Oxonium ylides Stevens -shift

The addition of /i-toluenesulfmate to the silyl ether of l-hydroxybut-3-yn-4-yl(phenyl)iodonium triflate triggers a sequence of reactions that ultimately delivers 2-substituted-3-(/i-toluenesulfonyl)dihydrofuran products in variable yields (Scheme 31). A putative 1,2-group shift within an unsaturated oxonium ylide (Stevens rearrangement) accounts for the oxygen-to-carbon transfer of the ether substituent <2000JOC8659>. 

The other major reaction pathway for oxonium ylide is [l,2]-shift (Stevens rearrangement). Compared with [2,3]-sigmatropic rearrangement, which is an orbital symmetry-allowed concerted process, the [l,2]-shift has higher activation barrier, [1,2]-Shift is generally considered as stepwise process with radical pair as possible intermediates. 

Cyclic mixed acetals with pendant diazo ketone side-chains undergo rearrangement to ether-bridged cycloheptane ring systems on treatment with Cu(hfacac)2.118 A Stevens [1,2]-shift of an oxonium ylide gives the major product (62), in some cases accompanied by minor amounts of a product (63) resulting from a [l,2]-shift of a sulfonium ylide. 

Feldman, K. S., Wrobleski, M. L. Alkynyliodonium salts in organic synthesis. Dihydrofuran formation via a formal stevens shift of a carbon substituent within a disubstituted-carbon oxonium ylide. J. Org. Chem. 2000, 65, 8659-8668. 

Vanecko, J. A., West, F. G. A Novel, Stereoselective Silyl-Directed Stevens [1,2]-Shift of Ammonium Ylides. Org. Lett. 2002,4, 2813-2816. Marmsaeter, F. P., Murphy, G. K., West, F. G. Cyclooclanoid Ring Systems from Mixed Acetals via Heteroatom-AssIsted [1,2]-Shlft of Oxonium Ylides. J. Am. Chem. Soc. 2003,125,14724-14725. 

The rhodium acetate-catalyzed decomposition of l-diazo-4-(benzyloxy)butan-2-one and related compounds offers a way to substituted tetrahydrofuranones a Stevens [ 1,2]-shift of a cyclic oxonium ylide is involved in this reaction (Equation (89)) <92J0C3479>. 

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