Synthesis and Synthetic Application of Phosphonoketene Dithioacetals. New Synthesis of Dithioallenes and (α-Dithiocarboxyvinyl)phosphonates

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Abstract
Phosphonoketene dithioacetals 3a−e were obtained in good yields by the reaction of ethyl phosphonoacetates 1a,b with 2−4 equiv of thiols 2a−c in the presence of an alkylaluminum dichloride or dialkylaluminum chlorides. Reaction of 2,2-dithio-1-phosphonovinyl anions with aldehydes afforded allylic alcohols 4−7, 11−18 in good to moderate yields. Treatment of the alcohols 4−6 with t-BuOK in THF led to symmetrical [2 + 2] cycloadducts 20−22 of 1,1-(ethylenedithio)allenes in moderate yields, while a similar reaction of the alcohols 11−13 produced a mixture of symmetrical and unsymmetrical [2 + 2] cycloadducts of 1,1-(trimethylenedithio)allenes, 23a−25a and 23b−25b, in 55−94% yields. The alcohol 15 on a similar treatment gave 3-tert-butyl-1,1-bis(ethylthio)allene (26) in quantitative yield. The structures of 20 and 23b were determined by X-ray analysis. Treatment of the alcohols 15 and 18 with trifluoromethanesulfonic acid/n-Bu4NX (X = Br, I) or triphenylphosphine/CBr4 in CH2Cl2 afforded α-phosphonodithioacryclic acid esters 34 and 35 in 25−52% yields. The tandem Michael−Wittig reaction of 35 with sodium salt of 2-pyrrolecarbaldehyde in DMF gave ethyl 3-phenyl-3H-cyclopenta[a]pyrrole-2-dithiocarboxylate (36) in 25% yield.