A steric scale of 20 recurrent groups was established from comparison of rotational barriers on N-(o-substituted aryl)thiazoline-2-thione atropisomers. The resulting energy of activation ΔG reflects the spatial requirement of the ortho substituent borne by the aryl moiety, electronic aspects and external parameters (temperature and solvent) generating negligible contributions. Concerning divergent rankings reported in the literature, the great sensitivity of this model allowed us to show unambiguously that a methyl appears bigger than a chlorine and gave the following order in size: CN > OMe > OH. For the very bulky CF and iPr groups, constraints in the ground state decreased the expected ΔG values resulting in a minimization of their apparent sizes.
A number of cyclic derivatives of 3-amino-2,4-dihydroxybutanoic acid are known in the literature but they are often prepared from other cyclic precursors. This study showed that the title compound too may serve as a convenient substrate for cyclization reactions. Using orthogonally-protected linear derivatives, regioselective cyclizations were performed, leading to original and highly-functionalized γ-lactones, oxazolidinones, oxazolines and aziridines. In these reactions a key role was played by the C3 nitrogen group function, while the C2 alcohol function showed no propensity for participation in cyclization reactions.
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