Revisited after 50 Years: The ‘Stereochemical Interpretation of the Biogenetic Isoprene Rule for the Triterpenes’

Total synthesis of eudesmane terpenes by site-selective C-H oxidations

From menthol to cholesterol to Taxol, terpenes are a ubiquitous group of molecules (over 55,000 members isolated so far) that have long provided humans with flavours, fragrances, hormones, medicines and even commercial products such as rubber. Although they possess a seemingly endless variety of architectural complexities, the biosynthesis of terpenes often occurs in a unified fashion as a 'two-phase' process. In the first phase (the cyclase phase), simple linear hydrocarbon phosphate building blocks are stitched together by means of 'prenyl coupling', followed by enzymatically controlled molecular cyclizations and rearrangements. In the second phase (the oxidase phase), oxidation of alkenes and carbon-hydrogen bonds results in a large array of structural diversity. Although organic chemists have made great progress in developing the logic needed for the cyclase phase of terpene synthesis, particularly in the area of polyene cyclizations, much remains to be learned if the oxidase phase is to be mimicked in the laboratory. Here we show how the logic of terpene biosynthesis has inspired the highly efficient and stereocontrolled syntheses of five oxidized members of the eudesmane family of terpenes in a modicum of steps by a series of simple carbocycle-forming reactions followed by multiple site-selective inter- and intramolecular carbon-hydrogen oxidations. This work establishes an intellectual framework in which to conceive the laboratory synthesis of other complex terpenes using a 'two-phase' approach.

Unusually cyclized triterpenes: occurrence, biosynthesis and chemical synthesis

The biosynthetic origin of most of triterpenes lies in cascade cyclizations and rearrangements of the acyclic precursors squalene (S) and 2,3-oxidosqualene (OS), processes leading to tetra- and pentacyclic triterpene skeleta. Apart from these, a number of triterpenoid structures derived from cyclization processes, that are different from those leading to tetra- and pentacyclic triterpenes, are also found in Nature. We have defined these processes as unusual cyclizations, and grouped them in three blocks, namely, incomplete cyclizations of the corresponding S-derived precursors, cyclizations of S or OS towards polycyclic triterpenes and subsequent cleavage of the preformed ring systems, and two independent cyclizations of the S- or OS-derived precursor. Apart from the molecules obtained from intact organisms, we will also consider the compounds obtained from in vitro cyclizations promoted by enzyme systems. After establishing which compounds could unambiguously be grouped under the term 'unusually cyclized triterpenes', this review moves on to the advances achieved in this kind of structure during the last ten years. These advances are presented in three parts. The first one presents the structure and biological properties of the unusual triterpenes reported in the last decade. The second part considers the main biosynthetic pathways which justify the formation of these triterpenes from their corresponding acyclic precursors. Finally, we look at the achievements made in different synthetic strategies directed at some of these molecules. One hundred and twenty-three references are cited.

Protostadienol biosynthesis and metabolism in the pathogenic fungus Aspergillus fumigatus

Details of the fungal biosynthetic pathway to helvolic acid and other fusidane antibiotics remain obscure. During product characterization of oxidosqualene cyclases in Aspergillus fumigatus, we found the long-sought cyclase that makes (17Z)-protosta-17(20),24-dien-3beta-ol, the precursor of helvolic acid. We then identified a gene cluster encoding the pathway to helvolic acid, which is controlled by a transcription regulator (LaeA) associated with fungal virulence. Evidence regarding the evolutionary origin and taxonomic distribution of fusidane biosynthesis is also presented.

A simple route for renewable nano-sized arjunolic and asiatic acids and self-assembly of arjuna-bromolactone

While separating two natural nano-sized triterpenic acids via bromolactonization, we serendipitously discovered that arjuna-bromolactone is an excellent gelator of various organic solvents. A simple and efficient method for the separation of two triterpenic acids and the gelation ability and solid state 1D-helical self-assembly of nano-sized arjuna-bromolactone are reported.

Triterpenoids

This review covers the isolation and structure determination of triterpenoids including squalene derivatives, protostanes, lanostanes, holostanes, cycloartanes, dammaranes, euphanes, tirucallanes, tetranortriterpenoids, quassinoids, lupanes, oleananes, friedelanes, ursanes, hopanes, isomalabaricanes and saponins. The literature from January 2005 to December 2006 is reviewed and 478 references are cited.

Conformational energetics of cationic backbone rearrangements in triterpenoid biosynthesis provide an insight into enzymatic control of product

2,3-( S)-Oxidosqualene (C 30H 50O) serves as a versatile starting point for the remarkable biosynthesis of many isomeric naturally occurring triterpenoids of formula C 30H 50O. These biosyntheses all involve polycyclization via cationic intermediates. The fully cyclized primary products then are converted to various structures by cationic rearrangements involving the polycyclic backbone. The energetics of these rearrangements has been examined by B3LYP 6-31 G* DFT calculations and by ab initio Hartree-Fock calculations at the 6-31G* or 3-21G(*) level. The results have led to the conclusion that the biosynthesis of friedelin, the most drastically rearranged of the pentacyclic triterpenes, involves a complex nonstop process, with no stable intermediates between 2,3-( S)-oxidosqualene and friedelin. It is proposed that this single-reaction biosynthesis consists of pentacyclization to the lupanyl cation followed directly by a sequence of 10 suprafacial 1,2-shifts of carbon and hydrogen, driven by the large exergonicity of the pentacyclization and electrostatic acceleration of the rearrangement steps.

An Oxidosqualene Cyclase Makes Numerous Products by Diverse Mechanisms: A Challenge to Prevailing Concepts of Triterpene Biosynthesis

The genome of the model plant Arabidopsis thaliana encodes 13 oxidosqualene cyclases, 9 of which have been characterized by heterologous expression in yeast. Here we describe another cyclase, baruol synthase (BARS1), which makes baruol (90%) and 22 minor products (0.02-3% each). This represents as many triterpenes as have been reported for all other Arabidopsis cyclases combined. By accessing an extraordinary repertoire of mechanistic pathways, BARS1 makes numerous skeletal types and deprotonates the carbocation intermediates at 14 different sites around rings A, B, C, D, and E. This undercurrent of structural and mechanistic diversity in a superficially accurate enzyme is incompatible with prevailing concepts of triterpene biosynthesis, which posit tight control over the mechanistic pathway through cation-pi interactions, with a single proton acceptor in a hydrophobic active site. Our findings suggest that mechanistic diversity is the default for triterpene biosynthesis and that product accuracy results from exclusion of alternative pathways.

Cation-pi interaction in the polyolefin cyclization cascade uncovered by incorporating unnatural amino acids into the catalytic sites of squalene cyclase

It has been assumed that the pi-electrons of aromatic residues in the catalytic sites of triterpene cyclases stabilize the cationic intermediates formed during the polycyclization cascade of squalene or oxidosqualene, but no definitive experimental evidence has been given. To validate this cation-pi interaction, natural and unnatural aromatic amino acids were site-specifically incorporated into squalene-hopene cyclase (SHC) from Alicyclobacillus acidocaldarius and the kinetic data of the mutants were compared with that of the wild-type SHC. The catalytic sites of Phe365 and Phe605 were substituted with O-methyltyrosine, tyrosine, and tryptophan, which have higher cation-pi binding energies than phenylalanine. These replacements actually increased the SHC activity at low temperature, but decreased the activity at high temperature, as compared with the wild-type SHC. This decreased activity is due to the disorganization of the protein architecture caused by the introduction of the amino acids more bulky than phenylalanine. Then, mono-, di-, and trifluorophenylalanines were incorporated at positions 365 and 605; these amino acids reduce cation-pi binding energies but have van der Waals radii similar to that of phenylalanine. The activities of the SHC variants with fluorophenylalanines were found to be inversely proportional to the number of the fluorine atoms on the aromatic ring and clearly correlated with the cation-pi binding energies of the ring moiety. No serious structural alteration was observed for these variants even at high temperature. These results unambiguously show that the pi-electron density of residues 365 and 605 has a crucial role for the efficient polycyclization reaction by SHC. This is the first report to demonstrate experimentally the involvement of cation-pi interaction in triterpene biosynthesis.

Modern synthetic efforts toward biologically active terpenes

Terpenes represent one of the largest and most diverse classes of secondary metabolites, with over 55,000 members isolated to date. The terpene cyclase enzymes used in nature convert simple, linear hydrocarbon phosphates into an exotic array of chiral, carbocyclic skeletons. Further oxidation and rearrangement results in an almost endless number of conceivable structures. The enormous structural diversity presented by this class of natural products ensures a broad range of biological properties-ranging from anti-cancer and anti-malarial activities to tumor promotion and ion-channel binding. The marked structural differences of terpenes also largely thwart the development of any truly general strategies for their synthetic construction. This review focuses on synthetic strategies directed toward some of the most complex, biologically relevant terpenes prepared by total synthesis within the past decade. Of crucial importance are both the obstacles that modern synthetic chemists must confront when trying to construct such natural products and the key chemical transformations and strategies that have been developed to meet these challenges.

Highly selective synthesis of oxabicycloalkanes by indium tribromide-mediated cyclization reactions of epoxyalkenes

The cyclization of epoxyalkenes to oxabicycloalkanes is catalyzed by stoichiometric quantities of indium tribromide which exhibits excellent selectivity giving the oxabicyclic product in high yield in preference to other cyclized or rearrangement products.

Antigiardial activity of triterpenoids from root bark of Hippocratea excelsa

Two new triterpenoids, 21 beta-hydroxyolean-12-en-3-one (1) and a seco-dinor derivative of pristimerine named dzununcanone (2), were isolated from the root bark of Hippocratea excelsa. Their structures were assigned on the basis of spectroscopic evidence, mainly 1H and 13C 1D and 2D NMR including DEPT, COSY, ROESY, HSQC, and HMBC experiments, as well as EIMS and HREIMS. The known 21alpha-hydroxy-3-oxofriedelane (3), a compound new to the species, and the known methide quinones pristimerine (4) tingenone (5), and xuxuarine Ebeta (7) were also isolated. The antiprotozoal activities were determined against Giardia intestinalis. Pristimerine and tingenone were the most active antigiardial compounds, with IC50 values of 0.11 and 0.74 microM, respectively, compared with metronidazole, the current drug of choice (IC50 1.23 microM).

Cyclization of 1,5-dienes: an efficient synthesis of beta-Georgywood

In the acid-promoted 1,5-diene cyclization of pseudo- to beta-Georgywood, the cyclization product is obtained with high selectivity in spite of an unfavorable substituent at the C(2)-position of the diene precursor. Preisomerization of the cyclohexene double bond, which occurs in the presence of Brønsted acids, is suppressed with >1 equiv of MXn-type Lewis acids, whereas RAlX2-type Lewis acids such as >2 equiv of MeAlCl2 have the additional benefit of steering the double bond of the cyclized product into the desired beta-position. Mechanistic studies revealed a crucial participation or nonparticipation of the carbonyl group in the cyclization reaction, depending on the acid family employed, and allowed finally the development of a cyclization reaction catalyzed by MeAlCl2 that can be generated in situ from precatalyst AlMe3.1.

Origin of structural diversity in natural triterpenes: direct synthesis of seco-triterpene skeletons by oxidosqualene cyclase

At1g78500, one of the oxidosqualene cyclase (OSC) homologues from Arabidopsis thaliana, was expressed in a lanosterol synthase-deficient yeast strain and the products were analyzed. In addition to the known triterpenes, this OSC was found to produce two new triterpenes, the structures of which were determined by NMR and MS analyses. The new triterpenes are C-ring-seco-beta-amyrin (1) and C-ring-seco-alpha-amyrin (2) and named beta-seco-amyrin and alpha-seco-amyrin, respectively. beta-seco-Amyrin is produced from the oleanyl cation through bond cleavage between C8 and C14, and alpha-seco-amyrin is produced from the ursanyl cation in the same manner. Together with Grob fragmentation catalyzed by another OSC (marneral synthase) from A. thaliana, the formation of seco-amyrins by this OSC revealed that OSCs not only catalyze carbon-carbon bond formations and Wagner-Meerwein rearrangements but also cleave preformed ring systems in cationic intermediates. Based on this information, direct production of other natural seco-triterpenes by OSCs is proposed.

Selective Monocyclization of Epoxy Terpenoids Promoted by Zeolite NaY. A Short Biomimetic Synthesis of Elegansidiol and Farnesiferols B−D

Epoxy terpenes cyclize readily, by confinement within zeolite NaY, to form exomethylenic cyclohexanols as the major products. The selective monocyclization of 10,11-epoxyfarnesyl acetate within NaY provides a short and efficient biomimetic route to (+/-)-elengasidiol and (+/-)-farnesiferols B-D. [reaction: see text].

Chemical Constituents of the Aerial Parts of Kalidium foliatum

Studies on the chemical constituents of the aerial parts of Kalidium foliatum have led to the isolation of three new and one known compounds. The structures of new constituents have been elucidated through spectral studies including 2D-NMR experiments (HMQC, HMBC, COSY, NOESY and J-resolved) and MS/MS fragmentation using Q-TOF mass spectrometer equipped with an ESI source as kalidiumoside C (=3beta-hydroxy-29-methylmalonoxy-olean-12-en-23,28-dioic acid-23-methyl-28-beta-D-glucopyranosyl ester; 1), kalidiunin (=3beta,23,29-trihydroxy-olean-12-en-28-methyl-oate; 2) and kalidiumoside D (=3beta,23,29-trihydroxyolean-12-en-28-oic acid-beta-D-glucopyranosyl ester; 3). The known compound was identified as 3beta,23,29-trihydroxy-olean-12-en-28-oic acid 4) through comparison of its spectral data with those reported in literature. Acid hydrolysis of both 2 and 3 yielded the known compound 4 providing a conclusive evidence of the proposed structures.