Esters, benzyl hydrogenation

Monobenzyl ester Benzyl hydrogen 2-formylmalonate C11H10O5 M 222.197 Needles (hexane). Mp 54-56°. 

S,N-Ditrityl-L-cysteine diethylamine selt L-Tyrosine lower alkyl ester L-lsoleucine lower alkyl ester Benzyl-L-proline hydrochloride L-Leucine lower alkyl ester Ammonia Hydrogen chloride Glycine lower alkyl ester... 

Although these Boc derivatives underwent methylation with poor selectivity (compared to 3-amino-N-benzoyl butanoates [106] and Z-protected methyl 4-phen-yl-3-aminobutanoate [107]), epimers were succesfully separated by preparative HPLC or by flash chromatography. However, saponification of the methyl ester caused partial epimerization of the a-stereocenter and a two-step (epimerization free) procedure involving titanate-mediated transesterification to the corresponding benzyl esters and hydrogenation was used instead to recover the required Boc-y9 -amino acids in enantiomerically pure form [104, 105]. N-Boc-protected amino acids 19 and 20 for incorporation into water-soluble /9-peptides were pre-... 

Concomitant hydrogenolysis of a benzyl ester and hydrogenation of a diene were performed in one step over Pt02 in AcOEt.207 In contrast, 10% Pd(OH)2/C in AcOEt or EtOH at room temperature for 1 hour under hydrogen was used for the hydrogenolysis of a benzyl group without the saturation of a cyclic carbon-carbon double bond.208... 

The most convincing evidence in favor of a uniform 3,5-diester linkage between nucleotides has been obtained by the action of various enzymes on synthetic diesters of known constitution.218 217 Ribonuclease and spleen extracts were found to act only on nucleoside 3-(benzyl hydrogen phosphates), but not on other isomers, to give nucleoside cyclic phosphates which are broken down further to give nucleoside 3-phosphates. It is concluded, by analogy, that polynucleotides, which are substrates for these enzymes, also possess ester groupings at the 3-positions, rather than at the... 

A method has been developed which is designed to remove nucleotide residues singly from a polynucleotide chain, and it is anticipated that more precise information will shortly be forthcoming regarding the order in which the nucleotides are linked.220 The method is based on the fact that esters of 3-oxo alcohols are unstable toward alkali. In agreement with this, it is found that the products of the action of periodate on adenosine 5-phosphate (XXX) or adenosine 5-(benzyl hydrogen phosphate) are hydrolyzed under very mild conditions. Thus, after removal of terminal, singly-esterified phosphoryl residues from a polynucleotide chain, it is anticipated that periodate oxidation and hydrolysis will result in the removal of the... 

Thus, the best compromises for Boc and Fmoc chemistries seem to be cyclohexyl and 2,4-dimethylpent-3-yl (Dmpn), which is of intermediate stability, and the removal of which by trifluoromethanesulfonic acid with the aid of thioanisole (see Section 6.22) leads to minimal imide formation (see Section 6.13). Points to note are that acidolysis of esters by hydrogen fluoride can lead to fission at the oxy-car-bonyl bond instead of the alkyl-oxy bond, thus generating acylium ions that can react with nucleophiles (see Sections 6.16 and 6.22), and that benzyl esters may undergo transesterification if left in methanol. The side reactions of cyclization (see Section 6.16) and acylation of anisole (see Section 6.22) caused by acylium ion formation do not occur at the side chain of aspartic acid.47-51... 

A number of substrates having a benzylic ether moiety were reacted with 51 to afford the corresponding benzylic esters in good yields (equation 84). For evaluating the effects of p-substiments on the oxidation of a series of benzylic ethers, a competitive oxidation of p-substimted benzylic propyl ethers with 51 was carried out. The Hammett correlation plot for the oxidation reaction gave a better correlation of the relative ratio factors with the a rather than with the a+ substituent constants and afforded a reaction constant p+ = —0.57 (r = 0.99). This p+ value shows that 51 is an electrophilic species and appears to be comparable to the p+ value of —0.65 for benzylic hydrogen abstraction from dibenzyl ethers by the benzoyloxy radicaP . 

Keratinocytes, cosmetics, 613 a-Ketoalkyl radicals, 252 y-Ketoalkyl radicals, 252 of-Keto esters, benzylic oxidation, 518 2-Ketoglutarate, hydrogen peroxide determination, 655... 

Reduction of (S)-pinanediol [phenyl(chloro)methyl]boronate with commercially available lithium triethylborodeuteride yields a chirally deuterated benzylboronic ester. Deboronation of the deuterated benzylboronic ester with hydrogen peroxide yields chirally deuterated benzyl alcohol in 96-98% ee70. The conversion of the deuterated benzylboronic ester to chirally deuterated phenylalanine has also been accomplished (Section 1.1.2.1.4.2.). 

Abnormal olefin arylation reactions which are of interest mechanistically and preparatively occur with some allylically substituted compounds. The ailylic esters and ethers appear normal and produce cinnamyl derivatives exclusively while ailylic alcohols and chlorides are abnormal. Ailylic alcohols and "arylpalladium acetates form 3-arylaldehydes from primary ailylic alcohols and 3-arylketones from secondary alcohols 3°). The mechanism of reaction apparently involves anti-Markovnikov addition of the palladium compound to the double bond followed by elimination of the hydrogen atom on the hydroxyl-bearing carbon rather than the benzylic hydrogen. This again would be elimination of the more electronegative hydrogen atom. 

Photolysis of diethyl bromomalonate in benzene in the presence of compounds with a benzylic hydrogen as possible H-atom source leads to reduction of the ester and bromina-tion of the benzylic compound263. The malonyl radical and not the Br atom turns out to be the H-atom abstracting agent. 

Other esters are sometimes used to protect carboxylic acids, especially when there is a desire to deprotect the acid by using different conditions from those available for methyl esters. Benzyl esters are prepared in the usual manner but can be cleaved by reaction with hydrogen and a catalyst. Again it is the benzylic carbon-oxygen bond that is broken in the hydrogenolysis reaction ... 

As described in the previous section, substitution at the benzylic position always takes place together with nuclear substitution if the aromatic substrates possess replaceable benzylic hydrogens (equation 39). Benzylic alcohols, esters and ethers can all be oxidized at the benzylic position to yield the corresponding carbonyl compounds. ... 

The synthesis of the racemic dihydropyrones has previously been described.24,30,31 The chiral synthesis of the dihydropyrones is shown in Scheme 1 as reported by Christopher Gajda.16,17 The ketone 131 was reacted with the appropriate acetate enolate to afford 132,134, or 136. The resulting ester was converted into the acid 133 by either hydrolyzing the alkyl ester or hydrogenating the benzyl ester. The resulting racemic acid was resolved by classical resolution with a variety of chiral amines to afford resolved 135. An alternative route to chiral acid 138 was achieved by resolution of the intermediate ester using chiral columns and... 

Dibenzyl Phosphite. Pbosphonic acid bhiphenyt-methyl) ester dihenzyl hydrogen phosphiie benzyl phosphite. C,4Hl503P mol wt 262,24. C 64,12%, H 5-77%, O 18,30%, P 11.81%, (HO)P(OCH2C4Hs)2- Prepd by adding drop wise a mixrure of dimethylan time and benzyl ale to a SOln Of phosphorus trichloride in benzene Atherton et a/, J, Chem- Soc, 1945, 382 U.S. pat. 2,490 573 <1949 to Hoffmann La Roche). 

Cleavage of henzylic esters. Benzylic esters are cleaved in 80-98 7o yield by nitrosonium tetrafluoroborate. The reaction involves a hydride transfer (equation 1). Benzylic esters have usually been cleaved by hydrogenation. ... 

---