Boronic acid amide esters, cyclic

The cyclic boron esters (99) resulted from the three-component reaction between benzil (96), urea (97), and boric acid (Scheme 21). The stereoisomeric products arose as a consequence of boric acid addition to the intermediate cyclic amides (98) <89PHA110>. Carbon-13 NMR spectroscopy and mass spectrometry are consistent with structure (99). 

Boron or tin (II) Z-enolates are generated by reaction with the corresponding triflates with a carbonyl compound in the presence of tertiary amines like r-P NEt or. M-ethylpiperidine (except when using dicyclopentylboron triflate [407]). E-Enolates are prepared by using dicyclohexyl- or other cyclic chloroboranes in the presence of Et3N or Me NEt [407, 685, 686, 1246, 1247, 1248], Because enolization does not take place under such conditions with esters or aliphatic tertiary amides, thiophenyl esters RGH COSPh have been used as ester/amide substitutes. Furthermore, Z-boron enolates of ketones can be prepared by conjugate addition of acid derivatives of dialkylboranes to a-enones [687],... 

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