Organic molecules homochirality

As we have seen, one of the main reasons why reactions in crystals lead to high levels of asymmetric induction is that the constituent molecules can be organized in homochiral fixed conformations and intermolecular orientations that are predisposed to formation of a single product enantiomer. With this in mind, it was natural to seek other ways of preorganizing molecules in restricted environments for the purpose of asymmetric synthesis, and one approach that has shown a good deal of promise is the use of chirally modified zeohtes. The great majority of this work has been carried out by Ramamurthy and coworkers at Tulane University [23], and a brief summary is given below. 

In view of the importance currently attached to the synthesis of homochiral organic molecules, examples which illustrate the use of organoboron and organosilicon compounds in this area are included where appropriate. 

Podlech J (2001) Origin of organic molecules and biomolecular homochirality. Cell Mol Life Sci 58 44-60... 

Chapter 1 considers the possible relationships of earthly clays and other minerals to the origin of chirality in organic molecules. Attempts to establish experimental evidence of asymmetric adsorption on clays were unsuccessfiil, but die search for chirality did find naturally occurring enantiomorphic crystals like quartz. Asymmetric adsorption of organic molecules on quartz crystals such as separation of racemic mixtures, like Co or Cr complexes, alcohols and other compounds, allowed for the conclusion that quartz crystals can serve as possible sources of chirality but not of homochirality. This latter conclusion results fi om the finding that all studied locations of quartz crystals contain equal amounts of d- and /-forms. The preparations of synthetic adsorbents such as imprinting silica gels are also considered. More than 130 references are analyzed. 

In similar ways circular polarized light irradiation of very low efficacy combined with enantiomorphic crystals and autocatalytic amplification reactions, could be the origin of homochirality of organic molecules (Bonner ). 

Subsequently, several research groups including Lin, Rosseinsky, Xiong, Fedin and others have designed and synthesized several homochiral porous metal-organic materials capable of enantioselective sorption of chiral organic molecules, which are listed in Table 1. 

The possibility that small organic molecules could catalyze organic transformations in a stereoselective way can even be considered as a key element in the origins of life, as it is widely accepted that the source of homochirality in biological systems should be attributed to the presence of enantiomerically enriched a-amino acids in meteorites, and especially once it was demonstrated that these a-amino acids were able to catalyze aldol reactions in a stereoselective way under prebiotic conditions conducing to sugar-type products. ... 

Many of the molecules that constitute living organisms, such as amino acids, enzymes, proteins, and DNA, are chiral and present in only one of the two enantiomeric forms. When small, chiral organic molecules interact with the homochiral molecules of biological... 

Lin and co-workers [177] have studied self-constructing arrays of individual Fe-l,2,4-benzenetricarboxyhc acid (Fe-tmla) complexes on a Cu(lll) substrate, produced by OMBE deposition of precursor tmla adlayers and iron atoms in ultrahigh vacuum. These selfconstructed metal-organic arrays (of four tmla molecules per one Fe atom) form large homochiral monolayers (i.e. both S and R chirality are found) extending laterally up to 200 nm. 

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