Silanediol peptidomimetics. Evaluation of four diastereomeric ACE inhibitors

Article abstract of DOI:10.1016/j.bmcl.2004.03.042

Four diastereomers of a Phe-Ala peptide mimic incorporating a central silanediol group have been individually prepared and tested as inhibitors of angiotensin-converting enzyme (ACE). Three of the silanediols exhibit levels of inhibition that are similar

Full text of DOI:10.1016/j.bmcl.2004.03.042

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2853–2856
Silanediol peptidomimetics. Evaluation of four diastereomeric
ACE inhibitors
Jaeseung Kima and Scott McN. Sieburtha,b,
*
aDepartment of Chemistry, State University of New York at Stony Brook, Stony Brook, NY 11794-3400, USA
bDepartment of Chemistry, Temple University, 1901 N. 13th Street, Philadelphia, PA19122, USA
Received 25 January 2004; accepted 16 March 2004
Abstract—Four diastereomers of a Phe-Ala peptide mimic incorporating a central silanediol group have been individually prepared
and tested as inhibitors of angiotensin-converting enzyme (ACE). Three of the silanediols exhibit levels of inhibition that are similar
to those of corresponding ketones reported by Almquist. For the fourth diastereomer, with both stereogenic carbons inverted
relative to the most active isomer, the ketone gives the least enzyme inhibition whereas the silanediol shows a surprisingly low IC50
value.
Ó 2004 Elsevier Ltd. All rights reserved.
Silanediols are a new central subunit for design of pro-
tease inhibitors, replacing for the scissile amide carbonyl
of the protease substrate. This nonhydrolyzable mimic
of a carbonyl hydrate has been incorporated into several
inhibitor structures, replacing ketone,1;2 hydroxyl,3 and
phosphinic acid groups,4 and the resulting structures
have been effective inhibitors of both metallo and
aspartic proteases. We report here the first set of
silanediol structures, four diastereomers that are directly
comparable to ACE inhibitor 2 and its diastereomers.
first silanediol protease inhibitor 1 was assembled as an
analogue of Almquist’s ketone-based ACE inhibitor 2.13
While 1 was an effective inhibitor, direct comparison
with 2 was difficult because it contained an isobutyl
groupwhere 2 has a benzyl groupand was an insepa-
rable mixture of four diastereomers (Fig. 1).
Inhibition of ACE is an established therapy for hyper-
tension.14;15 All commercial ACE inhibitors incorporate
a groupto interact with the active site zinc ion, such as
the thiol found in captopril 3 or an anionic carboxylate
or a phosphinate.14 Ketone 2 presumably chelates the
active site zinc as the tetrahedral hydrate,5 with an IC50
of 1 nM,13 and the silanediol was expected to act in the
same way. Mixture 1 was found to inhibit ACE with
a IC50 of 14 nM.2 Presumably only one of the four
diastereomers of 1 was responsible for most of this
As a transition state analogue, the silanediol presents
two hydroxyl groups corresponding to a hydrated amide
carbonyl at the enzyme active site. In this way it is
similar to a fluorinated ketone hydrate,5–7 although
slightly larger,8 and comparable to a phosphinic acid,9;10
but would not be ionized at physiological pH.11;12 The
Figure 1. Three inhibitors of ACE.
Keywords: Protease; Inhibitor; Silane; ACE.
Corresponding author. Tel.: +1-215-204-7916; fax: +1-215-204-1532; e-mail: scott.sieburth@temple.edu
ꢀ
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.03.042

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J. Kim, S. McN. Sieburth / Bioorg. Med. Chem. Lett. 14 (2004) 2853–2856
Scheme 1. Assembly of enantiomerically pure intermediates. Reagents and conditions: (a) BnOC(NH)CCl3, TfOH, 0 °C; (b) LiAlH4, 0 °C; (c) I2,
Ph3P, imidazole, 0 °C; (d) 2 tert-butyllithium, ꢁ78 °C, 8, to rt; (e) n-butyllithium, ꢁ78 °C, Ph2SiF2, to rt.
inhibition. The IC50 data for 1 and related silanols was
consistent with an interaction with ACE in a manner
similar to that of ketone 2.2 For example, replacement of
one silanol of 1 with a methyl group(to give a methyl-
silanol) eliminated the enzyme inhibition. To probe this
relationshipfurther, we have prepared four silanediol
diastereomers that more closely resemble 2, inhibitors
that could be directly compared to diastereomers
reported by Almquist.
with iodine and triphenylphosphine (85%) was followed
by metal–halogen exchange at ꢁ78 °C using tert-butyl-
lithium,18;19 yielding lithium reagents 5 and 10. These
reagents were coupled with fluorosilane 8 to produce 6
and 11 (65%). Fluorosilane 8 was readily prepared by
the reaction of difluorodiphenylsilane with the anion of
2-benzyl-1,3-dithiane 720 (83%).
Elaboration of the dithianes 6 and 11 to the desired a-
aminosilane groups was accomplished by hydrolysis of
the dithiane to sensitive silylketones21–23 (Scheme 2,
73%), which were then reduced with lithium aluminum
hydride to give a 1:1 mixture of diastereomeric alcohols
(quantitative). These diastereomeric mixtures were car-
ried through several steps without separation: the alco-
hol was displaced with phthalimide using Mitsunobu
conditions24 (63%), followed by removal of the phthal-
imide with hydrazine; treatment with benzoyl chloride
gave the benzamide (91%, two steps), and benzyl ether
cleavage with boron tribromide led to the corresponding
The synthesis of silanediol-based analogues of 2 was
predicated on the use of a diphenylsilane as an acid-
hydrolyzable silanediol precursor.2;16 The synthesis of
the diastereomers began with the commercially available
(S)-3-hydroxy-2-methyl propionate 4 and its enantiomer
(R)-9, Scheme 1. Following the method of White and
Kawasaki,17 the alcohol was converted to a benzyl ether
with benzyl 2,2,2-trichloroacetimidate and triflic acid
(82%), and the resulting ester was then reduced to an
alcohol (87%). Conversion of the alcohol to an iodide
Scheme 2. Synthesis of diastereomerically pure silanediols. Reagents and conditions: (a) HgCl2, MeCN, H2O, rt; (b) LiAlH4, 0 °C; (c) PhthNH,
ꢀ
Ph3P, DEAD, 0 °C to rt; (d) N2H4, ethanol, reflux; (e) PhCOCl, NaHCO3, 0 °C; (f) BBr3, CH2Cl2, ꢁ78 °C to rt; (g) TPAP, NMO, 4 A molecular
sieves, CH2Cl2; (h) NaClO2, NaH2PO4, 2-methyl-2-butene, tert-butanol; (i) -proline-O-tert-Bu, DEPC, Et3N, DMF; (j) (i) TfOH, CH2Cl2, 0 °C;
L
(ii) NH4OH; (iii) 48% HF; (k) 3 NaOH.

J. Kim, S. McN. Sieburth / Bioorg. Med. Chem. Lett. 14 (2004) 2853–2856
2855
alcohols (90%). Separation of the diastereomers using
standard column chromatography25 was most readily
accomplished after removal of the benzyl ether, giv-
ing stereochemically pure isomers 12, 13, and their
enantiomers. The identity of 12 was secured by X-ray
crystallography,26 which thereby defined all four dia-
stereomers, based on their origin from 4 and 9 and their
chromatographic properties.
toring by 19F NMR spectroscopy indicated that the
conversion of the silyl fluorides to fluoride ion was
complete within several minutes. The resulting silanediol
carboxylate 15 and its diastereomers were monomeric,
stable species, readily soluble in water.
Evaluation of the four silanediols was conducted using
Holmquist’s modification29 of the enzyme assay
described by Cushman and Cheung,30 employing com-
mercially available enzyme and fluorescent substrate.31
For three of the silanediols, there is a close correlation of
the IC50 data with the published values for the corre-
sponding ketones, Table 1.13 The most potent inhibitors,
ketone 2, and silanediol 15, have the same absolute
configurations. A similar relationshipholds for diaste-
reomeric ketones 16 and 18, and their corresponding
silanediols 17 and 19. For these three inhibitor pairs, the
relative potencies vary within a rather narrow range of
2.3–4.5, suggesting a similar mechanism of inhibition for
the two structural types. Inverting the C2 stereogenic
center of 15 to give 19 has a much greater effect (54x)
than inverting the C5 stereochemistry (17, 5x).
Oxidation of the alcohols to acids using TPAP27 and
sodium chlorite28 (77%) was then followed by coupling
with the tert-butyl ester of proline, yielding 13a and the
three other diastereomers (72–80%). With only depro-
tection of the ester and diphenylsilane remaining, we
turned our attention to this final step. This hydrolysis is
potentially problematic because of the well-known pre-
dilection of silanediols to undergo dehydration/poly-
merization, promoted by both acidic and basic
conditions. Our original work utilized triflic acid for
hydrolysis, followed by addition of ammonium
hydroxide.2;3 Ammonium hydroxide neutralizes the
acid, provides water for silanediol formation, and
ensures hydrolysis of any cyclized intermediates that
may have resulted from amide participation in cleavage
of the silicon–phenyl bonds. We found, during our
preparation of a thermolysin inhibitor,4 that including
aqueous HF in the workuprpoduced an isolable,
monomeric difluorosilane intermediate, which subse-
quently undergoes a facile hydrolysis with aqueous base.
Surprisingly, reversing both stereogenic centers of 2 and
15 gave very different results. Inhibition of ACE by
ketone 20 is less effective by a factor of 3200 relative to
ketone 2, consistent with a mutually antagonistic effect
of the two stereocenters. In contrast, the inhibition by
silanediol 21, relative to 15, is reduced by only a factor
of 20. Inverting the C2 and C5 stereochemistry is three
times better than inverting only the C2 stereocenter. The
anomalous level of inhibition of ACE by 21 may indi-
cate that a conformation of this silane not accessible to
the ketone can inhibit the enzyme, perhaps as a conse-
quence of the longer C–Si bonds relative to C–C bonds
and the slightly different bond angles surrounding a
silanediol.2 Tacke et al. have noted an attenuated effect
Treatment of 14a and its diastereomers with 0.5 M triflic
acid in methylene chloride at 0 °C for 1 h, followed by
neutralization with ammonium hydroxide for 30 min,
and then addition of 48% HF followed by evaporation
gave a crystalline homogenous product (92%). When
this difluoride in water was treated with 3 equiv of
NaOH, both silyl fluorides rapidly hydrolyzed. Moni-
Table 1. Comparison of ketone and silanediol inhibitor IC50 values
IC50 (nM), compound #
Relative potency
3.8
1, 2
3.8, 15
19, 17
207, 19
72, 21
8.2, 16
46, 18
3200, 20
2.3
4.5
0.023

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J. Kim, S. McN. Sieburth / Bioorg. Med. Chem. Lett. 14 (2004) 2853–2856
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This study is the first assessment of the consequences of
stereochemistry with a silanediol-based protease inhibi-
tor. The most active silane 15 exhibits a good level of
enzyme inhibition, and two epimers show the expected
stereochemical dependence. Silanediol 21 has an unex-
pectedly low IC50 value, suggesting a special ability for
the silanediol to adapt its conformation, a result that
may become explicable with the very recent publication
of an ACE crystal structure.33 Additional studies are in
progress.
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Acknowledgements
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Support for this research was provided by National
Institutes of Health to whom we are grateful. We thank
Dr. Patrick J. Carroll, University of Pennsylvania, for
the crystal structure.
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P21=n with a ¼ 13:8822 (8) A, b ¼ 8:4270 (6) A,
3
ꢀ
ꢀ
ꢀ
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