Bacteria rely on chemical communication or quorum sensing to coordinate activities necessary for their survival in colonies. Among the numerous processes mediated by this intercellular communication is the formation of biofilms. The prevalence of biofilms in many different environments can be problematic. Their association to infectious diseases and their inherent ability increase antibiotic resistance in bacteria has led to a groundswell of research focused on new methods to control them. Their dependence on quorum sensing has made those signaling systems within bacteria an attractive target for the design of new therapeutic agents. Compounds that can disrupt this process are termed quorumsensing inhibitors (QSIs). By disrupting the biofilms, thereby making the bacteria more susceptible to traditional antibiotics, these QSIs may provide the newest weapon in the therapeutic arsenal against infections involving drug-resistant bacteria. These QSIs can come from a variety of sources and have a wide array of structures. This review will cover the scope of QSIs that have been reported in the literature, in particular those that have been shown, or may have potential, to inhibit biofilm formation and development.

Diels-Alder cycloaddition of s-trans-1,3-butadiene (1) should yield trans-cyclohexene (7), just as reaction of the s-cis conformer gives cis-cyclohexene (9). Investigation of this long-overlooked process with Hartree-Fock, Moller-Plesset, CASSCF, and DFT methods yielded in every case a C(2)-symmetric concerted transition state. At the B3LYP/6-31G (+ZPVE) level, this structure is predicted to be 42.6 kcal/mol above reactants, while the overall reaction is endothermic by 16.7 kcal/mol. A stepwise diradical process has been studied by UBLYP/6-31G theory and found to have barriers of 35.5 and 17.7 kcal/mol for the two steps. Spin correction lowers these values to 30.1 and 13.0 kcal/mol. The barrier to pi-bond rotation in cis-cyclohexene (9) is predicted (B3LYP theory) to be 62.4 kcal/mol, with trans-cyclohexene (7) lying 53.3 kcal/mol above cis isomer 9. Results suggest that pi-bond isomerization and concerted reaction may provide competitive routes for Diels-Alder cycloreversion. It is concluded that full understanding of the Diels-Alder reaction requires consideration of both conformers of 1,3-butadiene.

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