Vicinal dissymmetry

The following discussion is divided into three subsections the ketone chromophore (Section 4.4,1.1.), for which configurational assignments are based on the effect of ring dissymmetry and substitution pattern on the rotatory power of the n-rt transition. For conjugated chro-mophores (Section 4.4.1.2.) it is both the helicity of the chromophore and the vicinal substituent effect that determines the rotatory power of the 71-71 transition. Finally, the versatile stereochemical method, exciton chirality method (Section 4.4.2.), is based on the chiral interaction between the electric dipoles of the allowed transitions in two or more chromophores. 

In the literature on synthesis and reactions, almost all provide NMR data along with other structural evidences, even though they are not categorized under NMR or experimental methods keyword search in Chemical Abstracts. The presence of two vicinal heteroatoms causes dissymmetry in the ring nucleus, thus affects the chemical shifts of both proton and carbon nuclei in the ring. The differences in the chemical shifts almost always enable one to elucidate the structure of an unknown isoxazole, on the basis of NMR alone. Of course one needs to consider the effects of substituents on the chemical shift. 

The chromophoric derivatives (1) have a chromophore that is optically inactive by nature but becomes optically active in the vicinity of the asymmetric a-carbon atom (2) absorb in the near-uv or visible spectral range (above 300 nm, where the aromatic and other chromophoric substituents are transparent) and (3) preferably have a rather high Kuhn s dissymmetry factor... 

---