A six-step synthetic approach to marine natural product (+)-aureol

A concise synthetic approach to the marine natural product (+)-aureol has been achieved from readily available starting materials using obviously fewer steps in comparison to the related report in literature (6 steps versus 12 steps from (+)-sclareolide). Key steps of this protocol include a boron trifluoride-catalyzed domino 1,2-H and 1,2-methyl shifts and a nickel(II)-catalyzed cross-coupling reaction between an alkyl iodide and an aryl Grignard reagent.     

Improved synthesis of a functionalized 2,8-ethanonoradamantane derivative

An improved preparation of a functionalized 2,8-ethanonoradamantane derivative, which avoids two protection/deprotection steps while deprotection of a benzyloxymethyl protecting group takes place during an acid-catalyzed dehydration, is described. Altogether the synthesis has been reduced in five steps (from 16 to 11 steps) with an increase in the global yield from 5.8 to 7.2%.     

Total synthesis of lamellarins D, H, and R and ningalin B

A concise total synthesis of lamellarins D (7 steps), H (7 steps), and R (5 steps) and ningalin B (5 steps) is achieved starting from the corresponding aldehydes and amines. The synthesis features three oxidative reactions as key steps in a biomimetic manner, involving an AgOAc-mediated oxidative coupling reaction to construct the pyrrole core, a Pb(OAc)(4)-induced oxidative cyclization to form the lactone, and Kita's oxidation reaction to form the pyrrole-arene C-C bond.     

Syntheses of strychnine, norfluorocurarine, dehydrodesacetylretuline, and valparicine enabled by intramolecular cycloadditions of Zincke aldehydes

A full account of the development of the base-mediated intramolecular Diels-Alder cycloadditions of tryptamine-derived Zincke aldehydes is described. This important complexity-generating transformation provides the tetracyclic core of many indole monoterpene alkaloids in only three steps from commercially available starting materials and played a key role in short syntheses of norfluorocurarine (five steps), dehydrodesacetylretuline (six steps), valparicine (seven steps), and strychnine (six steps). Reasonable mechanistic possibilities for this reaction, a surprisingly facile dimerization of the products, and an unexpected cycloreversion to regenerate Zincke aldehydes under specific conditions are also discussed.     

Short diastereoselective synthesis of the C1-C13 (AB spiroacetal) and C17-C28 fragments (CD spiroacetal) of spongistatin 1 and 2 through double chain-elongation reactions

A unique and practical synthetic sequence for rapid access to polyketides and to further the spiroacetals derived from them, which utilizes a bidirectional Hosomi-Sakurai allylation approach around key allylsilanes in the synthesis of the AB and CD ring systems of spongistatin 1 and 2, is reported. The synthesis of the AB spiroacetal 9 requires 13 steps, with a longest linear sequence of seven steps in an overall yield of 27%. The synthesis of the CD spiroacetal 13 requires 15 steps, with a longest linear sequence of 11 steps in an overall yield of 30%. Both syntheses start from but-3-enol.     

Enantioselective syntheses of colletodiol,colletol, and grahamimycin A

[reaction: see text] The enantioselective synthesis of colletodiol has been achieved in 11 steps from methyl 1,3,5-octatrienoate and 16 total steps from both ethyl sorbate and methyl 1,3,5-octatrienoate. The route relies upon an enantio- and regioselective Sharpless dihydroxylation and a palladium-catalyzed reduction to form a 5-hydroxy-1-enoate and an 7-hydroxy-1,3-dienoate. These esters were further functionalized, coupled, and macrolactonized to provide colletodiol after deprotection. Grahamimycin A and colletol were synthesized in one and two steps, respectively, from colletodiol.     

Expedient total synthesis of pyrrothine natural products and analogs

This paper describes an expedient and straightforward total synthesis of the two pyrrothine natural products holomycin (7 steps, 11% overall) and xenorhabdin I (7 steps, 11% overall) and analogs thereof via a common late-stage intermediate. The pathway proceeds via the pyrrothine hydrochloride intermediate (6 steps, 17% overall) which also gave access to very fast synthesis of analogs as demonstrated by the synthesis of , and (7 steps, 11-12% overall).     

Enantioselective Total Synthesis of the Lignan (+)‐Linoxepin

An enantioselective total synthesis of the natural (+)‐linoxepin ( 1 ) was accomplished in eleven steps from bromovanin ( 24 ). Key steps are a domino carbopalladation/ Mizoroki–Heck reaction with the formation of a pentacyclic system, an asymmetric hydroboration as well as an oxidative lactonization.     

Concise total synthesis of (-)-8-epigrosheimin

A highly efficient route was developed to synthesize (-)-8-epigrosheimin in four steps from aldehyde 2 based on a substrate-controlled method. The key steps of the synthesis included (1) a stereo- and regioselective allylation addition, (2) an intramolecular translactonization, and (3) an aldehyde-ene cyclization.     

Evaluation of the carbene hydride mechanism in the carbon-carbon bond formation process of alkane metathesis through a DFT study

Olefin metathesis on a silica supported tantalumhydridocarbene complex, the key carbon-carbon making process in alkane metathesis, requires a large number of elementary steps in contrast to the known olefin metathesis pathway, which corresponds to successive [2 + 2]-cycloaddition and cycloreversion steps. The direct pathway is forbidden because it requires the formation of a high energy reaction intermediates, an olefin adduct of trigonal bipyramid (TBP) geometry, where the carbene is trans to an hydride ligand. Extra low-energy steps are therefore necessary to connect the reactants to products, the key being a turnstile interconversion at the metallacyclobutane intermediates.     

An approach to the skeleton of the securinega alkaloids. The total synthesis of (+/-)-securinine

[structure: see text]. The concise total synthesis of securinine in nine steps from readily available starting materials is described. Key steps of the synthesis include an addition of a silyloxyfuran to an in situ generated iminium ion and a novel ring closing metathesis reaction.     

2-氰基-2,3,3-三甲基-5-硫代-吡咯烷的合成新法

以3,3-二甲基-4-戊烯酸甲酯为原料, 经过6步反应合成了目标产物2-氰基-2,3,3-三甲基-5-硫酮-吡咯烷, 反应的总收率约为37%, 其中关键步骤为分子内环化和碘化氢消除反应. 通过元素分析, ¹H NMR, IR和MS对所合成化合物的结构进行了表征.     

A Unified Approach for the Assembly of Atisine‐ and Hetidine‐type Diterpenoid Alkaloids: Total Syntheses of Azitine and the Proposed Structure of Navirine C

A tetracyclic dinitrile was synthesized in twelve steps from cyclohex‐2‐en‐1‐one by using a chelation‐triggered conjugate addition to a γ‐hydroxy‐substituted α,β‐unsaturated nitrile and an oxidative dearomatization/Diels–Alder cycloaddition cascade as the key steps. The first total synthesis of azitine (in 17 steps) was achieved through a simple reductive cyclization of this intermediate and subsequent transformations while the total synthesis of the proposed structure of navirine C (in 19 steps) was accomplished by a hydrogen‐atom‐transfer reaction of the tetracyclic dinitrile, Pd/C‐catalyzed reductive cyclization, and subsequent functional group manipulation.     

Total synthesis of cis-solamin: exploiting the RuO4-catalyzed oxidative cyclization of dienes

[structure: see text] An enantioselective total synthesis of cis-solamin has been accomplished using a highly diastereoselective ruthenium tetroxide catalyzed oxidative cyclization as a crucial transformation. Further key steps involved an enzymatic desymmetrization, a TPAP-catalyzed oxidative termini differentiation, and a ruthenium-catalyzed Alder-ene reaction. Thus, the total synthesis of cis-solamin was achieved in 11 steps with an overall yield of 7.5%.     

Asymmetric total synthesis of (-)-saframycin A from L-tyrosine

The asymmetric total synthesis of (-)-saframycin A, a natural antitumor product of the tetrahydroisoquinoline antitumor antibiotics family, has been accomplished by employing L-tyrosine as the starting chiral building block in 24 steps for the longest linear sequence in an overall yield of 9.7%. The key steps in the synthesis involve stereoselective intermolecular and intramolecular Pictet-Spengler reactions, which induced the correct stereochemistry at C-1 and C-11, respectively. The selective protection-deprotection protocol of an amino group in the two-step transformation from intermediate 10 to 12 and a hydroxyl group in the first two steps resulted in both high selectivity and efficiency of the synthetic route.     

A Concise Total Synthesis of the Fungal Isoquinoline Alkaloid TMC-120B

Recent reports of antiepileptic activity of the fungal alkaloid TMC-120B have renewed the interest in this natural product. Previous total syntheses of TMC-120B comprise many steps and have low overall yields (11–17 steps, 1.5–2.9% yield). Thus, to access this compound more efficiently, we herein present a concise and significantly improved total synthesis of the natural product. Our short synthesis relies on two key cyclization steps to assemble the central scaffold: isoquinoline formation via an ethynyl-imino cyclization and an intramolecular Friedel-Crafts reaction to form the furanone.     

Stereoselective approach to the DEF ring system of terpendole E

Diastereoselective construction of a DEF ring model of terpendole E, a M phase-specific antimitotic natural product, has been achieved from a known bicyclic keto ester by a 10-step (or 12-step) sequence that features an efficient one-pot intramolecular etherification of an olefinic alcohol with mCPBA. The synthesis proceeded in an excellent overall yield of 36% through 10 steps or in 56% via 12 steps which involved a highly diastereoselective reduction of a ketone derived from an undesired diastereomeric alcohol intermediate.     

C5-methylation of cytosine in B-DNA thermodynamically and kinetically stabilizes BI

We have examined the backbone dynamics of two alternating purine-pyrimidine dodecamers. One sequence consists of "pure" GC bases; the other one contains 5-methylcytosines. The effect of the methyl groups on the backbone substates BI/BII was investigated by means of molecular dynamics. The methylation influences, on one hand, the transition barrier between BI and BII and, on the other hand, the state of equilibrium. The kinetic consequences are an increase of the DeltaG of Gp5mC steps by 1.5 kcal/mol and a decrease of the DeltaG of 5mCpG steps by 0.8 kcal/mol (compared with the nonmethylated DNA). Thus, the additive group differentiates between the two occurring dinucleotide steps and renders the phosphate of the 5-methylcytosine more rigid, as proposed by experimental studies. The thermodynamic consequences are an increase of the DeltaG of Gp5mC steps by 1.1 kcal/mol and a decrease of the DeltaG of 5mCpG steps by 0.8 kcal/mol. The reason for this shift in equilibrium is still not completely clear on a molecular basis. But we can conclude that the indirect readout of DNA is influenced by methylation.     

Enantioselective allyltitanations and metathesis reactions. Application to the synthesis of piperidine alkaloids (+)-sedamine and (-)-prosophylline

An enantioselective synthesis of the piperidine alkaloids (+)-sedamine and (-)-prosophylline is reported. The synthesis of (+)-sedamine has been achieved in 12 steps with an overall yield of 20% from benzaldehyde, and (-)-prosophylline was obtained in 15 steps with an overall yield of 9.2%, starting from D-glyceraldehyde acetonide 14. The key steps are enantioselective allyltitanation reactions and ring-closing or cross-metathesis reactions.     

Comparison of time constants for liquid ion-exchange membrane electrode responses determined by an impedance method and an activity step method

Linear current-step perturbations and resulting membrane potential responses of liquid ion-exchange membranes have been analyzed and the numerous possible time constants examined and catalogued. Ideal activity steps at a single membrane interface produce potential—time responses similar to those resulting from current steps. The common time constants have nearly identical values. However, fewer time constants are observed for activity steps than for current steps. Slow processes at the active interface dominate and obscure short time processes. For liquid ion-exchange membranes it is concluded that an activity step on one side is not identical to a current step because processes involving bulk membrane transport and the far side of the membrane are not involved to the same extent for the two excitation modes. Two analog circuit models (one active, one passive) are proposed for the potential—time response to an activity step. Systems containing neutral carriers are not considered.