determinations. Coumarin derivative 2 has a skeleton re-
sembling that of p-dimethylaminocinnamate.10 However, the
exo double bond of this chromophore is locked by a lactone
ring, making it absorb at a longer wavelength than its
cinnamate counterpart (406 nm vs 360 nm) in addition to
its fluorescent property.11 Its applicability in the CD exciton
chirality method is demonstrated as follows.
The bischromophoric derivative of (1R,2R)-1,2-diami-
nocyclohexane (3)12 displays an absorption maximum at 406
nm (ꢀ 73 400) and an emission at 462 nm in acetonitrile. Its
CD spectrum shows an intense couplet at 429 nm (∆ꢀ -134)
and 392 nm (∆ꢀ +69). This gives rise to a high A value of
-203. The sign of this CD couplet is consistent with the
chirality of the starting chiral diamine (Figure 1).
chiral diol on a six-membered ring, compound 5 from a 2,3-
diol of a glucopyranoside derivative, and compound 6 from
ponasterone A.14 Their UV/vis, fluorescence, and CD data
are shown in Table 1.
Table 1. UV/Vis, Fluorescence, and CD Data of Compounds
3-6 in Acetonitrile; Wavelengths in Nanometers (nm)
UV/vis:
emission:
CD:
compd
λmax (ꢀ)
λmax
λext (∆ꢀ)
A
3
4
5
6
406 (73 400)
414 (79 300)
417 (80 900)
412 (78 900)
462
462
463
462
429 (-134), 392 (+69) -203
431 (-100), 394 (+61) -161
435 (+118), 398 (-77) +195
432 (-72), 394 (+38)
-110
All four bischromophoric derivatives (3-6) display fluo-
rescence at λem 462-463 nm (λex 406-417 nm), which is
an important feature for highly sensitive detection of biologi-
cal samples. The CD Cotton effects resulting from these
bischromophoric derivatives faithfully represent the stereo-
chemistry of the parent compounds with no ambiguity in
the structural determination. For example, bis-ester 4 gives
a negative Cotton effect (A ) -161) similar to that of the
bis-amide 3. The effects of solvents (MeOH, MeCN, and
CH2Cl2) on the UV/vis and CD spectra of compounds 3 and
4 were also studied. The results show only minor solvent
dependence; wavelength shifts within 10 nm and A values
within 15%. Compound 5, derived from methyl 4,6-O-
benzylidene-R-D-glucopyranoside, has a strongly positive
couplet (A ) +195). This A value is much higher than those
from the p-bromobenzoate (GlcBBAA, +69)15 and p-
methoxycinnamate (GlcCCAA, +95)15 chromophores used
in carbohydrate studies.16
Figure 1. The structure of bischromophoric derivative 3 and its
Newman projection showing negative handedness.
High isolated yields of bischromophoric derivatives 4-6
(Figure 2) are achieved when the parent hydroxyl compounds
(9) To a stirred solution of 7-diethylaminocoumarin-3-carboxylic acid
(2, 1.00 g, 3.83 mmol) in CH2Cl2 (35 mL) was slowly added a solution of
1,1′-carbonyldiimidazole in CH2Cl2 (5 mL). The reaction mixture was stirred
at rt for 14 h and then concentrated at reduced pressure to remove most of
the solvent. The desired imidazolide 1 was obtained after recrystallization
from benzene (805 mg, 68%): mp 128-130 °C; 1H NMR (400 MHz,
CDCl3) δ 8.13 (s, 1 H), 8.08 (s, 1 H), 7.47 (s, 1 H), 7.36 (d, J ) 9.0 Hz,
1 H), 7.06 (s, 1 H), 6.64 (dd, J ) 9.0, 2.4 Hz, 1 H), 6.48 (d, J ) 2.4 Hz,
1 H), 3.45 (q, J ) 7.1 Hz, 4 H), 1.23 (t, J ) 7.1 Hz, 6 H); 13C NMR (100
MHz, CDCl3) δ 162.5, 158.6, 158.5, 153.4, 149.4, 137.8, 131.3, 130.4,
117.4, 110.7, 110.1, 107.5, 96.9, 45.2, 12.4; FAB-HRMS for C17H18O3N3
(M+ + 1) calcd 312.1348, found 312.1319.
(10) Verdine, G. L.; Nakanishi, K. Chem. Commun. 1985, 1093.
(11) 7-Diethylaminocoumarin-3-carboxylic acid (2) is a commercially
available reagent widely used in the fluorescent labeling of bioconjugates
with the surface accessible amino acid side chains. Haugland, R. P.
Handbook of Fluorescent Probes and Research Chemicals, 6th ed.;
Molecular Probes: Eugene, OR, 1996.
(12) Compound 3 was prepared by the carbodiimide coupling method
(EDC/HOBt): 1H NMR (300 MHz, CDCl3) δ 8.82 (bs, 2 H), 8.63 (s, 2 H),
7.34 (d, J ) 9.0 Hz, 2 H), 6.55 (dd, J ) 9.0, 2.1 Hz, 2 H), 6.41 (d, J ) 2.1
Hz, 2 H), 4.05 (bs, 2 H), 3.39 (q, J ) 7.0 Hz, 8 H), 2.15 (bs, 2 H), 1.74
(bs, 2 H), 1.42 (bs, 4 H), 1.18 (t, J ) 7.0 Hz, 12 H); 13C NMR (75 MHz,
CDCl3) δ 162.9, 162.3, 157.5, 152.2, 147.9, 131.0, 110.8, 109.6, 108.4,
96.6, 52.7, 45.0, 32.3, 24.6, 12.4.
(13) General procedure for derivatization of diols using imidazolide
1: To a solution of the diol substrate (0.1 mmol) and imidazolide 1 (0.25
mmol) in anhydrous CH3CN (5 mL) was slowly added 0.5 mL of a 0.5 M
DBU/CH3CN solution. After being stirred at rt for 5 h, the mixture was
concentrated and the desired product was purified by silica gel column
chromatography.
Figure 2. Structures of the bischromophoric derivatives 4-6.
are subjected to imidazolide reagent 1 in the presence of
DBU (5 h of stirring in CH3CN solution), making this
derivatization procedure well-suited for microscale reac-
tions.13 Compound 4 exemplifies the derivative from a simple
(7) Gargiulo, D.; Derguini, F.; Berova, N.; Nakanishi, K.; Harada, N. J.
Am. Chem. Soc. 1991, 113, 7046.
(8) Berova, N.; Gargiulo, D.; Derguini, F.; Nakanishi, K.; Harada, N. J.
Am. Chem. Soc. 1993, 115, 4769.
(14) Nakanishi, K.; Koreeda, M.; Sasaki, S.; Chang, M. L.; Hsu, H. Y.
Chem. Commun. 1966, 915.
684
Org. Lett., Vol. 2, No. 5, 2000