Enantioselective and Collective Syntheses of Xanthanolides Involving a Controllable Dyotropic Rearrangement of cis-β-Lactones

Xanthanolides in Xanthium L.: Structures, Synthesis and Bioactivity

Xanthanolides were particularly characteristic of the genus Xanthium, which exhibited broad biological effects and have drawn much attention in pharmacological application. The review surveyed the structures and bioactivities of the xanthanolides in the genus Xanthium, and summarized the synthesis tactics of xanthanolides. The results indicated that over 30 naturally occurring xanthanolides have been isolated from the genus Xanthium in monomeric, dimeric and trimeric forms. The bioassay-guided fractionation studies suggested that the effective fractions on antitumor activities were mostly from weak polar solvents, and xanthatin (1) was the most effective and well-studied xanthanolide. The varieties of structures and structure-activity relationships of the xanthanolides had provided the promising skeleton for the further study. The review aimed at providing guidance for the efficient preparation and the potential prospects of the xanthanolides in the medicinal industry.

Elucidating the selectivity of dyotropic rearrangements of β-lactones: a computational survey

A comprehensive computational study on dyotropic rearrangements of β-lactones has been performed for the first time, which provides a valuable guidance for predicting and controlling the selectivity of the reactions.

Total Synthesis of Mycinamicin IV as Integral Part of a Collective Approach to Macrolide Antibiotics

The total synthesis of the 16‐membered macrolide mycinamicin IV is outlined, which complements our previously disclosed, largely catalysis‐based route to the aglycone. This work must also be seen in the context of our recent conquest of aldgamycin N, a related antibiotic featuring a similar core but a distinctly different functionalization pattern. Taken together, these projects prove that the underlying blueprint is integrative and hence qualifies for a collective approach to this prominent class of natural products. In both cases, the final glycosylation phase mandated close attention and was accomplished only after robust de novo syntheses of the (di)deoxy sugars of the desosamine, chalcose, mycinose and aldgarose types had been established. Systematic screening of the glycosidation promoter was also critically important for success.

Tailored Synthesis of Skeletally Diverse Stemona Alkaloids through Chemoselective Dyotropic Rearrangements of β-Lactones

The collective synthesis of skeletally diverse Stemona alkaloids featuring tailored dyotropic rearrangements of β-lactones as key elements is described. Specifically, three typical 5/7/5 tricyclic skeletons associated with stemoamide, tuberostemospiroline and parvistemonine were first accessed through chemoselective dyotropic rearrangements of β-lactones involving alkyl, hydrogen, and aryl migration, respectively. By the rational manipulation of substrate structures and reaction conditions, these dyotropic rearrangements proceeded with excellent efficiency, good chemoselectivity and high stereospecificity. Furthermore, several polycyclic Stemona alkaloids, including saxorumamide, isosaxorumamide, stemonine and bisdehydroneostemoninine, were obtained from the aforementioned tricyclic skeletons through late-stage derivatizations. A novel visible-light photoredox-catalyzed formal [3+2] cycloaddition was also developed, which offers a valuable tool for accessing oxaspirobutenolide and related scaffolds.

Strain-Driven Dyotropic Rearrangement: A Unified Ring-Expansion Approach to α-Methylene-γ-butyrolactones

An unprecedented strain-driven dyotropic rearrangement of α-methylene-β-lactones has been realized, which enables the efficient access of a wide range of α-methylene-γ-butyrolactones displaying remarkable structural diversity. Several appealing features of the reaction, including excellent efficiency, high stereospecificity, predictable chemoselectivity and broad substrate scope, render it a powerful tool for the synthesis of MBL-containing molecules of either natural or synthetic origin. Both experimental and computational evidences suggest that the new variant of dyotropic rearrangements proceed in a dualistic pattern: while an asynchronous concerted mechanism most likely accounts for the reactions featuring hydrogen migration, a stepwise process involving a phenonium ion intermediate is favored in the cases of aryl migration. The great synthetic potential of the title reaction is exemplified by its application to the efficient construction of several natural products and relevant scaffolds.

Enantioselective synthesis of xanthatin

The enantioselective synthesis of cytostatic and antibiotic xanthatin (1 a) is reported. As a key intermediate, a bicyclic compound 2 was identified, which can be readily synthesized from methyl-2-furoic acid in diastereo- and enantiomerically pure form. Compound 2 can be functionalized regio- and stereoselectively at C-6 and C-7, allowing the facile introduction of the functionalities found in xanthatin, as well as the synthesis of derivatives thereof. Moreover, a robust strategy for the introduction of the exo-methylene group at C-3, commonly found in many sesquiterpenes, was developed that makes use of masking the alkene in the α,β-unsaturated carbonyl system by O-pivaoyl, which is stable under acidic and mild basic conditions but eliminated upon treatment with strong bases.