Lipase-mediated resolution of 2-cyclohexen-1-ols as chiral buildingblocks en route to eburnane alkaloids

Retrosynthetic strategies and their impact on synthesis of arcutane natural products

Decisions, decisions, decisions: the interplay between different retrosynthetic strategies in the synthesis of the highly bridged, polycyclic arcutane natural products.

Retrosynthetic analysis is a cornerstone of modern natural product synthesis, providing an array of tools for disconnecting structures. However, discussion of retrosynthesis is often limited to the reactions used to form selected bonds in the forward synthesis. This review details three strategies for retrosynthesis, focusing on how they can be combined to plan the synthesis of polycyclic natural products, such as atropurpuran and the related arcutane alkaloids. Recent syntheses of natural products containing the arcutane framework showcase how these strategies for retrosynthesis can be combined to plan the total synthesis of highly caged scaffolds. Comparison of multiple syntheses of the same target provides a unique opportunity for detailed analysis of the impact of retrosynthetic disconnections on synthesis outcomes.

The role of biocatalysis in the asymmetric synthesis of alkaloids

Alkaloids are not only one of the most intensively studied classes of natural products, their wide spectrum of pharmacological activities also makes them indispensable drug ingredients in both traditional and modern medicine. Among the methods for their production, biotechnological approaches are gaining importance, and biocatalysis has emerged as an essential tool in this context. A number of chemo-enzymatic strategies for alkaloid synthesis have been developed over the years, in which the biotransformations nowadays take an increasingly 'central' role. This review summarises different applications of biocatalysis in the asymmetric synthesis of alkaloids and discusses how recent developments and novel enzymes render innovative and efficient chemo-enzymatic production routes possible.

Stereospecific and stereodivergent construction of tertiary and quaternary carbon centers through switchable directed/nondirected allylic substitution

This study introduces the ortho-diphenylphosphanylbenzoate (o-DPPB)/o-DPPB oxide system as a switchable directing/nondirecting leaving group in a copper-mediated allylic substitution with Grignard and organozinc reagents. With this system, the regioselective, stereospecific, and stereodivergent construction of quaternary as well as tertiary carbon centers is possible in a reliable and predictable fashion. Starting from one substrate enantiomer, both optical antipodes of the substitution products are readily available. Hence, this methodology features reversed polarity in comparison to established enolate alkylation chemistry and may be an interesting alternative, particularly for the construction of quaternary stereogenic carbon centers.

Enantioselective total synthesis of guanacastepene N using an uncommon 7-endo Heck cyclization as a pivotal step

A convergent, enantioselective total synthesis of (+)-guanacastepene N was developed that features a 7-endo Heck cyclization as the key step. In the course of this synthesis, short syntheses of the enantiomerically pure cyclopentenone and cyclohexene building blocks 5 and 6, which constitute A and C ring fragments of guanacastepene N, were developed. These fragments were linked by a challenging conjugate addition reaction that also generated the C11 quaternary carbon stereocenter. Regioselective 7-endo Heck cyclization gave rise to a tricyclic intermediate, which was elaborated to complete the first total synthesis of guanacastepene N and the second enantioselective total synthesis of a guanacastepene natural product.

Mass transport limitations reduce the effective stereospecificity in enzyme-catalyzed kinetic resolution

[reaction-see text] The kinetic resolution of seudenol catalyzed by Candida antarctica lipase B in hexane was investigated. Large differences in reaction rate and stereospecificity were observed when different enzyme preparations were used. These differences were ascribed to mass transport limitations which reduced both reaction rate and stereospecificity. Lyophilized enzyme preparations were more apt to give this problem than immobilized preparations. Further, low substrate concentrations enhanced the effect. Thus, high alcohol concentrations and enzyme immobilization can be recommended.

Solvent-induced stereospecific isomerization of an allylic alcohol to a homoallylic alcohol catalyzed by a chiral lithium amide

Deprotonation of cyclohexene oxide, 1, by lithium (S)-2-(1-pyrrolidinylmethyl)pyrrolidide, 2-Li, on changing the solvent from tetrahydrofuran (THF) to, for example, 2,5-dimethyltetrahydrofuran (DMTHF) or diethyl ether (DEE) has been shown to yield, besides the lithium alkoxide of 2-cyclohexene-1-ol, 3-Li, the lithium alkoxide of the homoallylic alcohol 3-cyclohexene-1-ol, 4-Li. It was shown that compound 4-Li is formed from 3-Li. No such rearrangement has been observed in THF. We have now shown that the solvent-induced isomerization of the lithium alkoxide of (S)-3-methyl-2-cyclohexene-1-ol, (S)-5-Li, catalyzed by 2-Li to the lithium alkoxide of (S)-3-methyl-3-cyclohexene-1-ol, (S)-6-Li, is 100% stereospecific. Furthermore, deuterium-labeling experiments suggest that the rearrangement of the proton is close to 100% intramolecular.Key words: 1,3-proton transfer, chiral lithium amide, intramolecular, solvent-induced isomerization, stereospecific.