Aminoacids reduction

The acid-labile SC linker 1.29 (86) bonded to aminomethyl PS resin, has been used to support acids and C-terminal Boc protected aminoacids. Reductive acidolysis (typically SiCU-thioanisole-anisole/TFA at it over 3 h) reduces the sulfoxide in the activation step and releases the free acid into solution because of the enhanced reactivity of the sulfide towards acid. 

This method is widely applicable to the unambiguous synthesis of quinoxalin-2-ones." It involves the intermediate preparation of a l,2,3,4-tetrahydro-2-oxoquinoxaline by the reductive ring closure of the o-nitrophenyl derivative of an a-aminoacid. These derivatives are formed readily from the aminoacid and an o-nitrohalogenobenzene. The final step of oxidation of the tetrahydro- to the dihydro-quinoxa-line is carried out with potassium permanganate or hydrogen peroxide. The preparation of 7-nitroquinoxalin-2-one illustrates the application of this synthesis ... 

The N-substituted aminoacids required could be prepared by microwave-assisted reductive amination of aminoacid methyl esters with aldehydes, and although in the Westman report soluble NaBH(OAc)3 was used to perform this step, other reports have shown how this transformation can be performed in using polymer-supported borohydrides (such as polymer-supported cyanoborohydride) under microwave irradiation [90]. An additional point of diversity could be inserted by use of a palladium-catalyzed reaction if suitably substituted aldehydes had been used. Again, these transformations might eventually be accomplished using supported palladium catalysts under microwave irradiation, as reported by several groups [91-93]. 

An especially important case is the enantioselective hydrogenation of a-amidoacrylic acids, which leads to a-aminoacids.29 A particularly detailed study has been carried out on the mechanism of reduction of methyl Z-a-acetamidocinnamate by a rhodium catalyst with a chiral diphosphine ligand DIPAMP.30 It has been concluded that the reactant can bind reversibly to the catalyst to give either of two complexes. Addition of hydrogen at rhodium then leads to a reactive rhodium hydride and eventually to product. Interestingly, the addition of hydrogen occurs most rapidly in the minor isomeric complex, and the enantioselectivity is due to this kinetic preference. 

Naphtho[l,8-de][l,2,3]triazine 114 can be alkylated with a variety of alkyl halides and lithium diisopropylamide (LDA) to give alkylated derivatives 115. Reduction of 115 with aluminium amalgam cleaves the naphthotriazine moiety to afford substituted a-aminoacids 116 in good overall yields <00TL6665>. 

From both a biochemical and a synthetical point of view the synthesis of hydrogenated uracils by direct reduction of the pyrimidine ring is a reaction of considerable importance. These reduced uracils bear a similar relationship to /3-aminoacids as the hydantoins do to a-aminoacids. A practical method of reducing uracil combinations, therefore, opens up a new method of synthesizing representatives of this important class of... 

The redox-sensitive linker 1.34 (91), obtained in several steps from Merrifield resin and a lactone precursor, was charged with a N-protected aminoacid, treated with NBS to debenzylate and oxidize the linker to quinone, and submitted to SPS. The quinone linker was reductively activated to dihydroquinone with NaBH4 in THF/MeOH for 30 min at rt, then cleaved by treatment with anhydrous TBAF in THF for 20 h at rt to provide the free acidic peptide via intramolecular cyclization of the linker moiety. 

Kawahata and Goodman utilized a chiral aziridine 166 as a simple precursor for the synthesis of / -aminoacids <1999TL2271>. The chiral aziridine is prepared in five steps from the corresponding allylic alcohol via a Sharpless asymmetric epoxidation. A one-electron reduction of aziridine 166 with SmG provided the ring-opened aziridine. Protection of the resulting amine as the BOC-derivative provided a 1.6 1 mixture of the BOC-amino ester diaster-eomers 167a and 167b in 66% yield (Equation 50). 

A curious method for determining the enzymatic activity of trypsin and of the duodenal content has been demonstrated on the cleavage of p-nitroanilides of some aminoacids [180]. The half-wave potential of p-nitroaniline reduction is more negative than that of the substituted derivatives and so the hydrolysis of the nitroanilides can be measured by the decrease of its reduction wave. 

Prolonged (ca 3 h) irradiation of Winterstein esters leds to the formation of the corresponding phenylpropionates [41] (Scheme 4) The reaction is typical of Winterstein s acid derivatives and does not involve antenna-like effects from chromophores on the terpenoid moiety. Similar results were obtained in fact with simple aliphatic esters of this P-aminoacid [41]. Under these conditions (irradiation for several hours), alkaloids of the lS-oxo.A4 20 11-taxadiene-type (taxine B and comptonine-type) undergo photocyclization to 3,11-cyclotaxanes [40], The photocyclization of this type of alkaloids could be accomplished without reductive deamination using shorter (17 min) irradiation times [S3]. 

Reduction of 2-acylamino acrylates to give aminoacids Hydrogenation with C2-Symmetrical BINAP Rh and Ru Complexes Asymmetric Hydrogenation of Carbonyl Groups Regioselective asymmetric hydrogenation ofenones Asymmetric reduction of ketones with kinetic resolution A Commercial Synthesis of Menthol... 

Unnatural N-aryl dipeptides can be easily obtained using our procedure direct acylation of a-aminoacid esters with nitro-benzoyl chlorides followed by reductive alkylation leads to the preparation of derivatives of methyl prolinate 1 and methyl valinate 2 (Table 3). A small but significant diastereoisomeric excess (d.e.) of 20-30% is measured by NMR. Optimisation of the reaction conditions as well as of the nature of the inductor may generate better d.e.. These... 

Phenmenthol monomalonates 1.20 are transformed by LDA into dilithiated anions, which also suffer diastereoselective alkylations on their least hindered face [166, 1008, 1034], After UAIH4 reduction, enantioenriched 2,2-dialkyl-l,3-propanediols are obtained. Alternatively, Curtius reaction followed by hydrolysis gives rise to nonracemic a-substituted aminoacids (Figure 5.16). The most interesting selectivities are observed when R and R are PhCH-j and Me. The highly stereoselective alkylation of the menthyl monoester of (1R, 2R)-cydopen-tanedicarboxylic add by methaliyl bromide is the first step of the total synthesis of (-)-ptaquilosin [1035]. 

Oppolzer and Tamura [460, 861, 1076] have recommended 1-chloro-l-ni-trosocyclohexane as an electrophile for preparation of nonracemic a-aminoacids. a-Aminoacids are formed with an excellent enantiomeric excess from sodium eno-lates ofN-acylsultams 1.134 (R = R CH by a sequence of nitrosylation, hydrolysis and reduction of the intermediate hydroxylamine, and cleavage of the auxiliary with LiOH (Figure 5.39). The stereoselectivity of tins process is interpreted by attack of the electrophile on the face opposite to the nitrogen lone pair of the Z-chelated enolate 5.58 (Figure 5.39). 

The most important and universal type of chemical equilibria to which a heme protein is subjected in natural environments is pH equilibria, involving the protein aminoacid residues and the ionizable groups of the porphyrin, the propionate side chains. Beside the effects on the overall protein structure, which are outside the scope of this review, the major consequences of proton equilibria may be the changes of the heme reduction potential (electron affinity), called the redox-Bohr effect (see Sect. 3.2), by analogy to the pH effect on the oxygen affinity of hemoglobin (Bohr effect). 

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