Dispiroketal protection, trans- 1,2-diols

Scheme 1 Dispiroketal or dispoke protection of trans-1,2-diols...

In 1992, Ley and co-workers demonstrated the inherent selectivity of 3.3, 4.4 -tetrahydro-6,6 -bis-2//-pyran (bis-DHP) 1 for trans diequatorial vicinal diols in polyol systems in carbohydrate derivatives (Scheme 1) [26]. In a typical experiment, the carbohydrate polyol was reacted with an excess of bis-DHP 1 in refluxing chloroform and in the presence of a catalytic amount of camphorsulfonic acid to afford the corresponding dispiroketal. For representative examples see Table 1. The protection process proceeds in moderate to good yields and gives diequatorial diol protection as the major outcome in all cases. In a few cases some c/s-diol protection was noticed as a minor process when steric interactions were of lesser magnitude. 

The 3,4-cyclohexylidene diacetal, which was introduced for the selective protection of trans diequatorial vicinal diols [33], is resistant to the flattening of carbohydrate ring required for the generation of an oxocarbonium ion intermediate and thus a torsionally disarming protecting group [15]. Similarly, the dispiroketal [15, 34]... 

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