ENANTIOSELECTIVE AUTOCATALYSIS - SPONTANEOUS RESOLUTION AND THE PREBIOTIC GENERATION OF CHIRALITY

Theoretical and experimental models for autocatalytic systems leading to the prebiotic origin of chirality via the spontaneous symmetry breaking (resolution) of racemic substrates are reviewed. Of the experimental models so far studied, only 2nd order assymetric transformations during crystallization of optically labile enantiometers, leading to their spontaneous resolution under racemizing conditions (SRURC) have been successful. Our objective was to investigate in further detail the most promising of these systems from the point of view of its overall efficiency and its potential viability as a mechanism for the spontaneous generation of molecular chirality on the prebiotic Earth. To this end the 1,4-benzodiazepinooxazole derivative XI, having a single asymmetric carbon atom, has been synthesized. We here confirm a report in the literature that (+/-)-XI undergoes SRURC in methanol, both on crystallization and as a slurry. The 'total spontaneous resolution' of (+/-)-XI has been achieved in a yield of 99%, of which 80% had an optical purity of ca. 93%. Arguments are presented that SRURC of racemic substrates, while thus demonstrably effective in laboratory experiments, was probably not of major importance for the origin or amplification of molecular chirality on the primitive earth.