Medium-ring (7-9 membered) oxasilacycles were synthesized by sequential electrophile-promoted rearrangement of diallylsilanes, etherification with an alkene-containing alcohol, and ring-closing metathesis (RCM). Depending on the choice of catalyst for the RCM, different oxasilacycle products could be obtained. The Grubbs first-generation catalyst cleanly afforded allyl ether oxasilacycles whereas the second-generation Grubbs catalyst selected for the regioisomeric cyclic enol ether, resulting from RCM followed by isomerization.Isomerization during RCM was suppressed by substitution at the allyl ether position.The use of homoallyl ether-or non-ether substrates, and/or the addition of benzoquinone also prevented isomerization during RCM, suggestive of a ruthenium hydride-based double bond migration mechanism. Both product subclasses represent useful synthetic intermediates. As an initial demonstration, this sequence was used to prepare the side-chain of the natural product psymberin, as well as a ulosonic acid analogue.
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