3.2. QSAR of SBO

logVmax = 1.485(0.022) + 0.096(0.030) + 0.353(0.046)m + 0.098(0.021)Es ............ (3)

where n=18, r=0.9134, s=0.0453, and F=23.489.

Chart 2. Observed logVmax vs Calculated value from eqn. (3) for QSAR of SBOs

The correlation of calculated logVmax from eqn. (3) and observed value from bioassay was examined and shown in Chart 2. According to eqn. (3) , the electronic nature of a m-substituent was the most important followed by the steric and hydrophobic effects for SBO. The positive and m terms mean that the more hydrophobic and the more electron-withdrawing the substituent, the greater the activity. 3,5-F2 derivative 41 had the highest Vmax, followed by 3-Cl (29) and 2,5-F2 (40) derivatives for stimulating adenylate cyclase prepared from thoracic nerve cords of P. americana (Table 3).

Table3. QSAR of SBO(<i>V</i>max)

pKa = 2.528(0.675) + 1.962(0.446)logVmax ......... (4)

where n=8, r=0.8739, s=0.1613, and F=19.386.

According to eqn. (4), the positive logVmax term means that the larger Vmax value, the higher pKa value. The observation that relationship between Vmax and pKa for SBOs is apparently not matched with that for SBTs implies there may be remarkable variations in the agonist-receptor interaction mechanism for these types of compounds. In addition, the presence of alkyl groups at the p-position or of an alkyl group at m-position of SBO seemed particularly important (Table 4).

Table4. QSAR of SBO(<i>pKa</i>)

Oxazoline 3-Cl-SBO (29) was more potent in terms of both Ka (0.52 M) and Vmax (40% relative to OA) than its thiazoline derivative (Ka=1.61 M, Vmax=23% relative to OA). Similarly, 2,5-F2-SB0 (40) (Ka=0.55 M, Vmax=33%) were more potent than thiazoline 16 (Ka=3.5 M, Vmax=27%). 2-Cl- (25), 3-CF3- (31), 4-Cl-, 4-F-(33), 4-CH3-, 4-OCH3- (36), 2,3-(OCH3)2-, 3-Cl,4-F-, 3,4-F2-, and 3,5-F2-SBO (41) had higher Vmax values (32, 32, 44, 27, 47, 32, 25, 16, 21, and 54%, relative to OA) than thiazolines [24, 26 (3-CF3) (8), 22 (4-Cl) (9), 20, 26, 29 (4-OCH3) (11), 8, 12 (3-Cl,4-F) (21), 19 (3,4-F2) (22), and 33% (3,5-F2) (35) relative to OA] suggesting that SBOs are more potent than their thiazoline derivatives (SBTs).

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