Conformationally restricted peptidomimetics

Abstract

Peptidomimetic compounds are becoming increasing important as medicinal agents. This thesis describes the design and synthesis of a novel class of non-hydrolysable peptidomimetic isosteres, the tetrazole-based amide bond isosteres. These new structural mimics have been developed for incorporation into enzyme inhibitors and biological probes where a non-hydrolysable cis- amide bond mimic is required. Chapter one provides a general outline of conformational restriction and how this concept has been advantageously applied to the design of bioactive peptidomimetics. Conformational restriction, when used to pre-organise peptidomimetic ligands into a desired bioactive conformation, gives ligands that bind to target receptors with a greater affinity. The tetrazole heterocycle has obvious structural similarities to the cis-conformation of the amide bond and has been used to lock amide bonds in an equivalent conformation. Hence, the 1,5-disubstituted tetrazole ring is a popular constrained, planar cis- amide bond mimic and its use as such is reviewed in chapter one. Chapter two outlines the design of the tetrazole-based isosteres, the α-methylene tetrazole (2.5.1), α-hydroxymethylene tetrazole (2.5.2), and α-keto tetrazole isostere (2.5.3). The tetrazole-based isosteres incorporate the design features of a non-hydrolysable amide bond isostere and the conformational restriction of the 1,5-disubstituted tetrazole ring, to generate the first examples of non-hydrolysable tetrazole-based cis- amide bond mimics. Chapter three reviews the synthesis of N-Z-(2RS,3S)-3-amino-2-hydroxy-4-phenylbutanoic acid [N-Z-AHPBA, 3.1.1], a key amide bond isostere used in potent protease inhibitors and a synthetic building block of the α-hydroxymethylene tetrazole isostere (2.5.2). We have investigated the synthesis of 3.1.1 by functionalisation of N-Z-L- phenylalaninal, 3.2.3. Due to shortcomings in this published procedure we have developed a new, generally applicable synthesis of this important amide bond isostere by hydroxylation of an enolate derived from methyl (3S)-N-Z-3-amino-4-phenylbutanoate (3.4.2) with the oxodiperoxymolybdenum (pyridine) (hexamethyl phosphoric triamide) complex (MoOPH). Chapter four describes the synthesis and spectral analysis of diastereomeric tetrazole-based compounds. We have synthesised two series of α-methylene tetrazolebased compounds, 4.1.4a-4.1.7a and 4.1.4b-4.1.7b, derived from L-alanine and D-alanine, respectively. We have used this analysis to establish trends in the 1H and 13C NMR of the diastereomeric series as an aid to the assignment of configuration of tetrazole-based ligands. We have also been able to monitor epimerisation of the C6 stereocentre adjacent to the tetrazole by 1H NMR under the conditions of peptide coupling. Chapter five outlines the synthesis of (2S)-1-(benzyl ethanoate)-5-[2-(N-benzyloxycarbonylamino)-3-phenylpropane]-tetrazole, 5.2.1, an α-methylene tetrazolebased dipeptide mimic. We have incorporated 5.2.1 into extended substrate sequences of HIVp and tested these compounds for in vitro activity against HIVp. The modestly potent HlVp inhibitors 5.4.1, 5.4.2, and 5.4.3 represent a step-wise elongation of the C-terminal, and gave IC50 values against HIVp of 94 µM, 47 µM and 18 µM, respectively. Chapter six describes the synthesis of the α-keto tetrazole isostere, by direct alkylation of (1RS,2S)-5-[2-(N-tert-butyloxycarbonylamino)-1-hydroxyl-3-phenylpropane]-1H-tetrazole, 6.3.13, with benzyl bromoacetate, followed by a TEMPO oxidation. The desired 1,5-disubstituted dipeptide mimic, (2S)-1-(benzyl ethanoate)-5-[2-(N-tert-butyloxycarbonylamino)-1-oxo-3-phenylpropane]-tetrazole, 6.3.16, was isolated from the 2,5-disubstitiuted dipeptide mimic, (2S)-2-(benzyl ethanoate)-5-[2-(N-tertbutyloxycarbonylamino)-1-oxo-3-phenylpropane J-tetrazole, 6.3.17. This outlines a new and generally applicable synthesis of the tetrazole-based isosteres. Chapter seven describes the solid state structures of three tetrazole-based dipeptide mimics. The α-methylene tetrazole based isostere has been observed in the X-ray structure of the dipeptide mimic, (2S)-1-(benzyl ethanoate)-5-[2-(N-benzyloxycarbonylamino)-3-phenylpropane]-tetrazole and the cyclic tetrazolodiazepine analogue, cyclo-{[(2S)-5-(2-amino-3-phenylpropane)-1-ethanamide] tetrazole}, These are the first examples of the solid state structure of the α-methylene tetrazole isostere. We have also observed the solid state structure of the 2,5-disubstituted tetrazole, (2S)-2-(Benzyl ethanoate )-5-[2-(N-tertbutyloxycarbonylamino)-1-oxo-3-phenylpropane]-tetrazole, 6.3.17.