stereochemistry

CARLO THILGEN, ISABELLE GOSSE, AND FRANÇOIS DIEDERICH 27
HO
HO
T
T = tether
EtO2CCH2COCl, C5H5N, CH2Cl2, 0-20°C, 60-70%
O
O
O
O
O
O
O
O
T
C60, DBU, I2, PhMe, 20°C
O
O
O
O
O O
Scheme 1.3. General synthetic scheme for the regio- and stereoselective macrocyclization of
C60 by Bingel addition of tethered bis-malonates.
these syntheses used the versatile Bingel type macrocyclization between C60
and a bis-malonate in a double nucleophilic cyclopropanation (Scheme 1.3).
Using a 1,10-phenanthroline-2,9-diylbis[(4-phenyl)methyl] unit as a tether
in such a reaction, C1-symmetric bis-adduct (±)-28 is obtained as the main
product, together with C2-symmetric bis-adduct (±)-29 (Figure 1.15). 125 In
addition to the inherently chiral trans-3 addition pattern, both compounds
include two stereogenic centers represented by the methano bridge C-atoms.
Because of the steric constraints imposed by the tether, however, these centers
cannot adopt all possible combinations of configurations: Depending on the
relative orientation of the ethyl ester residues at the methano-C-atoms (=
bridgeheads of a macropolycyclic structure including the tether as one of the
bridges), one distinguishes between the out,out ((±)-29) andthein-out ((±)-
28) diastereoisomers. The theoretically possible third form, the in-in isomer
was not observed. 125 Because of the additional configurational freedom, two
untethered, e type methano bridges with two different achiral substituents at
each methano-C-atom, 126 can, on the other hand, exist as two diastereoisomeric
pairs of enantiomers.
Transition to shorter tethers led to the formation of the e addition pattern in
(±)-30, as well as in a similar bis-adduct containing a p-xylylene tether (both
obtained next to the trans-4 adducts), 117,125 in a tethered bis-β-ketoester-adduct
of C60, 127 and in (±)-31 (Figure 1.15). 125 Although the e addition pattern itself
is not chiral, (±)-30 contains a stereogenic center in one of the two methano
bridges which are constitutionally heterotopic. The stereogenic center in (±)-
30 results from the malonate addition to the eface bond 104 and lowers the
overall symmetry of the molecule to C1 in comparison to the Cs-symmetry
of an e bis-adduct with C2v-symmetric addends (cf. Figure 1.12b). 102 The
steric constraints in (±)-30 and in similar (vide infra) compounds require the
tether to adopt a helical conformation which adds another (conformational)
chiral unit to the molecule. The helicity of the tether depends on the configuration
of the stereogenic center in the methano bridge. In C1-symmetric
T
O
O