Azaborolidines and other Boron Heterocycles

Amino acids can condense with boronic adds to form 1 1 chelates of type 62 [146]. The tetracoordinate structure of these adducts is very apparent by NMR due to the formation of a new stereocenter at boron. Interestingly, 4-boronophenylalanine (63), 

there are several other examples of internal heterocyclic derivatives in which an ortho substituent of an arylboronic acid doses onto the boronic acid with either a dative or a covalent bond [157]. For example, ortho-anilide derivatives 69 and the corresponding ureas (70), in a putative internally chelated form A, were shown to 

16 Hemi-heterocydic internal boronic ester derivatives. 

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