The Lewis acid catalyzed cyclization of 1,5-bis(trimethylsilyloxy)-1,3,5-hexatrienes with oxalyl chloride resulted in formation of polyunsaturated butenolides. Polyunsaturated g-alkylidenebutenolides, including prominent natural products such as freelingyne, dihydrofreelingyne, lissoclinolide, and dihydroxerulin, are of high pharmacological relevance. 1 Lissoclinolide, for instance, exhibits antibiotic activity against Gram-positive bacteria 2 and dihydroxerulin has proven to be an important nontoxic inhibitor in the biosynthesis of cholesterol. 3 g-Alkylidenebutenolides have been prepared so far by Wittig reactions, 4 stereospecific b-eliminations, 5 or lactonizations of 2-en-4-ynoic acids and related methods. 6 We have recently reported 7 a new approach to g-alkylidenebutenolides based on cyclizations of 1,3-dicarbonyl dianions and 1,3-bis(trimethylsilyloxy)-1,3-butadienes, electroneutral dianion equivalents, 8 with oxalic acid dielectrophiles. Herein, we wish to report an extension of this methodology to the synthesis of polyunsaturated g-alkylidenebutenolides which relies on the first cyclizations of 1,5-bis(trimethylsilyloxy)-1,3,5-hexatrienes, a previously unknown substance class, with oxalyl chloride. The butenolides prepared represent important analogues and synthetic precursors to pharmacologically relevant natural products such as lissoclinolide.Despite many potential applications, 1,5-bis(trimethylsilyloxy)-1,3,5-hexatrienes have to our knowledge not been previously prepared. These new synthetic building blocks were prepared as follows (Scheme 1): acetalization of the corresponding b-ketoesters afforded the acetals 1a-d;reduction of the ester group and subsequent Swern oxidation of the resulting alcohols afforded the aldehydes 3a-d in good yields. Wittig reaction afforded the protected 1,5-ketoesters 4a-d 9 which were deprotected and transformed into the novel silyl dienol ethers 6a-d in high yields. Deprotonation of 6a-d with LDA/HMPTA at -78°C and subsequent addition of Me 3 SiCl resulted in formation of the desired 1,5-bis(trimethylsilyloxy)-1,3,5-hexatrienes 7a-d in good yields. For the synthesis of 7a-c a spirocyclic acetal was used as the protecting group. In case of the synthesis of 7d, this group could not be chemoselectively removed without destruction of the molecule. This problem was eventually solved by the use of the dimethylacetal protecting group.