Studies on the preparation of camphorylidene derivatives of α-amino acids

摘要

The preparation of imines derived from camphor and α-aminonacids was required as part of a project directed to the synthesisnof peptides in aqueous phase. In contrast to aldimines that arenreadily prepared, ketimines are difficult to synthesise,1 particularlynfrom camphor derivatives.2 The method which has beennemployed with varying degrees of success is the additionelimination of camphor and a primary amine. Reagents suchnas TiCl4,3 BF3u0001Et2O,4,5 molecular sieves,6 anhydrous ZnCl2,n7nand tetraethyl orthosilicate with acid catalysis,8 have beennemployed. Generally forcing conditions have been used, such asnhigh temperatures, non-stoichiometric quantities of reagents,nand stringently anhydrous conditions, including azeotropicndistillation.3 Additionally the enhanced ‘carbonyl’ addition ofnthiocamphor9 and the addition–elimination of thiocamphornS-oxide 10 have been employed.nOf these methods, 3 Å molecular sieves in methanolnhave been used to prepare stable ketimines derived fromnortho-hydroxyaryl ketones and a range of α-amino acid tetramethylammoniumnsalts,11 and thiocamphor has been refluxednin toluene with tert-butyl glycinate in the presence of 1,4-ndiazabicyclo[2.2.2]octane (DABCO) to give the desired imine.12