Pivalate ester

Preparation and Properties of Free-Amine Pivalate Ester (FAPE) 45... 

Nieman and Keay198 reported the use of c/.v,c/s-spiro 4,4]nonanc-1, 6-diol as a new chiral auxiliary to be used in asymmetric Diels-Alder reactions. Their best results in a series of reactions between chiral acrylates and cyclopentadiene were obtained when the pivalate ester of ds,ds-spiro[4,4]nonane-l,6-diol was used as the chiral auxiliary. When 318 was treated with cyclopentadiene, the expected endo adduct 319 was obtained with more than 97% de (equation 88). 

Scheme 16. Other useful pivalate esters of sucrose obtained by Hough, Richardson, and Chowdhary (1984).

THIA-1-AZABICYCLO(3.2.0)HEPTANE-2-CARB0XYLIC ACID, 6-( ((HEXAHYDRO-lH-AZEPIM-l-YL)METHYLENE)AMINO)-3.3-DIMETHYL-7-OXO-, HYDROXYMETHYL ESTER. PIVALATE (ester).MONOHYDROCHLORIDE, ( )-... 

There are a very large number of different ester protecting groups available and we have selected only five of the more common representatives which exemplify the breadth of graduated hydrolytic lability that is available by simple steric or electronic tuning. Thus pivalate esters, with their greater steric hin-... 

Reactions with Oxygen Nucleophiles. The hist report of the reaction of oxygen nucleophiles was for the deracemization of cyclic allylic ethers, for example, the palladium(0)-catalyzed reaction of 2-cyclohexeny 1-1 -methyl carbonate with sodium pivalate afforded the pivalate ester in 94% yield and 92% ee. This reaction was extended to other cyclic allylic carbonates. 

A simple, convenient method for the selective acylation of a primary OH in the presence of a secondary OH group is its conversion into the t-butanoyl ester (also known as a pivalate ester, OPv). Moreover, the steric bulk of the t-butyl group makes these esters resistant to nucleophilic attack, including hydrolysis under mild basic conditions. The pivalate ester can be cleaved using metal hydride reagents. 

This group is a pivalate ester equivalent that still has the steric advantage associated with pivalic acid but can be removed after the olefin is converted to an alcohol by hydroboration. 

Radical trapping by a nitroxide was used to determine the ratio between (3-fragmentation and hydrogen abstraction by alkoxy radicals generated by thermal decomposition of peroxy pivalate esters.The ratio of fragmentation hydrogen abstraction increased sharply with radical substitution and stability. 

The pivalate ester 31 reacted unexpectedly readily with Grignard reagents 1139. The regio- and stereoselectivity found in the product mixtures was very similar from either rv. v or trims reactants, but depended upon the Grignard reagent used. Some homo-coupling product from 31 was also formed. 

Alkylation. The tolerance of functional groups in these organometallics makes them very valuable in synthesis. Thus their formation and reactivity are not affected by the presence of a pivalate ester at the a-position. ... 

Ley [44] performed deprotection of pivalic esters on alumina under the action of microwave activation in the absence of solvent. It was found that selective deprotection of the 6-position could be achieved without migration of groups or isomerization of the anomeric center (Scheme 5.7). Yields of the order of 90% were obtained such levels cannot be achieved by classical heating. 

The successful implementation of this strategy is shown in Scheme 1.370. The optically pure stannane 1441 was transmetalated in situ with 1442, preparing a borane reagent that condensed with 1440. After methylation, 1443 was isolated in excellent yield with 10.5 1 diastereoselection. At this point, 1443 was elaborated to incoprorate the pyran moiety (five steps), and the side chain was converted to an aldehyde (two steps). Overall, this reaction sequence to 1444 was accomplished in 46% yield. The skipped triene fragment was then incorporated via the anion of sidfone 1445. Saponification of the pivalate ester then completed the synthesis of 1191a. 

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