Thiophene-derived cyclopropanes

In this reaction only the cyclopropane ring is opened to form III/66. This compound is in equilibrium with the y-lactol III/67, which, in case of R=H, was transformed oxidatively (pyridinium chlorochromate) to the furanone III/68. The synthesis of thiophene derivatives in a similar reaction is shown in Scheme III/ll, also. 

By analogy, 2-vinylthiophene reacts with tetrabromocyclopropene by [4 + 2] cycloaddition subsequent loss of HBr with concomitant opening of the cyclopropane ring leads to the benzo[A]thiophene derivative 350. 

Several [4 -I- 2] cycloadditions of 2-vinyl and 3-vinylthiophene with dienophiles have been reported <8572435, 87T269, 87T99i>. Earlier, similar reactions of the silyl enol ether of 2-acetylthiophene with several dienophiles had been recorded <84JCR(S)2i8>. 2-Vinylthiophene also reacts with tetra-bromocyclopropene by [4 -I- 2] cycloaddition subsequent loss of HBr with concomitant opening of the cyclopropane ring leads to the benzo[6]thiophene derivative (168) in 32% yield <90TL4581>. 

Furan and thiophene undergo addition reactions with carbenes. Thus cyclopropane derivatives are obtained from these heterocycles on copper(I) bromide-catalyzed reaction with diazomethane and light-promoted reaction with diazoacetic acid ester (Scheme 41). The copper-catalyzed reaction of pyrrole with diazoacetic acid ester, however, gives a 2-substituted product (Scheme 42). 

The formation of cyclopropane derivatives by photolysis of diazoalkanes in the presence of alkenes is believed to occur by photolytic decomposition of the diazoalkane to yield the carbene, followed by addition of this carbene to the alkene. Cycloaddition of this type has been reported in furan, dihydrofuran, and thiophene.198 Thus, photolysis of ethyl diazoacetate in thiophene yields the bicyclic sulfur heterocycle (215). Alternatively, photolysis of 3-diazo-l-methyl-oxindole (216) in cyclohexene leads to the formation of two isomers which are thought to have the spirocyclopropyl structure (217) photolysis in ethanol yields 3-ethoxy-1-methyloxindole.194... 

Carbenoid insertion into the C-S bond of 2-substituted thiophenes competes with cyclopropanation at the C=C bonds. Formation of the ylide 402 and hence a thiopyran by way of a Stevens rearrangement is the dominant pathway for 2-(methylthio)thiophene, but is less significant for the 2-methyl and 2-trimethylsilyl derivatives (Scheme 115) <1998T15499>. 

In another example, direct irradiation of compound 38 yields cyclopropane 39 and the thiopyrane derivative 40, arising from secondary photolysis of 39 (Sch. 13). The formation of 39 is rationalized based on a di-7r-methane rearrangement involving the participation of two benzo [6] thiophene rings present in 38 [29]. 

Cyclopropane derivatives of type III/64 (Scheme III/ll) have been shown to be useful starting materials for a smooth transformation to furanones [63] and thiophenes [64]. The aldol reaction of III/64 and a ketone or aldehyde yielded III/65, which forms, on desilylation, an ester diol (by a retro aldol reaction). 

Formally, the insertion of a carbene(oid) into the 2,3-doubIe bond of the thiophene ring should result in the formation of the 2-thiabicyclo [3.1.0] hex-3-ene ring system. Copper(II)-catalyzed reaction of thiophene with diazomethane results in the formation of 24 (R = H) in modest yield (63TL1047). Analogously, the reaction of thiophene with ethyl diazoacetate yields 24 (R = COjEt) (22LA154). Although these reactions appear to be simple carbene insertion reactions, it is probable that this simple mechanism is not in operation. Rather, the cyclopropane derivatives 24 probably result from the initial formation of the ylid (e.g., 18), which subsequently rearranges. 

A theoretical study of the photochemical isomerization of thiophene as well as of its 2-cyano and 2-methyl derivatives has been published <91J0C129>. Photolysis of the thiophenetriptycenes (6) proceeds by the expected di-m-methane rearrangement to give (8) via the biradical (7) ( heme 2). The cyclopropane ring in (8) then opens up to give products depending on the substituents R and... 

Additionally, Charette and coworkers disclosed the synthesis of biologically active 3,3 -cyclopropyl oxindole 44 via a Pd-catalyzed, Ag-promoted C-H functionalization of cyclopropanecarboxamide 43 derived from 2-bromoaniline (Scheme 10) [31]. Substitution on the aryl ring or on the cyclopropane, including heterocycles such as furan or thiophene, was well tolerated. A mixmre of diaste-reomers was obtained when aryl substitution was present on the cyclopropane. X-ray crystallography confirmed the structure of both diastereomers. 

Thiophen-fused Tropylium Ions and Related Compounds.— The reaction of some thieno-tropones such as (241) with dimethylsulphoxonium methylide gave cyclopropane derivatives (242). The reaction of (241) with electrophilic reagents has been studied. The synthesis of compounds of type (241) has been modified... 

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