A Mild and General Solid-Phase Method for the Synthesis of Chiral Polyamines. Solution Studies on the Cleavage of Borane−Amine Intermediates from the Reduction of Secondary Amides

A mild oxidative workup protocol using iodine in an acetic acid-acetate buffer solution is described for the cleavage of borane-amine adducts arising from the borane-promoted reduction of polyamides supported onto practical trityl-based resins. Chiral polyamines with diverse side-chain functionalities can be generated as free bases without premature release from the solid support and with essentially no racemization using this method. A series of model oligomeric secondary diamides 6 containing various alpha-amino acid residues (Val, Phe, Tyr, Ser, Cys, Met, Gln, Trp) provided triamine products 8 in high yields and good to excellent purity. On the other hand, a substrate containing a tertiary amide (15) formed a rather unusual triaminoborane intermediate that required more stringent workup conditions to liberate the polyamine product 20. The reduction of oligomeric tertiary amides such as 9 was found sluggish, but these compounds could nonetheless be obtained in high purity from in situ reductive amination of the corresponding secondary amines. Control studies, carried out in solution with model secondary amide 23, confirmed the efficiency of the buffered iodine solution and highlighted several advantages (no heating necessary, no need for strong bases or acids) over existing methods for the cleavage of borane-amine adducts. A possible mechanism involving all buffer components (iodine, acetic acid, and acetate ion) is proposed in which borane-amine adducts are transformed first to the monoiodoborane-amine and then to the corresponding acetoxyborane-amine adduct of much weaker coordination affinity. The latter would dissociate readily and get trapped by the acetic acid to provide the desired secondary amine. This reduction/oxidative workup protocol is useful as a general method for the facile solid-phase synthesis of polyamines for eventual release in solution and use in various applications. It is also potentially very useful toward the synthesis and screening of bead-supported libraries of free oligoamines assembled through split-pool methods.