Mild, orthogonal solid‐phase peptide synthesis: use of Nα‐dithiasuccinoyl (Dts) amino acids and N‐(iso‐propyldithio)carbonylproline, together with p‐alkoxybenzyl ester anchoring linkages*

Abstract

Several Nα‐dithiasuccinoyl (Dts) amino acids (1) have been esterified without race‐mization by use of either N,N'‐dicyclohexylcarbodiimide or 1‐(3‐dimethylamino‐propyl)‐3‐ethylcarbodiimide hydrochloride, each in the presence of 4‐dimethylaminopyridine (0.1 equiv.), to 2, 4, 5‐trichlorophenyl 4′‐hydroxymethylphenoxyacetate (10) or the corresponding propionate (11). The resultant handle derivatives (8,9) were purified and then quantitatively attached onto aminomethyl supports by couplings using as solvent N,N‐dimethylformamide containing 1‐hydroxybenzotriazole (0.1 M). This methodology facilitates anchoring of Dts‐amino acids as p‐alkoxybenzyl esters, which can be cleaved in good yields by trifluoroacetic acid‐dichloromethane (1:1) at 25°. Model experiments established that quantitative thiolytic removal (>99.8%) of the Dts group occurs with (i) β‐mercaptoethanol (0.5M)‐N,N‐diisopropylethylamine (0.5M) in dichloromethane, 2 times 2min; (ii) N‐methylmercaptoacetamide (0.5M)‐N‐methylmorpholine (0.5M) in dichloromethane, 2 times 2min; and (iii) N‐methylmercaptoacetamide (0.5M)‐1‐hydroxybenzotriazole (0.1M) in N,N‐dimethylformamide, 2 times 2 min. The susceptibility of the Dts functionality to nucleophiles was also defined, including demonstration of tertiary amine‐catalyzed hydantoin formation from Dts‐dipeptidyl units, but side reactions from these processes are entirely avoided under appropriate conditions relevant to peptide synthesis. These observations were exploited to devise efficient, racemization‐free solid‐phase syntheses of a number of model peptides in high yields and purities, including L‐leucyl‐L‐alanylglycyl‐L‐valine, H‐Gly6‐Val‐OH, H‐Met‐Ala‐Gly‐OH, methionine‐enkephalin, and bradykinin.

Orthogonal Solid-Phase Peptide Synthesis

“… Primary amino groups that can be protected by Dts include not only the N α -amino group of -amino acids [1,20,21], but also the building blocks for peptide nucleic acids (PNA) [22] and the side-chains of amino sugar building blocks for certain glycopeptides [23].  Obviously, Dts cannot be used to protect the N α -imino group of proline, but fortunately we were able to optimize open-chain carbamoyl disulfide protection for this residue [24].…”

“… We can "invert" the fundamental mechanism of thiolytic deprotection to develop protecting groups for the sulfhydryl side-chain of cysteine, and for "directed" syntheses of interand intramolecular disulfides [25,26].  The fact that Dts derivatives are "masked" isocyanates [21] can be very important, given all the difficulties and complications of working directly with isocyanates. Moreover, Dtsamino acids are desulfurized to N-carboxyanhydrides (NCA's), and Dts-dipeptides give rise to hydantoins, all under unusually mild conditions [20,21].…”

“… The fact that Dts derivatives are "masked" isocyanates [21] can be very important, given all the difficulties and complications of working directly with isocyanates. Moreover, Dtsamino acids are desulfurized to N-carboxyanhydrides (NCA's), and Dts-dipeptides give rise to hydantoins, all under unusually mild conditions [20,21].  Focusing on the trivalent phosphorus that can act to desulfurize Dts (and a related compound we call "EDITH"), this chemistry serves as an excellent entry to phosphorothioate DNA and RNA for "anti-sense" applications [27,28].…”

Unexpectedly Stable (Chlorocarbonyl)(N-ethoxycarbonylcarbamoyl)disulfane, and Related Compounds That Model the Zumach–Weiss–Kühle (ZWK) Reaction for Synthesis of 1,2,4-Dithiazolidine-3,5-diones

“…A 1966 patent by Zumach, Weiss, and Kühle − described a general method (ZWK reaction) for preparation of 1,2,4-dithiazolidine-3,5-diones ( 1 ) by the facile and rapid reaction of O -ethyl thiocarbamates ( 2 ) plus (chlorocarbonyl)­sulfenyl chloride ( 3 ) (Scheme ). The heterocyclic system 1 − was subsequently adopted as the basis of the orthogonally removable dithiasuccinoyl (Dts) amino protecting group for peptide synthesis, ,, and can be exploited for a myriad of additional applications. ,− …”

Mechanism of sulfur transfer from 1,2,4-dithiazolidine-3,5-diones to triphenylphosphines

“…4‐Substituted 1,2,4‐dithiazolidine‐3,5‐diones are known from 1966 when Zumach and his co‐workers prepared a modest series of compounds containing this skeleton. Ten years later, Barany and Merrifield found the 1,2,4‐dithiazolidine‐3,5‐dione ring to be a very efficient orthogonal amino protecting group and since that time it has been extensively used in the synthesis of peptides, aminosugars in glycopeptides and peptide nucleic acids . More recently 1,2,4‐dithiazolidine‐3,5‐diones were recognized as a nucleophilic isocyanate building block and used in the Mitsunobu reaction or in the synthesis of linear polyurethanes and sulfonyl ureas .…”