Hikmet Agirbas

Synthesis, characterization, antimicrobial activity, and QSAR studies on substituted oxadiazaboroles

This paper presents the synthesis and in vitro antimicrobial activity studies of 3,4,5-trisubstituted 4,5-dihydro-1,2,4,5-oxadiazaboroles (4) and 3,5-disubstituted 4,5-dihydro-1,2,4,5-oxadiazaboroles (7). The antimicrobial activities of the compounds were assessed against a panel of microorganisms including Staphylococcus aureus, Enterococcus faecalis, Pseudomonas aeruginosa, Escherichia coli, Streptococcus mutans, and Candida albicans. Some of the oxadiazaboroles exhibited fair activities against these microorganisms. The pMIC values of the compounds were first correlated with Hammett polar substituent constant (σ) and lipophilic constant (π) and statistically significant correlations were obtained. Additionally, the pMIC values of the compounds were correlated with σ, π, and some theoretical descriptors and fair 2D-quantitative structure–activity relationship models with clogP, surface area approx, ELUMO, µ, and EHOMO as independent variables were obtained. Application of training and test sets to quantitative structure–activity relationship models gave good results. Squared correlation matrix of the theoretical descriptors used in the quantitative structure–activity relationship study showed no correlation between the descriptors.

13C NMR STUDY OF SUBSTITUENT EFFECTS IN 1,2,4-OXADIAZOLE AND 1,2,4-THIADIAZOLE DERIVATIVES

13C chemical shifts of C═N, C═O and C═S carbons of 3,4-disubstituted-1,2,4-oxadiazole-5-ones(thiones) and 3,4-disubstituted-1,2,4-thiadiazole-5-ones have been determined in CDCl3 solution. Exceptionally good Hammett correlations of 13C NMR chemical shifts of these carbons with σ were obtained. The negative ρ values observed (inverse substituent effects) indicate π-polarization of the C═N, C═O and C═S bonds. As expected, the long distance C═O and C═S 13C chemical shifts were found less susceptible to substituent-induced electronic changes.

Substituent effects on thermal rearrangement of 3- (substituted phenyl)-4-(p-tolyl)-1,2,4-oxadiazole-5(4H)-thiones

The kinetics of the rearrangement of 3-(substituted phenyl)-4-(p-tolyl)-1,2,4-oxadiazole-5(4 H)-thiones in solution were determined in the temperature range 160-200°C. From the correlation of logk against s, it was found that for m-Me, p-Cl and p-CN, the compounds rearrange with the homolytic cleavage of the N—O bond, whereas for p-Me, H and m-NO2, the rearrangement occurs with the heterolytic cleavage of the N—O bond. In comparison with the uncatalysed rearrangement, Cu catalysis greatly increased the rate of the rearrangement of 3-( m-chlorophenyl)-4- (p-tolyl)-1,2,4-oxadiazole-5(4 H)-thione at 166 °C. Copyright