Vinyllithium, ring expansion

Thiophthalide (452) is the oxo form of l-hydroxybenzo[c]thiophene. It is a stable compound, being intermediate in oxidation level between o-xylene and phthalic acid. It can be formed from o-xylene by oxidation with a mixture of sulfur and water at high temperatures it can also be formed by reduction of phthalic anhydride with H2S + H2 (72AHC( 14)331). It reacts with vinyllithium to form a complex which on hydrolysis undergoes ring expansion to 4,5,6,7-tetrahydro-2//-benzo[c]thiepin-5-one (453), as shown in Scheme 154. 

Thies ° has shown that the 1,3-oxy-Cope rearrangement can be used to advantage as a two-carbon ring expansion method (equation 9). ° Addition of vinyllithium to a cyclic p. -unsaturated ketone (6), followed by pyrolysis of the TMS ether (7) of the resulting alcohol, gives predominandy the 1,3-rearrangement product (8). 

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