... Tetrahedron Letters. Volume 40, Issue 31, 30 July 1999, Pages 5661–5665. Cover image Cover image. Sequential mono-N-arylation of piperazine nitrogens. Part 2: The role of hydrogen bonding. ... References. 1. M. Hepperle, J. Eckert, D. Gala; Tetrahedron Lett., 40 (1999), p. 5655 ...