Glycals nucleophilic attack

It was known from solution-based studies that an iodonium electrophile adds to the glycal linkage, in the presence of a sulfonamide, in a transdiaxial fashion.24 This results in formation of a 1 -a-sulfonamido-2-p-iodo product. Furthermore, displacement of iodine can be induced by nucleophilic attack of a thiolate nucleophile... 

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... 

The glycals react with water, alcohols, phenols, carboxylic acids, and certain bases, in the presence of an acidic catalyst, in the same way as does 2,3-dihydro-4/f-pyran (which gives, for example, 2-hydroxy-, 2-alkoxy-, and 2-aryloxy-tetrahydropyrans in high yield ). (Tetrahydro-pyranyl ethers are useful for protecting alcoholic groupings during reactions in basic media, and are readily hydrolyzed with acid,) The first step of these additions would be expected to be protonation at C-2, followed by attack of the nucleophilic reagent on the resonance-stabilized, C-1 carbonium ion. 

The 0X0 reaction, which involves heating an olefin with hydrogen and carbon monoxide under pressure in the presence of dicobalt octacarbonyl, with the introduction of a formyl or (hydroxymethyl) group at one carbon atom, is believed to proceed by the addition of the hydride HCo(CO)4 across the double bond, followed by insertion of CO into the newly formed carbon-cobalt bond. It would be anticipated that the cobalt ion would be nucleophilic and would therefore mainly attack glycals at C-1. In accordance with this view, the products of the reaction with the model glycal 2,3-dihydro-4Ff-pyran have been found to contain 78% of 2-(hy-... 

---