A simple approach to several derivatives of pipecolic acid is via a multicomponent reaction starting from cyclic imines 2, which are synthesized on a large scale and with different substitution patterns. The protected amino acids 3 are formed in high yields. In cases where chiral imines are used the target compounds are obtained with remarkable diastereoselectivity. Bisamides 3 serve as versatile precursors for the preparation of a wide range of amino acid derivatives. Different methods of hydrolysis of 3 lead to the free pipecolic acids or its derivatives. Employment of methanol or ethanethiol as a nucleophile in the acid-mediated conversion of enamides 3 results in N-acylated amino acid esters 5. Furthermore a method for the resolution of the obtained racemic α-amino acids via diastereomeric salt formation is described.
The synthesis of alkyl- and aryl-substituted derivatives of pipecolic acid (piperidine-2-carboxylic acid) by a modified Strecker protocol is described. Addition of trimethylsilyl cyanide (TMSCN) to various tetrahydropyridines 1 and subsequent hydrolysis of the obtained α-amino nitriles 2 yielded pipecolic acid derivatives 3. Addition of TMSCN to cyclic imines 1 proceeded rapidly and led to α-amino nitriles in excellent yields without any necessary further purification. Almost quantitative diastereoselectivity for the formation of 2,6-trans-substituted amino nitrile 2g was observed at low temperature, when chiral imine 1g was used as reactant. Acidic hydrolysis of α-amino nitriles 2 yielded the corresponding amino acids 3 in good to excellent yields. Furthermore, an efficient protocol for the optical resolution of N-formylated derivatives of pipecolic acid by separation of diastereomeric norephedrinium salts is described.
homopipecolic acid are prepared by α-amino alkylation of malonic acid with cyclic imines 6 and 7. These are prepared on a large scale and with different substitution patterns. The β-aminoacids 8 and 9 were formed in high yield and with remarkable diastereoselectivity if chiral imines are used as starting materials. The diastereoselectivity of the amino alkylation leading to homopipecolic acid analogues