Stereoselective Intramolecular Hetero Diels–Alder Reactions of Cyclic Benzylidenesulfoxides and DFT Calculations on the Transition Structures

Abstract

The 4-benzylidene-3-oxo[1,3]oxathiolan-5-ones 13–15, which were derived from 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 with high yield and excellent endo/exo as well as induced diastereoselectivity to give the hetero Diels–Alder adducts 16–18. The preferred formation of the Knoevenagel products 13–15 with a (Z) configuration was predicted with DFT calculations (B3LYP-6-311+G*) using the model systems 28 and 29. In addition B3LYP-6-31G*/sB3LYP/3-21G(*) calculations on transition structures for 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 dienophile syn to the S–O group in 13–15 leads to the main products.