Macrocyclic thia-alkenolides formed by intramolecular ‘ene’ reactions of thioaldehydes

Abstract

The crystalline acids which are formally cycloadducts of cyclopentadiene and the thioaldehyde thioxo-acetic acid [(thioformyl)formic acid](HO₂C·CHS) have been converted into a series of esters with the alkenols CH₂CH[CH₂]_nOH. These cycloadduct esters, 8 and 9, when subjected to flash vacuum pyrolysis (FVP) at ∼500 °C, liberated the corresponding alkenyl thioxoacetates, 10, which underwent intramolecular ‘ene’ reactions to give a series of thia-alkenolides having 6- to 11-membered rings. The 3-thianon-5-en-9-olide 11d has been transformed, by standard reactions, into 3-vinylhex-2-en-6-olide, 20, with removal of sulfur. The sulfoxides 28 of the cyclopentadiene adducts have been employed similarly as precursors for the corresponding thioxoacetate ester S-oxides (sulfines). Under FVP conditions, intramolecular cyclisation of the allyl 33 and the homoallyl 29 sulfines gave furan-2(5H)-one 35 and 5,6-dihydro-2-pyrone, 31, respectively. The prenyl derivative 37 gave a mixture of the simple product furan-2(5H)-one 35 and its 4-isopropenyl derivative 39. Generally, fumarate esters were formed from sulfines that had failed to cyclise.