More than half of all naturally-occurring compounds incorporate heterocyclic rings in their structures. Included in this category are alkaloids, antibiotics, coenzymes, nucleic acids, most drugs, some vitamins, inter alia. This course presents the organic chemistry of aromatic, heterocyclic compounds containing N, O, and/or S atoms. The focus is on the structures, properties, preparations, and reactivities of five- and six-membered ring systems. Each week a different heterocyclic system is studied: on Thursdays the instructor presents the lectures and on Tuesdays two students each present 30-minute lectures. The maximum number of such presentations per student during the semester is four. Laboratory sessions involve original research projects as team endeavors. At the end of the semester each student selects a recent total synthesis of a biologically-active natural product containing heterocyclic unit(s) and analyzes the strategies and reactions in a term paper. Format: lecture, three hours per week; laboratory, four hours per week. Evaluation will be based on class presentations, problem sets, two hour tests, laboratory work, and a term paper. Prerequisites: Chemistry 251/255. May be taken concurrently with Chemistry 256 with permission of instructor. No enrollment limit (expected: 8).