and Lewis acids. The first approach employs chiral metal catalysts to modulate the endo and exo stereoselectivity. The chiral catalysts, in particular Rh2[(S)-DOSP]4 and Rh2[(S)-BPTV]4 can induce modest asymmetric induction and was used to synthesize the antifungal agent pseudolaric acid A. This is a result of the chiral metal catalyst remaining associated with the carbonyl ylide during the cycloaddition, which confers facial selectivity. However, the exact mechanisms are not yet fully understood. The second approach employs a chiral Lewis acid catalyst to induce facial stereoselectivity after the generation of the carbonyl ylide using an achiral metal catalyst. The chiral Lewis acid catalyst is believed to coordinate to the dipolarophile, which lowers the LUMO of the dipolarophile while also leading to enantioselectivity. Azomethine ylides 1,3-Dipolar cycloaddition between an azomethine ylide and an alkene furnishes an azacyclic structure, such as pyrrolidine. This strategy has been applied to the synthesis of spirotryprostatin A. Ozone Ozonolysis is a very important organic reaction. Alkenes and alkynes can be cleaved by ozonolysis to give aldehyde, ketone or carboxylic acid products. Biological applications The 1,3-dipolar cycloaddition between organic azides and terminal alkynes (i.e., the Huisgen cycloaddition) has been widely utilized for bioconjugation. Copper catalysis The Huisgen reaction generally does not proceed readily under mild conditions. Meldal et al. and Sharpless et al. independently developed a copper(I)-catalyzed version of the Huisgen reaction, CuAAC (for Copper-catalyzed Azide-Alkyne Cycloaddition), which proceeds very readily in mild, including physiological, conditions (neutral pH, room temperature and water solution). This reaction is also bioorthogonal: azides and alkynes are both generally absent from biological systems and therefore these functionalities can be chemoselectively reacted even in the cellular context. They also do not react with other functional groups found in nature, so they do not perturb biological systems. The reaction is so versatile that it is termed the "Click" chemistry. Although copper(I) is toxic, many protective ligands have been developed to both reduce cytotoxicity and improve rate of CuAAC, allowing it to be used in in vivo studies. For example, Bertozzi et al. reported the metabolic incorporation of azide-functionalized saccharides into the glycan of the cell membrane, and subsequent labeling with fluorophore-alkyne conjugate. The result is that the cell membrane is fluorescently labeled, and can therefore be imaged using a fluorescence microscope. Strain-promoted cycloaddition To avoid toxicity of copper(I), Bertozzi et al. developed the strain-promoted azide-alkyne cycloaddition (SPAAC) between organic azide and strained cyclooctyne. The angle