Isoxazolidines nitrogen inversion

Subsequently, this interpretation has been commonly accepted however, it does not account for the factors responsible for a decrease in the rate of the fast classical ring inversion. Hence, in our opinion, it is more reasonable to consider the stereodynamics of isoxazolidines as a result of superposition of two processes, a fast ring inversion (IR) and slow nitrogen inversion (shown in the lower portion of Scheme 3.166) (418). 

On the basis of all the above mentioned results, Russian researchers provided an explanation for the stereodynamics of isoxazolidines shown in Scheme 3.166. There is a so-called combined inversion-buckling process (417), which occurs through a planar transition state A with an unhybridized p electron pair at the nitrogen atom (see Scheme 3.166). 

Proton NMR (sometimes low temperature NMR or NOE) is the major reliable method for conformational analysis of isoxazolidines which undergo pyramidal inversion at the ring nitrogen atom (82JOC4397, 840MR(22)399>. 

Since the separation of the cyclic 7-chloro-7-azabicyclo[4,l,0]heptane (45) into its isomers there has been much interest in slow inversion at nitrogen in heterocyclic compounds. Slow inversion in a diaziridine (43) has already been mentioned. A -Methoxyaziridines, e.g. (46), also invert slowly,as do oxazetidines (47), the isoxazolidine (48), and the oxazolidine (49). Solvent effects on inversion rates were investigated for the compound (48). Z-Ray experiments have confirmed the existence of two isomers of the compound (49). Further examples of studies of intramolecular processes in oxygen-nitrogen ring compounds include those of the three-membered ring series (50). ... 

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