C O M M U N I C A T I O N S
Scheme 3. Synthesis of â-Piperidinoethylsulfide 2a
a Reagents and conditions: (a) (i) Boc2O, NEt3, dioxane/water, rt, (ii) 3
M HCl; (b) (i) SOCl2, DMF, THF, (ii) piperidine, THF, rt; (c) 13, TFA,
TFE, reflux; (d) LiAlH4, THF, rt.
Figure 2. Structure of 5h bound to CDK2/cyclin A. Dotted lines represent
polar contacts <3.4 Å.
positioned to interact with the ꢀ-NH2 group of Lys89 (the first
observation of such an interaction in this series) and the terminal
OH group with the side chain of Asp86. All these interactions result
in the extended anilino substituent adopting an ordered conformation
on the CDK2 surface. However, the lower potency for 5h compared
to that of 1c suggests that this network of polar contacts is
insufficient to compensate for the loss of more favorable interactions
between CDK2 and the purine and anilino rings of 1c. An overlay
of the two inhibitors bound to CDK2/cyclin A showed that their
relative binding orientations do differ.
Figure 1. Lead CDK2 inhibitors identified by a structure-based design
strategy.
existence of the equilibrium described, while providing a basis for
the application of similar chemistry to the purine scaffold.
We were gratified to find that the chemistry planned for 2
proceeded exactly as intended to provide sulfones 5a-5w (Scheme
1 and Supporting Information). To access the â-piperidinoethyl-
sulfide 2 (Scheme 3), commercially available 4-aminophenylthio-
acetic acid (10) was N-protected with t-butoxycarbonyl (Boc) to
give 11, which was converted into amide 12 by reaction of an
intermediate acid chloride with piperidine. On heating 12 with
fluoropurine 13 under the trifluoroacetic acid/2,2,2-trifluoroethanol
protocol previously described,13 in situ removal of the Boc group
released aniline 14, which reacted with 13 to furnish the amide 15.
Reduction of 15 afforded the corresponding tertiary amine 2.
Application of the methodology developed with 6 to the purine 2
(Scheme 1) gave compounds 5a-5c in acceptable yields (>65%)
and sufficiently pure after HPLC for biological evaluation. The
methodology was then implemented in a multiple-parallel format,
allowing the synthesis of compounds 5d-5w in high purity and
satisfactory yields. The “one-pot” procedure developed is applicable
to numerous scenarios in the context of drug development.
Inhibitors 5a-5w displayed a range of activities against CDK2,
the structure-activity relationship (SAR) appearing to favor the
incorporation of smaller, less sterically hindered groups (e.g., 5h;
IC50 ) 45 nM). Increasingly bulky groups (e.g., 5t; IC50 ) 1.78
µM) largely abolished activity. The structure of the CDK2/cyclin
A/5h complex provided a starting point from which to rationalize
the observed SAR (Figure 2). There are three hydrogen bonds
between the guanine N9, N3, and the NH of the C2-substituted
anilino group and the backbone carbonyl of Glu81 and the amide
NH and carbonyl group of Leu83, respectively.8-10 The position
of the anilino group is similar to that of 1c bound to CDK2/cyclin
A9 and enables the 3-hydroxypropyl group of the aminoethylsul-
fonyl substituent to form multiple polar contacts, notably with
Asp86 and Lys89. Mimicking the interactions of the 1c sulfonamide
group, the backbone amide NH and side chain carboxylate of Asp86
can hydrogen bond to a sulfone oxygen and the NH of the
ethylamino group, respectively. The other sulfone oxygen is then
Application of the methodology described to the development
of inhibitors of other CDKs is in progress.
Acknowledgment. We thank Cancer Research UK, BBSRC
(A.H.), and EU FP 6 (A.E.) for financial support.
Supporting Information Available: Details of preparative proce-
dures/spectroscopic data for compounds 5a-5w and 9a-9d, CDK2
inhibitory data for 5a-5w; crystallization of a T160pCDK2/cyclin A/5h
complex, X-ray crystallography data collection and processing; complete
refs 8-10. The coordinates of the CDK2/cyclin A/5h complex have
been deposited in the Protein Data Bank under ID code 2G9X. This
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