On the halogenation of N(1),N(2)-di-t-Boc-5-hydroxy-piperazic acid esters and the conformational preferences of their 5-halo-piperazic acid products. The importance of A1,3 rotameric-strain in determining N(2)-acyl piperazic acid ring conformation

In this Letter, an unambiguous synthetic strategy is reported for the preparation of enantiomerically purecis-5-halo-piperazic acid derivatives in single diastereoisomer form. Contrary to the recent report by Shin and co-workers (Chem. Lett. 2001, 1172), in which it is claimed that the Ph₃P and N-chlorosuccinimide (NCS)-mediated chlorination of (3R,5S)-trans-N(1),N(2)-di-t-Boc-5-hydroxy-piperazic acid derivative 1proceeds with retention of configuration at C(5) to give 2, we now show that this and related Ph₃P-mediated halogenations all occur with S_N2 inversion at the alcohol center, as is customary for such reactions. Specifically, we demonstrate that the (3R,5S)-trans-5-Cl-piperazic acid derivative 2 claimed by Shin and co-workers (Chem. Lett. 2001, 1172) is in actual fact the chlorinated (3S,5R)-enantiomer 6, which must have been prepared from the cis-(3S,5S)-alcohol 3, a molecule whose synthesis is not formally described in the Shin paper. We further show here that the cis-(3R,5R)-5-Cl-Piz 13 claimed by Shin and co-workers inChem. Lett. 2001, 1172, is also (3S,5R)-trans-5-Cl-Piz 6. Authentic 13 has now been synthesized by us, for the very first time, here. Since Lindsley and Kennedy have recently utilized the now invalid Shin and co-workers’ retentive Ph₃P/NCS chlorination procedure on 1 in their synthetic approach to piperazimycin A (Tetrahedron Lett. 2010, 51, 2493), it follows that their claimed 5-Cl-Piz-containing dipeptide 25 probably has the alternate structure 26, where the 5-Cl-Piz residue has a 3,5-cis-configuration. The aforementioned stereochemical misassignments appear to have come from a mix-up of starting materials by Shin and co-workers (Chem. Lett. 2001, 1172), and an under-appreciation of the various steric and conformational effects that operate in N(2)-acylated piperazic acid systems, most especially rotameric A^1,3-strain. The latter has now been unambiguously delineated and defined here under the banner of the A^1,3-rotamer effect.