P-Bromobenzyl esters

The p-bromobenzyl ester has been used to protect the /3-COOH group in aspartic acid. It is cleaved by strong acidic hydrolysis (HF, 0°, 10 min, 100% yield), but is stable to 50% CF3COOH/CH2CI2 used to cleave /-butyl carbamates. It is 5-7 times more stable than a benzyl ester. ... 

Clavulanic acid, p-nitrobenzyl and p-bromobenzyl esters, Howarth et al. (1976)... 

The quaternization method is also highlighted by the short asymmetric synthesis of cell adhesion molecule BIRT-377 (Scheme 5.24), which is a potent inhibitor of the interaction between intercellular adhesion molecule-1 (ICAM-1) and lymphocyte function-associated antigen-1 (LFA-1) [16]. Thus, asymmetricp-bromobenzylation of the alanine derivative 42 (R1 = Me) with (S)-18 under similar phase-transfer conditions as described above gave rise to p-bromobenzylalanine ester 10 in 97% ee (83% yield). A similar asymmetric p-bromobenzylation of alanine ethyl ester 42 (R1 = Me, R= Et) gave the amino ester 47 (R= Et) in 90% ee (86% yield). The amino ester 47 (R = t-Bu or Et) was treated with 3,5-dichlorophenyl isocyanate in the presence of sodium carbonate in dimethylsulfoxide (DMSO) to furnish the hydantoin 48 in 86%... 

Thus the alkyl ester end groups of convergently grown poly(benzyl ether) dendrimers can be subjected to post-synthetic modification by hydrolysis [28], reduction [29], transesterification/amidation [28] in a variety of ways. Subsequent modification of poly(benzyl ether) dendrons bearing p-bromobenzyl end groups by palladium-catalysed coupling reactions permitted preparation of dendrons with phenyl, pyridinyl, or thiophenyl end groups [30]. 

Benzyl bromides 15 derived from either salicylaldehydes or 2-methylphenols by reaction with alkyl propynoates afford 277-1 -benzopyrans on treatment with tetrahydrothiophene and K2CO3. A sulfonium ylide is proposed which initiates a Michael addition - elimination -substitution sequence. When CS2CO3 is used as the base, 477-1-benzopyrans result almost exclusively, presumably through isomerisation of the 277-1-benzopyran. The protocol thus offers access to both chromene isomers through simple variation of the reaction conditions <06OL3853>. The Cu-catalysed intramolecular O-arylation of a-(2-bromobenzyl)-P-keto esters in refluxing THF offers a useful route to 2-substituted 477-l-benzopyran-3-carboxylates <06JOC6427>. 

A soln. of p-bromobenzaldehyde and pyridine borane in isopropyl ether refluxed 45 min. p-bromobenzyl alcohol. Y 94%.—The reducing power of pyridine borane is considerably less than that of those hydrides heretofore used for the reduction of carbonyl compounds to the alcohol stage. Acids and acid chlorides gave lower yields and esters could not be reduced. F. e. and reductions s. R. P. Barnes, J. H. Graham, and M. D. Taylor, J. Org. Ghem. 23, 1561 (1958). 

Alkylation of the enolate of 29d is stereospecific and takes place from the face opposite die biphenyl moiety. It is higMy diastereoselective and gives derivatives of 23 in >98% ee as determined by chiral HPLC after conversion to the methyl ester. In our procedure, LiHMDS (1.1 eq.) is added at -25 °C to a mixture of p-bromobenzyl bromide and 29d leading to 34 in virtually quantitative yield. The results are poorer when the enolate is first generated at -25 °C followed by addition of electrophile, indicating that the enolate itself is less stable at -25 °C. 

---