T. Yasuhara et al. / Bioorg. Med. Chem. Lett. 19 (2009) 2944–2946
2945
O
OH
Table 1
Effects of benzhydrol analogues on the release of b-hexosaminidase, TNF-
a
and IL-4
ii)
from RBL-2H3 cells
RO
RO
iii)
OAc
OR2
4 : R = H
5 : R = TBS
6 : R = TBS
3 : R = H
i)
R1O
1
2, 7-16
Scheme 1. Reagents and conditions: (i) TBSCl, imidazole, DMF, 0 °C, 1 h, 96%; (ii)
NaBH4, THF, 0 °C, 3 h, 60%; (iii) TBAF, THF, 0 °C, 0.5 h, 95%.
AcO
IC50
(
l
M) for release of
b-Hexosaminidase
TNF-
a
IL-4
12
1
10S-10-acetoxy chavicol acetate
17
17
ketotifen fumalate
158
OR
R1
R2
2 : R = Ac
9 : R = COOMe
i)
7 : R = COEt 10 : R = COCH=CH2
8 : R = COi-Pr 11 : R = CHO
3
2
7
8
9
10
11
12
13
14
15
16
Ac
Ac
COEt
14
38
>100
18
28
>100
>100
27
36
17
COEt
COiPr
COOMe
COCH@CH2
CHO
Ac
Ac
Ac
Ac
Ac
RO
COiPr
COOMe
COCH@CH2
CHO
Scheme 2. Reagents and conditions: (i) 2: Ac2O, Et3N, DMAP, CH2Cl2, 0 °C, 1 h, 97%;
7–9: EtCOCl, iPrCOCl or MeOCOCl, pyridine, 0 °C, 2 h, 90%, 93% and 86%,
respectively; 10: CH2@CHCOCl, NaH, THF, 0 °C, 0.5 h, 72%; 11: formic acid, DCC,
CH2Cl2, acetone, rt, 24 h, 90%.
H
COiPr
COtBu
COPh
COOMe
Treatment of 6 with tetrabutylammonium fluoride in THF yielded
4-hydroxybenzhydrol 3 in 55% overall yield from 4.
28
11
23
12
6.5
Treatment of 3 with acetic anhydride, appropriate acyl chlo-
rides or methyl chlorocarbonate under usual conditions afforded
corresponding five esters (2 and 7–10) in good yields (Scheme 2).
The formate 11 was prepared by condensation reaction of 3 with
formic acid activated by dicyclohexylcarbodiimide25 in 90% yield.
Selective acetylation of the phenolic hydroxyl of 3 leading to the
monoacetate 12 was accomplished with acetic anhydride in the
presence of triethylamine and dimethylaminopyridine in 97%
yield. Acylation of another hydroxyl by acyl chlorides or methyl
chlorocarbonate gave the corresponding 20-acyloxy or 20-meth-
oxycarbamate derivatives (13–16) in 95%, 93%, 88% and 77% yields,
respectively (Scheme 3).
The chemical stability of the most active compound 16 was
compared with that of 1. A solution of 10 mg of racemic mixture
of 1 in 0.5 mL CDCl3 had been kept at 50 °C, and the 1H NMR spec-
tra was taken after 24 h. Complete decomposition of 1 into a com-
plex mixture was observed. In a solution of DMSO-d6/D2O (4/1)
kept at 40 °C, ca. 50% of compound 1 decomposed in 48 h giving
a complex mixture concomitant with acetic acid. On the other
hand, no change was detected by 1H NMR spectroscopic studies
on 16 conducted at the same conditions as described above for 1.
The antiallergic activities of benzhydrol derivatives thus syn-
thesized were evaluated by indexing the inhibition of release of
b-hexosaminidase from RBL-2H3 cells.26 The simplest analogue 2,
in which the vinyl group of 1 was substituted by the phenyl group,
(12, R2 = H) caused considerable decrease of its activity
(>100 l
M). However, analogues 13–15, which have large R2 acyl
moieties such as COiPr, COtBu and COPh, gain the stronger inhibi-
tory activity, while 8 showed no activity. Finally acetoxybenzhy-
drol methylcarboxylate analogue 16 (R2 = COOMe) was found
more active than 1.
The immediate responses of the Type I allergy are known
mainly due to small molecule chemical mediators (e.g., histamine,
serotonin) from mast cells. Mast cells also produce cytokines
including TNF-a, IL-4, and IL-5, and these cytokines play an impor-
tant role in the late phase of the allergic reactions.20–23 In the pres-
ent study, effects of analogues 15 and 16, which exhibited strong
inhibitory effects against the release of b-hexosaminidase, on pro-
duction of TNF-
were also examined. As shown in Table 1, 15 and 16 inhibited
the production of TNF- and IL-4 as strong as 1 (12–17 M) with
IC50 values of 11–28 M. These findings suggest that these esters
a and IL-4 in RBL-2H3 cells 4 h after challenge,
a
l
l
15 and 16 are effective against not only the immediate phase but
the late phase of the Type I allergy reactions.
In conclusion, we developed stable and more potent antiallergic
agent 16 by the structural modification of natural inhibitor 10S-10-
acetoxychavicol acetate (1). The structural requirement of 10S-10-
acetoxychavicol acetate analogues for the inhibitory activity
against release of b-hexosaminidase were clarified as follows: (1)
Substitution of the vinyl group in 1 to the phenyl group keeps
the activity. (2) Phenyl acetate moiety was essential for the activ-
ity. (3) As for the ester moiety on 10 position smaller one was pre-
ferred for higher activity. In addition, benzhydrol derivatives 15
showed nearly the same inhibitory activity as 1 (IC50 14 lM (Table
1). However, the activity decreased along with the increased bulk
of the ester moieties, activities of propanoate (7) and isobutanoate
(8) being reduced to 38
(10) maintained the activity to some extent, IC50 of which being
28
M. Methylcarboxylate analogue (9, R1 = R2 = COOMe) have
nearly equal activity (18 M) to 1. Formate analogue 11
lM and >100 lM, respectively. Propenate
l
and 16 inhibited the antigen-IgE-mediated TNF-a and IL-4 produc-
l
(R1 = R2 = CHO) showed no inhibitory activity (>100
lM). Ana-
tion as strong as 1, thus these compounds were revealed to exert
their antiallergic activity on both the immediate and late phase
of the type I allergic reactions.
logues 12–16 have common phenyl acetate moiety (R1 = Ac), and
were modified only at R2 group. Removal of the ester group at R2
Acknowledgements
OH
OR
13 : R = COi-Pr
14 : R = COtBu
15 : R = COPh
16 : R = COOMe
This work was supported by a Grant-in Aid for Scientific Re-
search from ‘High-Tech Research Center’ Project for Private Univer-
sities: matching fund subsidy from MEXT (Ministry of Education,
Culture, Sports. Science and Technology), 2007–2011 and also sup-
ported partly by the 21st COE Program and Academic Frontier Pro-
ject from the Ministry of Education, Culture, Sports, Science and
Technology of Japan.
i)
ii)
3
AcO
AcO
12
Scheme 3. Reagents and conditions: (i) Ac2O, Et3N, DMAP, CH2Cl2, 0 °C, 1 h, 97%;
(ii) 13–16: iPrCOCl, tBuCOCl, PhCOCl or MeOCOCl, Et3N, DMAP, CH2Cl2, 0 °C, 1 h,
95%, 93%, 88% and 77%, respectively.