Synthesis of Locked meso-β-Substituted Chlorins via 1,3-Dipolar Cycloaddition

A novel approach toward “locked” chlorins with increased stability has been studied in detail. The chlorin
skeleton is assembled in a convergent fashion from two fragments via a porphyrin forming reaction,
followed by 1,3-dipolar cycloaddition of azomethine ylides, which are formed in situ. Central to the
success of the process is the presence of two electron-withdrawing groups in vicinal positions at the
perimeter of the porphyrin. As a result, the 1,3-dipolar cycloaddition took place regioselectively, on the
bond activated by two electron-withdrawing groups. Moreover, the chlorins formed are locked and hence
more stable because of the presence of two quaternary carbon atoms. Overall, in just six steps locked
chlorins were constructed from easily available materials. The large array of functionalities tolerated in
this approach validates it for a broad use in more advanced studies. The correlation between the results
of the 1,3-dipolar cycloaddition and dipolarophile (porphyrin) LUMO energy was extensively studied.
There was a definite correlation between the reaction time and the LUMO energy level, and a partial
correlation between the reaction yield and the distribution of the LUMO. Additionally, various approaches
toward crucial building blocks, namely 3,4-disubstituted-2,5-diformylpyrroles, were investigated.