Glycals anionic

The first step is a reversible dissociation of 48 into free collidine (49) and a reactive intermediate, coll-I+ (50). The electrophilic collidine-iodonium cation forms with glycal 16 the cyclic compound 52. The perchlorate anion of 48 is not nucleophilic enough to attack the sustained intermediate 52, but 52 is captured by nucleophilic attack of benzenesulfonamide (51). This attack proceeds from the back and leads to trans product 17. 

In a study of NIS-promoted additions to glycals it was noted that when the nu-cleophilicity of the alcohol was low, a competing reaction occurred whereby the succinimide anion itself acted as a nucleophile, attacking the anomeric carbon.76,77... 

Properly protected glycals can be selectively deprotonated at C(l) by very strong bases such as r-buty I lithium (Scheme 3.5d). The resulting anions can be quenched with electrophiles such as Bu3SnCl, I2 and PI1SO2CI.28 The anomeric tin and iodo derivatives have been used in... 

In other experiments, a range of nucleophilic agents were reacted with sulfonyl hex-2-enopy-ranoside 103, and corresponding glycals 104 and 105 were obtained stereoselectively (O Scheme 3S) [209]. In this case, C-3 stereochemistry was regulated by the nature of the incoming anion. 

Furanoid glycals are not stable to the acidic conditions used to prepare the pyranoid congeners. Scheme 4 illustrates a very mild method for preparing furanosyl chlorides, which undergo efficient reductive elimination on treatment with lithium in liquid ammonia. Alternatively, a Zn-Ag couple adsorbed on graphite will reduce furanosyl bromides and the corresponding chlorides react with the radical anion prepared from naphthalene and sodium. ... 

---