The Eschenmoser sulfide contraction is an organic reaction first described by Albert Eschenmoser for the synthesis of 1,3-dicarbonyl compounds from a thioester. The method requires a base and a tertiary phosphine.  The method is of some relevance to organic chemistry and has been notably applied in the vitamin B total synthesis. 
A base abstracts the labile hydrogen atom in the thioester, a sulfide anion is formed through an episulfide intermediate which is removed by the phosphine.