FMoc chloride, reaction with

An alternative to the above is esterification by reaction of the salt of the Fmoc-amino acid with the halomethylphenyl-support (see Section 3.17). It was established in the 1960s that this method of esterifying A-alkoxycarbonylamino acids, which does not involve electrophilic activation, is not accompanied by enan-tiomerization. Examples of supports with haloalkyl linkers are bromomethylphe-noxymethyl-polystyrene and 2-chlorotrityl chloride resin (see Section 5.23). 

FIGURE 5.22 (A) Reaction of an Fmoc-amino acid with 2-chlorotrityl chloride resin.56 The ester bond formed is cleavable by the mild acid, which does not affect tert-butyl-based protectors. (B) Generation of a protected peptide containing cystine by detachment of a chain, deprotection of cysteine residues, and oxidation of the sulfhydryls by the reagent containing iodine. The cations produced are trapped by CF3CH2OH. 

The Se-(4-methoxybenzyl)selenocysteine is obtained by reduction of selenocystine with NaBH4 and in situ reaction with 4-methoxybenzyl chloride. 7 The optimized procedure of Tanaka and Soda 32 is preferentially used for the synthesis of the starting selenocystine, which involves reaction of (1-chloroalanine with a 2.3-fold excess of disodium diselenide in aqueous solution at pH 9. Alternatively, the significantly less selenium demanding synthesis of Stocking et al. 33 is used for the preparation of expensive 77Se-selenocystine, this consists of the reaction of methyl (2R)-2-[(/ert-butoxycarbonyl)amino]-3-iodopropanoate with equivalent amounts of dilithium diselenide. Subsequent conversion of SeC(Mob) into the M -Fmoc derivative 7 and finally into the pentafluorophenyl ester 10 is performed following standard procedures. 

FMOC-protected amino acid fluorides afford the expected Reissert adducts 160 with a good stereoselectivities, the a-sulfonylamino acid fluorides undergo cycliza-tion to adduct 161 [47, 140, 141], Itho s protocol is amenable to using silyl enol ethers 157 as nucleophiles [142], Gibson has used bulky asymmetric acid chlorides as substrates in a Reissert reaction with TMS-CN the corresponding Reissert compound was then treated with aldehydes and sodium hydride to obtain the enantiopure adducts 4 (Scheme 3) [143],... 

Trityl-based resins are highly acid-labile. The steric hindrance of the linker prevents diketopiperazine formation and the resins are recommended for Pro and Gly C-terminal peptides. Extremely mild acidolysis conditions enable the cleavage of protected peptide segments from the resin. These resins are commercially available as their chloride or alcohol precursors. The trityl chloride resin is extremely moisture-sensitive, so reagents and glassware should be carefully dried before use to avoid hydrolysis into the alcohol form. It is necessary to activate the trityl alcohol precursor and it is highly recommended to reactivate the chloride just before use see Note 4). After activation, attachment of the first residue occurs by reaction with the Fmoc amino acid derivative in the presence of a base. This reaction does not involve an activated species, so it is free from epimerization. Special precautions should be taken for Cys and His residues that are particularly sensitive to epimerization during activation (Table 2). 

While there is evidence from IR investigations for the formation of traces of oxazol-5(4//)-one by treatment of Fmoc-protected amino acid fluorides, e.g. Fmoc-Val-F, with DIPEA, this reaction is not rapid enough to compete with the acylation process. In contrast to Fmoc amino acid chlorides, the related fluorides can be used in homogeneous systems with tertiary amines as efficient agents in peptide synthesis. 

Starting from L-tryptophan, immobilized on hydroxyethyl polystyrene through its carboxylic group, the intermediate a,[3-unsaturated imine (554) was formed by reaction with 3-methylcrotonaldehyde in pure trimethyl orthoformate (TMOF). The imine was then allowed to react with Fmoc chloride in the presence of pyridine to afford the required tetrahydro-[3-carboline (555) through an N-acylimin-ium ion mediated Pictet-Spengler-type cyclization. Further manipulation of the Pictet-Spengler product afforded the desired demethoxy-FTC as the minor cis isomer, along with its C-3 trans epimer (556) (diastereoisomeric ratio 1 3) (Scheme 115). 

Both primary and secondary ammo acids react with 9-fluorenylmethyl chloroformate (FMOC chlonde) under alkaline, aqueous conditions (Anson-Moye and Boning, 1979). The reaction is complete in 30 s. However, although FMOC chloride is fluorescent, the excess reagent and the byproducts are easily removed by extraction prior to separation (Ejnarsson et al., 1983). 

The instability and chemical conversion of some OPA derivatives imply that a denvatized compound may, in fact, result in one fluorescent and two radioactive peaks (Simson and Johnson, 1976, Fig. 1). The chemical rearrangement of the derivatives may, however, be a minor factor with respect to retention and the fluorescent and nonfluorescent derivatives may coelute. The use of more chemically stable amino acid derivatives, i.e. those formed by reaction with FMOC chloride, eliminates this problem. When the radioactivity of an amino acid is measured, it is often desirable and necessary to inject larger concentrations of amino acids than in a routine expenment. With the OPA method it is then critical to (a) make sure that OPA is present in the required molar excess (Lindroth and Mopper, 1979), (b) lower the pH of the reagent mixture to spare the top of the column, and (c) use the same or lower proportion of organic solvent in the sample as in the beginning of the gradient in order to obtain a concentration of the derivatives on the column top. 

Bisphosphonates were prepared and from naturally occurring 1-amino acids [141], In this case, initial protection of the amino acid-amine moiety was required. Insertion of a phosphorus atom was divided into two steps initial reaction with a suitable phosphorus reagent to produce the acylphosphonate, and a second phosphorus attack to form the bis-phosphonate. Since conversion to acid chloride served as the acid activation process, the amine-protecting group had to be either Fmoc or phtaUmide, both of which are stable in acidic conditions. Prolonged reaction times always resulted in partial rearrangement and formation of by-products. 

Amino groups are often protected as their terf-butyloxycarbonyl amide (Boc) or fluorenylmethyloxycarbonyl amide (Fmoc) derivatives. The Boc protecting group is introduced by reaction of the amino acid with di-fi rt-butyl dicarbonate in a nucleophilic acyl substitution reaction and is removed by brief treatment with a strong acid such as trifluoroacetic acid, CF3CO2H. The Fmoc protecting group is introduced by reaction with an acid chloride and is removed by treatment with base. 

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