A facile procedure for tert-butoxycarbonylation of amines promoted by yttria-zirconia based strong Lewis acid catalyst (original) (raw)
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A rate enhancement of tert-butoxycarbonylation of aromatic amines with Boc2O in alcoholic solvents
Tetrahedron Letters, 2006
A rate enhancement of tert-butoxycarbonylation of aromatic amines by Boc 2 O in alcohols compared to aprotic solvents was demonstrated. Kinetic analysis by NMR suggested that the reaction in CD 3 OD was faster than in CDCl 3 by a factor of 70. Reactions between Boc 2 O and various aliphatic and aromatic amines in ethanol provided the N-Boc derivatives in good to excellent yields in short reaction times.
ISRN Organic Chemistry, 2012
A simple, efficient, and eco-friendly protocol for the N-Boc protection of the amine moiety in a variety of compounds with ditert-butyl dicarbonate under water-acetone catalyst-free conditions is described. The corresponding monocarbamate is obtained in excellent yields on short reaction times. No competitive side reactions such as isocyanate urea and O-Boc were observed. This method represents a reasonable alternative to the previous reported protection procedures.
Journal of Molecular Catalysis A: Chemical, 2002
A simple and efficient method has been developed for the acylation of alcohols, amines and thiols using yttria-zirconia-based Lewis acid as catalyst and carboxylic acids as acylating agent. The reaction was found to be chemoselective for the amino alcohol, 2-mercapto ethanol and 1,2-diol. Thus, using this method, acylation of an amino group in the presence of OH group, of OH group in the presence of SH, and of a primary OH group in the presence of secondary OH group has been achieved with excellent selectivity.
RSC Advances, 2014
A simple, rapid and solvent-free protocol is described for the chemoselective transformation of amines to tert-butyloxycarbonyl protected derivatives (NHBoc) using Boc 2 O and imidazolium trifluoroacetate protic ionic liquid (5-20 mol%). Unwanted side products such as isocyanate, urea or N,N-di-Boc were not detected. The scope of the protection strategy was successfully explored for substrate alcohols, phenols and thiol at elevated temperatures. Optically pure amino acids, amino acid esters and amino alcohols were efficiently converted to the corresponding N-Boc protected derivatives in excellent yields without racemization at the chiral center. The distinct advantages of this method are: operational simplicity, cleaner reaction, high selectivity, excellent yield, rapid reaction convergence, easy preparation and recyclability of the catalyst.
The Journal of Organic Chemistry, 2008
Carbon tetrabromide (CBr 4) catalyzes O-tert-butoxycarbonylation of functionalized phenols without any side reactions (bromination, addition of CBr 3 to a double bond, and formation of symmetrical diaryl carbonates, cyclic carbonates, or carbonic-carbonic anhydrides). The parent phenols are regenerated from the O-t-Boc derivatives by the catalyst system CBr 4-PPh 3 without affecting other protecting groups (aryl alkyl ether, alkyl ester, and thioacetal) or competitive side reaction such as bromination, nitrene (from NO 2) and R,R-dibromoolefine (with CHO/COMe) formation, and transesterification (with CO 2 Me/ Et) taking place. The protection and deprotection of hydroxyl group are encountered with 30% and 14% frequency, respectively, in the preparation of drug candidates. 1 Acylation 2,3 is a common approach to protect hydroxyl groups, but the regeneration of the parent compounds requires hydrolysis under strong alkaline conditions 2 incompatible with other functional groups. Although chemoselective deacylation is achievable by "demand-based thiolate anion generation" 4 under neutral or virtually neutral and nonaqueous conditions, O-tert-butoxycarbonylation is a suitable alternative 2 as the O-t-Boc moiety is compatible with reaction conditions routinely adopted in organic synthesis. The limited methodologies 5 available for the preparation of O-t-Boc phenols have several drawbacks: the requirement of long reaction times, 5d,e low/ high temperatures, 5d,e special efforts to prepare the tertbutoxycarbonylation reagents, 5d,e and auxiliary substances (e.g., solvents, bases, etc.). 5 In analogy with the recent trend of Lewis acid catalyzed N-tert-butoxycarbonylation, 6 O-t-Boc formation does not become feasible as phenols form the tert-butyl ethers with Boc 2 O in the presence of a strong Lewis acid 7 and a mild Lewis acid requires heating under reflux in DCM for 5-19 h. 5f Therefore, the need to use/develop organocatalytic procedures was realized. Only a few reports are available for O-t-Boc formation using nucleophilic organocatalyst. 5b-e,8 Some of these give side products such as symmetrical carbonates, cyclic carbonate, and carbonic-carbonic anhydrides. 5c In our search for an effective organocatalyst, we choose 4-nitrophenol 1 as a model substrate and treated it with Boc 2 O in the presence of CBr 4 , NBS, NCS, and I 2 (Table 1). The best results
Journal of the Chilean Chemical Society, 2013
An efficient, environmentally benign, highly facile and convenient synthetic protocol for the selective t-butyl carboxylation of aliphatic, aromatic and heterocyclic amines using Amberlyst R A 21 catalyst; a mild basic solid resin under solvent free conditions is reported. This method explores several advantages such as reusability of the heterogeneous catalyst, cleaner reaction profile, mild and solvent free system, short reaction time, operational simplicity, high conversions , excellent product yields and low cost of the catalyst. Furthermore since the catalyst is mild basic, decomposition of the carbamate formed is not observed if the reaction is continued for prolonged time as in the case of Lewis acid catalyzed N-Boc protection. This makes the present protocol a useful and attractive for N-Boc protection of amines.