2
L. Xu et al. / Bioorg. Med. Chem. Lett. xxx (2014) xxx–xxx
R3
Ph
O
Ph
O
H
N
b
S
N
H2N
Ph
S
N
O
O
O
N
H
BocHN
COOH
N
H
R3
O
2a
Ph
3a
BocHN
O
4
R3
R3
c
O
R1
OH
a
COOMe
b
N
N
H
6
H2N
R2
O
O
R3
Ph
5
O
H
N
R1
S
N
O
N
N
H
N
H
R2
O
Ph
1a
Scheme 1. Synthesis of compound 1a. Reagents and conditions: (a) (1) CDI/iPr2NEt; (2) R1R2NH; 3. NaOH/CH3OH; (b) EDCI, HOBt, iPr2NEt; (c) (1) TFA or HCl/dioxane; (2) CDI,
iPr2NEt; (3) R1R2NH.
In the preceding letter, we identified R,R- and S,S-1,4-diamine
cores (exemplified by 1a and 1b Fig. 1) as our optimal central
pieces. We herein report the synthesis and exploration of P2 and
P3 regions of the diamine CYP3A inhibitors; these investigations
led to the discovery of a novel pharmacoenhancer cobicistat
(compound 35, GS-9350).
simultaneously studied the SAR at the P3 and P2 portions of the
lead compound 15.
To establish SAR on P2/P3, our initial goal was to use modifica-
tions at P3 and/or P2 portion to dissociate the anti-HIV activity
from the inhibitory potency against CYP3A possessed by the lead
compound. Combination of the optimized P2 and P3 moieties with
the best core would then allow us to identify an analog that is ac-
tive against CYP3A but devoid of anti-HIV activity. The results are
summarized in Table 1.
A general sequence for the synthesis of compound 1a analogs is
outlined in Scheme 1. Compounds with formula 1b were prepared
using the same procedure except replacing the R,R-core of 2a with
the corresponding S,S-core. Compound 1a can be constructed using
two different routes. Amine 2a6 was coupled with Boc-protected
amino acid 4 to give amide 3a. Removal of the Boc-group yielded
the corresponding amine, which was then acetylated with one
equivalent CDI and subsequently reacted with amine R1R2NH to af-
ford 1a. Alternatively, compound 1a can be prepared through cou-
pling of acid 6, which already contains a urea moiety, with amine
2a. Acid 6 was prepared by treatment of protected amino acid ester
5 with CDI followed by R1R2NH. Subsequent ester hydrolysis gen-
erates acid 6 which is then coupled to 2a using standard condi-
tions. The amino acid 4 or 5 with various R3 side chains were
either commercially available in protected forms suitable for use
in the described sequences, or prepared following conventional
methods for synthesis of protected amino acids.
Reducing the size or removing the 20-isopropyl group of the P3
portion 4-thiazolyl moiety (17, 18 vs 15), in attempts to minimize
interaction of the analogs with the HIV protease, only marginally
decreased the binding with HIV protease and antiviral activity.
Similarly, replacing the N-methyl with N-cPr (16 vs 15), aiming
to interrupt the interactions of the compound with HIV protease,
resulted in only minor reduction of anti-HIV activity. Compounds
16–18 with modified P3 possess comparable activity towards
CYP3A. Combining these two modifications provided compound
19 which had a ꢀ14-fold reduction of inhibitory activity against
HIV yet maintained potency against CYP3A. This represents an in-
crease in selectivity of approximately 80-fold compared to RTV.
Compound 19 now has clinically marginal anti-HIV EC50 of 4 lM,
but we desired a compound inactive against HIV. Using 5-thiazole
instead 4-thiazole at P3 (20 vs 18) did not offer any advantage in
terms of selectivity, but simplified synthesis, as it can be obtained
from the same intermediate as the right-side (P20) thiazole.
We then turned our attention to the P2 portion of the molecule.
Replacing valine with b-alanine at P2 (21) provided ꢀ8-fold reduc-
tion of anti-HIV activity to give a compound with micromolar
activity against HIV. Replacement of the valine at the 2-position
with a serine, containing a more polar moiety, offered 5-fold reduc-
tion in activity against HIV. With serine at P2, the SAR trend at the
P3 portion was consistent with that observed in the P2 valine ser-
ies (compounds 22–25 vs 15–19). Compound 25 has an EC50 of
As shown in Scheme 2, 40- or 50-thiazolyl methyl amines
(R1R2NH) 7 or 8 were synthesized from 4- or 5-chloromethyl
thiazoles 11 or 13, respectively, by treatment with primary amines.
2-Substituted 4-chloromethyl thiazole 11 was prepared from con-
densation of thioamide 9 with 1,3-dichloroacetone 10.7 5-Chloro-
methyl thiazole 13 was obtained from commercial available
5-hydroxymethyl thiazole 12 by reacting with methanesulfonyl
chloride.
As discussed in the previous communication,6 we identified
desoxy-RTV (15) as
a lead compound for structure-activity
relationship (SAR) studies. Although the antiviral potency of des-
oxy-RTV is greatly reduced compared to RTV, it still possesses
modest anti-HIV activity with an EC50 of 290 nM (Table 1). While
building our understanding of the SAR for the diamine core, we
20 lM and shows over 400-fold improvement of selectivity com-
pared to RTV. Further exploration at the P2 site showed that polar
and bulky moieties can be tolerated by CYP3A but are less well tol-
erated by HIV protease (compounds 27–29). The fact that that in-
creased steric bulk at P2 can significantly reduce inhibitory
activity against HIV protease and virus replication is exemplified
by comparing compound 27 to 26. The selectivity and activity of
compound 27 met the minimum criterion for a selective CYP3A
inhibitor we set at the outset of the program. However the physi-
cochemical properties of compound 27, especially its aqueous sol-
ubility, were not desirable.
Cl
R2
Cl
Cl
S
S
N
H
R'
NH2
+
a
c
O
N
N
S
S
R'
R'
Cl
9
10
11
7
8
R2
N
H
c
N
OH
N
N
b
S
S
12
After understanding the SAR for disassociating the HIV activity
from CYP inhibitory potency, our next goal was to improve other
properties including physicochemical properties. We thus incorpo-
rated moieties with potential to provide analogs with desirable
13
Scheme 2. Synthesis of compounds 7 and 8. Reagents and conditions: (a) MgSO4/
acetone/reflux. (b) MsCl/Et3N/MeCN; (c) R2NH2.