Synthesis.

Biosynthesis.1

Early studies showed that the biosynthesis of prodigiosin proceeds by a bifurcated pathway culminating in the enzymic condensation of the terminal products of the two pathways, 2-methyl-3-pentyl-pyrrole ( 2-methyl-3-n-amyl-pyrrole , MAP) and 4-methoxy-2,2'-bipyrrole-5-carbaldehyde (MBC) to form prodigiosin. (FIG.6)

Each biosynthetic pathway contains proteins with similarity to other natural product synthases, such as the adenylation domains and peptidyl carrier protein (PCP) domains of non-ribosomal peptide synthases (NRPS), and acyl carrier proteins (ACP) and ketosynthase domains of polyketide synthases (PKS). Specific prodigiosin enzymes have been assigned to every step of the two convergent pathways in the biosynthesis of MAP, MBC and condensation to form the pigment. The initial precursor for the biosynthesis of MAP, 2-octenal, might be generated by fatty acid biosynthesis enzymes or might be derived from the autooxidation of unsaturated fatty acids.


The final step in the biosynthesis of prodigiosin is the condensation of the monopyrroles with MBC to form the tripyrrole pigments. This condensation reaction can occur chemically at low pH. However, in bacteria, the condensation of the monopyrrole and MBC is catalysed by PigC.
Like many secondary metabolites, the biosynthesis of prodiginines is dependent on growth phase, with maximal expression occurring under conditions of nutrient deprivation or stress, such as when cells enter stationary phase. Prodiginine synthesis responds to multiple environmental and physiological cues, which can include temperature, oxygen supply, pH, light and ionic strength. In addition to variations in carbon and nitrogen source, the availability of inorganic phosphate (Pi), salt, detergent and various cations and anions all affect prodiginine biosynthesis.

Total synthesis.6

The first total synthesis of “prodigiosin” was reported by Rapoport and Holden in the early 1960s. It was based on the formation of the azafulvene motif by an acid-catalyzed condensation of 2-methyl-3-pentylpyrrole with aldehyde, mimicking the final stages of the biosynthesis. (FIG.7)

Although in the original publication of Rapoport and Holden ring C of this compound was supposed to be the azafulvene residue, recent X-ray crystallographic investigations of several prodigiosin derivatives have shown that the structure of these alkaloids is best described by the tautomer in which the central B-ring incorporates the basic site. However, it must be kept in mind that all prodigiosins can exist in two stable isomeric forms A and B in solution, with the equilibrium distribution being determined by the pH value of the medium, that is, by the degree of protonation of the basic nitrogen atom. (FIG.8) An E/Z isomerization of the exocyclic double bond of the azafulvene is responsible for this process.

Wasserman and co-workers developed two different approaches to the bipyrrole aldehyde, one of which is based on the cyclization of the vicinal tricarbonyl intermediate (FIG.9), while the other involves the oxidation of pyrrolecarboxylic acid ester with singlet oxygen (FIG.10). The overall yield of both routes, however, remained poor, not least because of the inefficiency of the final McFayden–Stevens reduction protocol.

An alternative route to bipyrrole aldehyde was devised by Boger and Patel as part of their SAR studies of the prodigiosin alkaloids (FIG.11). An inverse-electron-demand Diels–Alder reaction of 1,2,4,5-tetrazine with 1,1-dimethoxyethene gave cycloadduct in excellent yield, which underwent reductive ring contraction upon treatment with zinc dust in HOAc to give pyrrole dicarboxylate.

A more convenient and scalable alternative to this “biomimetic” route was pioneered by D'Alessio et al. as part of their studies towards a synthesis-driven mapping of the SAR (Structure-Activity Relationships) profile of the prodigiosins and the optimization of their immunosuppressive properties. This route relies on a Suzuki reaction for the formation of the bipyrrole axis, as shown in (FIG.12) for the parent undecylprodigiosin.

The underlying concept is highly flexible and was easily adapted to the synthesis of many different prodigiosin derivatives.