Asymmetric synthesis of naturally occurring nonenolide xyolide through cross metathesis and macrolactonization reaction

The Role of Total Synthesis in Structure Revision and Elucidation of Decanolides (Nonanolides)

Ten-membered lactones are commonly observed structures of natural products. They are mostly fungal metabolites, which often act as plant pathogens, but recently ten-membered lactones were identified as pheromones of frogs and termites. Although modern spectroscopic methods are nowadays routinely used to elucidate the structure of natural products, structural assignments of ten-membered lactones often remain incomplete or are surprisingly often erroneous. Most errors concern the absolute configuration. The examples discussed in this chapter demonstrate that enantioselective total synthesis is not only an efficient tool for corroborating or revising a proposed structure, but that the synthesis of different stereoisomers as references for gas chromatographic investigations can be a vital part of the structure elucidation process if only minute amounts of material are available. As a method of outstanding importance for the synthesis of ten-membered lactones olefin metathesis has emerged. Most of the examples discussed herein use one or more olefin metathesis reactions as key steps.

A Desymmetrization-Based Total Synthesis of Reserpine

Reported herein is a desymmetrization-based synthetic approach to the fused polycyclic indole alkaloid reserpine. The centerpiece of the developed strategy features an internal desymmetrization process that enabled the use of a readily accessible and nonstereogenic reserpine E-ring precursor, in contrast to the synthesis-intensive and stereodefined E-ring intermediates employed in all past reserpine syntheses. Utilization of inexpensive reagents through an orchestrated sequence of carefully selected chemical transformations further highlight the overall effectiveness of the developed pathway.

Studies Directed toward the Stereoselective Synthesis of Cytospolide E

Our exhaustive effort toward the total synthesis of cytotoxic marine nonanolide cytospolide E has been detailed. To achieve this synthesis, we have explored both the ring-closing metathesis and lactonization-based macrocyclization strategies using a variety of precursors. Unfortunately, none of them provided the desired product. The ring-closing metathesis approach provided mainly the macrocycle with Z-olefin, whereas the macrolactonization strategy culminated in 8-epi-9-epi-cytospolide E following the regioselective formation of a 10-membered macrocycle over a 9-membered macrocycle.