Benzaldehyde ethylene dithioacetal

Benzylidene acetals can be formed under essentially neutral conditions using the bis-sulphonium ion (62) obtained by methylation of benzaldehyde ethylene dithioacetal with methyl fluorosulphonate (Magic Methyl ). Thus, methyl -d-glucopyranoside afforded a 4,6-0-benzylidene derivative (25%) when treated with (62) in pyridine. In the absence of an acid catalyst, it is unlikely that the thermodynamically favoured acetal would be obtained in high yield. 

---