Thiochromans intermediates

Similar reactions were carried out on 17,29 18,30 and 19.30 The last gave thiochromans and 2,3-dihydrothiophenes from an apparent cyclization at the ortho methyl substituent, but in fact involving a variety of hydrogen and methyl shifts. The route to the two main products [Eq. (7)] of the thio-Claisen rearrangement has been suggested to be via the thiirane intermediate (20).31... 

Z/-l-Benzothiopyrans with catalytic amounts of acid undergo disproportionation to thiochromans and benzothiopyrylium salts. In the case of 3,4-dimethyl-2//-l-benzothiopyran, intermolecular hydride transfer yields an 85 15 mixture of cis- and intermediate bridged sulfonium ion has been suggested to be responsible for the stereochemical control of this reaction.276 Bromination of 2H-1-... 

Optically pure (+)-(i )-3-phenylsulfanyl-l,3-diarylpropan-l-ones are readily available by the enantioselective Michael addition of thiophenols to chalcones. After reduction, acidic dehydration of the racemic alcohol affords a mixture of the racemic cis- and trans- 2,4-disubstituted thiochromans (Scheme 164). A detailed consideration of the stereochemical outcome of the reaction with unsymmetrically substituted diaryl derivatives suggests the involvement of a [1,3] PhS shift via a four-membered sulfonium intermediate and this is backed up by theoretical calculations <2003T3621>. 

From this context it is interesting to note that isothiochroman-4-ones— which essentially represent (bi)cyclic derivatives of phenacylsulfides—behave differently on irradiation as they photoisomerize to thiochroman-3-ones, albeit in moderate yield [32]. As a reasonable proposition, l-methylene-9-thia-spiro[5,3]nona-2,4-dien-7-ones were assumed to be the intermediates, which would then undergo a consecutive light induced a-cleavage to give the final products (Scheme 6). 

Synthesis of tetramethylchroman analogue (64) was similar to that of thiochroman (63) except that the key ethynyl intermediate (72) was prepared from acetophenone (70). Formation of a phosphoester from the lithium enolate salt of (70) was followed by elimination of the phosphate group to give acetylene (72). Coupling of (72) with ethyl 4-iodobenzoate proceeded as with (71) and gave (64). 

In the presence of o-iodothiophenol as the starting compound, it is conceivable that the reaction may proceed through oxidative addition of the o-iodothiophenol to palladium(O) followed by the insertion of allene to produce a a-allylpalladium intermediate. Reductive elimination of Pd(0) would give an iodothioether. Oxidative addition of the latter to Pd(0) and subsequent CO insertion and intramolecular cyclization would afford thiochroman-4-ones 48 and regenerate the catalyst (Scheme 13.31). 

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