Stereoselective intramolecular hetero Diels-Alder reactions of cyclic benzylidenesulfoxides and DFT calculations on the transition structures

The 4-benzylidene-3-oxo[1,3]oxathiolan-5-ones 13-15, which were derived fro m the 3-oxo[1,3]oxathiolan-5-ones 9-11 by Knoevenagel condensation with the aldehydes 12, cyclize in an intramolecular hetero Diels-Alder reaction wit h high yield and excellent endo/exo as well as induced diastereoselectivity to give the hetero Diels-Alder adducts 16-18. The preferred formation of t he Knoevenagel products 13-15 with a (Z) configuration was predicted with D FT calculations (B3LYP/6-311+G*) using the model systems 28 and 29. In addi tion B3LYP/G-31G*//B3LYP/3-21G((*)) calculations on transition structures f or the hetero Diels-Alder reaction of 29 and 30 allowed a good correlation with the experimental results, which show that an endo attack of the dienop hile syn to the S-O group in 13-15 leads to the main products.