I-butyl ester

Methyl esters react more rapidly with lithium iodide than do ethyl esters, which in turn react more rapidly than esters of secondary alcohols. On the other hand, i-butyl esters are cleaved very readily with a catalytic amount of lithium iodide. 

Treatment of the i-butyl ester (103) with dry hydrogen chloride in cold nitromethane gave the hydrochloride (114), from which the free... 

The esters differ from each other in stability. To decompose the isopropyl ester, higher temperatures and higher acid strengths are needed than for decomposition of the i-butyl ester. It is claimed that the resulting carbenium ions are stabilized by solvation through the acid (25-27). Branched alkenes do not form esters. It is beheved that they are easily protonated and polymerized (28). 

The reagent is also for asymmetric reduction of n-keto esters, particularly a-keto I butyl esters, "nius r-bulyl pyruvate is reduced to (S)-f-buiyl lactate in 100% ee. ... 

Azetidines. (12, 333) These cyclic t directly, but can be obtained by reduction of 5-lactams with cleavage of the 1,2-bond to al ut monochloroalane (AIH2CI) or dichlortnli a high yield (85-100%). Azido groups, i -Butyl ester groups are reduced to hydroxyl... 

Esters of tertiary alcohols may not be prepared from carboxylic acids containing acidic a-protons using this modified procedure, since deprotonation and subsequent condensation, competes. However, the use of stoichiometric 1,8-Diazabicyclo[5.4.0]-undec-7-ene as base has been shown to provide good yields of i-butyl esters even for acids with acidic a-protons (eq 3). This procedure was unsuccessful for pivaUc acid or for IV-acyl-a-amino acids. 

An alternative approach to increasing the rate of esterification is to activate further the intermediate (2). N-Bromosuccinimide has been used for this purpose, but unsaturation in the carboxylic acid or alcohol is not tolerated. More generally useful is the addition of an activated halide, usually A//y/ Bromide, to a chloroform solution of (1) and a carboxylic acid, resulting in formation of the acylimidazolium salt (3) (eq 4). Addition of the alcohol and stirring for 1-10 h at room temperature or at reflux affords good yields of ester in a one-pot procedure. These conditions work well for the formation of methyl, ethyl, and i-butyl esters of aliphatic, aromatic, and a,/3-unsaturated acids. Hindered esters such as i-butyl pivalate can be prepared cleanly (90% yield). The only limitation is that substrates must not contain functionality that can be alkylated by the excess of the reactive halide. 

Martin et developed a biogenetic entry into Ala-methylvellosimine (168) which employed an iminium ion-mediated cyclization similar to van Tamelen s original proposal. As shown in Scheme 4, dihydrocar-boline 169, which was readily obtained from commercially available D-tryptophan, was allowed to react with the vinyl ketene acetal 170 to afford a single product which was directly converted into the i-butyl ester 171. The Nb acylation of amine 171 with diketene furnished an intermediate P-keto amide that underwent facile cyclization via an intramolecular... 

The syntheses of the pyrimidine with S-methylisothiourea and of the methane-sulfonamide by oxidation and substitution proceed along standard methods. After reduction of the ester with DIBAIH to the alcohol, the latter is converted into the phosphonium salt. The Wittig reaction leads first to the protected rosu-vastatin i-butyl ester, from which the protecting groups are cleaved off in acid, and the active compound is finally precipitated as its calcium salt. 

The i-butyl ester derivatives of the d- and /-A -acetyl amino acids, S- benzylcys-teine, N-i-butyltryptophan, phenylglycine, and phenylalanine were well resolved (a > 1.3) with k values <2.5 using 70/30 hexane/DCM or 70/30 hexane/chloro-form mobile phases on a (iV-formyl-L-valinylamino)propyl bonded phase (2 = 230 nm). The use of aprotic mobile phase modifiers, instead of the more commonly used alcohols, resulted in greater separation between enantiomers (as determined by... 

The change of mechanism with tertiary alkyl esters is valuable in synthetic methodology because it permits certain esters to be hydrolyzed very selectively. The usual situation involves the use of i-butyl esters, which can be cleaved to carboxylic acids by action of acids such as />-toluenesulfonic acid or trifluoroacetic acid under anhydrous conditions where other esters are stable. 

---