Isocyanides protecting group

In a three-component synthesis the amide (86-1) obtained from the ester (85-5) and benzyl isocyanide is reacted with the piperdone (86-2). The product from this transform consists of the addition product (86-3) where amide nitrogen in (86-1) as well as the carbon from the isocyanide have added to the carbonyl group on the piperidine. Treatment of the adduct (86-3) with a strong acid hydrolyzes the urethane function on the fert-butyloxycarbonic protecting group, leaving behind the primary... 

An alternative cleavage methodology of the a-amino acids being generated via Ugi reactions with 150 and 156 was reported by Linderman et al. [122], who made use of isocyanide 157 (Scheme 10.51). In this approach, the A-glycosidic bond and the silyl protecting group of the... 

L. Banfi and co-workers utilized the Passerini three component reaction to prepare a 9600 member hit generation library of nor-statines. " ° These compounds are potential transition state mimetics for the inhibitors of aspartyl proteases. The authors produced the library by starting out from eight A/-Boc-a-aminoaldehydes, twenty isocyanides and sixty carboxylic acids. The key Passerini reaction occurred under mild conditions. This transformation was followed by removal of the Boc protecting group and acyl transfer. Three representative examples of the library are shown. 

The peptide-based isocyanides 50a-e were successfully polymerized by Ni(II) catalysts under an inert atmosphere [63-66]. An alanine-derived isocyanide, whose isocyano group was labeled using 13C and 15N, was prepared and successfully polymerized for structure elucidation [67]. Several solvent systems were used in the polymerization of the peptide-based isocyanides, depending upon the solubility of the isocyanides. As mentioned earlier, the use of alcohols as a solvent or co-solvent can accelerate the polymerization. The protective groups on the ester, hydroxy, and imidazole groups were removed after polymerization by treatment with aqueous NaOH to yield poly(isocyanide)s bearing unprotected peptide side chains. 

The Ugi reaction is also limited in its postcondensation modification. Commercially available isocyanides introduce a secondary amide functionality into the reaction that can be difficult to hydrolyze in the absence of harsh conditions, which represents a problem for highly functionalized compounds. Thus, in recent years, there has been a growing interest among researchers to develop convertible isocyanides (CICs), or isocyanides with an easily removable protecting group (PG). These CICs (at times referred to as universal isocyanides ) eliminate the need to... 

Formamides are useful protecting groups in organic synthesis and serve as key building blocks in the synthesis of isocyanides and heterocycles. Current methods to access formamides rely on the use of moisture sensitive or toxic reagents. Williams and co-workers reported the synthesis of formamides... 

Convertible isocyanide reagent 66 allows a mild and chemoselective in situ post-Ugi activation of the isonitrile bom amide with simultaneous deprotection of the nucleophilic amine, that is, liberation and activation of two Ugi-reactive groups, if desired also under subsequent lactam formation [33]. Another recently introduced convertible isocyanide, l-isocyano-2-(2,2-dimethoxyethyl)-benzene 73, was shown effective by Rhoden et al. In the course of this short sequence, a hydrolytically labile W-acylindole 78 is formed, which is displaced intramolecularly by the amine portion of the former Boc-protected amino acid 75 (Scheme 13). 

Recently, a combination of the UAC and UDC protocols was reported by Wessjohann s group [40]. In this work, the acid-activated, well-behaved 1-iso-cyano-2-(2,2-dimethoxyethyl)-benzene was employed as convertible isocyanide in an Ugi-4CR [41]. The advantage of this one-pot procedure is the in situ deprotection of the W-protected amino acid along with the simultaneous activation of the isonitrile-bom carboxylate, enabling the nucleophilic attack of the free amine... 

Another approach to a-ketoamide peptide mimetics was employed by Xu et al. [33] for the preparation of a human cytomegalovirus protease inhibitor library. In this case the oxidizable -OH group, protected as formate, belonged to the starting isocyanides. Thus, the reaction between N-acylated a-amino acids, amines, aldehydes, and isocyanides 42 afforded the a-hydroxyamides 43 in modest yields. Cleavage of the O-formyl bond was accomplished during the reaction by employing two... 

The Hulme group reported an efficient three-step, one-pot solution-phase synthesis of 2-imidazolines employing the UDC strategy [62], The reaction between N-Boc-protected a-aminoaldehydes, amines, acids, and isocyanides afforded the N-Boc-protected a-acylamino amides 94 which, upon heating in addic medium, underwent N-deprotection and cyclization to 2-imidazolines 95 (Scheme 2.34). This procedure was adapted to combinatorial synthesis in a rack of 96 reaction vials. 

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