Isotetronic acids and butenolides are of great pharmacological relevance and occur in a number of natural products: 1 for example, (+)-leptosphaerin represents a metabolite of the marine ascomycete Leptosphaeria oraemaris 2a and compound WF-3681 represents an aldose reductase inhibitor produced by Chaetomella raphigera. 2b Isotetronic acid derivatives have been used also as key intermediates during the synthesis of (−)-tetrodotoxin, 3a 6-thiosialic and neuraminic ...