Kurze Synthesen der Spirotryprostatine

Natural products synthesis: enabling tools to penetrate Nature's secrets of biogenesis and biomechanism

Selected examples from our laboratory of how synthetic technology platforms developed for the total synthesis of several disparate families of natural products was harnessed to penetrate biomechanistic and/or biosynthetic queries is discussed. Unexpected discoveries of biomechanistic reactivity and/or penetrating the biogenesis of naturally occurring substances were made possible through access to substances available only through chemical synthesis. Hypothesis-driven total synthesis programs are emerging as very useful conceptual templates for penetrating and exploiting the inherent reactivity of biologically active natural substances. In many instances, new enabling synthetic technologies were required to be developed. The examples demonstrate the often untapped richness of complex molecule synthesis to provide powerful tools to understand, manipulate and exploit Nature's vast and creative palette of secondary metabolites.

Terminating Catalytic Asymmetric Heck Cyclizations by Stereoselective Intramolecular Capture of η-Allylpalladium Intermediates: Total Synthesis of (-)-Spirotryprostatin B and Three Stereoisomers

A catalytic intramolecular Heck reaction, followed by capture of the resulting η(3)-allylpalladium intermediate by a tethered diketopiperazine, is the central step in a concise synthetic route to (-)-spirotryprostatin B and three stereoisomers. This study demonstrates that an acyclic, chiral η(3)-allylpalladium fragment generated in a catalytic asymmetric Heck cyclization can be trapped by even a weakly nucleophilic diketopiperazine more rapidly than it undergoes diastereomeric equilibration.

Concise synthesis of (+/-)-horsfiline and (+/-)-coerulescine by tandem cyclisation of iodoaryl alkenyl azides

A brief, efficient and economical synthesis of the spiropyrrolidinyloxindoles, horsfiline and coerulescine, has been achieved, starting from itaconic acid and, respectively, p-anisidine or o-iodoaniline. Tandem radical cyclisation of iodoaryl alkenyl azides is the key step in both syntheses.

Alkyne metathesis: development of a novel molybdenum-based catalyst system and its application to the total synthesis of epothilone A and C

Sterically hindered molybdenum(III) amido complexes of the general type [Mo[(tBu)(Ar)N]3] (1), upon treatment with CH2Cl2 or other halogen donors, have been converted into highly effective catalysts for all kinds of alkyne metathesis reactions. Although the actual nature of the propagating species formed in situ is still elusive, halogen transfer to the Mo center of 1 plays a decisive role in the activation of such precatalysts. It was possible to isolate and characterize by X-ray crystallography some of the resulting molybdenum halide derivatives such as 15, 16 and 20 which themselves were shown to be catalytically active. Numerous applications illustrate the performance of the catalytic system 1/CH2Cl2 which operates under mild conditions and tolerates an array of polar functional groups. The wide scope allows the method to be implemented into the total synthesis of sensitive and polyfunctional natural products. Most notable among them is a concise entry into the potent anticancer agents epothilone A (86) and C (88). The macrolide core of these targets is forged by ring closing alkyne metathesis (RCAM) of diyne 113, followed by Lindlar hydrogenation of cycloalkyne 114 thus formed. Since this strategy opens a stereoselective entry into (Z)-alkene 115, the approach is inherently more efficient than previous syntheses based on conventional RCM.