Sulfonium ylides ring expansions

Ring expansion of activated aziridines (43) with sulfur ylides also provides a synthesis of azetidines (75JOC2990, 58BSF345, 81CC417). The highly reactive sulfonium methylide (44 R = R = H) undergoes further reaction with the azetidines (46), but the reaction is satisfactory for substituted methylides. The less reactive sulfoxonium methylide (45 R = R = H)... 

Other synthetic routes to benzazepines involving ring expansion of six-membered heterocycles include the action of diazomethane (77CPB321), sulfonium ylides (77H(7)37> or acyl halides (75T1991) on quaternary 3,4-dihydroisoquinolines that of sulfoxonium ylides on quaternary quinolines (74IJC(B)1238) and the zinc-acetic acid reduction of quaternary 1-acyltetrahydroisoquinolines (77BSF893). Photoaddition of acyl- or aryl-nitrenes to the exocyclic alkene bond of 2-methylene-1,2-dihydroquinolines results in ring expansion to... 

Dihydrothiophene and its 1-oxide undergo [2 + 3] cycloaddition with DMAD to give unstable sulfonium ylides (e.g. 232). The latter undergoes ring expansion to give the thiepin 1-oxide (233) (80AG(E)833). 

Reactions of carbocyclic P-keto esters, sulfonium ylides and enamines with activated alkynes such as DMAD are known to result in formation of (n + 2) ring expanded products. In a study of the analogous reactions of carbocyclic p-keto phosphonates, it was found that in the cases of the simple cyclic P-keto phosphonates 1, ring expansion occurred to give 2 in reasonable yield. Extension of the method to the tetralone 3, however, led to formation of two products, the "expected" (n + 2) ring expansion product analogous to 2 (37%), and the lactone 4 (29%). 

Compounds 128 were prepared in high yields either by thermal [1 Ire-arrangement of sulfonium ylides 126 to intermediates 127 which were then converted into the thiazepine derivatives (128) by ring expansion on photolysis in methanol, or directly from the ylides 126 by ultraviolet irradiation, which causes a photo[l,2]rearrangement followed by ring expansion (Scheme 39) (77CPB292). 

Comparable cis/trans selectivities and similar conformational effects on them were studied in ring enlargements of cyclic sulfonium ylides. ° Rearrangements of sulfur ylides performed in repetition were used for stepwise ring expansion 5- 8—> 11- —> 14- - 17-membered. ° Synthesis applications of... 

The formation of free carbene 425 was postulated in the reaction of sulfides 426 with methyllithium. It is stabilized via cyclization into a four-membered sulfonium ylide 427, followed by rearrangement with ring expansion into thiabicyclo[3.1.0]hexane 428, which is isolated as a mixture of endo (72%) and exo (28%) isomers in an overall yield of 17-38%. However, simultaneous occurrence of side processes makes this reaction synthetically inappropriate [82ACS(B)593],... 

The epoxy sulfones were prepared by exhaustive peracid oxidation of the corresponding alkene sulfides. These were generated by ring expansion of cyclic 1-methyl-2-vinyl sulfonium salts 1 via 2,3-sigmatropic rearrangement of the methanides. From the five-membered sulfonium ylide la, (Z)- and ( )-thiacyclooct-4-enes 2 were obtained as an 85 15 mixture. Their chromatographic separation turned out not to be feasible due to concomitant EjZ isomerization on the silica gel column. Since the separation of these epoxy sulfone mixtures obtained by exhaustive oxidation also proved unsuccessful, it was found expedient to first oxidize the mixture of sulfides to an alkene sulfone mixture ( )-3, which could be separated and eventually epoxidized into epoxides cis- and trans-4 and cis- and trans-5. 

Recently, the sulfonium-ylide pathway has been used repeatedly to perform one-carbon ring expansion by formal insertion of a carbene moiety into a C—S or N—S bond of a cyclic sulfur compound. Examples are compiled in Scheme 37. It can be seen that ring enlargement was successful with thiochroman-4-ones 345 and 348,... 

Ring Expansion. Sulfonium ylides can be prepared from sulfides and dimethyl diazomalonate under rhodium(II) or copper... 

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