Stereoselective synthesis of a butenolide sugar derivative was possible by reaction of the appropriate sugar epoxide with the dilithium salt of phenylselenoacetic acid, followed by oxidation of the a-phenylselenobutanolide obtained with hydrogen peroxide in the presence of catalytic amounts of acetic acid. On the other hand, synthesis of an exocyclic a, (3-unsaturated lactone was accomplished by Reformatsky reaction on the appropriate sugar carbonyl groups with ethyl bromomethylacrylate and activated zinc, leading to the introduction of this ring at position 2 or 4 of a firanose ring. Nuclear Overhauser effect studies led to the unambiguous determination of the configuration of the new chiral centre formed by the Reformatsky reaction. The fungicidal efficacy of some unsaturated lactone sugar derivatives is given. © 1995, Taylor & Francis Group, LLC. All rights reserved.