S-ALKYLATION OF SULFIDES BY AN ACTIVATED CARBOHYDRATE EPIMINE UNDER ACIDIC CATALYSIS - FORMATION OF ALPHA-ACETAMIDO-SULFIDES .2. REACTIONS WITH CYCLIC SULFIDES AND WITH SULFIDES BEARING AN ADDITIONAL NUCLEOPHILIC SULFUR SUBSTITUENT

Methyl 2,3,4-tri-O-acetyl-6,7-acetylepimino-6,7,8-trideoxy-1-thio-D-erythro-α-D- gaLacto-octopyranoside (1) reacts with cyclic sulphides in the presence of acetic acid to give 7(S)-(ω-acetoxyalkyl)thioamides via attack by acetate ion on the intermediate cyclic sulphonium salts. The co-formation of the 7(S)-{[3-(3-acetoxypropyl)-thio]propyl}thio-derivative (8) from the reaction with thietan demonstrated that collapse of the cyclic sulphonium salt could be occasioned by nucleophilic attack by a sulphide sulphur atom also. αω-Bis(methylthio)alkanes yield monosulphonium salts which collapse to the 7(S)-methylthio-derivative (11) by neighbouring group participation of the sulphide sulphur atom; this participation is inefficient with a 1,3-disubstituted alkane. With ω-(alkylthio)alkanethiols, the efficiency of participation of the thiol sulphur atom again controls the manner of collapse of the intermediate sulphonium salt. With these sulphide-thiol reagents, the coformation of [(ω-alkylthio)alkyl]thio-derivatives is unexpected.