2- ethylidene acetal protect diols

Benzylidene acetals are frequently used for the protection of the 4- and 6-hydroxyl groups of pyranoses, Reclamation of the diol can be accomplished by many methods that will be discussed below, but Scheme 3.53 shows an unusual cleavage of benzylidene acetals protecting the glucose moieties in 53.1 using 1% iodine in methanol (w/v),86 The reaction works equally well with isopropylidene, ethylidene and dithioacetals,87... 

In a related process, triethylsilane plus SnCLr can expediently convert appropriately protected aldol products to fully protected 1,3-diols. Moreover, the synthesis of iyn-l,3-ethylidene acetals from l-(2-methoxyethoxy)ethyl-protected -hydroxy ketones with SnCLr and EtsSiH can occur with very high levels of diastereocontrol (eq 33). ... 

Vicinal diols in sugar substrates can also be protected as their 2-(ferf-butoxycarbonyl)-ethylidene ( Bocdene ) or 2-(methoxycarbonyl)-ethylidene ( Mocdene ) derivatives in the reaction with ferf-butyl or methyl propynoate. The acetal-like structures of these protecting groups is of interest because they are stable under acidic conditions, which allows their selective deprotection versus other acetals, and can be removed under basic conditions via an addition-elimination mechanism (O Scheme 21) [140]. The procedure is not suitable for 1,3-or 1,4 diols. 

---