Perspective Journal of Medicinal Chemistry,1994,Vol 37,No.8 1045 Table 4 compound IO(nM)HCAII K(nM)HCAⅡ 05 0.61 心0 A-7700 ethe The irst involved the N- A-7879 1709 m chair gauch 1 kcal/mo dif of th easure for the 100- 13) up to orientedn igher ner able 5 ty In approach. hat of co ation pr n of -SO.NH calculatio suggest that this sh H.C li hilicityin ompound Is (mM K:(M)angle (deg)NSCS (kcal/mol ers were prepared and coo HCA I by. talle MK-417S 61 76 05 n gaine ed from X-rav crvstallog 6. ng the size o th e subs tuent phe ide N the tr R'form (Figure14 the ns at the 3-21 G abinitiole evel su h potency in the trans seri is likely due to over its antiomer This feature al atehavnpeneonteract of the ccoun ditte tBedmenta a balance bet n the incre ed lipophi intera affin between th NSCS can estimated to 1.5 kcal/mol by ab initio alculation sis indicated that the 6 tathat the-ahe 0 6 ctions in thel Perspective Table 4 Journal of Medicinal Chemistry, 1994, Vol. 37, No. 8 1045 14sab compound I5o(nM) HCAII Ki (nM) HCA I1 Senantiomer MK-417 0.54 45 0.61 71 between the enantiomers. The first involved the N-S￾C-S dihedral angle which was 150' for the S-enantiomer and 170' for the R. This represents a 20' twist of the thiophene ring in the R-isomer relative to the S. Ab initio molecular orbital calculations at the 3-21 G* (Gaussian 88) level suggest that the preferred dihedral angle is 72'; this implies that the angle formed in the S-enantiomer, while not ideal, is preferred by a AH of 1 kcal/mol. A second conformational difference between the two enan￾tiomers involved the geometry of the 4-isobutylamino substituent. The side chain is trans in the S-enantiomer and gauche in the R. Ab initio calculations at the 3-21 G* level suggest that the trans geometry should be preferred by 1 kcal/mol. These two conformational features, dif￾ferences in the NSCS dihedral angles and the trans vs gauche geometry of the side chain, can account in large measure for the 100-fold difference in affinity and potency observed experimentally for the two enantiomers (Figure 13).71 Optimization of Inhibitors. Since accommodation within the active-site cavity requires the isobutylamino group to be oriented in the higher energy pseudoaxial conformation, enhanced affinity could presumably be obtained by decreasing this energic penalty. In one such approach, that of conformation preference, a methyl group was introduced into the 6-position of the thienothiopyran ring system; ab initio calculations suggest that this should eliminate the pseudoequatorial preference. To counter the enhanced lipophilicity introduced by the methyl substituent, the 4-isobutylamino group was modified to an ethylamino moiety. All of the four possible optical isomers were prepared and cocrystallized with HCA 11, and the structure of each complex was determined by X-ray crystallography. As shown in Table 5, the isomer with the greatest inhibitor potency and affinity had the trans S,S￾configuration and a Ki value of 0.37 nM. The structural information gained from X-ray crystallography established that the major difference in the conformation of the four optical isomers as bound within the active site was the thiophenesulfonamide NSCS dihedral angle. This torsion angle ranged from 140' for the S,S-isomer to 175' for the R,R form (Figure 14). Calculations at the 3-21 G* ab initio level suggests this 35' difference amounts to - 1.5 kcal/mol favoring the S,S￾isomer over its R$-enantiomer. This feature alone accounts for the 20-fold difference found experimentally between the two optical isomers. Likewise, the difference in affinity observed between the two cis isomers can be accounted for by differences in the NSCS dihedral angles. X-ray crystallographic analysis indicated that the 6- methyl group was immediately above a lipophilic groove, one wall of which is formed by Phe 131. In theory, A-77003 A-78791 140 17 Figure 10. Comparison of A-77003 with its deshydroxy analogue, A-78791: (A, top) chemical structures and inhibition constants for HIV PR; (B, bottom) overlay of inhibitor structures after superposition of the enzyme active sites from the structures of the two inhibitor-enzyme complexes; the two inhibitor structures agree to within 0.15 8, rms for all non-hydrogen atoms.% Atoms are color-coded by type. Table 5 #J HN HCA I1 dihedral AH compound 1x1 (nM) Ki (nM) angle (deg) NSCS (kcal/mol) trans S,S 0.23 0.37 140 cis S,R 1.1 2.0 153 0.5 transR,R 7.1 5.5 175 1.5 cis R,S 29.5 15.3 168 1.5 MK-417s 0.54 0.61 150 0.5 extending the size of this alkyl substituent should increase the nonbonded interaction with the enzyme. As indicated in Table 6, increasing the size of the substituent from methyl to ethyl to propyl results in only a modest improvement in affinity for the trans S,S-enantiomers whereas a 10-fold enhancement in Ki was observed with the cis S$-isomers. Modeling studies suggest that failure to improve potency in the trans series is likely due to negative 1,5-synpentane interaction that occurs between the side chain and the lower oxygen of the ring sulfone. Thus, a balance between the increased lipophilic interac￾tion is compensated by a negative steric interaction estimated to be -1.5 kcal/mol by ab initio calculations using a 3-21 G* basis set. With the cis S,R-isomers, modeling suggests that the 6-alkyl substituent can engage in nonbonded interactions in the lipophilic groove without unfavorable steric interaction with the sulfone oxygen