Polylithiation of thioethers: a versatile route for polyanionic synthons

Abstract

The successive reaction of phenyl vinyl thioether ( 1) with n-butyllithium and an electophile [E 1=PhCHO, (CH 2) 4CO, (CH 2) 5CO] in THF at −78°C gives, after hydrolysis, the expected methylenic hydroxy thioethers ( 2). Deprotonation of 2 with n-butyllithium followed by a DTBB-catalysed lithiation and reaction with a second electrophile [E 2= t BuCHO, PhCHO, Me 2CO, (CH 2) 5CO], at −78°C, gives after hydrolysis the corresponding methylenic diols 3. The same diols can be prepared starting from 1 in a one-pot process without isolation of the hydroxy thioether 2. The same methodology was applied to the cyclopropyl phenyl thioether ( 4), cyclopropyl 1,3-diols 5 {E 1= t BuCHO, PhCHO, [Me(CH 2) 4] 2CO, (CH 2) 5CO, (CH 2) 7CO; E 2= t BuCHO, Me 2CO, (CH 2) 5CO} being isolated in moderate yields. The successive treatment of bis(phenylthio)methane ( 7) with: (a) n-butyllithium at 0°C, (b) a carbonyl compound [E 1= t BuCHO, Me 2CO, Et 2CO, (CH 2) 5CO] at −40°C, (c) lithium and catalytic amount of DTBB (5%) and (d) a second carbonyl compound [E 2= i PrCHO, t BuCHO, Me 2CO, Et 2CO, (CH 2) 5CO] both at −78°C leads, after hydrolysis, to the expected dihydroxy thioethers 8. When after step (d), a second DTBB-catalysed lithiation is performed at temperatures ranging between −78 and 20°C, the corresponding allylic alcohols 9 were isolated. Finally, treatment of alcohols 9 with a few drops of 6 M hydrochloric acid gives dienes 10 in almost quantitative yields.