Reversing the stereoselectivity of the intermolecular Pauson-Khand reaction: formation of endo-fused norbornadiene adducts

[reaction: see text] An unprecedented endo-selective and regioselective intermolecular Pauson-Khand reaction takes place when heterobimetallic (Mo-Co) complexes derived from N-(2-alkynoyl)oxazolidinones or sultams are heated in the presence of norbornadiene.     

Enantioselective and diastereoselective syntheses of cyanohydrin carbonates

A new and general synthesis of alkyl cyanoformates is presented starting from the appropriate alcohol and oxalyl chloride. This is used to prepare enantiomerically pure cyanoformates from enantiomerically pure primary and secondary alcohols. Optimal conditions for the addition of various achiral cyanoformates to aldehydes catalysed by an enantiomerically pure titanium(salen) catalyst in the presence of potassium cyanide as a cocatalyst are developed. Under these conditions, two chiral cyanoformates also reacted with aldehydes to give cyanohydrin carbonates. The stereochemistry of this process is predominantly determined by the stereochemistry of the titanium(salen) catalyst and the stereochemistry of two of the cyanohydrin carbonates was confirmed by X-ray crystallography. In a further extension of the chemistry, a homogeneous system in which the potassium cyanide/18-crown-6 complex is used as the cyanide cocatalyst has been developed and the kinetics of this reaction show that it displays first order kinetics, provided at least 2 mol % of the potassium cyanide complex are employed.     

Enantioselective syntheses of tremulenediol A and tremulenolide A

A concise entry to the skeleton of the tremulane sesquiterpenes is described that culminated in the first enantioselective syntheses of tremulenediol A and tremulenolide A. The approach features a series of efficient transition metal-catalyzed reactions commencing with an enantioselective rhodium(II)-catalyzed intramolecular cyclopropanation followed by a regioselective allylic alkylation and a diastereoselective rhodium(I)-catalyzed [5+2] cycloaddition.     

Enantioselective syntheses of tremulenediol A and tremulenolide A

[reaction: see text] An enantioselective entry to the skeleton of the tremulane sesquiterpenes is described. The approach features a series of efficient transition metal-catalyzed reactions commencing with an enantioselective rhodium(II)-catalyzed intramolecular cyclopropanation followed by a regioselective allylic alkylation and a diastereoselective rhodium(I)-catalyzed [5 + 2] cycloaddition. This strategy was applied to the first enantioselective syntheses of tremulenediol A and tremulenolide A.     

Enantioselective total syntheses of ropivacaine and its analogues

An alternative asymmetric synthesis of ropivacaine and analogues employing the ‘cation pool’ strategy and host/guest supramolecular co-catalysis approach is presented. In this study, chiral auxiliaries, several soft nucleophiles as well as one-pot conditions for anodic oxidation, followed by nucleophilic addition, have been applied.     

Enantioselective total syntheses of trichodermamides A and B

Starting from commercially available (-)-quinic acid, the enantioselective total syntheses of trichodermamides A and B were achieved. The key reactions involve the stereoselective construction of the oxazine ring via an intramolecular epoxide ring opening reaction and an EDCI-assisted peptide coupling reaction.     

Enantioselective syntheses of 2,5-disubstituted pyrrolidines based on iridium-catalyzed allylic aminations--total syntheses of alkaloids from amphibian skins

A broadly applicable route to trans‐2,5‐disubstituted pyrrolidines has been developed. Key steps are an asymmetric iridium‐catalyzed allylic amination, a Suzuki–Miyaura coupling, and an intramolecular aza‐Michael addition. Enantiomeric excesses in the range of 93–99 % ee have been achieved. Total syntheses of the alkaloids (?)‐ 225 C , (+)‐ and (?)‐ 223 H (xenovenine), (+)‐ 223 AB , (+)‐ 195 B , and (+)‐ 223 R have been carried out as applications.     

Catalytic syntheses of polycyclic compounds based on norbornadiene in the presence of nickel catalysts

The cycloadditions of olefins containing an electron-withdrawing substituent to norbornadiene are reported. The influence of the substituent and solvent nature on the [2+2+2]-cycloadduct yield and stereoselectivity is elucidated. The competitive cycloaddition of pairs of olefins to norbornadiene is discussed. Hypotheses about the structure of key intermediates of the process are considered. The consistent generalized mechanism for the cycloaddition of unsaturated compounds of various classes to norbornadiene is suggested.     

Enantioselective syntheses of monotetrahydrofuran Annonaceous acetogenins tonkinecin and annonacin starting from carbohydrates.

The total synthesis of two mono-THF acetogenins, tonkinecin (1) and annonacin (2), is reported in full detail. Terminal acetylene 3 prepared from D-glucono-delta-lactone and asymmetric dihydroxylation was employed as a common intermediate for both targets 1 and 2. Pd(0)-catalyzed coupling reaction of 3 with vinyl iodides 4 and 5, the chiral centers of which were taken from D-xylose and S-(-)-ethyl lactate, afforded enyne 26 and 27, respectively. Selective hydrogenation of 26 or 27 with diimide followed by removal of MOM ethers completed the synthesis of 1. A coupling reaction between the lithium derivative of 3 and epoxide 6 in the presence of boron trifluoride etherate gave 42. Both chiral centers in epoxide 6 were taken from L-ascorbic acid. Subsequent catalytic hydrogenation and MOM protection led to 43b. Introduction of the butenolide moiety by aldol condensation of protected S-lactal followed by cleavage of all MOM ethers completed the synthesis of 2.     

Enantioselective biomimetic total syntheses of katsumadain and katsumadain C

Enantioselective total syntheses of katsumadain and katsumadain C were achieved concisely through a biomimetic approach. Assembly of styryl-2-pyranone (3) and monoterpene 6 via acid-promoted regio- and stereoselective C-C bond formation afforded katsumadain (2), which underwent the photoinduced [2 + 2] dimerization in a head-to-tail mode to furnish katsumadain C (1).     

Enantioselective total syntheses of slagenins A-C and their antipodes

Full details of the total syntheses of slagenins A-C (1a-c) and their antipodes (2a-c), novel bromopyrrole alkaloids with a unique tetrahydrofuro[2,3-d]imidazolidin-2-one moiety, are described in which their absolute stereochemistry was established. The key step in the syntheses involves the efficient condensation of dihydrofuran-3-one or glyoxal with urea to construct the slagenin bicycle core.     

Enantioselective syntheses of (+)-xylarenal A and ent-xylarenal A

[reaction: see text] The total synthesis of the sesquiterpenoid xylarenal A is reported. This first synthetic entry to an eremophilane terpenoid with an exocyclic vinyl aldehyde unit involves the use of the bicyclic enone (+)-3, which after a gamma-oxidation and alpha'-allylation leads to the formation of the ketone (+)-8. After its acylation, an oxidative cleavage of the allyl side chain followed by alpha-methylenation of the resulting aldehyde gives (+)-xylarenal A (1). The synthesis of (-)-xylarenal A from (-)-3 is also reported. Moreover, the first total synthesis of the trinoreremophilane (+)-1alpha-hydroxyisoondetianone (5) is described.     

Enantioselective total syntheses of manzamine a and related alkaloids

As a prelude to undertaking the total syntheses of the complex manzamine alkaloids, a series of model studies were conducted to establish the scope and limitations of intramolecular [4 + 2] cycloadditions of N-acylated vinylogous ureas with the trienic substrates 17a,b, 28a,b, and 34. These experiments clearly demonstrated that the geometry of the internal double bond and the presence of an electron-withdrawing group on the diene moiety were essential for the facile and stereoselective formation of the desired cycloadducts. The enantioselective syntheses of the manzamine alkaloids ircinol A (75), ircinal A (5), and manzamine A (1) were then completed by employing a convergent strategy that featured a novel domino Stille/Diels-Alder reaction to construct the tricyclic ABC ring core embodied in these alkaloids. Thus, the readily accessible chiral dihydropyrrole 58 was first converted in a single chemical operation into the key tricyclic intermediate 60. Two ring-closing metathesis reactions were then used to form the 13- and 8-membered rings leading to Z-72 and 74, the latter of which was quickly elaborated into ircinal A (5) via ircinol A (75). The synthetic 5 thus obtained was converted into manzamine A (1) following literature precedent. This concise synthesis of ircinal A required a total of 24 operations from commercially available starting materials with the longest linear sequence being 21 steps.