Preparation of a D-(+)-biotin intermediate

An improved process for the preparation of a D-(+)-biotin intermediate of the formula I ##STR4## in which R is H or C.sub.1-6 -alkyl,

• by reacting a thiolactone of the formula II ##STR5## in which R has the meaning indicated, with 1,4-dimagnesiumchlorobutane and carbon dioxide in an inert solvent and subsequently eliminating water using a mineral acid, wherein the following steps are carried out successively:
• a) reacting 1,4-dichlorobutane with magnesium turnings in the presence of catalytic amounts of 1,2-dibromoethane in tetrahydrofuran as a solvent,
• b) adding thereto the thiolactone of the formula II, dissolved in tetrahydrofuran at temperatures between -25.degree. and -15.degree. C., and, if appropriate, subsequently stirring,
• c) passing over CO.sub.2 at temperatures between -25.degree. and -15.degree. C.,
• d) adding sulfuric acid for neutralization and dehydration,
• e) separating the phases and concentrating under reduced pressure,
• f) dissolving the residue in a hydrocarbon solvent and washing the organic phases with aqueous alkali,
• g) acidifying the aqueous phase with a mineral acid and extracting the aqueous phase with a hydrocarbon, and
• h) concentrating the organic phase remaining from g) under reduced pressure.