Further explorations at the ortho and meta positions

Further explorations at the ortho- and meta-positions of the phenyl in the tail proceeded on the basis of the preliminary results. First, we investigated the fluorine group at the meta- (13p) and ortho-positions (13q) of the phenyl group, with both compounds being slightly less potent than 13o. Surprisingly, conversion of fluorine into the strong electron withdrawing nitro group at the ortho-position (13s) displayed a potency similar to that of 13q, while 13r (nitro group at the meta-position) was less active than 13s but more potent than 13m. We further examined chlorine and methoxy substitutions ortho and meta to the phenyl, and a similar trend of potency to that of 13r and 13s (data not shown). These results imply that the ortho-position is well tolerated with electron-withdrawing/donating and sterically hindered substituents, while the length and size of the substituents at the meta-position may impact activity. In addition, we assessed the viability of 1,2,3,4-tetrahydroquinoline (13t) and 1,2,3,4-tetrahydroisoquinoline (13u) as the tail on account of our previous SAR results. To our delight, 13t and 13u each had comparable activity to that of 13c, with 13u being slightly more potent than 13c. Further structural expansion of 13t led to 13v, a compound with slightly decreased potency. These result indicated that a linear fused-ring tail was more favorable. Altogether, these SAR results revealed that the potency was strongly impacted by the size of the substituents and steric hindrance around the phenyl ring in the tail. The incorporation of a small fluorine group at para-position of the tail gave the most potent compound 13o. Further exploration of the tail led to identification of another potent compound, 13u, with a shorter fused-ring as the tail. These results were followed by continued structural modifications to the head and linker of the chemical structure. Modifications to the β-substituted benzenepropanoic KN-92 mg in the head of 13o (Table 4) did not yield any improvement in potency. For example, replacement of the 1-propynyl group with a cyclopropyl group (14b), direct removal of the propynyl group (14a) and further alteration of the head moiety into 3-((2,3-dihydrobenzofuran-3-yl))acetate acid (14c) all resulted in markedly eroded potency. Next, the stereochemistry at the β-position was examined on account of previous reports that the S-isomer was more favorable [23]. The S-isomer (14d) indeed had a slightly improved potency over 13o (racemate), while the R-isomer (14e) had markedly lower potency. These results supported that (S)-β-propynyl substituted 3-benzenepropanoic acid was an optimal head for the amide structure. The SAR exploration of the linker aimed to replace the thiophene ring with bioisosteres. Some moieties found to be favorable for potency in our previous SAR findings were combined with the linker in an effort to produce the most potent compound(s). (S)-β-Propynyl substituted 3-benzenepropanoic acid was chosen as the head and, for the tail, 4-fluorine phenylpiperazine, 1,2,3,4-tetrahydroisoquinoline and 1,2,3,4-tetrahydro-1-naphthylamine were selected, with focus on both potency and structural diversity. Thiazole, benzene and furan rings were introduced to replace the thiophene ring of the linker (Table 5). The thiazole ring was deleterious for potency with three types of tails (15b, 15e, 15h). The combinations of a 1,2,3,4-tetrahydroisoquinoline and a furan linker (15f) or 1,2,3,4-tetrahydro-1-naphthylamine and a benzene linker (15g) afforded high potency, with 15g exhibiting the best activity. 15g, which was the most active agonist for hGPR40 among the analogues we synthesized, has two chiral centers and hence exists as four diastereomers. Considering the more favorable S-configuration at the β-position of the head, the impact of the absolute configuration at the tail was evaluated (Fig. 3). The S,R-isomers (15k) had 2-fold promotion in potency over the S,S-isomers (15j) and the racemate (15g). Furthermore, 15k, 15j and 14d exhibited good agonistic activities for both rat and mouse GPR40, indicating low species-dependent differences (Table 6).