Nafion-TMS- and TMS-trifluoroacetate-induced rearrangements of cyclopropyl ketones - a procedure for the regioselective conversion of semibullvalenes to barrelenes

Abstract

Trimethylsilyl trifluoroacetate (TMSTFA) and the polymer-supported sulfonate Nafion-TMS exhibit selective reaction modes in the opening of cyclopropyl ketones. The yields are generally high. Nafion-TMS rearranges the tricyclooctanones 1a and 1b to the bicyclooctenones 4a and 4b (while TMSTFA gives ring-opened adducts 6a,b) with high regioselectivity. Aro-semibullvalenes (8a, 14, 17a,b) are efficiently rearranged to arobarrelenes (7a, 13, 20a,b) by both reagents. The latter rearrangements have also been achieved in mixtures of trifluoroacetic acid and tetramethylsilane (TMS), where the acid combines with an unidentified impurity of commercial TMS to form a strong electrophilic agent. The electrophile-assisted rearrangement of the naphtho-semibullvalenes 14a+b resulted in conversion to the naphthobarrelenes 13a+b with the opposite regioselectivity to that observed for the thermal equilibration at 220°.