Biosynthesis of the Polycyclic System in the Antifungal HSAF and Analogues from Lysobacter enzymogenes

The biocontrol agent Lysobacter enzymogenes produces polycyclic tetramate macrolactams (PoTeMs), including the antifungal HSAF. To elucidate the biosynthesis of the cyclic systems, we identified eleven HSAF precursors/analogues with zero, one, two, or three rings through heterologous expression of the HSAF gene cluster. A series of combinatorial gene expression and deletion experiments showed that OX3 is the “gatekeeper” responsible for the formation of the first 5‐membered ring from lysobacterene A, OX1 and OX2 are responsible for formation of the second ring but with different selectivity, and OX4 is responsible for formation of the 6‐membered ring. In vitro experiments showed that OX4 is an NADPH‐dependent enzyme that catalyzes the reductive cyclization of 3‐dehydroxy alteramide C to form 3‐dehydroxy HSAF. Thus, the multiplicity of OX genes is the basis for the structural diversity of the HSAF family, which is the only characterized PoTeM cluster that involves four redox enzymes in the formation of the cyclic system.     

Synthesis of some fused β‐carbolines including the first example of the pyrrolo[3,2‐c]‐β‐carboline system

Condensation of 1‐methyl‐β‐carboline‐3‐carbaldehyde with ethyl azidoacetate and subsequent thermolysis of the resulting azidopropenoate was used to [c] annulate a pyrrole ring onto the β‐carboline moiety, thus producing the first example of the pyrrolo[3,2‐c]‐β‐carboline ring system. The latter ring system results from cyclization at the C‐4 carbon, whereas cyclization at the N‐2 nitrogen atom also occurs to form a pyrazolo[3,2‐c]‐β‐carboline ring system. Condensation of β‐carboline‐1‐carbaldehyde with ethyl azidoacetate produced a non‐isolable intermediate, which immediately underwent cyclization, however in this case cyclization occurred via attack at the ester and the azide remained intact. The resulting 5‐azidocanthin‐6‐one was transformed to the first examples of 5‐aminocanthin‐6‐ones. β‐Carboline‐1,3‐dicarbaldehyde failed to give an acceptable reaction with ethyl azidoacetate, but did undergo selective condensation with dimethyl acetylene dicarboxylate at the C‐1 carbaldehyde with concomitant cyclization to form a highly functionalized 2‐formyl‐canthine derivative.     

Total synthesis of the 6-silasteroid ring system

The total synthesis of the first¹ silasteroid ring system is described. A bidirectional construction was employed, starting from precursors of rings A and D. Metal-organic substitutions on dimethyldichlorosilane gave the allyl-m-methoxyphenyl derivative 2, as elements of rings AB. Hydroboration-oxidations led to the arylsilylpropionic acid 5. Activation and cyclization gave the key intermediate, 4,4 - dimethyl - 4 - sila - 6 - methoxy - 1 - tetralone, 7. Attachment of ring D and closure of ring C followed the Torgov-Smith outline, but required important differences in implementation, to give D,L-6, 6 - dimethyl - 6 - sila - 3 - methoxy - estra - 1,3,5(10),8,14 - pentaene - 17 - one, 11. Extensive physical data are presented.     

Asymmetric Formal Synthesis of Azadirachtin

An asymmetric formal synthesis of azadirachtin, a potent insect antifeedant, was accomplished in 30 steps to Ley’s synthetic intermediate (longest linear sequence). The synthesis features: 1) rapid access to the optically active right‐hand segment starting from the known 5‐hydroxymethyl‐2‐cyclopentenone scaffold; 2) construction of the B and E rings by a key intramolecular tandem radical cyclization; 3) formation of the hemiacetal moiety in the C ring through the α‐oxidation of the six‐membered lactone followed by methanolysis.     

Synthesis of Some New 1,2,4‐Triazole Derivatives by Mitsunobu Chemistry

The reactive 1 : 1 intermediate produced in the reaction between triphenylphosphine and diisopropyl azodicarboxylate has been trapped by isocyanates or isothiocyanates to yield 1,2,4‐triazole derivatives 2 (Scheme 1). The structures of the highly functionalized compounds 2 were corroborated spectroscopically (IR, ¹H‐ and ¹³C‐NMR, EI‐MS) and by elemental analyses. A mechanism for this type of cyclization is proposed (Scheme 2).     

Functionalization of Intramolecular Frustrated Lewis Pairs by 1,1‐Carboboration with Conjugated Enynes

The vicinal P/B frustrated Lewis pair (FLP) Mes₂PCH₂CH₂B(C₆F₅)₂ undergoes 1,1‐carboboration reactions with the Me₃Si‐substituted enynes to give ring‐enlarged functionalized C₃‐bridged P/B FLPs. These serve as active FLPs in the activation of dihydrogen to give the respective zwitterionic [P]H⁺/[B]H^? products. One such product shows activity as a metal‐free catalyst for the hydrogenation of enamines or a bulky imine. The ring‐enlarged FLPs contain dienylborane functionalities that undergo “bora‐Nazarov”‐type ring‐closing rearrangements upon photolysis. A DFT study had shown that the dienylborane cyclization of such systems itself is endothermic, but a subsequent C₆F₅ migration is very favorable. Furthermore, substituted 2,5‐dihydroborole products are derived from cyclization and C₆F₅ migration from the photolysis reaction. In the case of the six‐membered annulation product, a subsequent stereoisomerization reaction takes place and the resultant compound undergoes a P/B FLP 1,2‐addition reaction with a terminal alkyne with rearrangement.     

Mechanistic Study of Silver‐Mediated Furan Formation by Oxidative Coupling

Density functional calculations and experiments have been carried out to unravel the mechanism of a silver‐mediated furan formation by oxidative coupling. Various possible reaction paths were considered and the most favorable channel has been identified on the basis of the calculated solvent‐corrected Gibbs free‐energy profiles. The mechanism represented by this route consists of a radical and a subsequent ionic route. The silver cation has a double role in the mechanism: it is the oxidant in the radical steps and the catalyst for the ionic steps, which is in accordance with the experimental observations. The two most important aspects of the optimal route are the formation of a silver–acetylide, reacting subsequently with the enolate radical, and the aromatic furan‐ring formation in a single step at the latter, ionic segment of the reaction path. Our findings could explain several experimental observations, including the “key‐promoter role” of silver, the preference for ionic cyclization, and the reduced reactivity of internal acetylides.     

Total Synthesis of Talatisamine

Talatisamine ( 1 ) is a member of the C₁₉‐diterpenoid alkaloid family, and exhibits K⁺ channel inhibitory and antiarrhythmic activities. The formidable synthetic challenge that 1 presents is due to its highly oxidized and intricately fused hexacyclic 6/7/5/6/6/5‐membered‐ring structure (ABCDEF‐ring) with 12 contiguous stereocenters. Here we report an efficient synthetic route to 1 by the assembly of two structurally simple fragments, chiral 6/6‐membered AE‐ring 7 and aromatic 6‐membered D‐ring 6 . AE‐ring 7 was constructed from 2‐cyclohexenone ( 8 ) through fusing an N‐ethylpiperidine ring by a double Mannich reaction. After coupling 6 with 7 , an oxidative dearomatization/Diels–Alder reaction sequence generated fused pentacycle 4 b . The newly formed 6/6‐membered ring system was then stereospecifically reorganized into the 7/5‐membered BC‐ring of 3 via a Wagner–Meerwein rearrangement. Finally, Hg(OAc)₂ induced an oxidative aza‐Prins cyclization of 2 , thereby forging the remaining 5‐membered F‐ring. The total synthesis of 1 was thus accomplished by optimizing and orchestrating 33 transformations from 8 .     

Trangmolins A–F with an Unprecedented Structural Plasticity of the Rings A and B: New Insight into Limonoid Biosynthesis

The absolute stereostructures of trangmolins A–F ( 1 – 6 ), limonoids with three new and one known topologies of the rings A and B, were unambiguously determined by NMR spectroscopic investigations, single‐crystal XRD analysis, and quantum‐chemical electronic circular dichroism calculations. Compounds 1 – 3 contain a hexahydro‐1H‐inden‐4‐one motif, compound 4 comprises a hexahydro‐2,6‐methanobenzofuran‐7‐one cage, and compound 5 consists of a hexahydro‐2H‐2,8‐epoxychromene scaffold. The C1?C30 linkage in 1 – 3 and the C3?C30 connection in 4 form two unprecedented types of ring A/B‐fused carbobicyclic cores: viii and ix . The oxidative cleavage of the C2?C3 bond in 5 and heterocyclization in 4 and 5 constitute the unprecedented tricyclic 6/6/5 ring A/B¹/B²‐ and 6/5/6 ring A¹A²/B‐fused topologies, respectively, which are uncovered, for the first time, in the construction of limonoid architectures. The diverse cyclization patterns of 1 – 6 reveal an unparalleled structural plasticity of rings A and B in limonoid biosynthesis.     

Mechanistic insights into triterpene synthesis from quantum mechanical calculations. Detection of systematic errors in B3LYP cyclization energies

Most quantum mechanical studies of triterpene synthesis have been done on small models. We calculated mPW1PW91/6-311+G(2d,p)//B3LYP/6-31G* energies for many C30H51O+ intermediates to establish the first comprehensive energy profiles for the cationic cyclization of oxidosqualene to lanosterol, lupeol, and hopen-3beta-ol. Differences among these 3 profiles were attributed to ring strain, steric effects, and proton affinity. Modest activation energy barriers and the ample exothermicity of most annulations indicated that the cationic intermediates rarely need enzymatic stabilization. The course of reaction is guided by hyperconjugation of the carbocationic 2p orbital with parallel C-C and C-H bonds. Hyperconjugation for cations with a horizontal 2p orbital (in the plane of the ABCD ring system) leads to annulation and ring expansion. If the 2p orbital becomes vertical, hyperconjugation fosters 1,2-methyl and hydride shifts. Transition states leading to rings D and E were bridged cyclopropane/carbonium ions, which allow ring expansion/annulation to bypass formation of undesirable anti-Markovnikov cations. Similar bridged species are also involved in many cation rearrangements. Our calculations revealed systematic errors in DFT cyclization energies. A spectacular example was the B3LYP/6-311+G(2d,p)//B3LYP/6-31G* prediction of endothermicity for the strongly exothermic cyclization of squalene to hopene. DFT cyclization energies for the 6-311+G(2d,p) basis set ranged from reasonable accuracy (mPW1PW91, TPSSh with 25% HF exchange) to underestimation (B3LYP, HCTH, TPSS, O3LYP) or overestimation (MP2, MPW1K, PBE1PBE). Despite minor inaccuracies, B3LYP/6-31G* geometries usually gave credible mPW1PW91 single-point energies. Nevertheless, DFT energies should be used cautiously until broadly reliable methods are established.     

Catalytic Dibenzocyclooctene Synthesis via Cobalt(III)–Carbene Radical and ortho‐Quinodimethane Intermediates

The metalloradical activation of ortho‐benzallylaryl N‐tosyl hydrazones with [Co(TPP)] (TPP=tetraphenylporphyrin) as the catalyst enabled the controlled exploitation of the single‐electron reactivity of the redox non‐innocent carbene intermediate. This method offers a novel route to prepare eight‐membered rings, using base metal catalysis to construct a series of unique dibenzocyclooctenes through selective C_carbene?C_aryl cyclization. The desired eight‐membered‐ring products were obtained in good to excellent yields. A large variety of aromatic substituents are tolerated. The proposed reaction mechanism involves intramolecular hydrogen atom transfer (HAT) to Co^III–carbene radical intermediates followed by dissociation of an ortho‐quinodimethane that undergoes 8π cyclization. The mechanism is supported by DFT calculations, and the presence of radical‐type intermediates was confirmed by trapping experiments.     

Rhodium‐Catalyzed Arylative Cyclization for the Enantioselective Synthesis of (Trifluoromethyl)cyclobutanols

A rhodium‐catalyzed cyclization of 1‐(trifluoromethyl)‐4‐alkyn‐1‐ones with arylboronic acids is described to yield a novel class of small rings: (trifluoromethyl)cyclobutanols bearing an exocyclic double bond. The use of a rhodium/chiral diene complex allowed the reaction to proceed under mild conditions, often with high enantioselectivity. An X‐ray crystal structure was obtained confirming the formation of the four‐membered ring products.     

Rhodium(I)‐Catalyzed Cyclization of Allenynes with a Carbonyl Group through Unusual Insertion of a CO Bond into a Rhodacycle Intermediate

Rhodium(I)‐catalyzed cyclization of allenynes with a tethered carbonyl group was investigated. An unusual insertion of a C?O bond into the C(sp²)–rhodium bond of a rhodacycle intermediate occurs via a highly strained transition state. Direct reductive elimination from the obtained rhodacyle intermediate proceeds to give a tricyclic product containing an 8‐oxabicyclo[3.2.1]octane skeleton, while β‐hydride elimination from the same intermediate gives products that contain fused five‐ and seven‐membered rings in high yields.     

Tandem oxidative dearomatization/intramolecular Diels-Alder reaction for construction of the tricyclic core of palhinine A

A concise construction of the 6/6/5 tricyclic core of Lycopodium alkaloid palhinine A (1) has been accomplished. The developed synthetic strategy featured a tandem oxidative dearomatization/intramolecular Diels-Alder reaction to construct C/D rings and an intramolecular 5-exo-trig radical cyclization to install the B ring of palhinine A (1). The developed approach paves the way for the total synthesis of palhinine A (1).     

Saccharomyces cerevisiae oxidosqualene‐lanosterol cyclase: A chemistry–biology interdisciplinary study of the protein's structure–function–reaction mechanism relationships

The oxidosqualene cyclases (EC 5.4.99‐) constitute a family of enzymes that catalyze diverse cyclization/rearrangement reactions of (3S)‐2,3‐oxidosqualene into a distinct array of sterols and triterpenes. The relationship between the cyclization mechanism and the enzymatic structure is extremely complex and compelling. This review covers the historical achievements of biomimetic studies and current progress in structural biology, molecular genetics, and bioinformatics studies to elucidate the mechanistic and structure–function relationships of the Saccharomyces cerevisiae oxidosqualene‐lanosterol cyclase‐catalyzed cyclization/rearrangement reaction. © 2008 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 8: 302–325; 2008: Published online in Wiley InterScience ( www.interscience.wiley.com ) DOI 10.1002/tcr.20157     

Reaction Mechanism of 3,4‐Dinitrofuroxan Formation from Glyoxime: Dehydrogenation and Cyclization of Oxime

The reaction pathway of the formation of 3,4‐dinitrofuroxan from glyoxime is theoretically investigated under experimental conditions with 25 % nitric acid and dinitrogentetroxide reagents to clarify the mechanism of formation of a furoxan ring by glyoxime. The geometric configurations of minima and transition‐state species are optimized at the (U)B3LYP/6‐311++G** level. The CCSD(T) and CASSCF(10e,8o)/CASSCF(9e,8o) single‐point energy corrections at the same level are performed on top of the optimized geometries. A subsequent dynamic correlation by using NEVPT2/6‐311++G**‐level single‐point energy calculations based on the CASSCF results is also performed to obtain accurate energy values. The formation reaction is analyzed from two processes: glyoxime nitration and 3,4‐dinitroglyoxime (nitration product) oxidative cyclization. Calculation results indicate that the electrophilic substitution of nitronium ions from the protonated HNO₃ and the abstraction of hydrogen ions by HNO₃ molecules are requisites of glyoxime nitration. The formation of a furoxan ring from 3,4‐dinitroglyoxime involves two possible mechanisms: 1) oxydehydrogenation by NO₂ molecules and the subsequent torsion of NO radical groups to form a ring and 2) the alternation of dehydrogenation and cyclization. The intermediates and transition states in both routes exhibit monoradical and diradical characteristics. Singlet and triplet reactions are considered for the diradical species. Results show that the singlet reaction mechanism is more favorable for cyclization than the triplet reaction. The formation of a furoxan ring from oxime is in accordance with the stepwise intermolecular dehydrogenation and intramolecular torsion to the ring.     

Macrocyclization of polymers via ring‐closing metathesis and azide/alkyne‐“click”‐reactions: An approach to cyclic polyisobutylenes

The end‐to‐end cyclization of telechelic polyisobutylenes (PIB's) toward cyclic polyisobutylenes is reported, using either ring‐closing metathesis (RCM) or the azide/alkyne‐“click”‐reaction. The first approach uses bisallyl‐telchelic PIB's (M_n = 1650, 3680, 9770 g mol^?1) and Grubbs 1st‐, 2nd‐, and 3rd‐generation catalyst leading to cyclic PIB's in 60–80% yield, with narrow polydispersities (M_w/M_n = 1.25). Azide/alkyne‐“click”‐reactions of bisalkyne‐telechelic PIB's (M_n = 3840 and 9820 g mol^?1) with excess of 1,11‐diazido‐undecane leads to the formation of mixtures of linear/cyclic PIB's under formation of oligomeric cycles. Subsequent reaction of the residual azide‐moieties in the linear PIB's with excess of alkyne‐telechelic PEO enables the chromatographic removal of the resulting linear PEO‐PIB‐block copolymers by column chromatography. Thus pure cyclic PIB's can be obtained using this double‐“click”‐method, devoid of linear contaminants. © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 671–680, 2010     

A density functional theory investigation on the formation mechanisms of DNA interstrand crosslinks induced by chloroethylnitrosoureas

Chloroethylnitrosoureas (CENUs) are an important family of alkylating agents used in the clinical treatment of cancer. Their anticancer mechanism primarily involves the formation of DNA interstrand crosslinks (ICLs) induced by the chloroethyldiazonium ion derived from the decomposition of CENUs. In this work, the mechanism for the formation of ICLs was investigated by density functional theory (DFT) with B3LYP, wB97XD, and M062X functinoals using conductor‐like polarizable continuum model solvent model. Three pathways leading to the formation of three types of G–C crosslinks were compared. G(N1)–C(N3) crosslink is predicted to be the dominant crosslinking product other than G(O⁶)–C(N⁴) and G(N²)–C(O²) crosslinks, which is consistent with the previous results obtained from QM/MM computations. The results indicate that the formation of the G(N1)–C(N3) crosslink via pathway A is the most favorable mechanism from both kinetic and thermodynamic standpoints. In this pathway, the chloroethyldiazonium ion alkylates guanine on the O⁶ site followed by intramolecular cyclization to form O⁶,N1‐ethanoguanine ( 4 ). The cytosine then reacts with intermediate 4 on the C^α atom to yield the G(N1)–C(N3) crosslink. This work provides reasonable explanations for the supposed mechanism of CENUs‐induced ICLs formation obtained from experimental investigations. © 2012 Wiley Periodicals, Inc.     

Chemodivergent Metathesis of Dienynes Catalyzed by Ruthenium–Indenylidene Complexes: An Experimental and Computational Study

A study on the enyne metathesis reaction leading to the formation cyclic compounds using ruthenium–indenylidene complexes is presented. Several 1,11‐dien‐6‐ynes have been subjected to ruthenium metathesis cyclization by using ruthenium–indenylidene complexes bearing various phosphine and N‐heterocyclic carbene (NHC) ligands. Interestingly, for some substrates chemodivergent metathesis occurs and is a function of the catalyst employed. This led us to investigate the competing “ene‐then‐yne” or “yne‐then‐ene” reaction pathways apparently at play in these systems using both experimental observations and DFT calculations. Experimental and computational studies were found in good agreement and permit to conclude that for phosphine‐containing catalysts, the “ene‐then‐yne” pathway is exclusively adopted. On the other hand, for catalysts bearing NHC ligands, both pathways are possible.