2-Arylbenzoxazoles as CETP inhibitors:Substitution of the benzoxazole moiety
Cameron J.Smith a,*,Amjad Ali a ,Liya Chen a ,Milton L.Hammond a ,Matt S.Anderson b ,Ying Chen b ,Suzanne S.Eveland b ,Qiu Guo b ,Sheryl A.Hyland b ,Deni P.Milot b ,Carl P.Sparrow b ,Samuel D.Wright b ,Peter J.Sinclair a
a Department of Medicinal Chemistry,Merck Rearch Laboratories,Rahway NJ 07065,United States b
Department of Cardiovascular Dias,Merck Rearch Laboratories,Rahway NJ 07065,United States
a r t i c l e i n f o Article history:
Received 26June 2009Revid 23October 2009Accepted 26October 2009
Available online 29October 2009Keywords:
Cholesteryl ester transfer protein CETP
High density lipoprotein HDL
Benzoxazole
a b s t r a c t
A ries of 2-arylbenzoxazole inhibitors of the cholesterol ester transfer protein (CETP)is described.Structure–activity studies focud on variation of the substitution of the benzoxazole moiety.Substitu-tion at the 5-and 7-positions of the benzoxazole moiety was found to be beneficial for CETP inhibition.Compound 47was found to be the most potent inhibitor in this ries and inhibited CETP with an IC 50of 28nM.
Ó2009Elvier Ltd.All rights rerved.
Coronary heart dia (CHD)is now the leading cau of death of people in developed countries.Elevated levels of low-density lipoprotein-cholesterol (LDL-C)has been identified as a major risk factor for CHD.The development of the statins has significantly helped to reduce LDL-C levels in patients at risk for CHD.1,2There is now a growing body of epidemiological evidence linking in-cread levels of high density lipoprotein-cholesterol (HDL-C)with decread risk of development of CHD.3–6Some cholesterol lower-ing drugs,including niacin,fibrates and statins,have a modest ef-fect on increasing HDL-C levels.7–10Regardless,niacin remains the front line therapy for raising HD
L-C levels despite its modest effi-cacy ($20%increa).Conquently there is a need for better ther-apies to address this problem.
The beneficial effects of high density lipoprotein (HDL)are thought to ari from its participation in rever cholesterol trans-port (RCT)as well as its anti-inflammatory and anti-oxidant prop-erties.11,12Cholesteryl ester transfer protein (CETP)mediates the exchange of cholesteryl ester (CE)from HDL with triglycerides pri-marily from very low-density lipoprotein (VLDL).11,12Inhibition of CETP would therefore be expected to increa rum HDL-C levels.Clinical studies in humans with the CETP inhibitors,dalcetrapib (JTT-705),13,14torcetrapib,15–19and anacetrapib 20–22established that pharmacological inhibition of CETP leads to significant in-creas in HDL-C concentrations.Despite this obrvation,imaging
顺治三年
studies with torcetrapib showed the compound had no effect on the progression of atherosclerosis.23–25Additionally,the torcetra-pib pha III trial was prematurely halted after the obrvation of
N
O O F 3C
F 3C
CF 3
O O
F 3C
3
3O
F
S
NH
O
O
dalcetrapib (JTT-705)
torcetrapib
anacetrapib
0960-894X/$-
e front matter Ó2009Elvier Ltd.All rights rerved.doi:10.1016/j.bmcl.2009.10.099
*Corresponding author.Tel.:+17325942529;fax:+17325949545.E-mail address:cameron_ (C.J.Smith).Bioorganic &Medicinal Chemistry Letters 20(2010)
346–349
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incread mortality in patients receiving torcetrapib and atorva-statin relative to the atorvastatin-only group.19This adver effect may have been related to the obrvation of an increa in mean systolic blood pressure in the torcetrapib treated arm.There is cur-rently significant debate over whether the adver effects obrved with torcetrapib were caud by mechanism bad factors or off-target activities.26–28Despite the uncertainty regarding the viabil-ity of CETP inhibitors,there is continued interest in the develop-ment of a cardiovascular drug against this target.This communication details the identification and optimization of a r-ies of2-arylbenzoxazole bad CE
TP inhibitors.
Inhibition of CETP mediated CE transfer was characterized in vitro using afluorescence transfer assay.29The assay us syn-thetic HDL donor particles that contain lf-quenching BODIPY la-beled CE along with an additionalfluorescence quencher.As the BODIPY labeled CE is transferred from the donor particle to an acceptor lipoprotein by CETP,fluorescence is obrved and quanti-fied.Inhibition of CETP mediated CE transfer was characterized by a decrea in levels offluorescence obrved relative to control.
A high throughput screen of compounds in the Merck collection at2l M using the above assay was conducted and hits were con-firmed by titration.2-Arylbenzoxazole1was identified as a screen-ing hit.This lead class was also independently identified by rearchers at Bristol–Myers Squibb and published subquent to our studies.30Their work showed the importance of substitution at the benzoxazole5-position on potency of CETP inhibition.The work described in this publication is consistent with that described by BMS,but also shows that further substitution of the benzoxaz-ole moiety at the7-position leads to compounds with additional enhancement of CETP inhibition.
O NH O
O
N
1
Cl
The development of lead compound1began with the investiga-tion of the effect of variation of the substitution of the benzoxazole moiety.The synthetic approach ud is shown in Scheme1.Key carboxylic acid2was synthesized from phenoxyacetic acid3by first treating with oxalyl chloride to form the corresponding acid chloride4.Coupling with4-aminobenzoic acid afforded carboxylic acid2.Benzoxazoles were then synthesized by one of three meth-ods.31Activation of2as the corresponding acid chloride using oxa-lyl chloride followed by coupling with a range of substituted2-aminophenols afforded amides of the general structure6.A solu-tion of amides6in xylene was heated at reflux in a Dean–Stark apparatus with either p-tolenesulfonic acid or pyridinium p-tolu-enesulfonic acid,to afford2-arylbenzoxazoles of general structure 7.Alternatively,a solution of carboxylic acid2,a2-aminophenol and boric acid in xylene could be heated at reflux or subjected to microwave irradiation at270°C to afford the corresponding2-aryl-benzoxazole.2-Aminophenols were either commercially av
ailable or obtained by reduction of the corresponding2-nitrophenol using either heterogeneous palladium or platinum oxide catalyzed hydrogenation or treatment with tin(II)chloride(Scheme2).Some 2-nitrophenols were obtained by nitration of the corresponding phenols.
The CETP inhibition data for a ries of compounds of different benzoxazole phenyl ring substitution is shown in Table1and it can be en that substitution of this ring appreciably alters CETP inhib-itory activity.The unsubstituted benzoxazole(8)was10-fold less active than lead compound1.A survey of a ries of benzoxazole substituents(compounds9–22)showed a clear preference for sub-stitution at the5-position.In particular the5-nitro and5-cyano
Table1
SAR of2-arylbenzoxazoles
隔空取物口诀Data reported is derived from duplicate wells and three independent experi-
ments.Mean IC50values were determined from10-point,one-third log concen-
tration respon curves and standard errors were610%.
b IC
50
not determined if%max inhibition was<50%.
C.J.Smith et al./Bioorg.Med.Chem.Lett.20(2010)346–349347
derivatives 15and 20were found to be the most potent CETP inhibitors with IC 50s of 0.94and 0.13l M,respectively.Compounds 23–29(Table 2)reprent a ries of substitutions that are toler-ated at the 5-position with only methoxy (compound 24)showing better potency than the original lead 1.Other 5-substituents such as larger alkyl,trifluoromethyl,methyl ester,carboxylic acid,amides,carbamates,sulfonamides,sulfones,hydroxyl,anilino,amidine,tetrazole and substituted phenyl were found to have little or no inhibitory activity (data not shown).The best 5-substituted deriv
ative found was therefore 5-cyano derivative 20and this rep-rented an early benchmark compound.This was consistent with the work published by rearchers at Bristol–Myers Squibb.30
Holding the cyano group constant,the SAR of additional benz-oxazole substitution was then explored (Table 3).The three possi-ble regioisomeric methyl derivatives 31,33and 35were prepared via palladium catalyzed cyanation of the corresponding aryl ha-lides 30,32and 34,respectively.The CETP inhibition data clearly shows that incorporation of a methyl group is preferred at the 7-position and affords a twofold increa in potency (compare 35to 20and 34to 23).Compound 35has a CETP IC 50of 60nM.Com-pounds 36–41show that a number of other substituents are toler-ated at the 7-position in combination with either 5-cyano or 5-halo the most potent being 5-cyano-7-fluoro derivative 39with a CETP IC 50of 62nM.
The substitution of the 7-position was further investigated by the synthesis of a ries of alcohols (compounds 44–57,Table 4).The compounds were synthesized from acetophenone 40as shown in Scheme 3.Treatment of ketone 40with sodium borohy-dride or a Grignard reagent afforded condary or tertiary alcohols respectively of general structure 42.The compounds were then transformed into nitriles of general structure 43via palladium cat-alyzed cyanation.By and large,potency of CETP inhibition is inver-ly proportional to the size of the alkyl group added to acet
河北科技大学学报ophenone 40,the best compound being methyl derivative 47with a CETP IC 50of 28nM.Replacement of the 5-cyano group of 47with a hydrogen to give compound 57results in a 10-fold loss in potency of CETP inhibition confirming the importance of substi-tution at both the 5-and 7-positions.
Compounds 20,35and 47were evaluated in a pharmacody-namic model in mice expressing cynomolgus monkey CETP and
did not show an increa in HDL-C levels.This may be attributed to the lack of oral bioavailability obrved with the compounds in mou PK studies.
In summary,after high throughput screening of the Merck com-pound collection identified 2-arylbenzoxazole 1as a lead,it was developed into compounds 35and 47using a modular synthetic approach that showed the importance of substitution at the 7-po-sition for potency enhancement over the 5-position alone.The compounds reprent important leads for further development of this structure class as CETP inhibitors.Further modifications of the amide and aryloxy moieties will be reported.
Table 2
SAR of 5-substituted-2-arylbenzoxazoles
Data reported is derived from duplicate wells and three independent experi-ments.Mean IC 50values were determined from 10-point,one-third log concen-tration respon curves and standard errors were 610%.b
Synthesized by reduction of 26;NaBH 4,MeOH,room temperature,1h,quant.c
Synthesized from 23by Stille coupling;vinyltributyl tin.(Ph 3P)4Pd,DMF,80°C,12h,13%.d
Synthesized from 23by Sonagashira coupling;(i)TMS acetylene,Pd(PPh 3)2Cl 2,CuI,Ph 3P,Et 2NH,DMF,microwave irradiation,120°C,75min,(ii)aq NaOH,THF,room temperature,1h,32%.
Table 3
SAR of disubstituted-2-arylbenzoxazoles
Data reported is derived from duplicate wells and three independent experi-ments.Mean IC 50values were determined from 10-point,one-third log concen-tration respon curves and standard errors were 610%.
Table 4
SAR of 5,7-disubstituted-2-arylbenzoxazoles
Data reported is derived from duplicate wells and three independent experi-ments.Mean IC 50values were determined from 10-point,one-third log concen-tration respon curves and standard errors were 610%.b
Synthesized by reduction of 40;NaBH 4,MeOH,room temperature,1h,quant.c
Synthesized from 46;LiAlH 4,THF,room temperature,1.5h,14%.
348 C.J.Smith et al./Bioorg.Med.Chem.Lett.20(2010)346–349
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31.Benzoxazole synthesis—General method1:A mixture of2(1.05mmol),2-
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aminophenol derivative(1.05mmol)and boric acid(1.37mmol)in o-xylene (60mL)was heated at reflux under a Dean–Stark apparatus overnight.After this time the reaction mixture was diluted with EtOAc(50mL),washed successively with saturated NaHCO3(50mL),H2O(50mL),and brine(50mL), dried(Na2SO4)and concentrated in vacuo to afford the crude product.This was purified byflash chromatography and/or reverd pha HPLC to afford the desired benzoxazole.
General method2:A mixture of2(0.307mmol),2-aminophenol derivative
(0.430mmol)and boric acid(0.430mmol)in o-xylene(2.5mL)was subjected
to microwave irradiation(300W,270°C,60min).The reaction mixture was diluted with EtOAc(25mL),washed successively with saturated NaHCO3 (25mL),H2O(25mL),and brine(25mL),dried(MgSO4)and concentrated in vacuo to afford the crude product.This was purified byflash chromatography and/or reverd pha HPLC to afford the desired benzoxazole.
General method3:A solution of oxalyl chloride(2M in CH2Cl2,1.40mmol)was added to a stirred suspension of2(0.702mmol)in CH2Cl2(11mL)followed by
a few drops of DMF at room temperature under N2.The reaction was stirred at
room temperature for4h after which time the suspension dissolved.The reaction mixture was concentrated in vacuo and azeotroped with toluene (10mL).The crude acid chloride and2-aminophenol(1.05mmol)were dissolved in1,4-dioxane(20mL)and heated at reflux for4h under N2.The reaction was diluted with EtOAc(50mL)and water(50mL)and the aqueous layer was extracted with EtOAc(2Â50mL).The combined organic extracts were washed with brine(50mL),dried(Na2SO4)and concentrated in vacuo to afford the crude amide product.A mixture of the crude amide and pyridinium p-toluenesulfonate(0.0702mmol)in o-xylene(30mL)was heated at reflux under a Dean–Stark apparatus overnight under N2.The reaction was diluted with EtOAc(100mL)and washed successively with saturated NaHCO3(50mL), water(50mL)and brine(50mL),dried(Na2SO4)and concentrated in vacuo to afford the crude product.This was purified byflash chromatography and/or reverd pha HPLC to afford the desired benzoxazole.
C.J.Smith et al./Bioorg.Med.Chem.Lett.20(2010)346–349349
