Reaction Design of Medium-sized Ring Structure Based on the Working Hypothesis of Ethereal Oxonium Ylide Formation

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The ambiphilicity of cerbene-type molecules can be inherited in the form of onium ylides. While ethereal oxonium ylides still continue to elude spectroscopic identification, their formations have been demonstrated indirectly by kinetic measurements, e.g., in the reaction of (biphenyl-4-yl) chlorocarbene with THF.On this basis, we designed novel ring-expansion reactions via bi-and tricyclic oxonium ylides for synthesis.Bicyclooxonium ylides formed in the Rh-catalyzed reaction of diazoketones tethered to five-to eight-membered cyclic ethers, where the tethering chains are one methylene units, underwent a novel three-carbon ring-enlargement reaction to form effectively the corresponding eight-to eleven-membered cyclic ketoethers, respectively. Diazoketones tethered to five-to eight-membered cyclic acetals, where the tethering chains are one or two methylene units, also underwent the similar enlargement reaction. Not only rhodium ketocarbenoids but also free carbenes such as alkylidenecarb … More enes bearing cyclic ethers or acetals behaved similarly to give cyclic dienol ethers. In all the reactions, key intermediates are bicyclooxonium ylides whose reactivity is controlled by the stability of charges and ring-strain.The reaction of bicyclic orthoester bearing a diazocarbonyl function tethered by one methylene unit produced a 9-membered polyoxygenated cyclic ether quantitatively. The enlargement efficiency is not attenuated when the tethering methylene chain is elongated from one to four. This indicates that the formation of tricyclooxonium ylides and their ring-expansion are highly favored by not only the enolate-stabilization but also the presence of three ethereal oxygen atoms in orthoesters which delocalize the positive charge of the ylides to facilitate the formation of ring-enlarged zwitterion (1, n-ambiphile) structure.In addition to the ring-expansion reaction, it was unveiled that oxonium ylides of cyclic ethers show an effective novel ring-opening rearrangement reaction leading to the formation of alkenyloxyketenes known as highly reactive transient species. Less