T-Bu esters

Isobutylene, cat. coned. H2SO4, CH2CI2, 25°, 6-10 h, 93% yield. These conditions also convert carboxylic acids to t-Bu esters. 

From an acid chloride t-BuOH, AgCN, benzene, 20-80°, 60-100% yield.Alumina also promotes the conversion of an acid chloride to a t-Bu ester in 79-96% yield. ... 

Polymerization of t-butyl methacrylate initiated by lithium compounds in toluene yields 100% isotactic polymers 64,65), and significantly, of a nearly uniform molecular-weight, while the isotactic polymethyl methacrylate formed under these conditions has a bimodal distribution. Significantly, the propagation of the lithium pairs of the t-Bu ester carbanion, is faster in toluene than in THF. In hydrocarbon solvents the monomers seem to interact strongly with the Li+ cations in the transition state of the addition, while the conventional direct monomer interaction with carbanions, that requires partial dissociation of ion-pair in the transition state of propagation, governs the addition in ethereal solvents. 

The silanediol 21 has been prepared for use as a potential protease inhibitor by treatment of arylsilane 22 with CF3SO3H. This is a convenient method for the preparation of silyl triflates (which are readily hydrolysable, see below) but in this case it is thought that the amide carbonyl intercepts the protonated species 23 (loss of the t-Bu ester group also occurring) to give the spirocyclic silyl ether 24, which is then hydrolysed to give the diol, (Scheme 5)94. 

H3O- / t-Bu ester PG is stabie to basic hydroiysis Protecting a CarboxyUc Add as an Ester... 

MePhN(0)C-C6H4 57% (at 120 "C) 4-Me02C-CeH4 63% (obtained as t-Bu ester) 2-pyridyl 82%... 

Trimethylsilyl ketene acetals 92 react in a similar way (note that a bulky trialkylsilyl group works well) ketene acetals derived from t-Bu esters proved to exhibit slightly higher enantioselectivities than their Me, Et, or Ph counterparts (Scheme 15.18 and Table 15.5) [83]. Owing to the higher reactivity of ketene acetals, the catalyst loading could be reduced to 1-5 mol%. However, in contrast to the trichlo-rosilyl enolates, this reaction is believed to proceed via an open transition state, since both (E) and (Z) isomers 92b,c produce anti-aldol adducts 93 with high diastereo- and enantioselectivity (Scheme 15.18 and Table 15.6) [83]. 

ESTER, 43, 100 BenzOpyrazole, 43, 69 3-Benzoylpropionic acid, condensation with benzaldehyde to give a-benzylidene-y-phenyl-A t-bu-tenolide, 43, 3... 

Limited progress has been achieved in the enantioselective hydrogenation of a,/ -unsaturated carboxylic acid esters, amides, lactones, and ketones (Scheme 26.10). The Ru-BINAP system is efficient for the hydrogenation of 2-methy-lene-y-butyrolactone, and 2-methylene-cyclopentanone [98]. With a dicationic (S)-di-t-Bu-MeOBIPHEP-Ru complex under a high hydrogen pressure, 3-ethoxy pyr-rolidinone could be hydrogenated in isopropanol to give (R)-4-ethoxy-y-lactam in 98% ee [39]. 

Weakly nucleophilic amines such as 2,4-dinitroaniline are acylated in excellent yields by the mixed anhydride 301, prepared from the trimethylsilyl esters 299 (R = Me, i-Pr, t-Bu, PhCH2CH2, 2-MeCgH4 etc.) and 4-trifluoromethylbenzoic anhydride (300) under titanium(IV) catalysis 340... 

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