Total synthesis of a tyrosine -constrained analog of didemnin B and novel synthetic approaches to non-proteinogenic amino acids in cyclomarin A

Didemnin B is a metabolite produced by a marine organism called a tunicate or sea squirt as a defense mechanism against aquatic predators. Didemnin B is the lead member of a class of cyclic depsipeptides from the family Didemnidae (Trididemnun genus), isolated from a marine organism, a Caribbean tunicate. The didemnins are characterized by a 23-membered macrocyclic core. The natural congeners in the family vary by the side chain structure. Structural studies and chemical modifications at the N,O-diMeTyr of the didemnins led investigators to believe it was an important moiety for the didemnins\u27 biological activity. The tyrosine side chain in didemnin B is thought to be involved in binding to the receptor because it protrudes outward from the interior of the molecule both in solid state and in solution phase. The purpose of the research described herein was to synthesize an analog of didemnin B that might probe further the importance of the N,O-diMeTyr residue in the receptor-ligand binding event. The approach to analog design was to restrict rotation around certain bonds in order to minimize the number of low energy conformations that can be observed. This strategy aids in determining which low energy conformation is important for receptor binding, since the introduction of constraints in the ligand should reduce the entropy cost of the receptor-ligand complex and result in greater stability. The N,O-diMeTyr residue was to be replaced with the constrained amino acid 1,2,3,4-tetrahydro-7-methoxyisoquinoline-3-carboxylic acid (MeOTic) in order to determine the effect of constraining the side chain of this amino acid. However, in addition to constraining the amino acid, the introduction of the methylene tether also increased the substitution at the amino group to effectively mimic the N-methylated amino acid. The total synthesis of [MeOTic5]didemnin B is discussed in chapter one. In the process, we report a new preparation of 1,2,3,4-tetrahydro-7-methoxyisoquinoline-3-carboxylic acid in an efficient 5-step protocol. The total synthesis was completed in a convergent fashion in an overall yield of 1.3% for a 19-step linear sequence. An estaurine streptomycete, strain CNB982, collected in Mission Bay, CA was discovered to produce a novel family of cyclic heptapeptides, cyclomarins A, B, and C, under saline culture conditions exclusively. Cyclomarin A, the major metabolite, demonstrated mild (μM) in vitro cytotoxicity toward human cancer cell lines. More noteworthy however, cyclomarin A displayed potent antiinflammatory properties in both in vitro and in vivo assays. Four noncoded amino acids, N-(1,1-dimethyl-2,3-epoxypropyl)-β-hydroxytryptophan, threo-β-methoxyphenylalanine, N-methylhydroxyleucine, 2-amino-3,5-dimethylhex-4-enoic, acid and three common amino acids, alanine, valine, and N-methylleucine, constitute the target molecule