CONVENIENT SYNTHESIS OF 1-AMINOALKYLPHOSPHONATES UNDER SOLVENT-FREE CONDITIONS

1 -Aminophosphonic acids are probably the most important substitutes for the corresponding a-amino acids in biological systems.14 Indeed, a number of potent antibiotics? enzyme inhibitors? and pharmacological agents7 are 1-aminophosphonic acids, or derivatives, thereof. Of the number of methods for the synthesis of 1-aminoalkyl phosphonates developed during past two decades? the condensation of amines and carbonyl compounds followed by the nucleophilic addition of a dialkyl or diary1 phosphite to the resulting imine is the most convenient The formation of 1-hydroxyphosphonates or a product of its rearrangement frequently accompanies the formation of 1-aminoalkyl phosphonates.16 Lewis acids such as SnCl,, SnCl,, BF,*Et,O, ZnCI,, MgBr2, and InC1, have been used as catalyst. The most typical procedure for unsubstituted amino phosphonates is a Strecker-type reacti~nl~which involves the treatment of an aldehyde with ammonia and dialkyl phosphite. This reaction, however, does not give highyield nor can it be carried out on a large scale since it is performed in a sealed vessel at 100°C. In recent years, the use of reagents and catalysts immobilized on solid supports has received considerable attention. Such reagents not only simplify purification processes but also help prevent release of reaction residues into the environment. Reagents supported on organic polymers and within and/or on the surface of inorganic matrices have all been rep~rted. '~ The application of microwave energy to accelerate organic reactions is of increasing interest and offers several advantages over conventional techniques." It has been demonstrated that application of microwave irradiation (MWI) to organic reactions not only results in reduced reaction times but also in improved yield compared to those obtained under conventional conditions. Under microwave conditions, reactions are performed using minimal amounts of solvent or, in some cases, in the absence of solvent. Consequently such processes result in formation of reduced quantities of waste and combined with the rapid reaction times and improved yields normally observed, may therefore be considered as more environmentally acceptable. In the Al2O1(a)/NH40Ac I R-?WOHOEt)z I I . p-Ts _. 2 RCHO + HP(O)(OEt)2 1 I 1 MWI