Absolute Asymmetric Synthesis of a Homochiral Polymer from an Achiral Monomer

A compelling hypothesis for the origin of homochirality that led to the origin of life is that a chiral informational polymer, formed somehow from achiral monomers, biased the early chemical events, leading to the exclusive synthesis of chiral building blocks of life. However, the formation of a chiral polymer from achiral monomers continues to be a mystery. We exploited the ability of achiral molecules to rarely crystallize in chiral space groups to design achiral monomers for topochemical polymerization to obtain chiral polymers. One such achiral monomer synthesized, viz. a tripeptide monomer containing alkene and azide at its termini, crystallized in a chiral space group, forming a conglomerate as revealed by single-crystal X-ray diffraction. Upon heating at a temperature above 70 °C, each enantiomorphic crystal underwent a regiospecific and stereospecific topochemical ene-azide cycloaddition reaction in a single-crystal-to-single-crystal fashion to give a triazoline-linked homochiral helical polymer. We characterized several homochiral polymer crystals at atomic resolution either by SCXRD or by 3D electron diffraction.