Investigations in amine chemistry: Mn-Mediated radical addition approach toward gamma amino esters and synthetic studies of the tubulysins

Mn-Mediated radical addition has been developed within the Friestad laboratory as a versatile method toward addition to C=N bonds. N-Acylhydrazones generated by condensation between an aldehyde and an N-acylamine serves as the substrate toward radical addition. A bulky directed group attached with the N-acyl moiety and restricted rotation around N-N bond due to a three point chelation with a Lewis acid differentiates the faces of the C=N bond of the N-acylhydrazones. Radical generation initiated by photolysis of Mn2(CO)10 causing homolysis of C-X bond in alkyl halide serves as the radical donor to the N-acylhydrazones. Radical addition thereafter occurs stereoselectively from the less hindered face of the C=N bond of the N-acylhydrazones. The product N-acylhydrazines can be effectively transformed to α-chiral amines. In this thesis, a new protocol toward generation of non-proteogenic γ-amino esters using Mn-mediated radical addition has been described. Moreover, the utility of the Mn-mediated radical addition has been demonstrated through studies toward synthesis of tubulysin U and V.

Chapter 3 describes a new strategy for asymmetric synthesis of γ-amino esters starting from non-amino precursors. The α-substituted γ-amino esters are prevalent in drugs, drug candidates, and in peptidomimetics. As a part of progressing the Mn-mediated radical addition reaction, highly stereoselective reactions were devised for addition to N-acylhydrazonoesters in absence of Lewis acid. Spectroscopic investigations were carried out to decipher the Lewis acid chelation of N-acylhydrazones. Finally, a novel microwave mediated trifluoroacylation of N-acylhydrazinoesters facilitated the cleavage of N-N bond to liberate γ-aminoester.

Chapter 4 describes application of Mn-mediated radical addition toward synthesis of tubulysin natural products. Tubulysins are natural products, isolated from myxobacteria, that have exhibited potent cytotoxicity toward cancer cells in the picomolar regime. The Mn-mediated radical addition was used to prepare two chiral amine subunits in highly diastereoselective fashion. The subunits were then assembled after required manipulations into the tetrapeptide structure characteristic of tubulysins. This strategy to synthesize tubulysins is the most stereoselective of all efforts toward the synthesis of this molecule. Synthesis toward tubulysin was achieved in 18 steps as the longest linear sequence with a 31% overall yield to tubulysin V in benzyl protected form.

Chapter 5 describes a new strategy toward installation of N-hydroxymethyl unit into a peptide chain. N,O-Acetals are acid-base labile species that is present in some tubulysin natural analogs. This new approach exploits Fleming-Tamao oxidation and hence introduce the hydroxymethyl unit of the N,O-acetal in a masked form. Following peptide construction the masked hydroxy group is released to liberate the N-hydroxymethyl moiety. Acylation of the free hydroxy group furnishes the N,O-acetal moiety in a strategy that is potentially applicable toward synthesis of tubulysin D.