Bridgehead Nitrogen Heterocycles. Part-V. Syntheses of s-Triazolo[3,4-b][ 1,3,4,]benzothiadiazines, s-'Triazolo[3,4-b][1 ,3,4]thiadiazines, s-Triazolo[3,4-b][ 1 ,3,4]thiadiazino[5,6-b]quinoxaline, lmidazo[ 2, 1 -b ]benzothiazole and Benzimidazo[ 2,1-b] benzothiazole as Potential Antimicrobial A gents