## 3 Results and Discussion
### Structure-Activity Relationship Studies  
The structure-activity-relationship (SAR) studies revealed that the cyt-_bd_ of _M. bovis_ BCG was more sensitive to inhibition than _M. tuberculosis_ H37 Rv (Table 1). This mirrors the trend observed previously and is indicative that there is a greater expression of cyt-_bd_ in the lab-adapted _M. tuberculosis_ H37 Rv strain relative to the _M. bovis_ BCG or \"fresh\" Mtb clinical isolate N0145 [13]. This is further supported by the activity of aurachin D [7], a known synergistic cyt-_bd_ inhibitor of Mtb, which had greater activity against _M. bovis_ BCG in our ATP assay relative to _M. tuberculosis_ H37 Rv (2.9 \(\upmu\)M vs. 5.5 \(\upmu\)M, respectively; Table 1). In addition, based on our assay, compounds **6a-15a** are all specific inhibitors of cyt-_bd_ as they show no activity against wild-type (WT) _M. bovis_ BCG or _M. tuberculosis_ H37 Rv (Mth-H37 Rv), whereas the positive control bedaquiline (BDQ), a selective ATP synthase inhibitor, remains very potent against both strains (Table 1). Interestingly, aurachin D showed the expected synergy with Q203 against mycobacterial strains but was unexpectedly active against wild-type Mtb-H37 Rv (Table 1). SAR trends revealed that the pendant aryl group can accommodate a variety of steric and electronic changes and retain good activity (Table 1). Evaluation of the compounds with electron withdrawing substituents **6a** (_p_-CF\({}_{3}\)), **7a**, (_p_-OCF\({}_{3}\)), **8a** (_p_-CI), **9a** (_p_-SF\({}_{5}\)), indicated that **7a** is the most active compound with IC\({}_{50}\) values of 2.1 \(\upmu\)M against _M. bovis_ BCG and 11 \(\upmu\)M against Mtb H37 Rv. We explored electron withdrawing groups at the _meta_- and _ortho_-positions of the pendant aryl group by comparing compound sets **7a** (_para_-OCF\({}_{3}\)) and **13a** (_meta_-OCF\({}_{3}\)) and **6a** (_para_-CF\({}_{3}\)) and **15a** (_ortho_-CF\({}_{3}\)). We found that the _meta_-OCF\({}_{3}\) compound (**13a**) position was less active compared to the _para_-OCF\({}_{3}\) (**17a**). Whereas incorporation of substituents in the _ortho_-position gave slightly improved potency againstM. bovis_ but equal activity (within error) in Mtb-H37 Rv as demonstrated by comparing **6a** and **15a**. When evaluating the compounds with electron donating substituents, **10a** (_p_-CH\({}_{3}\)), **11a** (_p_-OCH\({}_{3}\)), and **12a** (_p_-_tert_-butyl), **12a** was found to be significantly more active with IC\({}_{50}\) values of 0.8 uM against _M. bovis_ BCG and 5.8 uM against Mtb H37 Rv. That is an improvement over thieno[3,2-_d_]pyrimidine (**1**) and activity on par with the optimized quinazoline ND-11992 (**2**). This is particularly interesting as it follows the Topliss aromatic substitution decision tree pattern [23] where one would first evaluate the phenyl (**3**), then _para_-chloro (**8a**) and if equally potent (as it is in our assays) then prepare the _tert_-butyl (**12a**) to derive the most potent compound. While the Topliss approach can often lead to compounds with improved activity it also directs the syntheses of compounds which often have metabolic liabilities (CH\({}_{3}\), OCH\({}_{3}\), _t_-butyl) or toxic pharmacophores (NH\({}_{2}\), NO\({}_{2}\), I). For this reason, we chose **7a** which bears a _para_-trifluorometoxy group (which is prevalent in various anti-TB drugs such as delamamaid, pretomandi, and telacebec) as the basis for our second set of compounds (Table 2) because it possesses good activity (IC\({}_{50}\)'s of 2.1 and 11 uM against BCG and Mtb-H37 Rv, respectively). The SAR trends for alteration of the quinazoline core suggest an interplay between both steric and electronic effects (Table 2). For instance, the 6-position was probed with three substituents-fluoro (**16a**), methyl (**17a**) and methoxy (**18a**)--which resulted in a wide spectrum of activity. The 6-methyl analog (**17a**) was the most active against _M. bovis_ BCG (IC\({}_{50}\) of 7.5 uM) but weakly active against Mtb H37 Rv (IC\({}_{50}\) of 22 uM).