Synthesis of Functionalized Bicyclo[3.2.1]octanes and Their Multiple Uses in Organic Chemistry

Efficient synthesis and subsequent transformations of phenylsulfanylbicyclo[2.2.2]octenones and phenylselenylbicyclo[2.2.2]octenones

Inverse-electron-demand Diels-Alder reactions of masked o-benzoquinones 2 with phenyl vinyl sulfide and phenyl vinyl selenide furnished highly functionalized bicyclo[2.2.2]octenone derivatives 3 and 4, respectively, in excellent regio- and stereoselectivities and yields up to 90%. The bicyclo[2.2.2]octenone derivatives 3 with the sulfur functionality were subjected to an oxidation-elimination process to furnish bicyclo[2.2.2]octadienone systems 7 in good yields. During the reduction process, the Diels-Alder adducts 3e and 4e led to 8, whereas the carbon-centered radicals generated from the other adducts 3a-d and 4a-d provided various rearranged products 9-13 depending on the substitution pattern and reagents utilized (Raney-Ni or n-Bu(3)SnH). Surprisingly these radicals showed preference for the carbonyl functionality to the olefinic double bond, leading to interesting rearrangement reactions of mechanistic importance and possible synthetic utility. Interestingly the alcohols obtained from the reduction of Diels-Alder adducts 3a-d underwent desulfurization smoothly to give desulfurized products in high yields; thus a detoured method of "reduction-desulfurization-oxidation" provides an entry to desulfurized bicyclo[2.2.2]octenones without rearrangement.

Aza- and Carbo-[3 + 3] Annulations of Exo-Cyclic Vinylogous Amides and Urethanes. Synthesis of Tetrahydroindolizidines and An Unexpected Formation of Hexahydroquinolines

[3 + 3] Annulations of exo-cyclic vinylogous amides and urethanes with vinyl iminium salts are described here. We observed an intriguing dichotomy in their reaction pathways. For pyrrolidine- and azepane-based vinylogous amides or urethanes, aza-[3 + 3] annulation would dominate to give tetrahydroindolizidines, whereas, unexpectedly, for piperidine-based vinylogous amides or urethanes, carbo-[3 + 3] annulation was the pathway, leading to hexahydroquinolines. The origin for such a contrast is likely associated with a switch in the initial reaction pathway between C-1,2-addition and C-1,4-addition.

Organocatalytic Asymmetric 1,6-Additions of β-Ketoesters and Glycine Imine

The first organocatalytic enantioselective 1,6-addition of beta-ketoesters and benzophenone imine to electron-poor delta-unsubstituted dienes using cinchona alkaloids under phase-transfer conditions is demonstrated. The scope of the reaction for the beta-ketoesters is outlined for reactions with different delta-unsubstituted dienes having ketones, esters, and sulfones as electron-withdrawing substituents giving the corresponding optically active products in good yields and enantioselectivities in the range of 90-99% ee. The 1,6-addition also proceeds with a number of cyclic beta-ketoesters having different ring sizes, ring systems and substituents in high yields and enantioselectivities. The potential of this new organocatalytic 1,6-addition for beta-ketoesters is demonstrated by a two-step synthesis of the bicyclo[3.2.1]octan-8-one structure, a bicyclic bridged skeleton occurring in a variety of natural compounds. Benzophenone imines also undergo the organocatalytic asymmetric 1,6-addition to the activated dienes in high yields and with enantioselectivities from 92% to 98% ee, except in one case. The synthetic utility of this asymmetric reaction is demonstrated by the two-step transformation of the allylated alpha-amino acid derivative to highly attractive optically active pyrrolidines.

5-Aminooxazole as an Internal Traceless Activator of C-Terminal Carboxylic Acid: Rapid Access to Diversely Functionalized Cyclodepsipeptides

A conceptually novel macrolactonization protocol has been developed. It is a domino process involving a sequence of: 1) protonation of 5-aminooxazole leading to the electrophilic iminium salt; 2) trapping of the iminium species by the neighboring C-terminal carboxylic acid leading to a putative spirolactone; and 3) intramolecular nucleophilic addition of the tethered alcohol to the spirolactone followed by fragmentation. The strategically incorporated 5-aminooxazole serves as an internal traceless activator of the neighboring C-terminal carboxylic acid, since it became an integral part of the peptide backbone after cyclization. No coupling reagent is required and the entire sequence is triggered by just a few equivalents of trifluoroacetic acid under very mild conditions (MeCN as the solvent at room temperature). The spirolactone as an activated form of the carboxylic acid has been evidenced by a sulfur-migration experiment. By combining with a three-component synthesis of 5-aminooxazole, a two-step synthesis of structurally complex cyclodepsipeptides from readily accessible starting materials was developed.

Expedient Synthesis of Potent Cannabinoid Receptor Agonist (−)-CP55,940

[reaction: see text] A stereocontrolled synthesis of (-)-CP55,940, a potent cannabinoid receptor agonist, has been attained using a novel aldolization/retro-aldolization interconversion strategy, in which a temporarily generated chiral aldol motif plays essential roles.

Tetronamides as latent acyclic vinylogous amides in formal aza-[3 + 3] cycloaddition reactions with alpha,beta-unsaturated iminium salts. An unexpected rearrangement and an approach to synthesis of substituted piperidines

A detailed account regarding formal aza-[3 + 3] cycloaddition reactions of tetronamides with alpha,beta-unsaturated iminium salts is described here. This investigation uncovers regioisomeric cycloadducts that were not found in previous studies involving this formal cycloaddition and an unexpected rearrangement that led to pyridines and dihydropyridines. Both stereochemical and regiochemical issues raised in this study provide further mechanistic insights into this cycloaddition. With careful control of reaction temperatures, the desired formal cycloadducts are obtained. Ensuing transformation of these cycloadducts into functionalized piperidines establishes the concept of employing tetronamides as latent acyclic vinylogous amides for the formal aza-[3 + 3] cycloaddition.

A Formal [3 + 3] Cycloaddition Reaction. 5. An Enantioselective Intramolecular Formal Aza-[3 + 3] Cycloaddition Reaction Promoted by Chiral Amine Salts

A detailed account on chiral secondary amine salt promoted enantioselective intramolecular formal aza-[3 + 3] cycloadditions is described here for the first time. The dependence of enantioselectivity on the structural feature of these chiral amines is thoroughly investigated. This study also reveals a very interesting reversal of the stereochemistry in the respective cycloadducts obtained using C(1)- and C(2)-symmetric amine salts. In addition, the influence of solvents, counteranions, and temperatures on the enantioselectivity is described, and a unified mechanistic model based on experimental results as well as semiempirical calculations is proposed.

Syntheses of Seven-Membered Rings: Ruthenium-Catalyzed Intramolecular [5+2] Cycloadditions

The Ru-catalyzed intramolecular [5+2] cycloaddition of cyclopropylenynes is investigated with respect to the regio- and diastereoselectivity as well as the functional group compatibility of the reaction. Evidence for the mechanism as occurring through a ruthenacyclopentene intermediate is elucidated from 1) the study of the diastereoselectivity of the cycloaddition; 2) the effect of variation of substituents on the regioselectivity of cyclopropyl bond cleavage in 1,2-trans- and 1,2-cis-disubstituted cyclopropanes and 3) examples that clearly do not involve ruthenacyclohexene as intermediates as products still incorporate the cyclopropyl moiety. The scope and limitations of the Ru-catalyzed cycloaddition are discussed and compared with the Rh-catalyzed reaction. The potential power of this methodology towards natural product total synthesis is demonstrated by the formation of several polycyclic systems with the chosen reaction conditions and readily available cyclopropylenyne substrates.

A Modular, One-Pot, Four-Component Synthesis of Polysubstituted 1,3-Oxazolidines

A modular, one-pot, two-step, four-component synthesis of polysubstituted 1,3-oxzolidines is described. The method comprises two linked domino processes: an organocatalyzed domino reaction of alkyl propiolate and an aliphatic aldehydes and a microwave-assisted amine addition cyclization domino process. An alternative modular, one-pot, three-step, four-component synthesis has also been developed by linking the organocatalyzed domino process to a sequential amine addition/Yb(OTf)(3)-catalyzed enamine cyclization reaction.

Synthesis and Photochemistry of ?,??-Di(2-furyl)-Substitutedo-Divinylbenzenes: Intra- and/or Intermolecular Cycloaddition as an Effect of Annelation

New heteroaryl-substituted o-divinylbenzenes, 2,2'-(1,2-phenylenedivinylene)difuran (9), 2,2'-(1,2-phenylenedivinylene)bisbenzo[b]furan (10), and 2,2'-(1,2-phenylenedivinylene)bisnaphtho[2,1-b]furan (11), were prepared and irradiated at various concentrations; intramolecular photocycloaddition and intermolecular [2+2] twofold photoaddition reactions took place to give bicyclo[3.2.1]octadiene derivatives 12-14 and cyclophane derivatives 15-17, respectively. Compound 11 was the most selective of these o-divinylbenzenes, which, owing to pi-pi intra- or intermolecular complexation, gave only the exo-bicyclo[3.2.1]octadiene derivative 14 at low concentrations, and only the cyclophane derivative 17 at high concentrations.

Stereoselective Ketal-Tethered Intramolecular Diels−Alder Cycloadditions. An Approach to the 2-Oxadecalin Spiroketal Core of Antifungal Agent Fusidilactone C

[reaction: see text] An approach toward the 2-oxadecalin spiroketal core of fusidilactone C via a rare ketal-tethered intramolecular Diels-Alder cycloaddition is described here. This intramolecular Diels-Alder cycloaddition is highly endo-selective and overall depended upon the nature of solvents and Lewis acids. We also observed some remarkable rate acceleration in MeOH.

Synthesis of highly functionalised enantiopure bicyclo[3.2.1]- octane systems from carvone

The commercially available monoterpene carvone has been efficiently converted into the tricyclo[3.2.1.0(2.7)]octane and bicyclo[3.2.1]octane systems characteristic of some biologically active compounds. The sequence used for this transformation involves as key features an intramolecular Diels-Alder reaction of a 5-vinyl-1,3-cyclohexadiene and a cyclopropane ring opening.

Bicyclo[3.2.1]octane Synthons from Cyclopropenes: Functionalization of Cycloadducts by Nucleophilic Additions

It has been known for several decades that a highly functionalized family of tetrahalobicyclo[3.2.1]octadienes are readily available through the cycloaddition of furan or cyclopentadiene with either tetrachloro- or tetrabromocyclopropene. However, the application of these highly functionalized building blocks in synthesis has remained relatively unexplored in relation to their better-known counterparts derived through oxyallyl cation additions. As a first step toward utilizing these highly versatile intermediates in synthesis, a study of the addition of various nucleophiles to the halogenated nucleus has been conducted. It has been found that these halogenated systems are amenable to a wide range of functionalizations in high yields and with good selectivities.

Nitroalkanes in Aqueous Medium as an Efficient and Eco-Friendly Source for the One-Pot Synthesis of 1,4-Diketones, 1,4-Diols, δ-Nitroalkanols, and Hydroxytetrahydrofurans

The Michael addition of primary aliphatic nitro compounds to alpha,beta-unsaturated enones, performed in aqueous media, provides the one-pot synthesis of 1,4-diketones, 1,4-diols, delta-nitroalkanols, and hydroxytetrahydrofurans, respectively, by the appropriate choice of the reaction conditions.

Formal [3 + 3] Cycloaddition Approach to Chromenes and Chromanes. Concise Total Syntheses of (±)-Rhododaurichromanic Acids A and B and Methyl (±)-Daurichromenic Ester

[reaction: see text] Total syntheses of (+/-)-rhododaurichromanic acids A and B and methyl (+/-)-daurichromenic ester are described here. Despite the complex appearances of these compounds, their syntheses are completed in six steps with a 15% overall yield as a mixture by featuring our formal oxa-[3 + 3] cycloaddition methodology.

An effective one-pot synthesis of 5-substituted tetronic acids

An expeditious one-pot synthesis of 5-substituted tetronic acids from aldehydes and terminal conjugated alkyne as starting materials is described. The entire process embodies two consecutive chemical events: a catalytic domino reaction to build the 1,3-dioxolane scaffolds 5 and a two-step acid-catalyzed trans-acetalization-lactonization reaction to furnish the tetronic acid derivatives 6.

A formal [3 + 3] cycloaddition reaction. Improved reactivity using alpha,beta-unsaturated iminium salts and evidence for reversibility of 6pi-electron electrocyclic ring closure of 1-oxatrienes

A detailed account regarding a formal [3 + 3] cycloaddition method using 4-hydroxy-2-pyrones and 1,3-diketones is described here. This formal cycloaddition reaction or annulation reaction is synthetically useful for constructing 2H-pyranyl heterocycles. The usage of alpha,beta-unsaturated iminium salts is significant in controlling competing reaction pathways to give exclusively 2H-pyrans. Most significantly, experimental evidence is provided to support the mechanism of this reaction that involves a sequential Knoevenagel condensation and a reversible 6pi-electron electrocyclic ring-closure of 1-oxatrienes.

Group 6 heteroatom- and non-heteroatom-stabilized carbene complexes. beta,beta'- and alpha,beta,beta'-annulation reactions of cyclic enamines

Cyclization reactions of group 6 Fischer carbene complexes with cyclopentanone and cyclohexanone enamines are described. Enamine 3a undergoes thermal alpha,beta,beta'-annulation with alkenylcarbene complexes 1 and 2 (THF, 60 degrees C), affording semibullvalenes 5. The metalate intermediates 6, resulting from beta,beta'-annulation of the enamines 3a and 4a, were quantitatively formed by running the reaction in hexane at room temperature. Acid-promoted demetalation of 6 afforded endo-2-bicyclo[3.2.1]octen-8-ones 7 and endo/exo-2-bicyclo[3.3.1]nonen-9-ones 8 (endo/exo = 5:1). Using (S)-methoxymethylpyrrolidine-derived enamines 3b and 4b,c allowed highly enantioenriched cycloadducts endo-(+)-7 as well as endo-(-)-8 and exo-(-)-8 to be accessed. The non-heteroatom-stabilized carbene complex 10 was formed from complex 6 by Me(3)SiOTf-promoted elimination of the methoxy group, characterized by (13)C NMR, and transformed into the organic compounds 7, 7-d, and 11 as well as into bicyclo[3.2.1]octan-2,8-diones 14 and cycloheptanones 15. On the basis of this sequence, enantioenriched cycloheptanones (+)-15 were efficiently prepared in one pot from carbene complexes 2 and enamine 3b (51-55% yield, 91-96% ee). Extension of this work to simple Fischer carbene complexes 16 allowed an appropriate way to generate the nonstabilized pentacarbonyl[(phenyl(alkyl)carbene]tungsten complex 17 to be designed, for which the thermal and chemical behavior leading to compounds 18-21 is described.

Stereoselective formal [3 + 3] cycloaddition approach to cis-1-azadecalins and synthesis of (-)-4a,8a-diepi-pumiliotoxin C. evidence for the first highly stereoselective 6pi-electron electrocyclic ring closures of 1-azatrienes

Evidence is described here to support that a highly stereoselective 6pi-electron electrocyclic ring closure of 1-azatrienes is a key step in formal [3 + 3] cycloaddition or annulation reactions of chiral vinylogous amides with alpha,beta-unsaturated iminium salts. This would represent the first highly stereoselective 6pi-electron electrocyclic ring closure of 1-azatrienes. We have also unambiguously demonstrated that these specific ring closures are reversible, leading to the major diastereomer that is also thermodynamically more stable, and that a rotation preference likely also plays a role. A synthetic application is illustrated here to stereoselectively transform the resulting dihydropyridines to cis-1-azadecalins with unique anti relative stereochemistry at C2 and C2a, leading to synthesis of epi isomers of (-)-pumiliotoxin C.

A novel and highly stereoselective approach to aza-spirocycles. A short total synthesis of 2-epi-(+/-)-perhydrohistrionicotoxin and an unprecedented decarboxylation of 2-pyrones

[reaction: see text] A novel and highly stereoselective synthesis of aza-spirocycles is described. An application of this methodology is illustrated as a short and concise total synthesis of 2-epi-(+/-)-perhydrohistrionicotoxin with high diastereomeric control at the aza-spirocenter. An unprecedented decarboxylation of the 2-pyrone ring is observed in this total synthesis effort.