Experimental and Theoretical Analysis of the Thiol-Promoted Fragmentation of 2-Halo-3-tosyl-oxanorbornadienes.

2-Halo-3-tosyl-oxanorbornadienes are able to accept two thiol molecules through an initial nucleophilic substitution, giving isolable oxabicyclic thiovinyl sulfones that, subsequently, can react with a second thiol molecule via thio-Michael addition. The resulting oxanorbornenic thioketals undergo retro-Diels-Alder (rDA) fragmentation to release a furan derivative and a ketene S,S-acetal. The substitution pattern of the oxanorbornadienic skeleton influences the rate of the rDA through electronic and steric factors examined by quantum mechanical calculations.

Transferring Substituents from Alkynes to Furans and Pyrroles through Heteronorbornadienes as Intermediates: Synthesis of ß-Substituted Pyrroles/Furans.

The use of 7-oxa/azanorbornadienes as synthetic intermediates for the preparation of 3/4-substituted (ß-substituted) furans/pyrroles is presented. The method lies in the inverse electron demand Diels-Alder (iEDDA) cycloaddition between a substituted heteronorbornadiene and an electron-poor tetrazine followed by spontaneous fragmentation of the resulting cycloadduct via two retro-Diels-Alder (rDA) reactions affording a ß-substituted furan/pyrrole. The scope of this tandem iEDDA/rDA/rDA reaction was explored in the preparation of 29 heterocycles. A one-pot procedure starting directly from the alkyne precursors of the heteronorbornadiene intermediates is also described.