Pivaloyl group

Pyrrolo[2,3-/ ]pyridinones react with diethyl cyanomethanylphosphonate to give two different products, depending on the substitution pattern of the starting material. For compounds without substituents in the 2-position, pyrrolo[2,3-3]-pyridinyl acetonitriles, 55, are produced. When the nitrogen atom is protected with a pivaloyl group, alkenyl nitriles, 56, are formed (Scheme 2) <2001T1995>. 

The N-pivaloyl group is held (85JOM1) in a planar conformation in derivative 134 and a planar chelate ring, 135, is also established in the presence of a magnesium cation. 

The second-order parameter to expresses the special "around the chain".repulsion between two pivaloyl groups in the racemic tt conformation. 

Ley, S.V. and Mynett, D.M., Microwave promoted hydrolysis of esters absorbed on alumina a new deprotection method for pivaloyl groups, Synlett, 1993, 793. 

The pivaloyl group which can be regioselectively introduced in carbohydrates [82], is photolyzed under preceding conditions to deoxygenated compounds [83], in better yields than for acetates or aromatic esters. 

Hydrolysis of ester 16 proceeds smoothly. Activation of the resulting acid is achieved via conversion into the mixed anhydride 60. The amino group of (S)-2-amino-l-propanol is more basic than the alcohol function therefore there is no need for protection. It attacks the anhydride at the carbonyl carbon of the former acid because of the steric interaction with the pivaloyl group and gives amide 17 in 71 % yield. 

An intermediate in the synthesis of laulimalide by Davidson8 illustrates the differential protection of alcohols. The starting materials 56 and 57 already have an alcohol protected as a TBDMS silyl ether and a diol protected as an acetal. The alcohol in 58 is protected as a p-methoxybenzyl ether and the acetal hydrolysed by acetal exchange to give the free diol 60. Selective protection of the primary alcohol by a bulky acyl group (pivaloyl, i-BuCO ) 61 allows silylation of the secondary alcohol with a TIPS group 62. Finally the pivaloyl group is selectively removed by DIBAL reduction to release just one free alcohol 63. 

Molecular models of 3 clearly indicate that the C(7)-0-pivaloyl group is appropriately positioned to engage in chelation with the alkynyllithium reagent in the manner shown overleaf. Chelation of this sort would obviously help guide in the nucleophile to the more-hindered top face of... 

The high stereoselectivity can be explained again by means of the zinc complex A (see Scheme 3). The nucleophilic isonitrile attacks the glycosyl imine from the steri-cally less shielded Re-side. The bulky pivaloyl group at the 2-position and the formed zinc complex block the Si-side efficiently. The exchange of the pivaloyl for the acetyl group decreases the selectivity of the reaction (10 1 instead of 30 1) [13]. 

Although discovered in the eariy 19(X)s, the oxidation of nitro compounds with potassium permanganate has become a truly useful reaction only within the last 23 years. An alkoxide base is used to form the nitronate salt, and aqueous potassium permanganate gives the oxidized product in excellent yield. Even aldehydes can be obtained as in equation (9) without loss of the pivaloyl group. Cetyltri-... 

One of the representative models is the picket fence iron porphyrin reported by Coltman and his coworkers (Figure 4). The picket fence porphyrin as a structural model for myoglobin has four bulky pivaloyl groups at the peripheral positions to prevent two of these iron porphyrins from coming together to form a /u.-oxo dimer. The first significant result in their study was to demonstrate the 3D structure of the stable... 

The stereochemical outcome of the addition was explained in terms of steric interactions, as illustrated in Scheme 10.64. The carbonyl groups of both the crotonyl and pivaloyl esters are co-ordinated by the magnesium organocuprate or the magnesium halides formed during the reaction. The crotonyl ester 191 probably prefers an -rran -conformation. As the /-face of the double bond is shielded by the bulky pivaloyl group, the cuprate reagent approaches from the less... 

The bulky pivaloyl group has been used as a protecting group in the synthesis of acylated nucleosides [247], monosaccharides, and disaccharides [248]. The pivaloyl esters are usually highly crystalline compounds, its position in a molecule is easily detectable by NMR, and it can be... 

---