Sugar enolones, XVIII. stereocontrolled functionalization at proanomeric centres by photobromination. – A novel efficient access to oxo‐ and oximinoglycosyl bromides

Abstract

Abstract Suitably O ‐blocked monosaccharides carrying an electron acceptor group such as an oxo or oximino function next to a proanomeric centre, readily homolyze the corresponding push‐pull‐substituted C–H bond to give under photobromination conditions the respective α‐glycosyl bromides in high yields and virtually stereospecific reactions. Accordingly, the tri‐ O ‐benzoyl derivatives of 1,5‐anhydro‐ D ‐fructose ( 7 ) and the anomeric L ‐ lyxo ‐hexos‐4‐uloses ( 12, 15 ) give the α‐bromides 8, 13 , and 16 in yields above 80%. The readily accessible benzoyloximino derivatives 29 – 32 of 1,5‐anhydro ketoses are photobrominated with particular ease to afford excellent yields of the stable, usually crystalline perbenzoyl(oximino)glycosyl bromides 33 – 36 which are versatile building blocks for the construction of amino sugar‐containing oligosaccharides. – In the dihydropyranones 22 and 37 the vinylogously activated proanomeric centre is preferentially photobrominated. – Alcoholysis of the glyculosyl bromides can be conducted to yield either β‐or α‐glycosiduloses which readily undergo β‐elimination of benzoic acid to give the respective enolones, e. g. 10, 11, 14 , and 17 which, as pyranoid derivatives with four distinctly differentiated carbonyl functions, are seen to be useful chiral building blocks.