9
34
M. S. Corbett et al. / Tetrahedron Letters 44 (2003) 931–935
3
ity in the cycloadditions of 1a, aligns the relatively
acidic CꢀH bond of the stereogenic center with the
anthracene orbitals, thereby allowing for maximum s-
donation and enhanced reactivity in the cycloaddition
References
1. Taken from: Sanyal, A. Ph.D. Dissertation, Boston
University, 2002.
by raising the HOMOdiene
.
2. (a) Lasne, M.-C.; Ripoll, J.-L. Synthesis 1985, 121–143;
(
b) Rickborn, B. Org. React. 1998, 52, 1–393; (c) Rick-
The incoming dienophile would approach the anthra-
cene from the top face, anti to the hydrogen atom, and
syn to the methoxy or the trifluoromethyl substituents.
The lone electron pairs on the oxygen atom of the
carbonyl group of the incoming dienophile will experi-
ence repulsive electrostatic interactions with both the
highly electronegative fluorine atoms of the tri-
fluoromethyl group and the lone electron pairs on the
oxygen of the methoxy group. It is well known that the
effective negative charge for a trifluoromethyl group is
born, B. Org. React. 1998, 53, 223–629; (d) Lertvora-
chon, J.; Thebtaranonth, Y.; Thongpanchang, T.;
Thongyoo, P. J. Org. Chem. 2001, 66, 4692–4694; (e)
Nanjappan, P.; Czarnik, A. W. J. Am. Chem. Soc.
1
987, 109, 1826–1833; (f) Helmchen, G.; Ihrig, K.;
Schindler, H. Tetrahedron Lett. 1987, 28, 183–186; (g)
Trost, B. M.; Patterson, D. E. Chem. Eur. J. 1999, 5,
3
279–3284.
. Sanyal, A.; Snyder, J. K. Org. Lett. 2000, 2, 2527–
530.
3
4
2
1
9
very large, and that this effective charge is responsible
. The photochemical version has just been reported:
Jones, S.; Atherton, J. C. C. Tetrahedron: Asymmetry
2
. (a) Stevens, B.; Perez, S. R.; Ors, J. A. J. Am. Chem.
Soc. 1974, 96, 6846–6850; (b) Sweger, R. W.; Czarnik,
A. W. J. Am. Chem. Soc. 1991, 113, 1523–1530.
. Furanone 6a was prepared by the acetylation (Ac O/
Et N in CH Cl , 88%, Ref. 1) of the corresponding
lactol prepared as described in the literature: Feringa,
B. L.; Lange, B. D. Tetrahedron 1988, 44, 7213–7222.
. Typical reaction condition: 4 equiv. 6a/equiv. 1b; 2 M
in 1b.
8. Mielert, A.; Braig, C.; Sauer, J.; Martelli, J.; Sustmann,
R. Liebigs Ann. Chem. 1980, 6, 954–970.
. Singh, R. P.; Kamble, R. M.; Chandra, K. L.; Sara-
for the seemingly abnormal ‘steric’ effects often associ-
20
ated with the CF group, which are perhaps better
3
001, 12, 1117–1119.
described as electrostatic repulsions. Thus, higher elec-
tron density around the trifluoromethyl group would
force the dienophile to approach with the carbonyl
pointed away from it (Fig. 2), to produce the observed
diastereoisomer. Similar electrostatic control of facial
selectivity by a perfluoroalkyl group (CF CF ) domi-
5
6
2
3
2
2
3
2
nating over a methoxyl group has been reported by
Wipf in the addition of methyllithium to the carbonyl
21
7
group of 4,4-disubstituted cross-conjugated dienones.
Kraus has also established the role of electrostatic
repulsions in controlling facial selectivity in
2
2
cycloadditions.
9
These studies indicate that C-10 methylated chiral
anthracene 12 is a promising template for asymmetric
synthesis. Subsequent manipulations of the cycload-
ducts elucidated the role played by the trifluoromethyl
group on the origin of diastereoselection. Further tun-
ing of this template, required to increase the regioselec-
vanan, P.; Singh, V. K. Tetrahedron 2001, 57, 241–247.
10. Simchen, G.; Schmidt, A. Synthesis 1996, 1093–1094.
11. Corey, E. J.; Noe, M. C. J. Am. Chem. Soc. 1996, 118,
11038–11053.
19
1
2. The enantiopurity of 11 was determined from
F
NMR spectra obtained in the presence of (R)-(+)-N-
methyl naphthylamine as a chiral solvating agent.
3. Paulin, M. S. Ph.D. Thesis, UIUC, 1977.
tivity
in
cycloaddition
with
nonsymmetrical
dienophiles, is currently underway.
1
1
4. [h] −87.3° (c 0.662 g/100 mL, CHCl ); Mp 96–98°C;
D
3
1
H NMR (400 MHz, CDCl ) l 8.99 (br d, J=8.8 Hz,
3
1
H), 8.39 (d, J=8.4 Hz, 1H), 8.36–8.34 (m, 1H), 8.21
Acknowledgements
3
(
d, J=8.4 Hz, 1H), 7.59–7.51 (m, 4H), 6.20 (q, JF,H=
13
8
.1 Hz, 1H), 3.35 (s, 3H), 3.15 (s, 3H); C NMR (75.5
MHz, CDCl ) l 134.2, 131.7, 130.6, 130.2, 129.7, 127.1
3
We thank the donors of the Petroleum Research Fund,
administered by the American Chemical Society, for
financial support (ACS-PRF 35222-AC2). We also
thank Pfizer for providing a PREPARE undergraduate
summer fellowship for M.S.C.
1
(
br), 126.5, 125.9, 125.7, 125.1, 125.04 (q, JF,H=282.2
2
Hz), 124.96, 124.6, 122.9, 120.2, 78.7 (q, JF,H=32.0
Hz), 57.9, 14.8; HRMS (CI, 140 eV, NH3) m/z
+
3
05.1156 ([M+1] , 13%), calcd for C H F O 305.1153.
18
15 3
1
5. NOEs between the anthracene-derived C10 methyl sin-
glet and the anomeric H4%, and H11 with H2% con-
firmed the regiochemical identity of 14a; 14b was con-
firmed by NOEs between H3% with H11, and the C10
methyl with H2%.
1
1
1
6. Wheeler, J. W.; Happ, G. M.; Araujo, J.; Pasteels, J.
M. Tetrahedron Lett. 1972, 4635–4638.
7. Bloch, R.; Gilbert, L. J. Org. Chem. 1987, 52, 4603–
4605.
8. [h]D −69.1 (c 0.825 g/100 mL, CHCl ) lit. −69.2 (diox-
3
Figure 2. Facial selectivity control by CF electrostatic repul-
sion.
ane, c 2): Vigneron, J. P.; Blanchard, J. M. Tetrahedron
Lett. 1980, 21, 1739–1742.
3