Selectivity in formation of protecting groups

B. Factors Governing Selectivity in Formation of Protecting Groups 1. Protection of carbonyl groups... 

Selectivity in formation of protective groups may also be achieved by a proper choice of reaction conditions and catalyst. Thus formation of the 3-monothioketal from 3,6-diketones is achieved by dilution of the ethane-dithiol-boron trifluoride reaction mixture with acetic acid. 3-Monocyanohydrins are obtained in good yield from 3,20-diketo-(5a)-pregnanes by diluting the exchange reaction with ethanol. Similarly, dilution of the... 

Factors governing choice of protecting groups / 375 Factors governing selectivity in formation of protecting groups / 375... 

Me3SiI, CH3CN, 25-50°, 100% yield. Selective removal of protective groups is possible with this reagent since a carbamate, =NCOOCMe3, is cleaved in 6 min at 25° an aryl benzyl ether is cleaved in 100% yield, with no formation of 3-benzyltyrosine, in 1 h at 50°, at which time a methyl ester begins to be cleaved. 

This revised strategy demonstrates that a simple change in hydroxyl protecting groups can markedly influence reactivity and selectivity. While formation of the Wittig salt 51 could now be achieved at atmospheric pressure, its coupling with aldehyde 45 only proceeded in moderate yield and selectivity. 

D. M. Clode, Cartxihydiate cyclic acetal formation and migration, Chem. Rev. 79 491 (1979). A. H. Haines, The selective removal of protecting groups in carirnhydrate chemistry, Adv. Carbohydn Chem. Biochem. 39 13 (1981). 

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