key: cord-0880838-m30w4o7d
authors: Ma, Dawei; Xie, Weiqing; Zou, Bin; Lei, Qiong; Pei, Duanqing
title: An efficient and tunable route to AG7088, a rhinovirus protease inhibitor
date: 2004-10-18
journal: Tetrahedron Lett
DOI: 10.1016/j.tetlet.2004.08.179
sha: afd02eef40ddcddd7af866ab707eb5f387cdb71a
doc_id: 880838
cord_uid: m30w4o7d

Aldol reaction of N,N-dibenzyl valinal with propiolic acid ethyl ester derived lithium reagent provides anti-aminoalcohol 8 and syn-aminoalcohol 9, which are converted into the lactone 6 via two different routes. Alkylation of 6 followed by lactone ring opening afford the acid 2a, which is coupled with the amine 3 and 5-methylisoxazole-3-carboxylate acid 1, respectively, to deliver AG7088.

AG7088 is a potent 3C protease inhibitor with an EC 90 of less than 0.1 lM against 48 different serotypes of human rhinovirus (HRV). 1, 2 This compound was discovered by Agouron researchers and is currently in human clinical trials to treat the common cold. 1, 2 This inhibitor was received more attention recently because it was proposed as a promising inhibitor to viral main proteinase (M pro , also called 3Cl pro ), a key enzyme for controlling the activities of the coronvirus replication complex. 3 Since the coronvirus is the causative agent of severe acute respiratory syndrome (SARS), development of potent inhibitors for this enzyme would be of benefit for SARS therapy.

By computer modeling, Hilgenfeld and co-workers indicated that the p-fluorobenzyl side chain at the P2 position of AG7088 might be too big for fitting into the S2 pocket of coronvirus M pro and those AG7088 analogues with smaller groups at this site might have more favored interaction with this enzyme thereby serving as more potent inhibitors. 3 In order to prove this hypothesis, as well as to discover potent coronvirus M pro inhibitors based on this hypothesis, we initiated a program to synthesize AG7088 analogues.

As the retrosynthetic analysis outlined in Figure 1 , AG7088 was prepared initially by Agouron researchers Tetrahedron Letters by coupling the ketomethylene dipeptide isostere 2 with 5-methylisoxazole-3-carboxylate acid 1 at its left hand and 3 at its right hand. 1 The ketoester 2 was obtained through a lactone 4 using diastereoselective alkylation of (À)-pseudoephedrine-derived amide 5 with p-fluorobenzyl bromide and subsequent lactonization promoted by NBS as the key steps. 4 Obviously, this protocol is not so efficient for quickly assembling AG7088 analogues with different side chains at the P2 site because installation of this side chain was set up at the early stage. This drawback, together with the requirement of (À)-pseudoephedrine as the chiral auxiliary, promoted us to develop a new route to the lactone 4 by direct alkylation of lactone 6. This lactone could be prepared using L Lvaline as the starting material.

Our synthesis for the lactone 6 was illustrated in Scheme 1. After L L-valine was treated with benzyl bromide under the assistance of potassium carbonate, the resultant ester was reduced with LAH to provide N,N-dibenzyl valinol 7. Swern oxidation of 7 followed by nucleophilic attack by a lithium reagent derived from propiolic acid ethyl ester provided anti-aminoalcohol 8 in 70% yield, together with syn-aminoalcohol 9 in 13.5% yield. The stereochemistry of 8 was assigned by transformation of it to known lactone 16 as described below. Since the diastereoselectivity (8:9 = 5.2) was not good at the aldol reaction step, we attempted to improve it using suitable additives. It was found low diastereoselectivity was still encountered when TiCl 4 , 5 Al(OPr-i) 3 , 5 or MgCl 2 6 was used. However, the ratio of 8 to 9 jumped to 9:1 when 2 equiv of HMPA 6 was added. Unfortunately, the yield was only 58% in this case. Consequently we still employed the initial condition to get aldol products when the reaction was scaled up.

After hydrogenation of the syn-aminoalcohol 9 catalyzed by Pd(OH) 2 /C along with in situ protection with di-tert-butyl dicarbonate, alcohol 12 was obtained. Treatment of 12 with acetic acid in toluene delivered the lactone 6 7 in 89% yield. In order to convert the anti-aminoalcohol 8 to 6, its C4 stereochemistry should be reversed. Accordingly, hydrogenolysis of 8 followed by in situ protection produced alcohol 10, which was transformed into its mesylate 11 with mesyl chloride. It was expected to obtain the lactone 6 by simple lactonization of 11 through an intramolecular S N 2 reaction. However, we found that a mixture of 6 and its 5-epimer 13 were isolated under various conditions such as exposure of 11 to NaOH in ethanol or DMF. The ratio of 6 and 13 was about 2:1-1:1, which implied that S N 1 reaction might also take place. Thus, we had to give up this protocol for assembling 6 from the alcohol 10.

Occasionally, we found that 11 was spontaneously transformed into oxazolidone 14 by standing at room temperature, which indicated that an intramolecular S N 2 reaction between mesylate unit and carbonyl moiety of Boc group occurred. Based on this observation, we developed a route from 11 to 6 as depicted in Scheme 2. Heating 11 in DMF at 65°C produced 14 quantitatively, which was treated with di-tert-butyl dicarbonate to afford 15 in 89% yield. After hydrolysis to cleavage the ester group and the oxazolidone ring, the resultant c-hydroxy acid was converted into the lactone 6 mediated with TsOH.

With the lactone 6 in hand, we finished the total synthesis of AG7088 starting from its alkylation. As depicted in Scheme 3, treatment of 6 with LiHMDS followed by trapping the anion with 4-fluorobenzyl bromide provided lactone 16 in 63% yield, together with its 2-epimer in 4.5% yield. Next, the lactone ring was opened with aqueous LiOH in DME, the liberated hydroxy group was oxidated with PCC to afford 2a in 61% yield. Finally, coupling of 2a with the amine 3 8 mediated with HATU produced 17 in 80% yield, which was exposed to trifluoroacetic acid to remove the Boc protecting group. The liberated amine was connected with 5methylisoxazole-3-carboxylic acid 1 assisted by HATU to furnish AG7088 9 in 73% yield. Its analytical data were all identical with those reported.

As a conclusion, we have developed a novel protocol to construct AG7088 using L L-valine as the chiral starting material. The key elements include a diastereoselective aldol reaction of N-protected valinal, transformation of the alcohol 8 into the lactone 6, and subsequent alkylation. This route is obviously efficient for tuning the substituents at the P2 position to obtain AG7088 analogues. Further synthetic work, as well as the biological evaluation of the resultant compounds, are underway and will be reported in due course. 

Selected data: ½a 15

m, 2H), 1.90-1.79 (m, 1H)

40 (s, 1H), 5.92 (s, 1H), 5.50 (d, J = 15.6 Hz, 1H)

The authors are grateful to the Chinese Academy of Sciences, National Natural Science Foundation of China (Grant 20132030 and 20342002), and Science and Technology Commission of Shanghai Municipality (Grants 02JC14032 and 03XD14001) for their financial support.