Facile Synthetic Method for the Preparation of Dithioacetals by the Double Conjugate Addition of Acetylenes Bearing Electron-Withdrawing Groups with Thiols

β, β-Bis(alkylmercapto)propionates were obtained by the reaction of methyl propiolates with 2 equiv. of mercaptanes in the presence of tri-n-butylphosphine in high yield. The corresponding dithioacetals could be also obtained by the conjugate addition of mercaptanes toward methyl β-alkylmercaptoenoates, in which, tri-n-butylphosphine also catalyzed the addition of thiols.