Boron enolatc

Relatively moderate diastcrcosclectivity is also obtained when the boron enolatc of (5S,6R)-4-(benzyloxyearbonyl)-5,6-diphenyl-2,3,5,6-tctra-hydro-4ff-l, 4-oxa/in-2-one is reacted with achiral aldehydes D. S. Reno, B. T. Lotz, M. J. Miller. Tetrahedron Lett. 31, 827 (1990). 

Boron enolatcs 6, which arc also used as activated derivatives of carbonyl sub-strates, " can be similarly obtained from silyl enolethers 4, - or from diazo-carbonyl compounds 7 (Fig. 7), as well as from a, P-unsaturated ketones. - ... 

Hexafluoroacetone has also demonstrated unusual reactivity when condensed with the boron enolatc of an optically active oxazolidinone3 or the boron enolate of the sultam derived from camphorsulfonic acid4 to give products 3 and 4, respectively. The absolute stereochemistry of the products 3 and 4 is the opposite of that formed on addition to nonfluorinated ketones and aldehydes. This change was attributed to the involvement of an open transition state in the aldol reaction, a consequence of the diminished basicity of fluorinated carbonyl oxygens. 

CB. The (Z)-boron enolatcs derived from 9-BBN undergo aldol reactions to provide aldols in >96% vyn-selectivity. However, (Z)-boron cnolates formed from CB show lower selectivity (syn/anti = 3 1). 

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