Cyclic Allenes Research Articles

Stereoselective Synthesis of Pyrrolo[1,2-a]indoles from Allenes in PEG-400 as the Reaction Medium

Base-catalyzed domino cyclization of allenes with 3-chloro-2-formylindoles in PEG-400 was investigated. Pyrroloindoles were formed stereoselectively as single products. The structure of the product depends on the type of allene used. Two compounds have been characterized by single crystal X-ray diffraction.

To Bend or Not to Bend! The Dilemma of Allenes

Allenes, though expected to be linear, are found to be bent in many examples, especially in the case of cyclic allenes. The bending of allene is attributed to the weakening of π-bond strength across the allene. However, tetrakis(dimethylamino)allene {((CH(3))N)(2)C═C═C(N(CH(3))(2))}, which is characterized by push-push interactions, has been shown to be linear, thus leading to doubts of the current understanding of the bent allenes. In this article, we report the ab initio MO/DFT, NBO based electronic structure analysis of R(2)C═C═CR(2) (R = H, NH(2)) with a gradual increase in the number of amino substituents. The results indicate that the allenic π-bond strength and bending potential decrease, with an increase in the amino substitution. Molecular orbital analysis provides necessary clues regarding the delicate balance between orthogonality of the π orbitals and the p orbitals on the central carbon, which dictates the bending potential of the allenes. The dilemma of to bend or not to bend is a unique feature of tetraaminoallenes (NH(2))(2)C═C═C(NH(2))(2) in comparison to isoelectronic heteroallenes (NH(2))(2)C═N═C(NH(2))(2)(+) and (NH(2))(2)C═B═C(NH(2))(2)(-).

ChemInform Abstract: Aluminum Carbenoids in Allene Cyclopropanation.

AbstractThe carbenoids react with allenes (I) and (IV) to give spiropentane products.

Fascinating reactivity in gold catalysis: synthesis of oxetenes through rare 4-exo-dig allene cyclization and infrequent β-hydride elimination

A novel reactivity in gold catalysis, namely the unusual preference for the 4-exo-dig cyclization in allene chemistry as well as the rare β-hydride elimination reaction, was uncovered starting from readily available allenols.

Aluminum carbenoids in allene cyclopropanation

The reactions of aluminum carbenoids with alkyl- and phenyl-substituted allenes and cyclic allenes are studied. An efficient method for the synthesis of substituted spiropentanes was developed. 1,2-Cyclononadiene was selectively converted into bicyclo[7.1.0]dec-1-ene. An unusual transformation of α-methylphenylallene into a spiroindane derivative under the action of Et 3Al–CH 2I 2 was found.

Synthesis of (−)-Sedinine by Allene Cyclization and Iminium Ion Chemistry

A synthesis of the sedum alkaloid sedinine has been achieved employing silver(I)-catalyzed allenic hydroxylamine cyclization and ring-closing metathesis to form a bicyclic N,O-acetal. Ring opening of this acetal with a silyl enol ether under Lewis acidic conditions is exclusively trans selective, leading to the natural product after reduction. On the other hand, conversion of the bicyclic N,O-acetal to a semicyclic N,O-acetal results in no stereoselectivity during such a reaction. The contrasting results can be rationalized by consideration of the conformation of the iminium ions.

ChemInform Abstract: Total Synthesis of Clavepictines A and B. Diastereoselective Cyclization of δ-Aminoallenes.

The stereocontrolled total synthesis of (−)-clavepictine A (1A) and (+)-clavepictine B (1B) has been accomplished in an enantioselective fashion, which has unequivocally established the absolute configuration of 1A and 1B. The pivotal step in the synthesis is diastereoselective silver(I)-promoted cyclization of δ-amino allenes. Another key method includes cross-coupling of enol triflates of N-acyl lactams, which allows stereocontrolled functionalization of otherwise unreactive lactams under mild conditions. The utility of Beak's α-lithiation-substitution chemistry of N-BOC piperidines involving a functionalized aldehyde as the electrophile is demonstrated in the preparation of highly substituted nitrogen heterocycles. These new synthetic strategies should be of general synthetic utility in the stereoselective syntheses of quinolizidines, indolizidines, and related aza-heterocylces. Also included is the unique conformational preference of the highly substituted cis-quinolizidine core of clavepictines; the ...

ChemInform Abstract: A Stereodivergent Approach to (-)-α-Kainic Acid and (+)-α-Allokainic Acid Utilizing the Complementarity of Alkyne and Allene Cyclizations.

A formal synthesis of (+)-α-allokainic acid and a total synthesis of (−)-α-kainic acid were carried out using a short, efficient, and highly stereoselective approach. From an alkyne precursor, a nickel-catalyzed cyclization and a palladium-catalyzed rearrangement were utilized in the synthesis of (+)-α-allokainic acid. From an allene precursor, a nickel-catalyzed cyclization was utilized in the synthesis of (−)-α-kainic acid. The allene cyclization used in the latter sequence was the first example of a metal-catalyzed cyclization of this type.

ChemInform Abstract: Recent Developments in Strained Cyclic Allenes

ChemInform Abstract: Recent Developments in Strained Cyclic Allenes

Regioselective intermolecular cycloalumination of cyclic allenes and ethylene with R n AlCl3−n , catalyzed by titanium and zirconium complexes

Intermolecular cycloalumiation of cyclic 1,2-dienes and ethylene with organoaluminum compounds R n AlCl3−n in the presence of zirconium- and titanium-based catalysts occurred regioselectively with formation of unsaturated bicyclic organoaluminum compounds in high yields.

Formation of Cyclic Allenes and Cumulenes by Cooperative Addition of Frustrated Lewis Pairs to Conjugated Enynes and Diynes

Formation of Cyclic Allenes and Cumulenes by Cooperative Addition of Frustrated Lewis Pairs to Conjugated Enynes and Diynes

Cyclotetramer of a Strained Cyclic Allene

The authors isolated compound 3 from the Doering-Moore-Skattebøl reaction of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene (1) through the strained cyclic allene intermediate 2. The reaction described is formally a [2+2+2+2] ­reaction; however, the authors propose that the product is more likely formed by two sequential dimerizations due to the entropic cost of the concerted mechanism. Such cyclotetramerizations of allenes are rare in the literature and have never been reported with a seven-membered cyclic ­allene.

Asymmetric Counteranion‐Directed Transition‐Metal Catalysis: Enantioselective Epoxidation of Alkenes with Manganese(III) Salen Phosphate Complexes

Inspired by pioneering contributions on chiral Bronsted acid mediated reactions and our own studies in aminocatalysis, we are currently exploring asymmetric counteranion-directed catalysis (ACDC) as a general strategy for asymmetric synthesis. According to this concept, catalytic reactions that proceed via cationic intermediates can be performed highly enantioselectively by the incorporation of a chiral counteranion into the catalyst. After our initial proof of concept with organocatalytic transfer hydrogenations and epoxidations, ACDC has recently been extended to transition-metal catalysis with Toste s gold-catalyzed allene cyclizations, and our palladium-catalyzed Tsuji–Trost-type a-allylation of aldehydes. The further exploration of the potential of ACDC in transition-metal catalysis, especially applied to redox reactions, is of great interest and significance. Here we report a highly enantioselective epoxidation of olefins that is catalyzed by a chiral ion pair consisting of an achiral Mn–salen cation and a chiral phosphate counteranion. Stimulated by an important contribution from Kochi et al. , Jacobsen and Katsuki have significantly advanced the catalytic asymmetric epoxidation of unfunctionalized alkenes by introducing chiral Mn–salen catalysts. These complexes display a broad substrate scope although certain olefin classes still fail to be converted with high enantioselectivity. Interestingly, cationic Mn–salen complexes are C2-symmetrical and inherently chiral—even when the salen ligand itself is achiral. In case of the Jacobsen–Kastuki epoxidation, the chiral backbone of the salen ligand fixes the complex in one of the two enantiomorphic confirmations. The neutral donor ligands typically added increase reactivity and enantioselectivity by displacing the apically coordinated anion of the Mn complex. Chiral neutral donor ligands such as sparteine and chiral N-oxides have been used in combination with an achiral Mn–salen complex and shown to shift the equilibrium of enantiomeric conformational isomers of the cationic metal complex to one side. Reasonably good enantioselectivities have been achieved in the corresponding epoxidations. We hypothesized that a chiral counteranion should also be able to induce a preference for one of the two enantiomorphic conformations. Specifically, chiral binol-derived phosphate anions are ideally suited for our purposes, because in addition to possibly inducing one enantiomorphic conformation of the cationic complex, these ions may also amplify the chiral microenvironment around the metal center with suitable substituents at the 3,3’-positions (Figure 1). Overall, this may lead to a new type of chiral Mn–salen catalyst with unique properties.

ChemInform Abstract: A Formal Synthesis of Swainsonine by Gold‐Catalyzed Allene Cyclization.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Photoannelation Reactions of 3‐(Alk‐1‐ynyl)cyclohept‐2‐en‐1‐ones

AbstractIrradiation (350 nm) of the newly synthesized 3‐(alk‐1‐ynyl)cyclohept‐2‐en‐1‐ones 1 and 2 leads to the selective formation of tricyclic head‐to‐head dimers. In the presence of 2,3‐dimethylbuta‐1,3‐diene, the (monocyclic) enone 1 affords trans‐fused 7‐alkynyl‐bicyclo[5.2.0]nonan‐2‐ones as major photoproducts, whereas photocycloaddition of benzocyclohept‐5‐en‐7‐one 2 to the same diene gives preferentially the eight‐membered cyclic allene 16 via ‘end‐to‐end’ cyclization of the intermediate allyl‐propargyl biradical 22. On contact with acid, cycloocta‐1,2,5‐triene 16 isomerizes to cycloocta‐1,3,5‐triene 18.

A Formal Synthesis of Porantheridine and an Epimer

A formal synthesis of porantheridine and a synthesis of its C6-epimer have been completed, employing silver-catalyzed allene cyclization to form a common cis-isoxazolidine intermediate and related N-acyl iminium ion intermediates for side-chain introduction. The stereochemistry of this step can be controlled by choice of the N-protection method.

Graphical Abstract: Eur. J. Org. Chem. 32/2009

AbstractThe cover picture shows a cyclohexallene‐based starry night over the cathedral of Santiago de Compostela in northwest Spain. 1,2‐Cyclohexadiene is a highly reactive cyclic allene that is usually generated by the use of strong bases or highly nucleophilic reagents, and this limits its scope. Herein we described an improved method to generate this molecule by fluoride‐induced β‐elimination of the corresponding 6‐(trimethylsilyl)cyclohexenyl triflate. This method allows the generation of this strained cyclic allene under mild reaction conditions and with higher efficiency than classical approaches, as demonstrated by trapping experiments based on cycloaddition reactions. Details are discussed in the article by D. Peña et al. on p. 5519 ff.

Generation and Reactivity of 1,2‐Cyclohexadiene under Mild Reaction Conditions (Eur. J. Org. Chem. 32/2009)

AbstractThe cover picture shows a cyclohexallene‐based starry night over the cathedral of Santiago de Compostela in northwest Spain. 1,2‐Cyclohexadiene is a highly reactive cyclic allene that is usually generated by the use of strong bases or highly nucleophilic reagents, and this limits its scope. Herein we described an improved method to generate this molecule by fluoride‐induced β‐elimination of the corresponding 6‐(trimethylsilyl)cyclohexenyl triflate. This method allows the generation of this strained cyclic allene under mild reaction conditions and with higher efficiency than classical approaches, as demonstrated by trapping experiments based on cycloaddition reactions. Details are discussed in the article by D. Peña et al. on p. 5519 ff.

Generation and Reactivity of 1,2‐Cyclohexadiene under Mild Reaction Conditions

AbstractAn improved method to generate 1,2‐cyclohexadienes by fluoride‐induced β‐elimination of 6‐(trimethylsilyl)cyclohexenyl triflates is presented. This method allows the generation of these highly strained cyclic allenes in good yields and under mild reaction conditions, as demonstrated by trapping experiments based on cycloaddition reactions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Crystal Structure of a Cyclotetramer from a Strained Cyclic Allene

The low-temperature treatment of 1,1-dibromo-1a,9b-cyclopropa[l]phenanthrene (1) with butyllithium and copper(II) chloride in THF affords a dibenzoannellated 1,2,4,6-cycloheptatetraene which undergoes a rare cyclotetramerization. The crystal structure of this "formal" 2 + 2 + 2 + 2 cyclotetramer (2) reveals a central eight-membered ring folded in a zigzag fashion with hydrogen atoms and exocyclic double bonds occupying axial positions. B3LYP/6-31+G** calculations indicate that the strained cyclic allene is significantly distorted and could be formed by ring expansion of a putative cyclopropylidene intermediate.