Hemiketals formation, intramolecular

Pretazettine (242) has a trans B-D ring juncture and seems a fairly strained molecule whereas tazettine (240) bears a cis B-D fusion allowing more flexibility, and the driving force for the B-ring opening is attributed to the relief of this internal strain. The reaction would be completed by an intramolecular crossed Cannizzaro reaction followed by hemiketal formation yielding the cis B-D fusion present in 242. 

Intramolecular hemiacetal and hemiketal formation is critical to understanding the structure and properties of carbohydrates (Chap. 18). 

The preference for the /3-silyl isomer product complements methods available for hydrostannation of alkynes, for which the a-stannyl regioisomer is formed preferentially.70 7011 70c In addition, the /3-silyl products serve as the platform for a tertiary alcohol synthesis (Scheme 15). Upon treatment of vinylsilanes such as B with tetrabutylam-monium fluoride (TBAF) in DMF at 0 °C, a 1,2 carbon-to-silicon migration occurs, affording the tertiary heterosilane E. Oxidation of the C-Si bond then provides the tertiary alcohol. Good 1,2-diastereocontrol has been demonstrated for y-alkoxy substrates, as in the example shown. The studies suggest that the oxidation of the sterically demanding silane intermediate is facilitated by the intramolecular formation of a silyl hemiketal or silyllactone for ketone or ester substrates, respectively.71... 

The proposed mechanism for these reactions is shown in Scheme 13.96 [106], The initial formation of hemiketal 262 is followed by loss of water and generation of oxonium cation 263. Subsequent intramolecular addition of the azide function onto the cation produces intermediate 264. Elimination of a proton and of N2 directly affords the heterocyclic products. An alternative mechanism involving a... 

Figure 11.5. Furanose Formation. The open-chain form of fructose cyclizes to a five-membered ring when the C-5 hydroxyl group attacks the C-2 ketone to form an intramolecular hemiketal. Two anomers are possible, but only the a anomer is shown.

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