Structure-based engineering of strictosidine synthase: auxiliary for alkaloid libraries

The highly substrate-specific strictosidine synthase (EC 4.3.3.2) catalyzes the biological Pictet-Spengler condensation between tryptamine and secologanin, leading to the synthesis of about 2000 monoterpenoid indole alkaloids in higher plants. The crystal structure of Rauvolfia serpentina strictosidine synthase (STR1) in complex with strictosidine has been elucidated here, allowing the rational site-directed mutation of the active center of STR1 and resulting in modulation of its substrate acceptance. Here, we report on the rational redesign of STR1 by generation of a Val208Ala mutant, further describing the influence on substrate acceptance and the enzyme-catalyzed synthesis of 10-methyl- and 10-methoxystrictosidines. Based on the addition of strictosidine to a crude strictosidine glucosidase preparation from Catharanthus cells, a combined chemoenzymatic approach to generating large alkaloid libraries for future pharmacological screenings is presented.     

Synthesis the Analog of Strictosidine and Vincoside from Geniposide

Strictosidine (1), a well-known monoterpene indole alkaloid glycoside^[1,2], is the precursor and building stone of nearly 2200 indole and related alkaloids and was first isolated by G. N. Smith from Rhazya^[3]. It is constructed in vivo from secologanin (2) and tryptamine (3) by plant species^[1], as well as in vitro in the presence of the enzyme strictosidine synthase, or under biomimetic conditions in aqueous solution at pH 4.5 (Scheme 1). In the coupling reaction, a new chiral center is formed with complete stereoselectivity in the presence of the enzyme, or together with vincoside (4) in a 1:1 ratio in the absence of the enzyme. Here we describe the preparation of the analog of strictosidine and vincoside from geniposide via the biomimetic conditions. 7 and 8 can be used as the starting material to synthesize other analogs of indole and related alkaloids.     

A Facile Chemoenzymatic Approach: One‐Step Syntheses of Monoterpenoid Indole Alkaloids

Facile chemoenzymatic syntheses of cytotoxic monoterpenoid indole alkaloids with novel skeletons and multiple chiral centers are described. Synthesis of these alkaloids was achieved by a simple one‐step reaction using strictosidine and 12‐aza‐strictosidine as the key intermediates. Strictosidines were prepared by coupling of secologanin with tryptamine and 7‐aza‐tryptamine, respectively, using the immobilized recombinant Rauvolfia strictosidine synthase. A detailed stereochemical analysis is presented herein. The results provide an opportunity for a chemoenzymatic approach that leads to an increased diversification of complex alkaloids with improved structures and activities.     

单萜吲哚生物碱的仿生合成

单萜吲哚生物碱因其骨架和官能团的丰富变化,加上它们的生物活性,多年来 一直哟引着一代又一代的化学家对其进行结构和合成研究,它们的共同生物合成前 体strictosidine是由色胺和单萜苷secologanin缩合形成的。自从secologanin可 以大量得到以后,以它为原料沿着可能的生物合成路线合成天然生物碱即仿生合成 就成为一个重要的研究领域。它对于理解和阐释生物碱的生物合成过程,为提供天 然来源极少的生物碱供药理试验及对促进有机合成化学的发展等都是有重要意义。 这方面的研究也取得了许多重要进展,成功合成了一些重要的单萜吲哚生物碱,如 育亨宾类、钩藤碱、异钩藤碱、卡得宾、利血平类似物、喜树碱等。     

A new type of monoterpenoid indole alkaloid precursor from Alstonia rostrata

Currently, all monoterpenoid indole alkaloids (MIAs) have been derived from strictosidine, which originates from the condensation of tryptophan with secologanin in a 1:1 ratio. However, our phytochemical research on Alstonia rostrata revealed a potential new precursor for these compounds. We isolated the alstrostines A and B, and it was determined that they were derived from tryptophan and secologanin in a 1:2 ratio, which supported the presence of a new type of MIA precursor.     

Comparative study of fourteen alkaloids from Uncaria rhynchophylla hooks and leaves using HPLC-diode array detection-atmospheric pressure chemical ionization/MS method

The purpose of the study is to compare alkaloid profile of Uncaria rhynchophylla hooks and leaves. Ten oxindole alkaloids and four glycosidic indole alkaloids were identified using HPLC-diode array detection (DAD) or LC-atmospheric pressure chemical ionization (APCI)-MS method, and a HPLC-UV method for simultaneous quantification of major alkaloids was validated. The hooks are characterized by high levels of four oxindole alkaloids rhynchophylline (R), isorhynchophylline (IR), corynoxeine (C) and isocorynoxeine (IC), while the leaves contained high level of two glycosidic indole alkaloids vincoside lactam (VL) and strictosidine (S). The presented methods have proven its usefulness in chemical characterization of U. rhynchophylla hooks and leaves.     

Subincanadines A-C,novel quaternary indole alkaloids from Aspidosperma subincanum

Three novel quaternary indole alkaloids with an unprecedented 1-azoniatricyclo[4.3.3.0(1,5)]undecane moiety, subincanadines A-C (1-3), as well as two new indole alkaloids with a 1-azabicyclo[5.2.2]undecane moiety, subincanadines D (4) and E (5), and a new indole alkaloid with a 1-azabicyclo[4.3.1]decane moiety, subincanadine F (6), have been isolated from the barks of Aspidosperma subincanum Mart, and the structures of 1-6 and the stereochemistry of 1-3 were elucidated by spectroscopic data and chemical means.     

Redesign of a central enzyme in alkaloid biosynthesis

Plant alkaloids exhibit a diverse array of structures and pharmaceutical activities, though metabolic engineering efforts in these eukaryotic pathways have been limited. Strictosidine synthase (STR) is the first committed step in the biosynthesis of over two thousand terpene indole alkaloids. We describe a rational redesign of the STR binding pocket to selectively accommodate secologanin substrate analogs. The mutant is selective for a substrate that can be chemoselectively derivatized. Evidence that this substrate can be processed by later steps of the terpene indole alkaloid pathway is provided. The work demonstrates that the central enzyme of this alkaloid pathway can be redesigned and that the pathway can turn over the unnatural intermediate that is generated. Modulation of the substrate specificity of enzymes of this complex pathway is therefore likely to enable metabolic engineering efforts of these alkaloids.     

Bioinspired Transformations Using Strictosidine Aglycones: Divergent Total Syntheses of Monoterpenoid Indole Alkaloids in the Early Stage of Biosynthesis

A series of bioinspired transformations that are applied to convert strictosidine aglycones into monoterpenoid indole alkaloids is reported. The highly reactive key intermediates, strictosidine aglycones, were prepared in situ by simple removal of a silyl protecting group from the silyl ether derivatives, and converted selectively via bioinspired transformations under substrate control into heteroyohimbine- and corynantheine-type, and akagerine and naucleaoral related alkaloids. Thus, concise, divergent total syntheses of 13 monoterpenoid indole alkaloids, (−)-cathenamine, (−)-tetrahydroalstonine, (+)-dihydrocorynantheine, (−)-corynantheidine, (−)-akagerine, (−)-dihydrocycloakagerine, (−)-naucleaoral B, (+)-naucleidinal, (−)-naucleofficines D and III, (−)-nauclefiline, and (−)-naucleamides A and E, were accomplished in fewer than 13 steps.     

Total Syntheses of (−)‐Strictosidine and Related Indole Alkaloid Glycosides

A collective synthesis of glycosylated monoterpenoid indole alkaloids is reported. A highly diastereoselective Pictet–Spengler reaction with α‐cyanotryptamine and secologanin tetraacetate as substrates, followed by a reductive decyanation reaction, was developed for the synthesis of (?)‐strictosidine, which is an important intermediate in biosynthesis. This two‐step chemical method was established as an alternative to the biosynthetically employed strictosidine synthase. Furthermore, after carrying out chemical and computational studies, a transition state for induction of diastereoselectivity in our newly discovered Pictet–Spengler reaction is proposed. Having achieved the first enantioselective total synthesis of (?)‐strictosidine in just 10 steps, subsequent bioinspired transformations resulted in the concise total syntheses of (?)‐strictosamide, (?)‐neonaucleoside A, (?)‐cymoside, and (?)‐3α‐dihydrocadambine.     

Rapid identification of enzyme variants for reengineered alkaloid biosynthesis in periwinkle

Monoterpene indole alkaloids from Catharanthus roseus (Madagascar periwinkle), such as the anticancer agents vinblastine and vincristine, have important pharmacological activities. Metabolic engineering of alkaloid biosynthesis can provide an efficient and environmentally friendly route to analogs of these synthetically challenging and pharmaceutically valuable natural products. However, the narrow substrate scope of strictosidine synthase, the enzyme at the entry point of the pathway, limits a pathway engineering approach. We demonstrate that with a different expression system and screening method it is possible to rapidly identify strictosidine synthase variants that accept tryptamine analogs not turned over by the wild-type enzyme. The variants are used in stereoselective synthesis of beta-carboline analogs and are assessed for biosynthetic competence within the terpene indole alkaloid pathway. These results present an opportunity to explore metabolic engineering of "unnatural" product production in the plant periwinkle.     

喜树碱的仿生合成：一个四分之一世纪的故事

喜树碱是从中国植物喜树(Camptotheca acuminata)中分离得到的抗癌生物碱 ，由于其独特的抗癌机理而成为抗癌药研究中的热门课题．喜树碱属于单萜吲哚生 物碱类，由色胺和secologanin缩合、衍生而来．它的仿生合成始于1972年，先后 有多人参加，直到1997年在经历了四分之一世纪之后才取得了成功．介绍喜树碱仿 生合成背后鲜为人知的故事．     

On the C-16 configuration of sitsirikine

The indole alkaloid sitsirikine has been synthesised by a biomimetic conversion of strictosidine and its C-16 configuration established as R by cyclisation to a 16,17-dihydroheteroyohimbine.     

Synthesis of bioactive nitrogen heterocycles from marine organisms

Recently many bioactive indole alkaloids have been isolated from marine organisms. Many of them have novel ring systems which are not found in the indole alkaloids isolated from higher plants and molds on the land. We have chosen β-carboline alkaloids, eudistomins and manzamines, as targets for total synthesis.     

Voacanga alkaloids. VII. Desoxovobasin

Acidic cleavage of vacamine under reductive conditions yields desoxovobasine which was correlated with a product obtained by C(3)-N fission of macusine B. Desoxovobasine can be transformed into indoline derivatives of a novel polycyclic system which has not yet been found among the naturally occurring indole alkaloids.     

Collective Total Synthesis of Aspidofractinine Alkaloids through the Development of a Bischler–Napieralski/Semipinacol Rearrangement Reaction

A tandem Bischler–Napieralski/semipinacol rearrangement reaction has been developed for the purpose of assembling a bis(spirocyclic) indole framework, a privileged structural unit of aspidofractinine-type monoterpenoid indole alkaloids, and was used in combination with a subsequent Mannich reaction to expeditiously construct the central bridged bicyclo[2.2.1]heptane ring system of these molecules with contiguous quaternary centers. The development of this novel strategy culminated in the collective total synthesis of four aspidofractinine alkaloids.     

Indole-Containing Natural Products 2019–2022: Isolations,Reappraisals, Syntheses,and Biological Activities

Indole alkaloids represent a large subset of natural products, with more than 4100 known compounds. The majority of these alkaloids are biologically active, with some exhibiting excellent antitumor, antibacterial, antiviral, antifungal, and antiplasmodial activities. Consequently, the natural products of this class have attracted considerable attention as potential leads for novel therapeutics and are routinely isolated, characterized, and profiled to gauge their biological potential. However, data on indole alkaloids, their various structures, and bioactivities are complex due to their diverse sources, such as plants, fungi, bacteria, sponges, tunicates, and bryozoans; thus, isolation methods produce an incredible trove of information. The situation is exacerbated when synthetic derivatives, as well as their structures, bioactivities, and synthetic schemes, are considered. Thus, to make such data comprehensive and inform researchers about the current field’s state, this review summarizes recent reports on novel indole alkaloids. It deals with the isolation and characterization of 250 novel indole alkaloids, a reappraisal of previously reported compounds, and total syntheses of indole alkaloids. In addition, several syntheses and semi-syntheses of indole-containing derivatives and their bioactivities are reported between January 2019 and July 2022.     

A Divergent Polyene Cyclization for the Total Synthesis of Greenwayodendrines,Greenwaylactams, Polysin and Polyveoline

We present a concise asymmetric total synthesis (5–8 steps) of nine sesquiterpenoid alkaloids featuring four different tetra-/pentacyclic scaffolds. To this end, a novel, bioinspired indole N-terminated cationic tricyclization has been developed, enabling the divergent synthesis of greenwayodendrines and polysin. Subtle variation of the C2-substituted indole cyclization precursor allowed switching between indole N- and C-termination. For the latter, a subsequent Witkop oxidation enabled conversion of the cyclopentene-fused indole into the eight-membered benzolactam to directly furnish the family of greenwaylactams. In addition, a diastereomeric C-termination product has been elaborated to provide access to polyveoline.     

Total Syntheses of (−)-Strictosidine and Related Indole Alkaloid Glycosides

A collective synthesis of glycosylated monoterpenoid indole alkaloids is reported. A highly diastereoselective Pictet–Spengler reaction with α-cyanotryptamine and secologanin tetraacetate as substrates, followed by a reductive decyanation reaction, was developed for the synthesis of (−)-strictosidine, which is an important intermediate in biosynthesis. This two-step chemical method was established as an alternative to the biosynthetically employed strictosidine synthase. Furthermore, after carrying out chemical and computational studies, a transition state for induction of diastereoselectivity in our newly discovered Pictet–Spengler reaction is proposed. Having achieved the first enantioselective total synthesis of (−)-strictosidine in just 10 steps, subsequent bioinspired transformations resulted in the concise total syntheses of (−)-strictosamide, (−)-neonaucleoside A, (−)-cymoside, and (−)-3α-dihydrocadambine.     

Total synthesis of marine sponge bis(indole) alkaloids of the topsentin class

The synthesis of four natural bis(indole) alkaloids of topsentin class 1 and 2 is described. Their bis(indole) alpha-carbonylimidazoline and subsequently bis(indole) alpha-carbonylimidazole moieties have been built via the condensation between indolic alpha-ketothioimidate salts 4 and 1-(indol-3'-yl)-1,2-diaminoethane 3. This compound results from the beta-amino indolic hydroxylamine 5 by a two-step sequence. This is the first total synthesis of compounds 1d, 2a, and 2b.