Cyclic iodonium ylides

The cyclic /J-dicarbonyl iodonium ylides can undergo [3 + 2] cycloaddition reactions with various substrates under catalytic or photochemical conditions, presumably via a stepwise mechanism [153-156]. In a recent example, iodonium ylide 211, derived from dimedone, undergoes dirhodium(II) catalyzed thermal [3+ 2]-cycloaddition with acetylenes 212 to form the corresponding furans 213 (Scheme 75). Under photochemical conditions ylide 211 reacts with various alkenes 214 to form dihydrofuran derivatives 215 [156]. 

Iodonium ylides of type 6 cannot be isolated, unless the carbanion center is substantially stabilized (83MI1, 92MI1, 97MI1, 02MI1). For example, although iodonium enolates 22 can be prepared by base-catalyzed condensations of DAIB or iodosylbenzene with /J-dicarbonyl compounds, they are not similarly available from unactivated ketones and esters. Indeed, cyclic and acyclic mono-ketones are converted to a-hydroxy dimethylketals 23 with DAIB (or 13) in KOH/MeOH (86ACR244, 99QR273). Other... 

A new method for the synthesis of cyclic iodonium ylides 484 has been developed via Rh2(OAc)4-catalyzed decomposition of a-diazodicarbonyl compounds and intramolecular capture of carbenes by aryl iodides (93MI2). 

Cyclic iodonium ylides, in which the iodonium atom is incorporated in a five-memijered ring, have much higher thermal stability. The unusually stable cyclic iodonium ylides 404 can be synthesized via intramolecular transylidation of a preformed acyclic ylide 403 (Scheme 2.118) [551],... 

Iodonium ylides derived from cyclic 1,3-dicarbonyl compounds, phenols, coumarin derivatives and hydrox-yquinones represent a particularly stable and synthetically important class of zwitterionic iodonium compounds [539,552], Several examples of these compounds are shown in Figure 2.15 in the major resonance form as enolate or phenolate zwitterionic structures 405-408. 

The unusually stable cyclic iodonium ylides 50 can be synthesized via the intramolecular transylidation of a preformed acycftc ylide 49 (Scheme 3 1992CC1487). X-ray stmctural analysis for cycftc yftde 50 shows a distorted... 

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