Total synthesis of (–)-irciniastatin B; Design and synthesis of analogues

The dissertation herein presents the first total synthesis of (-)-irciniastatin B in conjunction with the design and synthesis of analogues. Chapter One details the isolation and biological data of two potent cytotoxins (+)-irciniastatin A and (−)-irciniastatin B by Pettit and Crews. Also outlined in Chapter One are selected total syntheses and endgame strategies for (+)-irciniastatin A and reported structure activity relationship studies of the irciniastatin family of natural products. The synthetic strategy toward the construction of (-)-irciniastatin B is outlined in Chapter Two. A chemoselective deprotection/oxidation sequence was proposed to install the requisite oxidation state at C(11). To this end, a late-stage alcohol from the earlier Smith synthesis of (+)-irciniastatin A was employed. However, protection of the late-stage alcohol as an orthogonal SEM ether resulted in unexpected degradation. A modified protecting group strategy employing robust 3,4-dimethoxybenzyl ethers successfully led to the first total synthesis of (−)-irciniastatin B. This strategy also led to the construction of (+)-irciniastatin A from (-)-irciniastatin B, confirming the structural relationship of these two secondary metabolites. The design and synthesis of irciniastatin analogues are detailed in Chapter Three. Our synthetic strategy permits modification at C(11), which has been suggested to be a key structural element for the potent biological activity observed with the irciniastatins. Biological evaluation of C(11)-irciniastatin analogues will aid in the elucidation of the biological mode of action of the irciniastatin family of natural products

Total Synthesis of (−)-Irciniastatin B and Structural Confirmation via Chemical Conversion to (+)-Irciniastatin A (Psymberin)

The total synthesis and structural confirmation of the marine sponge cytotoxin (−)-irciniastatin B has been achieved via a unified strategy employing a late-stage, selective deprotection/oxidation sequence that provides access to both (+)-irciniastatin A (psymberin) and (−)-irciniastatin B

Reprogramming the specificity of sortase enzymes

Staphylococcusaureus sortase A catalyzes the transpeptidation of an LPXTG peptide acceptor and a glycine-linked peptide donor and has proven to be a powerful tool for site-specific protein modification. The substrate specificity of sortase A is stringent, limiting its broader utility. Here we report the laboratory evolution of two orthogonal sortase A variants that recognize each of two altered substrates, LAXTG and LPXSG, with high activity and specificity. Following nine rounds of yeast display screening integrated with negative selection, the evolved sortases exhibit specificity changes of up to 51,000-fold, relative to the starting sortase without substantial loss of catalytic activity, and with up to 24-fold specificity for their target substrates, relative to their next most active peptide substrate. The specificities of these altered sortases are sufficiently orthogonal to enable the simultaneous conjugation of multiple peptide substrates to their respective targets in a single solution. We demonstrated the utility of these evolved sortases by using them to effect the site-specific modification of endogenous fetuin A in human plasma, the synthesis of tandem fluorophore –protein–PEG conjugates for two therapeutically relevant fibroblast growth factor proteins (FGF1 and FGF2), and the orthogonal conjugation of fluorescent peptides onto surfaces.Chemistry and Chemical Biolog

Total Synthesis of (+)-Irciniastatin A (a.k.a. Psymberin) and (−)-Irciniastatin B

A unified synthetic strategy to access (+)-irciniastatin A (a.k.a. psymberin) and (−)-irciniastatin B, two cytotoxic secondary metabolites, has been achieved. Highlights of the convergent strategy comprise a boron-mediated aldol union to set the C(15)–C(17) <i>syn–syn</i> triad, reagent control to set the four stereocenters of the tetrahydropyran core, and a late-stage Curtius rearrangement to install the acid-sensitive stereogenic <i>N</i>,<i>O</i>-aminal. Having achieved the total synthesis of (+)-irciniastatin A, we devised an improved synthetic route to the tetrahydropyran core (13 steps) compared to the first-generation synthesis (22 steps). Construction of the structurally similar (−)-irciniastatin B was then achieved via modification of a late-stage (−)-irciniastatin A intermediate to implement a chemoselective deprotection/oxidation sequence to access the requisite oxidation state at C(11) of the tetrahydropyran core. Of particular significance, the unified strategy will permit late-stage diversification for analogue development, designed to explore the biological role of substitution at the C(11) position of these highly potent tumor cell growth inhibitory molecules

Design, Synthesis, and Evaluation of Irciniastatin Analogues: Simplification of the Tetrahydropyran Core and the C(11) Substituents

The design, synthesis, and biological evaluation of irciniastatin A (<b>1</b>) analogues, achieved by removal of three synthetically challenging structural units, as well as by functional group manipulation of the C(11) substituent of both irciniastatins A and B (<b>1</b> and <b>2</b>), has been achieved. To this end, we first designed a convergent synthetic route toward the diminutive analogue (+)-<i>C</i>(8)-desmethoxy-<i>C</i>(11)-deoxy-<i>C</i>(12)-didesmethylirciniastatin (<b>6</b>). Key transformations include an acid-catalyzed 6-<i>exo</i>-tet pyran cyclization, a chiral Lewis acid mediated aldol reaction, and a facile amide union. The absolute configuration of <b>6</b> was confirmed via spectroscopic analysis (CD spectrum, HSQC, COSY, and ROESY NMR experiments). Structure–activity relationship (SAR) studies of <b>6</b> demonstrate that the absence of the three native structural units permits access to analogues possessing cytotoxic activity in the nanomolar range. Second, manipulation of the C(11) position, employing late-stage synthetic intermediates from our irciniastatin syntheses, provides an additional five analogues (<b>7</b>–<b>11</b>). Biological evaluation of these analogues indicates a high functional group tolerance at position C(11)