Formation of (R)-configurated tetrahydroprotoberberine alkaloids and

Demystifying racemic natural products in the homochiral world

Natural products possess structural complexity, diversity and chirality with attractive functions and biological activities that have significantly impacted drug discovery initiatives. Chiral natural products are abundant in nature but rarely occur as racemates. The occurrence of natural products as racemates is very intriguing from a biosynthetic point of view; as enzymes are chiral molecules, enzymatic reactions generating natural products should be stereospecific and lead to single-enantiomer products. Despite several reports in the literature describing racemic mixtures of stereoisomers isolated from natural sources, there has not been a comprehensive review of these intriguing racemic natural products. The discovery of many more natural racemates and their potential enzymatic sources in recent years allows us to describe the distribution and chemical diversity of this 'class of natural products' to enrich discussions on biosynthesis. In this Review, we describe the chemical classes, occurrence and distribution of pairs of enantiomers in nature and provide insights about recent advances in analytical methods used for their characterization. Special emphasis is on the biosynthesis, including plausible enzymatic and non-enzymatic formation of natural racemates, and their pharmacological significance.

Homage to Professor Meinhart H. Zenk: Crowd accelerated research and innovation

Professor Meinhart H. Zenk has had an enormous impact within the plant biochemistry area. Throughout his entire career he was able to identify and address key scientific issues within chemistry and plant secondary metabolism. Meinhart H. Zenk and his research associates have provided seminal scientific contributions within a multitude of research topics. A hallmark in Meinhart H. Zenk's research has been to rapidly introduce and apply new technologies and to initiate cross-disciplinary collaborations to provide groundbreaking new knowledge within research areas that at the time appeared highly complex and inaccessible to experimentation. He strived and managed to reach scientific excellence. In this way, he was an eminent key mentor within the plant biochemistry research community. Today, few single individuals possess so much knowledge. However, web-based social platforms enable fast and global distribution and sharing of information also including science related matters, unfortunately often prior to assessment of its correctness. Thus the demand of scientific mentoring that Meinhart H. Zenk offered the science community is as important as ever. In the honor of Meinhart H. Zenk, let us keep up that tradition and widen our engagement to encompass the new social media and benefit from the opportunities offered by crowd accelerated innovation.

Enantiomeric natural products: occurrence and biogenesis

In nature, chiral natural products are usually produced in optically pure form-however, occasionally both enantiomers are formed. These enantiomeric natural products can arise from a single species or from different genera and/or species. Extensive research has been carried out over the years in an attempt to understand the biogenesis of naturally occurring enantiomers; however, many fascinating puzzles and stereochemical anomalies still remain.

Quaternary protoberberine alkaloids

This contribution reviews some general aspects of the quaternary iminium protoberberine alkaloids. The alkaloids represent a very extensive group of secondary metabolites with diverse structures, distribution in nature, and biological effects. The quaternary protoberberine alkaloids (QPA), derived from the 5,6-dihydrodibenzo[a,g]quinolizinium system, belong to a large class of isoquinoline alkaloids. Following a general introduction, the plant sources of QPA, their biosynthesis, and procedures for their isolation are discussed. Analytical methods and spectral data are summarized with emphasis on NMR spectroscopy. The reactivity of QPA is characterized by the sensitivity of the iminium bond CN(+) to nucleophilic attack. The addition of various nucleophiles to the protoberberine skeleton is discussed. An extended discussion of the principal chemical reactivity is included since this governs interactions with biological targets. Quaternary protoberberine alkaloids and some related compounds exhibit considerable biological activities. Recently reported structural studies indicate that the QPA interact with nucleic acids predominantly as intercalators or minor groove binders. Currently, investigations in many laboratories worldwide are focused on the antibacterial and antimalarial activity, cytotoxicity, and potential genotoxicity of QPA.

Biotransformation of alkaloids

Biotransformations of alkaloids over the last decade have continued to encompass a wide variety of substrates and enzymes. The elucidation of novel alkaloid biosynthetic and catabolic pathways will continue to furnish new biocatalysts for the synthetic organic chemist. Furthermore, an improved understanding of the genetic and biochemical basis of metabolic pathways will also permit the engineering of pathways in plants and other heterologous hosts for the production of therapeutically important alkaloids. The combination of increasing commercial interest and advances in molecular biology will facilitate the availability of robust biocatalysts which are a prerequsite to achieve economically feasible processes for the production of alkaloid-based therapeutics.

Biotransformation of alkaloids

Biotransformations of alkaloids over the last decade have continued to encompass a wide variety of substrates and enzymes. The elucidation of novel alkaloid biosynthetic and catabolic pathways will continue to furnish new biocatalysts for the synthetic organic chemist. Furthermore, an improved understanding of the genetic and biochemical basis of metabolic pathways will also permit the engineering of pathways in plants and other heterologous hosts for the production of therapeutically important alkaloids. The combination of increasing commercial interest and advances in molecular biology will facilitate the availability of robust biocatalysts which are a prerequsite to achieve economically feasible processes for the production of alkaloid-based therapeutics.