Five- and six-membered ring formation from the intramolecular reaction of a protonated oxirane and alkene

Computational studies of competing five- and six-membered cyclisation of alkenyloxiranes 1a-d show that intramolecular reaction of a protonated oxirane and alkene is a concerted, single-step, exothermic process. The reactions proceed via reactant-like transition states, but where the oxirane C-O bond is considerably stretched. Two factors are seen to affect the regiochemistry: (1) stabilisation of the transitory positive charge in the transition state favours cyclisation to the more highly substituted oxirane carbon; and (2) there is an inherent stereoelectronic preference for six-membered cyclisation over five-membered cyclisation. The inherent preference for six-membered cyclisation has a parallel in Baldwin's rules for six-membered ring closure of a carbocation with an alkene, rather than Baldwin's rule for intramolecular nucleophilic reaction of three-membered rings, suggesting that the protonated oxirane mimics a carbocation. The electronic and stereoelectronic effects for cyclisation are modified by steric interactions of axial methyl groups. These systems provide a model for the A-ring cyclisation of oxidosqualene.     

A synthesis of 2-epi-fagomine using gold(I)-catalysed allene cyclisation

A synthesis of 2-epi-fagomine via a highly stereoselective gold(I)-catalysed allene cyclisation is described. The stereochemical outcome of the cyclisation is opposite to that observed in previous studies. In contrast, gold(III)-catalysed cyclisation is inefficient and gives rise to double cyclisation by-products.     

7- and 8-membered ring sugars and related systems : 7-Membered rings from diolefins derived from dioxepans via double michael condensations

The possibilities of cyclisation to 7-membered ring sugars containing nitro- and cyano-functions via double Michael condensations of diolefins derived from sugar dioxepans was investigated. With nitromethane cyclised products were obtained while with the larger ethyl cyanoacetate and malononitrile only one double bond was attacked without cyclisation. Treatment of the diolefins with excess ammonia resulted in cyclisation to novel heterocyclic systems containing oxygen and nitrogen often of known stereochemistry; with butylamine no cyclisation occurred. Treatment with hydrogen sulphide also resulted in cyclisation to novel heterocyclic systems containing sulphur and oxygen often of known stereochemistry. In these systems cyclisation only occurs with the smaller reagents.     

On the cyclisation of N-alkylformazans to leucoverdazyls (1,2,3,4-tetrahydro-s-tetrazines)

A series of N-alkylformazans have been prepared and their cyclisation to leucoverdazyls studied. The cyclisation has been found to occur in the presence of strongly ionising bases, thermally, and in the presence of acid. The sensitivity of the cyclisation to the structure of the N-alkylformazans and to the cyclisation conditions are discussed in terms of mechanisms proposed for the reaction.     

Synthese de pyridines par cyclisation de systemes azatrieniques

The cyclisation of imines from α,β-unsaturated aldehydes and allylamine to pyridines has been studied. At 25°, the catalytic action of the basic system sodium-alloocimene gives a first type of cyclisation binding the α carbon of the amino chain to the δ carbon of the aldehydic one. Arguments in favour of a carbanionic cyclisation are advanced. A second type of cyclisation is observed in the case of a 3-azahexatrienic system which gives a pyridine at 120° in the absence of any catalytic system. This type of reaction is not described in the literature and seems purely thermal.     

Stereoselective synthesis of 3,4-disubstituted and 3,4,5-trisubstituted piperidines by Lewis acid-catalysed ene cyclisation of 4-aza-1,7-dienes

The thermal or Lewis acid-catalysed ene cyclisation of a variety of 4-aza-1,7-dienes afforded 3,4-disubstituted or 3,4,5-trisubstituted piperidines. Activation of the enophile with a single ester facilitated a thermal ene cyclisation, although the reaction was not amenable to Lewis acid catalysis. With other activating groups on the enophile it was found that Lewis acid catalysis was facile, although there was a fine balance between the desired ene cyclisation and the competing hetero-Diels-Alder reaction, with the product distribution being influenced by the activating group on the enophile, the nature of the ene component, and the Lewis acid used. Activation of the enophile with an oxazolidinone function facilitated Lewis acid-catalysed cyclisation to afford mixtures of ene and hetero-Diels-Alder products. Activating the enophile with two ester groups gave a substrate that underwent a very facile ene cyclisation catalysed by MeAlCl(2) to give the corresponding trans 3,4-disubstituted piperidines with diastereomeric ratios of >200 : 1.     

Synthesis of heteroarenes using cascade radical cyclisation via iminyl radicals

Cascade radical cyclisation involving homolytic aromatic substitution has been used to synthesise new tetracycles. Treatment of vinyl iodide radical precursors with Me(3)Sn. radicals (from hexamethylditin) yielded intermediate vinyl radicals which undergo 5-exo cyclisation onto suitably placed nitrile groups to yield intermediate iminyl radicals. The iminyl radicals undergo aromatic homolytic substitution via 6-endo cyclisation (or 5-exo cyclisation followed by neophyl rearrangement) with loss of hydrogen (H.) in a H-abstraction step. We propose that this abstraction was facilitated by tert-butoxyl (t-BuO.) radicals from di-tert-butyl peroxide or methyl radicals, generated from breakdown of trimethylstannyl radicals (Me(3)Sn.). The biologically active alkaloids mappicine and luotonin A were synthesised using the new methodology. A novel radical conversion of nitriles to primary amides is proposed.     

Radical cyclisations of methylenecyclopropyl azetidinones—synthesis of novel tricyclic β-lactams

Addition of lithium bis(methylenecyclopropyl)cuprates to acetoxy azetidinones gives methylenecyclopropyl azetidinones, which can be converted to various radical cyclisation precursors. Attempted 4-exo cyclisation of 3 led only to reduced product, while cyclisation of 5, using CuCl/bipy, gave a carbacephem, via a 5-exo cyclisation, but in low yield. Cyclisation of 6 and 7, however, gave novel tricyclic β-lactams, as the result of 7-endo cyclisation, in good yield, and a cyclisation of bromide 23 led to the tricyclic β-lactam 24, via a radical cascade sequence.     

Tandem intermolecular-intramolecular carbolithiation: application to the synthesis of silacyclopentanes

Silacyclopentanes have been prepared from vinyl(homoallyl)silanes or vinyl(homopropargyl)silanes and organolithium reagents by a tandem intermolecular-intramolecular sequence involving a 5-exo cyclisation process. The unexpected stereochemical outcome of the sequence involving a 5-exo-dig cyclisation is rationalised.     

Radical reactions with 3H-quinazolin-4-ones: synthesis of deoxyvasicinone, mackinazolinone, luotonin A, rutaecarpine and tryptanthrin

Alkyl, aryl, heteroaryl and acyl radicals have been cyclised onto the 2-position of 3H-quinazolin-4-one. The side chains containing the radical precursors were attached to the nitrogen atom in the 3-position. The cyclisations take place by aromatic homolytic substitution hence retain the aromaticity of the 3H-quinazolin-4-one ring. The highest yields were obtained using hexamethylditin to facilitate cyclisation rather than reduction without cyclisation. The alkaloids deoxyvasicinone , mackinazolinone , tryptanthrin , luotonin A and rutaecarpine were synthesised by radical cyclisation onto 3H-quinazolin-4-one.     

A one-pot, reductive amination/6-endo-trig cyclisation for the stereoselective synthesis of 6-substituted-4-oxopipecolic acids

The first stereoselective synthesis of 2,6-trans-6-substituted-4-oxo-L-pipecolic acids using a tandem reductive amination/6-endo-trig cyclisation process is described. The sequential reduction and cyclisation mediated by sodium cyanoborohydride allowed the preparation of a series of highly functionalised 6-alkyl and 6-aryl analogues.     

Synthetic approaches to the daucane sesquiterpene derivatives employing the intramolecular Buchner cyclisation of α-diazoketones

The use of the intramolecular Buchner cyclisation of an α-diazoketone as an approach to the synthesis of daucane sesquiterpenes is described; in particular the synthesis of the cis-fused analogue of dihydro CAF-603. The key step in the synthesis is the intramolecular Buchner cyclisation, which provides the bicyclo[5.3.0]decane framework with the required stereochemistry at the quaternary centre generated in the cyclisation. A synthetic route enabling access to an asymmetric synthesis is also outlined.     

CH activation and CH2 double activation of indolines by radical translocation: understanding the chemistry of the indolinyl radical

CH activation and CH(2) double activation of indolines at C2 may be achieved efficiently through radical translocation. The fate of the C2 indolinyl radical is dictated by the substitution at C3. Fragmentation, cyclisation and tandem cyclisation reactions leading to indole, azaheterocycle and azapropellane formation, respectively, are reported.     

Regio- and stereoselective 6-exo-trig radical cyclisations onto chiral perhydro-1,3-benzoxazines: synthesis of enantiopure 3-alkylpiperidines

Enantiopure 3-alkyl substituted piperidines are prepared by diastereoselective 6-exo-trig cyclisation of perhydro-1,3-benzoxazines derived from (−)-(8)-amino menthol. The diastereoselective cyclisation is promoted by tributyltin hydride, and the competitive 1,5-hydrogen migration depends on the position of the acceptor double bond and the radical site.     

Manganese(III)-mediated oxidative free-radical cyclisations of allenyl malonates

The regioselective synthesis of alkenylcyclanones by the oxidative radical cyclisation of malonyl radicals onto both allenes and an allylsilane is reported, along with the diastereoselective formation of [3.3.0]-bicyclic γ-lactones from two of these alkenylcyclanones. Furthermore, the cyclisation of 3-, 4- and 5-allenyl malonates on exposure to manganese(III) acetate under both oxidative and reductive termination conditions is reported.     

Stereoselective synthesis of 2,4,5-trisubstituted piperidines by carbonyl ene and Prins cyclisations

An approach to 2,4,5-trisubstituted piperidines is reported, in which the key step is the Prins or carbonyl ene cyclisation of aldehydes of the type 1. Prins cyclisation catalysed by concentrated hydrochloric acid in CH(2)Cl(2) at -78 degrees C afforded good yields of two of the four possible diastereomeric piperidines, with the 4,5-cis product 7 predominating in a diastereomeric ratio of up to 94:6. The diastereoselectivity of the cyclisation decreased as the 2-substituent increased in size, becoming unselective for very bulky 2-substituents. In contrast, cyclisation catalysed by MeAlCl(2) in CH(2)Cl(2) or CHCl(3) at temperatures of between 20-60 degrees C, favoured the 4,5-trans diastereomer 8, in a diastereomeric ratio of up to 99:1. The low-temperature cyclisations catalysed by HCl proceed under kinetic control via a mechanism involving the development of significant carbocationic character, in which the 4,5-cis cation is more stable than the 4,5-trans cation as a result of overlap with the neighbouring oxygen. The cyclisations catalysed by MeAlCl(2) proceed under thermodynamic control, affording the product in which both the 4- and 5-substituents are equatorial.     

A highly diastereoselective series of Nicholas cyclisation reactions of N-Boc-protected propargylamines

The reaction of N-Boc-protected propargylamine with salicylaldehyde derivatives and their subsequent Nicholas cyclisation reaction to provide a range of novel benzopyrans is reported. The cyclisation reactions proceeded with excellent levels of diastereoselectivity to afford compounds with cis-relative stereochemistry. As far as we are able to ascertain these are the first reported examples of Nicholas cyclisation reactions of propargyl alcohols that bear a terminal alkynyl N-protected amino motif.     

Stereoselective synthesis of (+)-cryptocarya diacetate by an iterative Prins cyclisation and reductive cleavage sequence

A highly stereoselective synthesis of (+)-cryptocarya diacetate is achieved through our recently developed strategy for the construction of 1,3-diols via Prins cyclisation. The route relies mainly on the reductive cleavage of allylic ethers, ozonolysis and Wittig olefination along with Prins cyclisation.     

The Baylis-Hillman approach to quinoline derivatives

Baylis-Hillman reactions of 2-nitrobenzaldehydes with various activated alkenes afford adducts that undergo reductive cyclisation to quinoline derivatives. The chemo- and regioselectivity of cyclisation appears to be influenced by the choice of both the substrate and the reagent system, and competing reactions have been observed.     

A flexible radical approach to 5-substituted 4,5-dihydro-3H-pyrido[4,3-b]azepin-2-ones. Some mechanistic observations on the radical cyclisation-aromatisation process

Variously substituted novel dihydropyridoazepinones have been prepared by an intermolecular radical addition followed by a radical cyclisation on a pyridine ring. The latter process involved the use of a combination of two different peroxides, an experimental contrivance resulting from a careful product analysis and a better understanding of the cyclisation step.