Process for the electrophilic substitution of thiazolidines or oxazolidines

A process for preparing compounds of the general formula (2)

• 15
• where
• X is S or O and
• R1 is selected from the group comprising hydrogen, metals of the first or second main group, linear or branched C1-C12-alkyl, C6-C15-aryl or C7-C21-aralkyl radicals, dialkylsilyl and trialkylsilyl, dialkylarylsilyl, diarylalkylsilyl, triarylsilyl radicals, and the organic radicals of the silyl radicals are in turn selected from C1-C12-alkyl and C6-C15-aryl radicals and
• R2 is selected from the group comprising linear or branched C1-C12-alkyl, C6-C15-aryl and C7-C21-aralkyl radicals and
• P is an amino protecting group and
• E is a radical selected from the group comprising optionally halogen, cyano, nitro or ester group-substituted, linear or branched C1-Cl2-alkyl, C3-C10-alkenyl, C6-C15-aryl and C7-C21-aralkyl radicals, or is an acyl or formyl group,
• comprising adding a base to a reaction mixture comprising a compound of the general formula (1)
• 16
• and an electrophile E-Y
• where
• Y is a leaving group,
• and carrying out the reaction at a temperature of greater than −40° C.