by retrosynthetic analysis, where an aldol-type disconnection
leads to enolate 3 (Figure 1). Functional group transforma-
Ozonolytic cleavage produced aldehyde 6, which was
subjected to enolizing conditions. Spiro-alcohol 7, the result
of a 5-exo-trig attack on the aldehyde, was recovered in
moderate yield from a complex mixture. Structural assign-
ment of this unusual spirocycle was made using 2D NMR,
which confirmed the syn stereochemistry about the 5,5 ring
junction and also indicated an endo configuration between
the two aromatic systems. The hydroxyl group at C21 is
assigned exo to the ring system (syn to the carbonyl group
at C8) on the basis of the observed strong ROSEY interaction
between H-1 and H-21.5 Preliminary evaluation of 7 binding
to DNA containing a two-nucleotide bulge using competition
experiments against 1 revealed a Kd of 50 µm (Table 1).6
Figure 1. Retrosynthetic analysis of spirocyclic mimic.
tions applied to 2 would offer access to a wide variety of
analogues and also provide a platform to modulate stereo-
chemical bias.
Accordingly, the synthesis of model compound 7 was
initiated from indanone (Scheme 1). Benzylic bromination
Table 1. Affinity of NCS-Chrom Cascade Mimics for Bulged
DNA Sequences
excitation
(nm)
emission
(nm)
bulgeb
duplexc
compd
Kd (µm)
Kd (µm)
1
(-7
(-13
ent1-13
ent2-13
(-12
390
nda
310
310
310
310
500
nda
450
450
450
450
2.2
≈50a
25
35
17
307
90
89
90
Scheme 1. Synthesis of Pentacyclic Model Compound
>500
a Competition assay against 1.
b Bulged DNA:
c Duplex DNA:
Unfortunately, inadequate fluorescence precluded quantitative
analysis using conventional protocols, thus necessitating the
introduction of additional chromophores to the molecule.
such as COSY, ROSEY, HMBC, and HMQC. MS: calcd for C21H18O3
318; found m/e 319.18 (M + H)+, 307.11, 289.12, 154.31, 136.32. 13: 1H
NMR (500 MHz, CDCl3) 8.28 (s, 1H, H-15), 7.93 (m, 1H, H-17), 7.46 (m,
2H, H-18, H-20), 7.37 (t, 1H, H-3), 7.34 (ddd, 1H, H-4), 7.25 (t, 7.25 Hz,
1H, H-2), 6.87 (t, 8 Hz, 1H, H-19), 6.55 (d, 7.5 Hz, 1H, H-1), 6.43 (s, 1H,
H-22), 4.41 (d, 8 Hz, 1H, H-11), 4.33 (ddd, 1H, H-25), 3.58 (ddd, 6.5 Hz,
12.5 Hz, 16 Hz, 1H, H-6), 3.36 (ddd, 1H, H-12), 3.32 (ddd, 2.0 Hz, 12.5
Hz, 1H, H-6′), 3.06 (ddd, 2.0 Hz, 6.0 Hz, 16.5 Hz, 1H, H-7), 2.82 (ddd,
12.5 Hz, 1H, H-7′), 2.75 (s, 1H, OH), 2.74 (dd, 11.5 Hz, 12.5 Hz, 1H,
H-24), 2.60 (ddd, 1.5 Hz, 7.5 Hz, 1H, H-24′). 13C NMR: 213.9, 208.5,
145.1, 138.6, 136.9, 135.5, 135.0, 132.6, 130.3, 128.8, 128.7, 128.3, 128.2,
127.9, 127.1, 127.0, 126.9, 124.5, 75.3, 64.6, 48.0, 46.5, 38.6, 35.2, 28.0.
MS: calcd forC25H20O3 368; found m/e 369.2 (M + H)+, 223.1, 195.1,
147.1, 99.1; calcd for C25H20O3 368.1412, found 368.1411.
followed by in situ elimination allowed Diels-Alder cy-
cloaddition with diene 4, in turn prepared from acetyl
tetralone,3 giving pentacyclic alkene 5 in moderate yield.4
(5) The highly unusual upfield shift of the H-1 (6.48 ppm) and H-18
(6.14 ppm) of 7 owing to the anisotropic effect of the two interacted aromatic
rings, the observed COSY interaction between H-11 and -12, and the
ROSEY interaction between H-1 and H-21 (strong), H-18 and H-11 (weak),
H-11 and H-6 (strong), and H-11 and H-7 (weak) aided in the assignment
of the stereochemistry of 7 as shown in Scheme 2. The stereochemistry in
13 was assigned similarly.
(6) Fluorescence quenching studies were carried out using SPEX Fluoro
Max-2 at 5 °C in a 10 mM phosphate buffer (pH 7.5). Dissociation constant
Kd was derived from curve-fitting with Kaleidagraph, using the equation
(3) Minuti, L.; Taticchi, A.; Gacs-Baitz, E.; Marrocchi, A. Tetrahedron
1995, 51, 8953-8958.
(4) All new compounds gave appropriate spectroscopic and analytical
1
data. 7: H NMR (300 MHz, CDCl3) 7.70 (d, 7.2 Hz, 1H, H-15), 7.29 (d,
7.5 Hz, 1H, H-4), 7.28 (t, 7.2 Hz, 1H, H-16), 7.21 (dt, 7.5 Hz, 1.2 Hz, 1H,
H-3), 7.16 (dt, 7.5 Hz, 1.5 Hz, 1H, H-2), 6.95 (t, 7.2 Hz, 1H, H-17), 6.48
(d, 7.5 Hz, 1H, H-1), 6.14 (dd, 7.5 Hz, 0.9 Hz, 1H, H-18), 4.29 (dd, 11.1
Hz, 7.5, Hz, 1H, H-21), 4.23 (d, 7.2 Hz, 1H, H-11), 3.43 (ddd, 5.7 Hz,
12.6 Hz, 15.9 Hz, 1H, H-6), 3.21 (ddd, 2.1 Hz, 7.5 Hz, 11.1 Hz, 1H, H-12),
3.15 (ddd, 2.4 Hz, 6.6 Hz, 15.9 Hz, 1H, H-6′), 2.97 (ddd, 2.4 Hz, 5.7 Hz,
16.5 Hz, 1H, H-7), 2.80 (s, 1H) OH, 2.77 (ddd, 6.6 Hz, 12.6 Hz, 16.5 Hz,
1H, H-7′), 2.62 (dt, 11.1 Hz, 12.9 Hz, 1H, H-20), 2.51 (ddd, 2.1 Hz, 7.5
Hz, 12.9 Hz, 1H, H-20′). 13C NMR: 213.8 (C-13), 208.2 (C-8), 152.7 (C-
14), 138.4 (C-5), 137.7 (C-19), 135.4 (C-10), 134.2 (C-2), 128.4 (C-1),
128.2 (C-16), 128.1 (C-4), 127.9 (C-18), 127.6 (C-3), 126.7 (C-17), 123.6
(C-15), 76.4 (C-21), 63.4 (C-9), 48.4 (C-12), 47.2 (C-11), 38.7 (C-7), 34.0
(C-20), 28.7 (C-6). All assignments were supported by 2D NMR analysis
i/i0 ) 1 + (∆i/2i0)*([T0] + [DNA] + Kd - (([T0] + [DNA] + Kd)2
-
4*[T0][DNA])1/2, wherein [T0] is the initial concentration of the fluorescent
probe, i is the intensity of the sample, i0 is the initial intensity of the sample,
[DNA] is the concentration of the DNA, and ∆i is the total change in
intensity per drug unit from the free state to the total binding state. The
competition assay against 1 was used for the binding assay of 7. The release
of the bulge DNA-bound 1, which is fluorescent (excitation at 390 nm,
emission at 500 nm), was monitored by addition of 7.
1376
Org. Lett., Vol. 1, No. 9, 1999