(A) New types of concerted domino acylation-cycloalkylation reactions promoted by methanesulfonic acid-phosphorus pentoxide, provide efficient, elegant, and expeditious routes for biologically active naturally occurring diterpenoids, namely (±)-ferruginol (3a), (±)-nimbidiol (3b), (±)-nimbiol (3c) (75-90%). This exceptionally short as well as stereoselective route to the total synthesis of diterpenes involve a one-pot reaction method unlike the literature methods commonly used for this type of compounds. (B) Microwave irradiation of oximes, semicarbazones, phenylhydrazones and tosylhydrazones of various aldehydes in the presence of phosphorus pentoxide provides a fast, efficient and simple method for regeneration of aldehydes in high yield (68-98%). (C) Microwave irradiation of phosphorus pentoxide supported in alumina was found to be an efficient reagent for the synthesis of 1,5-benzodiazepine derivatives from phenylenediamine and ketones. This method is an easy, rapid, solvent-free and high-yielding reaction for the synthesis of 1,5-benzodiazepines (75-85%). (D) Bromination of hydroxyheteroarenes using P 2O 5/Bu 4NBr proceeds under mild conditions to afford high yields of various bromoheteroarenes. In general, these compounds have been prepared with PBr 3 or POBr 3, but those bromination procedures have some disadvantages such as handling of hazardous reagents, exact reaction-temperature control and evolution of toxic HBr. The P 2O 5 procedure is successfully applied to large-scale synthesis of bromoheteroarenes (75%).