Synthesis of Endocyclic Cycloalkyne Amino Acids

"Click-ligation" is a widely adopted and valuable means to ligate biomolecules whereby two appended biologically inert moieties, such as alkynes and azides, link by cycloaddition. For terminal alkynes, Cu⁺¹ catalysis is required which degrades oligonucleotides by catalyzing their hydrolysis but is also physiologically incompatible. The smallest activated alkynes that do not require Cu⁺¹ catalysis are cyclooctynes or dibenzo-cyclooctynes. For this purpose, there are commercially available nucleosides and amino acids that are appended to these moieties. However, these structures are bulky, dissimilar to native amino acids, and when incorporated within biological molecules could likely perturb native structural configuration. Presented are the syntheses of structural analogues of proline with an inserted propargyl moiety within a series of ring sizes. Moreover, a synthetic pathway to medium-size ring heterocycloalkynes mediated by using mild Mitsunobu conditions in tandem with a Nicholas-related strategy for cyclization is introduced. Avoiding the usual harsh acidic conditions for the Nicholas reaction allows improved functional group compatibility.