Simple indole alkaloids and those with a nonrearranged monoterpenoid unit

Multicomponent reactions are flexible reactions for the rapid generation of complex molecules with often biologically relevant scaffold structures. Combined with the ease of parallelization and the exploratory power with regard to chemical space, multicomponent reactions have attracted significant attention from the medicinal chemistry community. In this Review, we present an overview of recent literature concerning this topic to provide insight into the applications of multicomponent reactions in this field.

Recent applications of multicomponent reactions in medicinal chemistry

Lignin, a phenolic polymer in the secondary wall, is the major cause of lignocellulosic biomass recalcitrance to efficient industrial processing. From an applications perspective, it is desirable that second-generation bioenergy crops have lignin that is readily degraded by chemical pretreatments but still fulfill its biological role in plants. Because plants can tolerate large variations in lignin composition, often without apparent adverse effects, substitution of some fraction of the traditional monolignols by alternative monomers through genetic engineering is a promising strategy to tailor lignin in bioenergy crops. However, successful engineering of lignin incorporating alternative monomers requires knowledge about phenolic metabolism in plants and about the coupling properties of these alternative monomers. Here, we review the current knowledge about lignin biosynthesis and the pathways towards the main phenolic classes. In addition, the minimal requirements are defined for molecules that, upon incorporation into the lignin polymer, make the latter more susceptible to biomass pretreatment. Numerous metabolites made by plants meet these requirements, and several have already been tested as monolignol substitutes in biomimetic systems. Finally, the status of detection and identification of compounds by phenolic profiling is discussed, as phenolic profiling serves in pathway elucidation and for the detection of incorporation of alternative lignin monomers.

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Metabolic engineering of novel lignin in biomass crops

Benzoxazinoid acetal glucosides are a unique class of natural products abundant in Gramineae, including the major agricultural crops maize, wheat, and rye. These secondary metabolites are also found in several dicotyledonous species. Benzoxazinoids serve as important factors of host plant resistance against microbial diseases and insects and as allelochemicals and endogenous ligands. Interdisciplinary investigations by biologists, biochemists, and chemists are stimulated by the intention to make agricultural use of the benzoxazinones as natural pesticides. These natural products are not only constituents of a plant defense system but also part of an active allelochemical system used in the competition with other plants. This review covers biological and chemical aspects of benzoxazinone research over the last decade with special emphasis on recent advances in the elucidation of the biosynthetic pathway.

Guenter Adam

Papers

Bisquinolinone alkaloids from Melicope ptelefolia

Benzopyrans from Melicope ptelefolia leaves

Benzopyranones and ferulic acid derivatives from Antidesma membranaceum

Alkaloidal, Megastigmane and Lignan Glucosides from Antidesma membranaceum (Euphorbiaceae)

β-Carboline alkaloids from Hedyotis capitellata