This project examined the utility and reactivity of S-cis-methylaziridine aldehyde, developed in Dr. Yudin’s lab, in peptide macrocyclization. Much of my work entailed finding more efficient chemoselective routes that produce high diastereoselectivity, or, in other words, one product is formed in excess over another. In place of the trans configuration of the aziridine aldehyde its cis congener was used in an Ugi four component condensation. The use of S-cis-methyl aziridine aldehyde, tert-butyl isocyanide and proline, either the D or L enantiomer, produced a single diastereomer. These preliminary results showed that using the cis configuration of the aziridine aldehyde was more effective than its trans congener. Furthermore, it provided further insight into the mechanism involved in peptide macrocyclization.