Enantioselective Total Synthesis of (+)‐Steenkrotin A and Determination of Its Absolute Configuration

The first enantioselective total synthesis of (+)‐steenkrotin A has been achieved in 18 steps and 4.2 % overall yield. The key features of the strategy entail a Rh‐catalyzed O?H bond insertion followed by an intramolecular carbonyl‐ene reaction, two sequential SmI₂‐mediated Ueno–Stork and ketyl–olefin cyclizations, and a cascade intramolecular aldol condensation/vinylogous retro‐aldol/aldol process with inversion of the relative configuration at the C7 position. The absolute configuration of (+)‐steenkrotin A was determined based on the stepwise construction of the stereocenters during the total synthesis.     

Total Synthesis of (−)‐Glaucocalyxin A

A practically useful method for the formation of the highly oxygenated bicyclo[3.2.1]octane ring system through Mn(OAc)₃‐mediated radical cyclization of alkynyl ketones was developed, which opens up a new avenue for the total synthesis of a number of highly oxidized diterpenoids. Application of this method enabled the first total synthesis of (?)‐glaucocalyxin A. Other salient features of the synthesis include a highly enantioselective conjugate addition/acylation cascade reaction, a Yamamoto aldol reaction, and an intramolecular Diels–Alder reaction to assemble the A/B ring system.     

Stereoselective Total Synthesis of Hetisine‐type C20‐Diterpenoid Alkaloids: Spirasine IV and XI

The first total synthesis of the architecturally complex hetisine‐type heptacyclic C₂₀‐diterpenoid alkaloids (±)‐spirasine IV and XI is reported. The A/F/G/C tetracyclic skeleton with the challenging N?C6 and C14?C20 linkages was efficiently constructed by an intramolecular azomethine‐ylide‐based 1,3‐dipolar cycloaddition with unusual regioselectivity. SmI₂‐mediated free‐radical addition to the arene moiety without prior dearomatization and a stereoselective intramolecular aldol reaction further enabled rapid access to the hetisine core, providing a bicyclo[2.2.2]octane ring with a new oxygen substitution pattern.     

Total Synthesis of Gelsedilam by Means of a Thiol‐Mediated Diastereoselective Conjugate Addition–Aldol Reaction

The total synthesis of gelsedilam, which features a highly diastereoselective thiol conjugate addition–intramolecular aldol reaction to install the strained and caged [3.2.2] bridged ring system and highly efficient NiCl₂/NaBH₄‐mediated four‐step transformation in one‐pot to construct its five‐membered lactam ring is reported. The synthesis requires only 18 linear steps from the known compounds, providing useful strategies for the construction of the intricate ring system in the synthesis of related gelsedine‐type alkaloids.     

Total Synthesis of (−)-Glaucocalyxin A

A practically useful method for the formation of the highly oxygenated bicyclo[3.2.1]octane ring system through Mn(OAc)₃-mediated radical cyclization of alkynyl ketones was developed, which opens up a new avenue for the total synthesis of a number of highly oxidized diterpenoids. Application of this method enabled the first total synthesis of (−)-glaucocalyxin A. Other salient features of the synthesis include a highly enantioselective conjugate addition/acylation cascade reaction, a Yamamoto aldol reaction, and an intramolecular Diels–Alder reaction to assemble the A/B ring system.     

Total Synthesis of Leucosceptroids A and B

Leucosceptroids A and B are sesterterpenoids with potent antifeedant and antifungal activities. A more efficient gram‐scale total synthesis of leucosceptroid B and the first total synthesis of leucosceptroid A are presented. The key transformations include an aldol reaction between a substituted dihydrofuranone and an (S)‐citronellal‐derived aldehyde, a SmI₂‐mediated intramolecular ketyl–olefin radical cyclization, and final‐stage alcohol oxidation.     

Synthesis of (±)‐Tetrapetalone A‐Me Aglycon

The first synthesis of (±)‐tetrapetalone A‐Me aglycon is described. Key bond‐forming reactions include Nazarov cyclization, a ring‐closing metathesis promoted with complete diastereoselectivity by a chiral molybdenum‐based complex, tandem conjugate reduction/intramolecular aldol cyclization, and oxidative dearomatization.     

Catalyst‐Free One‐Pot Synthesis of 2,4,6‐Triaryl‐1,4‐dihydropyridines in Ionic Liquid and Their Catalyzed Activity on Two Simple Diels–Alder Reactions

A series of 2,4,6‐triaryl‐1,4‐dihydropyridines bearing a hydroxyl group was synthesized greenly by the cascade aldol/Michael/addition reaction of aromatic aldehyde, acetophenone, and NH₄OAc (1:2:excessive) in ionic liquid [BmIm][BF₄] without any catalyst. The results of their catalyzed activity on two simple Diels–Alder reactions indicated that this kind of compound containing a poly aryl ring can be used as an effective catalyst on the Diels–Alder reaction. This method, because of its environmental friendliness, simplicity, mild conditions, effectiveness, and lower costs, is suitable for the synthesis of arrays of compounds.     

A Hydrolase‐Catalyzed Cyclization Forms the Fused Bicyclic β‐Lactone in Vibralactone

Vibralactone is isolated from the basidiomycete fungus Boreostereum vibrans as one of the strongest lipase inhibitors. Its unusual β‐lactone‐fused bicycle is derived from an aryl ring moiety by an oxidative ring‐expansion prior to an intramolecular cyclization. Herein, we report the discovery of the cyclase VibC which belongs to the α/β‐hydrolase superfamily and is involved in the vibralactone biosynthesis. Biochemical and crystal studies suggest that VibC may catalyze an aldol or an electrocyclic reaction initiated by the Ser‐His‐Asp catalytic triad. For the aldol and pericyclic chemistry in living cells, VibC is a unique hydrolase performing the carbocycle formation of an oxepinone to a fused bicyclic β‐lactone. This presents a naturally occurring, new enzymatic reaction in both aldol and hydrolase (bio)chemistry that will guide future exploitation of these enzymes in synthetic biology for chemical‐diversity expansion of natural products.     

Total Synthesis of (−)‐Daphenylline

A concise and highly stereoselective total synthesis of the Daphniphyllum alkaloids (?)‐daphenylline has been accomplished. The synthesis was started from (S)‐carvone and proceeded via a stereoselective Mg(ClO₄)₂‐catalyzed intramolecular amide addition cyclization, an intramolecular Diels–Alder reaction to construct the ABCD tetracyclic core architecture, and a Robinson annulation coupled with an oxidative aromatization sequence. Finally, the DF ring system was installed through an intramolecular Friedel–Crafts cyclization. The total synthesis of (?)‐daphenylline is achieved in 19 steps in the longest reaction sequence and in 7.6 % overall yield.     

Formal Synthesis of (−)-Englerin A and Cytotoxicity Studies of Truncated Englerins

An efficient formal synthesis of (−)-englerin A ( 1 ) is reported. The target molecule is a recently isolated guaiane sesquiterpene that possesses highly potent and selective activity against renal cancer cell-lines. Our enantioselective strategy involved the construction of the BC ring system of compound 1 through a Rh^II-catalyzed [4+3] cycloaddition reaction followed by subsequent attachment of the A ring through an intramolecular aldol condensation reaction. As such, this strategy allows the synthesis of truncated englerins. Evaluation of these analogues with the A498 renal cancer cell-line suggested that the A ring of englerin is crucial to its antiproliferative activity. Moreover, evaluation of these analogues led to the identification of potent growth-inhibitors of CEM cells with GI₅₀ values in the range 1–3 μM .     

Cyclobutane ring opening of allene-enone photoadducts by mercury (II) perchlorate

Cyclobutane ring opening of some allene-enone photoadducts by Hg(ClO₄)₂ in acetone gave diketones which led, in some cases, to bicyclic ketols through intramolecular aldol condensation.     

Total Synthesis of (+)‐MPC1001B

The first total synthesis of an epidithiodiketopiperazine alkaloid, (+)‐MPC1001B, was accomplished. This synthesis features a tetra‐n‐butylammonium fluoride mediated intramolecular aldol reaction for forming the 15‐membered macrolactone ring, and the construction of an epidithiodiketopiperazine substructure through a stepwise sulfenylation reaction involving a novel trityl trisulfide (TrSSS)‐group transfer.     

Asymmetric Synthesis of the Ring A Substructure of Genkwadane A

A practical approach to the asymmetric synthesis of the ring A substructure of genkwadane A, a daphnane diterpenoid, was developed. This synthesis features the Mukaiyama aldol reaction to introduce the quaternary stereogenic center at C4 and VO(acac)₂‐catalyzed Jackson‐Ellman‐Bolm sulfoxidation to deliver the corresponding sulfoxide. The Dieckmann‐type condensation was efficiently promoted by KHMDS and the ring A substructure was finally accomplished through Barton iodination condition.     

Hydrogen‐Bonding Interactions and Properties of Energetic Nitroamino[1,3,5]triazine‐Based Guanidinium Salts: DFT‐D and QTAIM Studies

The intramolecular hydrogen‐bonding interactions and properties of a series of nitroamino[1,3,5]triazine‐based guanidinium salts were studied by using the dispersion‐corrected density functional theory method (DFT‐D). Results show that there are evident LP(N or O; LP=lone pair)→σ*(N? H) orbital interactions related to O???H? N or N???H? N hydrogen bonds. Quantum theory of atoms in molecules (QTAIM) was applied to characterize the intramolecular hydrogen bonds. For the guanidinium salts studied, the intramolecular hydrogen bonds are associated with a seven‐ or eight‐membered pseudo‐ring. The guanylurea cation is more helpful for improving the thermal stabilities of the ionic salts than other guanidinium cations. The contributions of different substituents on the triazine ring to the thermal stability increase in the order of ? NO₂23 (? ONO₂)2. Energy decomposition analysis shows that the salts are stable owing to electrostatic and orbital interactions between the ions, whereas the dispersion energy has very small contributions. Moreover, the salts exhibit relatively high densities in the range of 1.62–1.89 g cm^?3. The detonation velocities and pressures lie in the range of 6.49–8.85 km s^?1 and 17.79–35.59 GPa, respectively, which makes most of them promising explosives.     

Stereoselective total synthesis of (-)-spirofungin A by utilising hydrogen-bond controlled spiroketalisation

The stereoselective total synthesis of the spiroketal containing Streptomyces metabolite (?)‐spirofungin A ( 1 ) is described. A key step involved a spiroketalisation controlled by an intramolecular H‐bond which favoured the desired spiroketal 4 (13:1 ratio). The presence of the intramolecular H‐bond in 4 is possibly due to a 1,5‐alkyne–oxygen interaction. Other key steps include an efficient cross‐metathesis to form the spiroketal precursor, a tin mediated syn‐aldol reaction and a Stille cross‐coupling reaction to create the C22? C23 bond. A final Wittig extension followed by deprotection gave (?)‐spirofungin A ( 1 ).     

Asymmetric Total Synthesis of (−)‐Kravanhin B

The first asymmetric total synthesis of kravanhin B has been accomplished with a linear reaction sequence of 13 steps starting from (R)‐(?)‐carvone. The synthesis features an intramolecular aldol cyclization to construct the desired cis‐fused decalin skeleton and an acid‐catalyzed dehydration and olefin isomerization to install the γ‐butenolide ring.     

Isotope Probing of the UDP‐Apiose/UDP‐Xylose Synthase Reaction: Evidence of a Mechanism via a Coupled Oxidation and Aldol Cleavage

The C‐branched sugar d ‐apiose (Api) is essential for plant cell‐wall development. An enzyme‐catalyzed decarboxylation/pyranoside ring‐contraction reaction leads from UDP‐α‐d ‐glucuronic acid (UDP‐GlcA) to the Api precursor UDP‐α‐d ‐apiose (UDP‐Api). We examined the mechanism of UDP‐Api/UDP‐α‐d ‐xylose synthase (UAXS) with site‐selectively ²H‐labeled and deoxygenated substrates. The analogue UDP‐2‐deoxy‐GlcA, which prevents C‐2/C‐3 aldol cleavage as the plausible initiating step of pyranoside‐to‐furanoside conversion, did not give the corresponding Api product. Kinetic isotope effects (KIEs) support an UAXS mechanism in which substrate oxidation by enzyme‐NAD⁺ and retro‐aldol sugar ring‐opening occur coupled in a single rate‐limiting step leading to decarboxylation. Rearrangement and ring‐contracting aldol addition in an open‐chain intermediate then give the UDP‐Api aldehyde, which is intercepted via reduction by enzyme‐NADH.     

Synthesis and Evaluation of Novel Analogues of Ripostatins

Ripostatins are polyene macrolactones isolated from the myxobacterium Sorangium cellulosum. They exhibit antibiotic activity by inhibiting bacterial RNA polymerase (RNAP) through a binding site and mechanism that are different from those of current antibacterial drugs. Thus, the ripostatins serve as starting points for the development of new anti‐infective agents with a novel mode of action. In this work, several derivatives of ripostatins were produced. 15‐Desoxyripostatin A was synthesized by using a one‐pot carboalumination/cross‐coupling. 5,6‐Dihydroripostatin A was constructed by utilizing an intramolecular Suzuki cross‐coupling macrolactonization approach. 14,14′‐Difluororipostatin A and both epimeric 14,14′‐difluororipostatins B were synthesized by using a Reformatsky type aldol addition of a haloketone, Stille cross‐coupling, and ring‐closing metathesis. The RNAP‐inhibitory and antibacterial activities are presented. Structure–activity relationships indicate that the monocyclic keto‐ol form of ripostatin A is the active form of ripostatin A, that the ripostatin C5–C6 unsaturation is important for activity, and that C14 geminal difluorination of ripostatin B results in no loss of activity.     

Total synthesis of (+)-quassin

A total synthesis of (+)-quassin from naturally occurring (S)-(+)-carvone is described. The total number of steps was 28, and the overall yield was about 2.6%. The synthetic strategy for the construction of the tetracyclic carbon framework was based on a C-->ABC-->ABCD ring annulation sequence, involving an aldol reaction, an intramolecular Diels-Alder reaction, and an intramolecular acylation as the key steps. Subsequent functionalization of ring A and ring C then afforded the target (+)-quassin.