Pivaloate group

The methyl group was introduced by a two-step procedure. Thus, the hydrazone Michael adducts 52 were converted into the enol pivaloates 53 in excellent yields and diastereomeric excesses de > 96%) by treatment with pivaloyl chloride and triethylamine. After treatment with lithium dimethylcuprate the chiral auxiliary was removed by addition of 6n HCl in order to obtain the 5-substituted 2-methylcyclopentene carboxylate 54 in good yields and with excellent stereoselectivity (de, ee > 96%). Finally, the asymmetric synthesis of dehydroiridodiol (55, R = Me, = H) and its analogues was accomplished by reduction of 54 with lithium aluminum hydride or L-selectride leading to the desired products in excellent yields, diastereo- and enantiomeric excesses (de, ee > 96%). 

In the first step tetrabutylammonium fluoride (TBAF) cleaves the silyl protecting group at C-2a (51). Secondly the pivaloate ester 53 is formed.24 In the last step the ketal was removed under acidic conditions by using trifluoroacetic acid (TFA) in water and CH2C12. 

There is a very large number of different ester protecting groups available and only the more common representatives in carbohydrate chemistry will be treated here. This includes acetate and substituted acetates such as chloroacetate, pivaloate, and levulinate groups. Aroyl groups are frequently used, such as benzoyl and substituted benzoyl e. g. p-phenylbenzoyl and 2,4,6-trimethylbenzoyl groups. 

Asymmetric borohydride reduction. Colonna and Fornasier have examined the reduction of ketones with sodium borohydride under phase-transfer conditions in the presence of optically active ammonium salts containing at least one hydroxyl group. Of the seven catalysts tested (-)-benzylquininium chloride (1) (7, 311) was the most effective for asymmetric reduction of r-butyl phenyl ketone (pivalo-phenone) to the corresponding carbinol with optical yields as high as 32%. Two factors would appear to be important for this asymmetric reduction the catalyst must be conformationally rigid and the hydroxyl group must be in the 8-position to the onium function. ... 

Esters (ROCOR ) can be easily prepared by reacting an alcohol (ROH) with an acid chloride (R COCl) and base. Deprotection is generally accomplished by basic hydrolysis or alcoholysis of the ester. Commonly used ester groups include acetate (-COCH3, or Ac) and benzoate (-COPh, or Bz).The pivaloate ester (-COr-Bu, or Pv) is useful for selective acylation of a primary alcohol in the presence of more hindered secondary or tertiary alcohols. 

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