Strained cyclic allenes are reactive species that can be trapped in a variety of complementary fashions that capitalize on their inherent high potential energy. 1,2,4-Cyclohexatrienes represent a subclass of allenes that, notably, can be conveniently generated by a net [4 + 2] cycloaddition within a 1,3-enyne bearing a tethered alkyne via a tetradehydro-Diels-Alder reaction. A limitation to the use of this type of thermally generated cyclic allene as a construct for the introduction of molecular complexity is their propensity to isomerize to benzenoids via a simple net 1,5-hydrogen atom migration. We have discovered that when the enyne component of the substrate is modified as an enol silyl ether (or an enol ester), migration of the silyl (or acyl) group can become the predominant event. Specifically, an appropriately electrophilic group can migrate from the O atom to the central allene carbon adjacent to the 1-siloxy(acyloxy) substituent. This process leads to highly substituted phenolic products (e.g., o-silyl phenols) following tautomerization of the intermediate cyclohexa-2,4-dienone. Experimental studies show that this novel mode of reactivity is general; DFT studies reveal the unimolecular nature of the group migration.
Bibliographical noteFunding Information:
This study was supported by a research grant from the National Institutes of General Medical Sciences (R35 GM127097) of the U.S. National Institutes of Health (NIH). Some NMR data were collected with an instrument partially funded by the Shared Instrumentation Grant program (S10OD011952) of the NIH. ESI HRMS data were obtained in the Analytical Biochemistry Shared Resource laboratory, part of the Masonic Cancer Center at the University of Minnesota; a portion of the instrumentation there was funded by the NIH National Cancer Institute (Cancer Center Support Grant CA-77598). DFT computational studies were done under the auspices of the University of Minnesota Supercomputing Institute (MSI). Alex Lovstedt performed the X-ray diffraction structure determination of 31 in the University of Minnesota X-Ray Crystallographic Laboratory.
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