Benzoylation of d-galactono ( 1a), d-glucono ( 2a), and d-mannono-1,4-lactones ( 3a), or alkaline treatment of their corresponding perbenzoylated derivatives ( 1b, 2b, and 3b) afforded the same product. 2,6-dibenzoyloxy-hexa-2,4-dien-4-olide ( 4). Depending on the conditions employed, compound 4 was obtained as the single 4- Z isomer, or as a 4- E and 4- Z diastereomeric mixture. Butenolide 4 arises from a double β-elimination process, which seems to involve an ElcB mechanism, as formation of 4 is not influenced by the relative orientation of the substituents in the starting aldonolactone. Hydrogenation of 4 gave stereoselectively the diastereoisomer having the d,l- threo configuration. Treatment of the lactonic disaccharide 2,3,5-tri- O-benzoyl-(2,3,5,6-tetra- O-benzoyl-β- d-galactofuranosyl)- d-galactono-1,4-lactone ( 7) with 20% triethylamine in dichloromethane afforded the diunsaturated derivative 8 as an E,Z mixture. Hydrogenation of 8 gave a 1:1 diastereomeric mixture of the 2( S),4( R) and 2( R),4( S)-3,5-dideoxylactones [ 9( S,R) and 9( R,S), respectively], which indicates that no asymmetric induction attributable to the galactofuranose residue, took place during the hydrogenation reaction. As compounds 9( S,R) and 9( R,S) could be separated by h.p.l.c., a glycosylation reaction was employed for the resolution of the racemic dideoxylactone derivative 6.