Observations on the state of synthesis: Some magic moments revisited

Pattern recognition analysis in complex molecule synthesis and the preparation of iso-Diels-Alder motifs

The identification of synthesizable substructural domains within more complex structural targets is of significant value in designing a workable plan of synthesis. We term this process "pattern recognition analysis" (PRA). In this paper we continued to build on the theme of PRA as a potential resource in retrosynthetic blueprints to reach highly challenging targets. The paper operates at two levels. First, there is provided a clear sense of definitions of categories by which patterns are related to hypothetical reaction types. Although the required reaction type may for the moment not exist, we believe that this method of analysis is likely to promote innovation that identifies unmet needs and opportunities to advance the cause of complex target synthesis. In addition, we describe reductions to practice in expanding the menu of achievable patterns. It is likely that the future value of PRA will be associated with its utility in leading the way to new and exploitable chemical innovation.

An Efficient One-Pot Asymmetric Synthesis of Biaryl Compounds via Diels−Alder/Retro-Diels−Alder Cascade Reactions

[reaction: see text] A single-step chirality transfer method for the synthesis of axially chiral biaryl compounds by construction of the second aromatic ring via a Diels-Alder/retro-Diels-Alder cascade reaction is reported. This methodology should find broad application in the synthesis of natural products and asymmetric catalysts.

Total synthesis of guanacastepene a: a route to enantiomeric control

[reaction: see text] The goal of the total synthesis of guanacastepene A served as a focus to bring together several chemical inquiries. One involved the synthesis of fused 5,7-hydrazulenones (see structure 20). Another issue had to do with the mechanistic intermediates in reductive cyclizations (see 17 to 18 and 19). The total synthesis required a mastery of an intramolecular Knoevenagel condensation of a beta,gamma-unsaturated ketone (see compound 41). Actually, cyclization was best accomplished when the terminal double bond of 41 was first converted to an epoxide. Further issues related to the stereochemistry at C5 and, rather surprisingly, the propensity for beta-face acetoxylation at C13. Crystallographic verification of the assigned beta-stereochemistry at C13 is provided. Finally, a route to optically active material is provided (see compound 20). A key element in this construction was an enantioselective addition of isopropenyl cuprate to 2-methylcyclopentenone (see compound 99).

Discovery through total synthesis: a retrospective on the himastatin problem

A total synthesis of a structure proposed for himastatin was accomplished. The non-identity of the fully synthetic material with himastatin necessitated a revision of the assigned structure. Confirmation of the revised stereostructure was subsequently confirmed through total synthesis. Among the achievements during this effort were i) stereospecific routes to both anti-cis and syn-cis pyrrolindoline substructures; ii) a practical synthesis to 5-hydroxypiperazic acid in enantiomerically pure form; iii) a Stille coupling leading to a complex bi-indole moiety, and iv) efficient protecting group management throughout the evolving depsipeptide domain. The outlines for a biological pharmacophore have been delineated. The alternating D- and L-substituents in the 6-mer as well as the biaryl linkage connecting the two identical subunits are critical for maintaining biological activity. This pattern is simulated in another antibiotic, and suggests a possible structural trend for future SAR investigations.