SYNTHESIS AND SYNTHETIC APPLICATION OF PHOSPHONOKETENE DITHIOACETALS - NEW SYNTHESIS OF DITHIOALLENES AND (ALPHA-DITHIOCARBOXYVINYL) PHOSPHONATES

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 ch lorides. Reaction of 2,2-dithio-1-phosphonovinyl anions with aldehydes afforded allylic alcohols 4-7, 11-18 in good to moderate yields. Trea tment 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-(trimethyle ne dithio)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 determine d by X-ray analysis. Treatment of the alcohols 15 and 18 with trifluor omethanesulfonic acid/n-Bu(4)NX (X = Br, I) or triphenylphosphine/CBr4 in CH2Cl2 afforded alpha-phosphonodithioacryclic acid esters 34 and 3 5 in 25-52% yields. The tandem Michael-Wittig reaction of 35 with sodi um salt of 2-pyrrole-carbaldehyde in DMF gave ethyl phenyl-3H-cyclopen ta[a]pyrrole-2-dithiocarboxylate (36) in 25% yield.