Diastereospecific synthesis of D- and L-allo-threonines and trichlorinated derivatives suitable for the preparation of tritium labelled material

Synthesis of the N-(tert-butyloxycarbonyl)-O-triisopropylsilyl-D-pyrrolosamine Glycal of Lomaiviticin A & B via Epimerization of L-Threonine

An efficient synthesis of the N-(tert-butyloxycarbonyl)-O-triisopropylsilyl-D-pyrrolosamine glycal of lomaiviticin A (1) and lomaiviticin B (2) is described. The synthesis is highlighted by the epimerization of the L-threonine-derived oxazolidine 10 to oxazolidine 11. This key epimerization reaction, which serves to establish the correct relative configuration of the carbohydrate unit, was made possible only after conformational analysis indicated that substituted oxazolidines may adopt conformations that preclude enolization.

Asymmetric synthesis of both enantiomers of anti-4,4,4-trifluorothreonine and 2-amino-4,4,4-trifluorobutanoic acid

A short and efficient enantioselective synthesis of both enantiomers of anti-4,4,4-trifluorothreonine and 2-amino-4,4,4-trifluorobutanoic acid was successfully developed. Trifluoromethylation of benzyl-protected bromoalkene 4 provided key intermediate trifluoromethylated trans-disubstituted alkene 2 in good yield. The sequence then involved Sharpless asymmetric dihydroxylation, nucleophilic opening of cyclic sulfate with NaN(3), palladium-catalyzed selective hydrogenation, and oxidation.

Total synthesis of the ramoplanin A2 and ramoplanose aglycon

Full details of a convergent total synthesis of the ramoplanin A2 and ramoplanose aglycon are disclosed. Three key subunits composed of residues 3-9 (heptapeptide 15), pentadepsipeptide 26 (residues 1, 2 and 15-17), and pentapeptide 34 (residues 10-14) were prepared, sequentially coupled, and cyclized to provide the 49-membered depsipeptide core of the aglycon. Key to the preparation of the pentadepsipeptide 26 incorporating the backbone ester was the asymmetric synthesis of an orthogonally protected l-threo-beta-hydroxyasparagine and the development of effective and near-racemization free conditions for esterification of its hindered alcohol (EDCI, DMAP, 0 degrees C). The coupling sites were chosen to maximize the convergency of the synthesis including that of the three subunits, to prevent late stage racemization of carboxylate-activated phenylglycine-derived residues, and to enlist beta-sheet preorganization of an acyclic macrocyclization substrate for 49-membered ring closure. By altering the order of final couplings, two macrocyclization sites, Phe(9)-d-Orn(10) and Gly(14)-Leu(15), were examined. Macrocyclization at the highly successful Phe(9)-d-Orn(10) site (89%) may benefit from both beta-sheet preorganization as well as closure at a d-amine terminus within the confines of a beta-turn at the end of the H-bonded antiparallel beta-strands. A more modest, but acceptable macrocyclization reaction at the Gly(14)-Leu(15) site (40-50%) found at the other end of the H-bonded antiparallel beta-strands within a small flexible loop may also benefit from preorganization of the cyclization substrate, is conducted on a substrate incapable of competitive racemization, and accommodates the convergent preparation of analogues bearing depsipeptide modifications. Deliberate late-stage incorporation of the subunit bearing the labile depsipeptide ester and a final stage Asn(1) side-chain introduction provides future access to analogues of the aglycons which themselves are equally potent or more potent than the natural products in antimicrobial assays.

Synthesis of a polymer-supported oxazolidine aldehyde for asymmetric chemistry

An expedient synthesis of the polymer-supported aldehyde 3, as a Garner aldehyde equivalent, is described. Oxazolidine 3 may be obtained through preformation of aldehyde linker 4 in solution and loaded onto amine-terminating resin under peptide-coupling conditions, or alternatively via oxidation of polymer-bound alcohol 14. The integrity of the serine-derived stereocenter is maintained through all steps of the synthesis.

Total synthesis of the ramoplanin A2 and ramoplanose aglycon

A convergent total synthesis of the ramoplanin A2 and ramoplanose aglycon is disclosed. Three key subunits composed of residues 3-9 (heptapeptide 15), pentadepsipeptide 26, and pentapeptide 34 (residues 10-14) were prepared, sequentially coupled, and cyclized to provide the 49-membered depsipeptide core of the aglycon. Key to the preparation of the pentadepsipeptide 26 incorporating the backbone ester was the asymmetric synthesis of an orthogonally protected L-threo-beta-hydroxyasparagine and the development of effective and near-racemization free conditions for esterification of its hindered alcohol (EDCI, DMAP, 0 degrees C). The coupling sites were chosen to maximize the convergency of the synthesis including that of the three subunits, to prevent late stage racemization of carboxylate-activated phenylglycine-derived residues, and to enlist beta-sheet preorganization of an acyclic macrocyclization substrate for 49-membered ring closure. As such, macrocyclization at the chosen Phe(9)-D-Orn(10) site may benefit from both beta-sheet preorganization as well as closure at a D-amine terminus. Deliberate late stage incorporation of the subunit bearing the labile depsipeptide ester and a final stage Asn(1) side chain introduction provides future access to analogues of the aglycons which themselves are reported to be equally potent or more potent than the natural products in antimicrobial assays.