Sugar enolones, XI. 2‐Halopentopyranosyl halides: Preparation, configurational elucidation, and conversion into enantiomeric dihydropyranones

Addition of chlorine or bromine to the enediol grouping of tribenzoyl‐pentenitol 2 exclusively yields cis ‐adducts, i.e. 1,2‐dihalides of α‐ D ‐ lyxo ( 4, 6 ) and β‐ D ‐ xylo configuration ( 5, 7 ). Configurational assignments were based on the titanium tetrahalide‐induced anomerization to the corresponding β‐ D ‐ lyxo ( 8, 10 ) and α‐ D ‐ xylo isomers ( 9, 11 ), on 1 HNMR spectroscopic determination of their conformations, and on an X ‐ray crystal structure analysis of the α‐ D ‐ xylo dichloride 9 . — The 1,2‐dibromides are partially hydrolyzed in contact with silica gel, clean hydrolysis being effected by silver carbonate in aqueous acetone, with the stereospecific formation of α‐ D ‐ threo ‐pentosulose 15 from D ‐ xylo dibromides ( 7, 11 ) and of the β‐ D ‐ threo anomer 25 from either of the lyxo derivatives ( 6, 10 ). The dichlorides are more stable requiring heating with sodium hydrogen carbonate in moist benzene, to give the enantiomeric dihydropyranones 16 (from D ‐ xylo dichlorides) and 26 (from D ‐ lyxo compounds). — Reactions at the anomeric center of 16 and 26 , such as replacement of the benzoyloxy group by halogen, hydroxy, or alkoxy substituents ( 20 – 23 ), lead to racemization or to elaboration of the γ‐pyrone system ( 24 ). Additions to the enolone grouping, however, proceed with high stereoselectivity as evidenced by the nearly exclusive formation of 4‐deoxy‐ D ‐ erythro ‐pentose ( 17 ) from α‐enolone 16 on hydrogenation and debenzoylation, whilst the β‐enolone affords the L ‐enantiomer 27 .