆ 320 Hz).
Scheme 2. Reaction of 2a with acrolein and 5. The trans configuration at
the epoxide ring was proven by the coupling constant for the trans protons
(7: 3J ˆ 2.5 Hz;[1] 8: 3J ˆ 2 Hz); the configuration at the cyclopropane ring in
8 is unclear.
diastereomerically pure trans-epoxide 7 (66%), although the
epoxycyclopropane 8 was also observed (34%) as a single
diastereomer. The latter was generated by epoxidation of the
already formed cyclopropane.
2b: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.75 (AB system, 4H, 3J ˆ
8.5 Hz, Dn ˆ 15 Hz), 5.53 (d, 1H, 2J ˆ 12 Hz), 4.96 (d, 1H, 2J ˆ 12 Hz), 4.80
(AB system, 2J ˆ 13 Hz, Dn ˆ 13 Hz), 3.76 (td, 1H, 3J ˆ 10.5, 10.5, 4 Hz), all
other signals are very similar to those of 2a.
2c: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.45 (brs, 4H), 5.62 (d, 1H, 2J ˆ
12 Hz), 4.86 (d, 1H, 2J ˆ 12 Hz), 4.57 (s, 2H), 3.80 (td, 1H, 3J ˆ 10, 10,
4 Hz), all other signals are similar to those of 2a. The 1H and 13C NMR
In all the cases the trans structure was assigned to the major
1
compound with the help of H NMR spectroscopy.[15] The
enantiomeric purities were determined by gas chromatogra-
phy on a CP-Chirasil-DEX CB column in the case of 4a and 6
or by HPLC on a Chiralcel OJ column in the case of 4a ± d.[16]
The 1R,2R absolute configurations were assigned to trans-
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spectra of 2c' are identical to those of 2c with the exception of the signal for
the carbon atom of the anion CF3SO2 at d ˆ 120.7 (q, J ˆ 320 Hz).
aldehydic proton in formylphenylcyclopropane: P. Scribe, J. Wied-
mann, Bull. Soc. Chim. Fr. 1971, 2268.
[16] The enantiomeric purities were determined by Dr. V. Vinkovic in the
group of Prof. V. Sunjic at the Ruder Boskovic Institut of Zagreb.
[17] Y. Inouye, T. Sugita, H. M. Walborsky, Tetrahedron 1964, 20, 1695.
[18] D. A. Evans, K. A. Woerpel, M. J. Scott, Angew. Chem. 1992, 104, 439;
Angew. Chem. Int. Ed. Engl. 1992, 31, 430.
2d: 1H NMR (200 MHz, CD2Cl2, TMS): d ˆ 7.95 (m, 4H), 7.56 (m, 3H),
5.43 (d, 1H, 2J ˆ 12 Hz), 4.90 (d, 1H, 2J ˆ 12), 4.77 (AB system, 2J ˆ 13 Hz,
Dn ˆ 15 Hz), 3.79 (td, 1H, 3J ˆ 10.5, 10.5, 4.5 Hz), 2.10 (m, 1H), 1.90 (m,
2H), 1.76 (s, 3H), 1.74 (s, 3H), 1.6 (m, 2H), 1.4 (q, 1H, J ˆ 11 Hz), 1.1 (m,
2H), 1.0 (d, 3H, 3J ˆ 6.5 Hz); 13C NMR (50 MHz, CDCl3, TMS): d ˆ 134.1,
133.8, 124.1, 71.5 (C), 131.4, 130.6, 128.5, 128.3, 128.2, 127.7, 127.0, 78.7, 43.7,
31.4 (CH), 58.5, 40.4, 36.9, 34.3, 24.1 (CH2), 26.2, 22.0, 21.1 (CH3).
4a: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.27 (m, 3H), 7.13 (m, 2H), 4.18
(q, 2H, 3J ˆ 7), 2.53 (ddd, 1H, J ˆ 9.5, 6.5, 4 Hz), 1.91 (ddd, 1H, J ˆ 8.5, 5.5,
4.5 Hz), 1.61 (ddd, 1H, J ˆ 9.5, 5.5, 4.5 Hz), 1.32 (ddd, 1H, J ˆ 8.5, 6.5,
4.5 Hz), 1.29 (t, 3H, 3J ˆ 7 Hz); 13C NMR (50 MHz, CDCl3, TMS): d ˆ
173.5, 140.2 (C), 128.5, 126.5, 126.2, 26.3, 24.2, (CH), 60.7, 17.1 (CH2), 14.4
(CH3).
Multiple Coordination of Metal Atoms to
Arenes: The Coordination of Six Ruthenium
Atoms to Naphthalene-1,8-diyl in
4b: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.50 (d, 2H, 3J ˆ 8 Hz), 7.13 (d,
2H, 3J ˆ 8Hz), 4.15 (q, 2H, 3J ˆ 7 Hz), 2.49 (ddd, 1H, J ˆ 10, 6.5, 4 Hz), 1.91
(ddd, 1H, J ˆ 8.5, 5.5, 4 Hz), 1.62 (ddd, 1H, J ˆ 10, 5.5, 4.5 Hz), 1.29 (ddd,
[Ru6(m6-C10H6)(m3-PPh)(CO)14]**
3
1H, J ˆ 8.5, 6.5, 4.5 Hz), 1.22 (t, 3H, J ˆ 7 Hz).
4c: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.33 (d, 2H, 3J ˆ 8.5 Hz), 7.06 (d,
2H, 3J ˆ 8.5 Hz), 4.16 (q, 2H, 3J ˆ 7 Hz), 2.51 (ddd, 1H, J ˆ 9.5, 6.5, 4 Hz),
1.90 (ddd, 1H, J ˆ 8.5, 5, 4 Hz), 1.60 (ddd, 1H, J ˆ 9.5, 5, 4 Hz), 1.34 (ddd,
Antony J. Deeming* and Caroline M. Martin
Research on the incorporation of arenes into clusters has
largely been stimulated by their potential as models for
chemisorption on metal surfaces and by a wish to modify
arene structure and reactivity.[1, 2] The chemistry of benzene
with metal clusters has been developed extensively in recent
years,[3] and dominant modes of coordination are h6 coordi-
nation at a single metal atom, m3-h2,h2,h2 coordination over a
triangular cluster face, and combinations of these two modes
with s-M C bonding, as observed in phenyl and ortho-
phenylene (1,2-didehydrobenzene) systems. Several of these
modes resemble chemisorption of benzene on a (111) metal
surface or on step-sites on such surfaces.[4] In contrast,
however, relatively few clusters that contain more complex,
polycyclic arenes such as naphthalene and anthracene have
been reported. Studies of such compounds are often hindered
by the difficulty of introducing the polycyclic arene into the
coordination sphere of the cluster, and has, so far, centered on
mono- and binuclear species, with interactions through either
s bonds, as in [Fe(C10H7)4][LiOEt2]2,[5] or more commonly by
p complexation through h2, h4, h6, or bis-allylic h3:h3 inter-
actions, as in, for example, the compounds [Rh2(C5Me5)2(1,2-
h2-3,4-h2-C10H8)(PMe3)2],[6] [RhCp(h4-C14H10)],[7] [Ru(h4-
C8H12)(h6-C10H8)],[8] and [Fe2(CO)6(m-h3:h3-C14H10)].[9] One
of our recent objectives has therefore been to extend the
coordination chemistry of naphthalene and anthracene by the
combination of the s and p M C interactions that are present
in the above molecules, to allow extensive metalation of each
ring of the polycyclic arene.
3
1H, J ˆ 8.5, 6.5, 4 Hz), 1.32 (s, 9H), 1.29 (t, 3H, J ˆ 7 Hz).
4d: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.78 (m, 3H), 7.57 (s, 1H), 7.45
(m, 2H), 7.21 (dd, 1H, 3J ˆ 8.5, 4J ˆ 2 Hz), 4.19 (q, 2H, 3J ˆ 7 Hz), 2.69
(ddd, 1H, J ˆ 9.5, 6.5, 4.5 Hz), 2.01 (ddd, 1H, J ˆ 8.5, 5, 4.5 Hz), 1.67 (ddd,
1H, J ˆ 9.5, 5, 4.5 Hz), 1.47 (ddd, 1H, J ˆ 8.5, 6.5, 4.5 Hz), 1.30 (t, 3H, 3J ˆ
7 Hz); 13C NMR (50 MHz, CDCl3, TMS): d ˆ 173.5, 137.6, 133.5, 132.4 ( C),
128.3, 127.7, 127.5, 126.4, 125.6, 124.9, 124.6, 26.5, 24.3 (CH), 60.9, 17.2
(CH2), 14.4 (CH3).
6: 1H NMR (200 MHz, CDCl3, TMS): d ˆ 7.30 (m, 3H), 7.10 (m, 2H), 2.53
(ddd, 1H, J ˆ 9, 6.5, 4 Hz), 2.32 (s, 3H), 2.23 (ddd, 1H, J ˆ 8, 5, 4 Hz), 1.70
(ddd, 1H, J ˆ 9, 5, 4 Hz), 1.39 (ddd, 1H, J ˆ 8, 6.5, 4 Hz); 13C NMR
(50 MHz, CDCl3, TMS): d ˆ 207.0, 140.4 (C), 127.5, 126.5, 126.1, 33.0, 30.9,
29.1 (CH, CH3), 19.2 (CH2).
Received: December 4, 1997 [Z11229IE]
German version: Angew. Chem. 1998, 110, 1824 ± 1827
Keywords: asymmetric synthesis ´ cyclopropanes ´ cyclo-
propanations ´ phosphazene bases ´ ylides
Â
[1] A. Solladie-Cavallo, A. Diep-Vohuule, V. Sunjic, V. Vinkovic,
Tetrahedron: Asymmetry 1996, 7, 1783.
[2] H. E. Simmons, R. D. Smith, J. Am. Chem. Soc. 1958, 80, 5323.
[3] A. B. Charette, S. Prescott, C. Brochu, J. Org. Chem. 1995, 60, 1081.
[4] A. B. Charette, C. Brochu, J. Am. Chem. Soc. 1995, 117, 11367.
[5] D. A. Evans, K. A. Woerpel, M. M. Hinman, M. M. Faul, J. Am. Chem.
Soc. 1991, 113, 726.
[6] S. E. Denmark, S. P. OꢁConnor, J. Org. Chem. 1997, 62, 584.
[7] H. Fritschi, U. Leutenegger, A. Pfaltz, Helv. Chim. Acta 1988, 71, 1553.
[8] E. J. Corey, M. Jautelat, J. Am. Chem. Soc. 1967, 89, 3912.
[9] M. Calmes, J. Daunis, F. Escale, Tetrahedron: Asymmetry 1996, 7, 395.
[10] M. J. De Vos, A. Krief, Tetrahedron Lett. 1983, 24, 103.
[11] a) E. Vedejs, D. A. Engler, Tetrahedron Lett. 1976, 3487; b) E. Vedejs,
D. A. Engler, M. J. Mullins, J. Org. Chem. 1977, 42, 3109.
We have introduced polycyclic arenes into the clusters by
the thermal degradation of tertiary phosphanes in the
presence of metal compounds. The thermolysis of [Ru3(CO)12]
Â
[12] A. Solladie-Cavallo, A. Adib, M. Schmitt, J. Fischer, A. DeCian;
Tetrahedron: Asymmetry 1992, 3, 1597.
[*] Prof. A. J. Deeming
[13] Such phosphazene bases provide very reactive naked anions. One
Department of Chemistry, University College London
20 Gordon Street, London, WC1H0AJ (UK)
Fax: (‡44)171-380-7463
‡
could expect the large Et ± P2H cation to be less tightly associated to
its anion and to the ylide, thus allowing an easy access to the ylide.
Furthermore, it should be geometrically able to help form the complex
of the ylide and the Michael substrate in the transition state.
[14] R. Schwesinger, H. Schlemper, Angew. Chem. 1987, 99, 1212; Angew.
Chem. Int. Ed. Engl. 1987, 26, 1167.
[15] In all cases the CH2 protons of the ester groups are more shielded (due
to the phenyl effect) in the cis isomer (d ꢀ 3.85, CDCl3) than in the
trans isomer (d ꢀ 4.20, CDCl3). This was already observed for the
E-mail: a.j.deeming@ucl.ac.uk
C. M. Martin
University Chemical Laboratory
Cambridge, CB21EW (UK)
[**] This work is supported by the EPSRC and the University of London
Central Research Fund.
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