Journal of Medicinal Chemistry Scheme 3. Modiﬁcation on the 6 Position of Imidazolopiperazine Corea Article aReagents and conditions: (a) methallyl chloride, K2CO3, KI, DMF, rt, 55%; (b) AcOH/MsOH (6:1), 210 °C, 39%; (c) N-Boc-glycine, HATU, DIEA, DMF, rt; 57%; (d) Br2, AcOH, CH2Cl2, rt; 55%; (e) ArNH2, Pd2(dba)3, xantphos, Cs2CO3, dioxane, 150 °C; (f) TFA, CH2Cl2, rt, 55% over 2 steps (for example 25, Ar = 4F-Ph). Scheme 4. Modiﬁcation on the 5 Position of Imidazolopiperazine Corea aReagents and conditions: (a) PMBCl, KOH, DMF, 0 °C to rt, 63%; (b) MeI, NaH, DMF, rt, 86%; (c) BH3·THF, THF, reﬂux, 100%; (d) TFA, 70 °C, quantitative; (e) N-Boc-glycine, HATU, TEA, DMF, rt, 49%; (f) Br2, AcOH, CH2Cl2, 90%; (g) ArNH2, Pd2(dba)3, xantphos, Cs2CO3, dioxane, 150 °C; (h) TFA, CH2Cl2, rt, 46% over 2 steps for example 26 where Ar = 4F-Ph. carbamate protecting group. Despite the harsh reaction condition, an acceptable yield of the product (39%) was obtained and the regiochemistry was conﬁrmed by 2D NMR of the intermediate. The cyclized core was subjected to amide coupling with N-Boc glycine to yield intermediate 13. The ﬁnal steps in the sequence are analogous to Scheme 2 to furnish analogue 14. The sequence involved six steps from inter- mediate 5a with an overall yield of 3.7%. The synthesis of the 5,5-dimethyl modiﬁed analogues diverged from their 8,8- dimethyl modiﬁed counterparts at intermediate 7a. N-PMB protection was necessary for lactam 7a before the NaH- mediated bis-methylation at the 5-position (Scheme 4). The lactam was then reduced using borane THF complex to aﬀord 7b. The bicyclic intermediate was then deprotected and amine was subjected to a HATU mediated amidation to obtain compound 7c. The ﬁnal sequence involved bromination followed by palladium-catalyzed aniline amination and Boc- deprotection to furnish 5,5-dimethyl modiﬁed compound 26. The sequence involved eight steps from intermediate 7a with an overall yield of 10.9%. ■ RESULTS AND DISCUSSION Structure−Activity Relationship. We began exploring the SAR on the piperazine ring by substituting at the 8-position ﬁrst, and we decided to use 4-F-aniline and 3-F-4-Cl aniline based on our previous work at the R6 position (Table 1).5 Adding a methyl group to the 8-position led to loss of potency by 10-fold (cmpd 17), but the compound was still equipotent to compound 15 (Scheme 5). Moreover, there was no pro- found eﬀect on the potency by changing the absolute conﬁguration of the methyl group. Increasing the size of substituent to isopropyl group led to dramatic loss in potency (examples 19 and 20). A dimethyl group on the 8-postion not only led to more favorable compound in terms of completely blocking the benzylic carbon as indicated by our metabolite studies but also removing the stereogenic carbon. Surprisingly, substitution of 3Cl-4F aniline in the 8-disubstitution case provided a less potent compound when compared to other anilines (21 vs 22, 23, or 24). Emboldened by the increase in the potency based on the substitution on the 8-position, we decided to study the eﬀect of diMe group on the ethylene carbons of the piperazine ring. On the basis of examples 25 and 26, it was abundantly clear that the dimethyl group on the 8-position had the most signiﬁcant positive eﬀect on potency. The substitution on the other positions of the piperazine ring only provided modest potencies when compared to 8-substitution, with activities in the double- digit nanomolar range. As the 5-substituted compounds and the 6-substituted compounds were less potent, we decided to focus our attention on the eﬀect of the amide substituent of the piperazine nitrogen with the 8,8-dimethyl group constant. Our previous eﬀorts demonstrated that dimethyl glycine had slightly inferior potency but oﬀered superior oral exposure. In the case of the substituted piperazines, installation of α methylalanine amide provided equipotent compounds (15 vs 27, 22 vs 28, and 23 vs 29). We proceeded to further understand the overall eﬀect of the dimethylpiperazine on the minimal pharmaco- phore. After studying the eﬀects of various substitutions on the piperazine ring and obtaining a signiﬁcant improvement in activity, we decided to reconﬁrm the minimal pharmacophore needed for antiplasmodial activity. On the basis of the earlier results, we were able to achieve antiplasmodial activity with EC50 in ∼160−400 nM range for the core devoid of the amino acid substituent on the piperazine nitrogen (compound 31, Table 2). Modiﬁcation of the piperazine ring into the corresponding lactam led to a ∼7-fold decrease in activity in the micromolar range (32). However, the antiplasmodial activity is regained by introduction of dimethyl substituent on the 8-position with a 7-fold improvement (33). The most optimal compound from this lactam subseries was a compound with 3Cl-4F aniline substitution with an activity around 50− 100 nM (34). The dimethyl substitution on the reduced piperazine core led to ∼10-fold improvement in potency (31 vs 35) with compounds to ∼10 nM range. These results led us to conclude that the amino acid substitution had minimal to no eﬀect on the potency of the compound with substitution on the 8-position of the imidazolopiperazine core and that the basicity 4246 dx.doi.org/10.1021/jm300041e | J. Med. Chem. 2012, 55, 4244−4273