A-Heteroatom-substituted hydroxamic esters

II. UNUSUAL CHARACTERISTICS OF A/-HETEROATOM-SUBSTITUTED HYDROXAMIC ESTERS... 

A-Heteroatom-substituted hydroxamic esters can be expected to have different properties to those displayed by simple hydroxamic esters in two respects. On the one hand, bisheteroatom substitution at nitrogen impacts strongly upon the nitrogen hybridization... 

A. Amide Properties in A/-Heteroatom-substituted Hydroxamic Esters... 

The barrier to inversion at nitrogen in A-heteroatom-substituted hydroxamic esters should be greater than that found for hydroxamic esters or simple amides. However, it is likely to be substantially reduced in anomeric amides relative to amines since the planar transition state in which nitrogen is sp hybridized, can benefit from jr-overlap with the carbonyl (Figure 3b) and this has been verified experimenfally Rudchenko has measured an inversion barrier for A,A-dialkoxyureas af AG = 9-11 kcalmoH and fhose of acyclic dialkoxyamines fypicaUy af AG = 20-22 kcalmoH ... 

Af,Af-Dialkoxyamides (Ic) are the least studied class of stable A-heteroatom-substituted hydroxamic esters. The combined electronegativity of two alkoxyl oxygen atoms is less than that of acyloxyl and alkoxyl oxygen in A-acyloxy compounds but properties are largely typical of anomeric amides. There is good spectroscopic evidence that the nitrogen... 

This chapter will cover the synthesis, structure and chemical reactivity of various N-heteroatom-substituted hydroxamic esters, anomeric amides in which at least one of the heteroatom substituents at nitrogen is an alkoxyl group. Throughout this review, these will either be referred to as A-substituted hydroxamic esters or as A-substituted-A-alkoxy amides. 

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