Iodonium ylides, benzene

A very simple synthesis of coumestrol (228) has been described by Kappe and coworkers (Scheme 46) (74ZN(B)292). It is based upon dehydrogenation of 4-hydroxy-3-phenyl-coumarins to coumestans (720PP233). A number of 2 -hydroxy 3-phenylcoumarins were oxidized with lead tetraacetate to the corresponding coumestans 3-(l-acetoxy-4-methoxy-2-oxo-3,5-cyclohexadienyl)coumarins were obtained as by-products (76BCJ1955). Coumes-tan itself (226) has been obtained by photolysis of the phenol ether (232), which is in turn available from 4-hydroxycoumarin (229) and (diacetoxyiodo)benzene (Scheme 47) (78CB3857) via an iodonium ylide (231). 

Oxidative methanolysis of azetidinone 176 followed by hydrogenolysis of compound 177 afforded /3-lactam 178, which was protected to obtain the protected amine 179. The best conditions for rearrangement of 179 were found using TFA. Conversion of compound 180 to carbacephem 183 was accomplished by ketone reduction, alcohol protection, and elimination of methanol. Synthesis of carbacephem derivative 186 has been performed by rhodium(n)-catalyzed cycliza-tion of iodonium ylide 185 <1997TL6981> (Scheme 33). The iodonium ylide 185 was easily prepared from the corresponding /3-keto ester 184 and [(diacetoxy)iodo]benzene in good yield. 

The reaction of alkenes with bis(phenylsulfonyl)carbene generated from the iodonium ylide photolytically (400-W low-pressure mercury lamp), or thermally [in the presence of bis(acetylacetonato)copper(ll), in chloroform or benzene], apart from the expected cyclopropanes 4, always affords bis(phenylsulfonyl)methane and phenyl benzenethiosulfonate. °... 

Most iodonium ylides have low thermal stability and can be handled only at low temperature or generated and used in situ. The relatively stable and practically important iodonium ylides, the dicarbonyl derivatives PhIC(COR)2 [535,540-543] andbis(organosulfonyl)(phenyliodonium)methanides, PhIC(S02R)2 [544-547], are prepared by the reaction of (diacetoxyiodo)benzene with the appropriate dicarbonyl compound or disulfone under basic conditions. A general procedure for the synthesis of phenyliodonium ylides 397 from malonate esters 396 is based on the treatment of esters 396 with (diacetoxyiodo)benzene in dichloromethane in the presence of potassium hydroxide (Scheme 2.115) [542]. An optimized method for preparing bis(methoxycarbonyl)(phenyliodonium)methanide (399) by using reaction of dimethyl malonate... 

The preparation of iodonium phenolates 410 was first reported in 1977 via a reaction of phenols 409 with (diacetoxyiodo)benzene followed by treatment with pyridine (Scheme 2.119) [553]. The system of an iodonium phenolate is stabilized by the presence of at least one electron-withdrawing substituent on the aromatic ring. Monosubstituted iodonium phenolates 410 are relatively unstable and easily rearrange to iodo ethers 411 under heating. Such a 1,4 aryl migration is a very common phenomenon for iodonium ylides of types 405-408 according to mechanistic and computational studies it is an intramolecular rearrangement via a concerted mechanism [554,555]. 

A relatively stable iodonium ylide (426) was prepared by the reaction of -ketosulfone 425 with [bis(trifluoroacetoxy)iodo]benzene (Scheme 2.124) [565]. Ylide 426 decomposes in a solution of dichloromethane/ethanol to form quantitatively, the trimer 427. 

Mixed phosphonium-iodonium ylides 432 represent a useful class of reagents that combine in one molecule the synthetic advantages of a phosphonium ylide and an iodonium salt. The preparation of the tetrafluoroborate derivatives 432 by the reaction of phosphonium ylides with (diacetoxyiodo)benzene in the presence of HBF4... 

Iodonium ylides 2 are nucleophiles and the C=N bond of the imines 3 is a good electrophile. In that case, flie simplest option to explain the second step of the reaction would consist in the nucleophilic attack of the carbanion to the imine to form zwitterions 12. Subsequent intramolecular nucleophilic attack (Sn2) by the nitrogen atom in 12 would yield aziridines 4 with concomitant liberation of iodo-benzene (Scheme 31.6). 

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