## 2 Chemistry and Structure-Activity Relationship of Styrylquinolines _in vitro_
### Modulation of the C-7 substituent on the 8,3',4'-trihydroxyyrylquinoline scaffold  
The introduction of a polar group at C-7 was the key breakthrough in the styrylquinoline class design story. Thus, when an additional nitrile group was introduced at the C-7 position in the quinoline half (compound **34**, Table 3) micromolar levels of anti-integrase inhibition were reached. The starting material in the preparation of nitrile **34** was the known 7-carbaldehyde-8-hydroxy-2-methylquinoline which was first converted into oxime **33**. Concomitant dehydration of the oxime function of **33** took place during the Perkin condensation with 3,4-dihydroxybenzaldehyde (**16**), affording carbonitrile **34** in 71% yield. Similarly, styrylquinoline **35** resulted from the condensation between quinaldine **3b** and aldehyde **16**, followed by hydrolysis of the crude product by using a pyridine-water mixture [14]. The introduction of a carboxylic acid group at C-7 (compound **35**) produced potent HIV-1 IN inhibitors. Furthermore, beside this good _in vitro_ potency, this compound gave rise to a significant antiviral effect as measured by \(\beta\)-gal assay, whereas no toxicity on either infected or non-infected cells was observed. In sharp contrast, compound **54** in which the 7-carboxyl group was replaced by a 7-carbomethoxy group, was completely devoid of biological activity [26]. The latter result clearly indicated that a free carboxylic group greatly improved integrase inhibition, suggesting that a Lewis base containing group at C-7 is need for good inhibition potency. Interestingly enough, an amide function is unable to play such a role, since styrylquinoline carboxamides **55** (Table 3), obtained through direct condensation of the requisite amine with **35**, exhibited no noticeable HIV-1 IN inhibition when tested up to 300 \(\mu\)M [19]. The significant loss of HIV-1 IN inhibitory activities of these compounds compared to **35** indicated that the increased size of such compounds seems not suitable to fit into the catalytic pocket of the active site although these compounds possess two pharmacophores that can bind with Mg\({}^{2+}\). The importance of the basic nitrogen in the styrylquinoline scaffold has been illustrated by comparison with the corresponding styrylbenzofuran **37** which was found to be 20-fold less potent that **35**. The latter was prepared with 20% yield through a Wittig type olefination using phosphonium **36** and 3,4-dihydroxybenzaldehyde (**16**) [27]. **Scheme 3. ** Synthesis of 8-hydroxy-3',4'-dihydroxystyrylquinoline 7-carbonitrile (**34**), 7-carboxylic acid **35** and benzofuran **37**. In line with the above results, well known isosters of carboxylic acid such as phosphonic acid and tetrazole group were introduced at the C-7 position of the styrylquinoline scaffold to evaluate their influence (compounds **43** and **46**). Furthermore, the distance between the quinoline ring and the acid function introduced at C-7 was modulated by introducing an ethylenic spacer as in **42**. The syntheses of **43** and **46** are depicted in Scheme 4. First, the crucial regioselective bromination of the C-7 position of 8-hydroxyquinaldine **2b** was carried out using the _tert_-BuNH\({}_{2}\):Br\({}_{2}\) complex [28]. Condensation of 7-bromo-8-hydroxyquinaldine (**38**) with 3,4-dihydroxybenzaldehyde (**16**) and protection of the hydroxyl groups gave SQLs **40** and **44** respectively. Stille cross-coupling reaction of bromide **40** with the stannyl tetrazole subunit **41**[29] followed by fluoride deprotection of the silyl groups provided the desired 7-tetrarole-styrylquinoline **43**, albeit in modest yield. Similarly the fully protected styrylquinoline **44** was metallated with \(n\)-butyllithium and treated with DMF. The resulting aldehyde **45** was condensed with carbomethoxytriphenylmethylenemosphorane and deprotected with BBr\({}_{3}\) to provide the unsaturated acid **42**[30].