Heating a 2,5-furanocyclic (2-azidoethyl)allene initiates a cascade reaction comprising azide–allene cycloaddition, loss of nitrogen, and azatrimethylenemethane (ATMM) diyl–furan transannular (4 + 3)-cycloaddition. The major product of this reaction contains the pentacyclic core common to guanacastepenes D and H and radianspenes J–L; in addition, the central oxa-bridged cycloheptene ring, flanked by two carbocyclic rings, is structurally related to the ABC-ring system found in the cortistatins. This is the first reported synthetic application of a “free” (nonconjugated) ATMM. The cyclization precursors were prepared via the first reported examples of the Ireland–Claisen rearrangement of an ethynyl lactone.

Covering: 2013 up to the end of 2015 This review covers the isolation and structure of new pyrrolizidines; pyrrolizidine biosynthesis; biological activity, including the occurrence of pyrrolizidines as toxic components or contaminants in foods and beverages; and formal and total syntheses of naturally-occurring pyrrolizidine alkaloids and closely related non-natural analogues.

Biocatalytic direct monohydroxylation of anilides has been achieved on preparative scale using mutant cytochrome P450BM3 enzymes. Representative mono- and disubstituted N-trifluoromethanesulfonyl anilides are shown to be converted in most cases to the corresponding 4-hydroxy derivatives, with substituent hydroxylation also occurring in two cases. By mutation variation, it is possible to achieve selective hydroxylation of either ring- or side-chain sites.

Expanding Nature’s catalytic repertoire to include reactions important in synthetic chemistry opens new opportunities for biocatalysis. An intramolecular C–H amination route to imidazolidin-4-ones via α-functionalization of 2-aminoacetamides catalyzed by evolved variants of cytochrome P450BM3 (CYP102A1) from Bacillus megaterium has been developed. Screening of a library of ca. 100 variants based on four template mutants with enhanced activity for the oxidation of unnatural substrates and preparative scale reactions in vitro and in vivo show that the enzymes give up to 98% isolated yield of cyclization products for diverse substrates. 2-Aminoacetamides with one- and two-ring cyclic amines bearing substituents and aliphatic, alicyclic, and substituted aromatic amides are cyclized. Regiodivergent C–H amination was achieved at benzylic and nonbenzylic positions in a tetrahydroisoquinolinyl substrate by the use of different mutants. This C–H amination reaction offers a scalable route to imidazolidin-4-ones with varied functionalized substituents that may have desirable biological activity.Keywords: C−H activation; C−H amination; heme monooxygenases; P450; protein engineering

Covering: July 2001 to December 2012 This review covers pyrrolizidine alkaloids isolated from natural sources. Topics include: aspects of structure, isolation, and biological/pharmacological studies; total syntheses of necic acids, necine bases and closely-related non-natural analogues.

The first total synthesis of pandamarilactone-1, an alkaloid of Pandanus amaryllifolius, is reported. The nine-step synthesis features furan oxidation with singlet oxygen and then spiro-N,O-acetalization and elimination to generate the natural product and further Pandanus alkaloids, pandamarilactonines A–D.

Aziridines formed upon treatment of allylic carbamates and homoallylic sulfamates with Rh(II) carboxylate catalysts under oxidative conditions are trapped by suitably-disposed hydroxyl groups to give functionalised tetrahydrofurans.

We describe the synthesis of a series of oxy-substituted butenolide spiroacetals and spiro-N,O-acetals by oxidative spirocyclisation of 2-[(4-hydroxy or 4-sulfonamido)butyl]furans. The axial–equatorial preference of each oxy-substituent is investigated (NMR) by an acid-catalysed thermodynamic relay of configuration between the spiro- and oxy-centres. The axial site is preferred for most oxy-substituents at synthetically useful levels. The potential origins of this preference are discussed in terms of a stabilising gauche effect combined with the influence of solvation. These results have relevance to the synthesis of bis(acetylenic)enol ether spiroacetals including AL-1 and related compounds.

An 11-step synthesis is described of two diastereomeric candidates for a bis(acetylenic) enol ether spiroacetal isolated from Chrysanthemum boreale. Key steps in the synthetic route include spiroacetal lactone alkylidenation and subseqent modified Cadiot–Chodkiewicz cross-coupling to install the bis(acetylenic) enol ether functionality. From NMR comparisons, neither of the candidates, whose structures were confirmed by single-crystal X-ray diffraction, correspond to the natural product, and a proposal for the correct structure is put forward.

An efficient synthesis of NP25302 is presented that relies on 5-endo-dig N-cyclization to establish the bicyclic core and Curtius rearrangement to install the N-acyl vinylogous urea functionality.

We describe a concise synthesis of (+)-isoaltholactonevia a Au-catalysed cyclisation of a monoallylic diol to form the tetrahydrofuranyl ring. Analogous cyclisations show that the stereochemical outcome is dictated by the stereochemistry of the diol substrate.

Rhodium-bound nitrenoids are trapped by tethered allenes generating acyloxy-enamines, aminocyclopropanes, and methylene aziridines. The aminocyclopropanes undergo substitution of the acetoxy group by a variety of nucleophiles.

Two new routes to the C(1−10) carboxylic acid core of taurospongin A are presented. In the first route, overall asymmetric hydration of a C(2)−C(3) alkene is achieved by Sharpless AD and selective deoxygenation at C(2); in the second route, the C(3) stereogenic center is set by Tietze asymmetric allylation. A short synthesis of the C(1′−25′) fatty acid combines with the product from the first route to complete the total synthesis of taurospongin A.

Two routes are described for the synthesis of the sawaranospirolides, stereoisomeric spirolactone ascorbigenins isolated from Chamaecyparis pisifera. Trapping of the keto enal formed by oxidation of a functionalised 2-(4-hydroxybutyl)furan affords a potential butenolide spiroacetal precursor to sawaranospirolides A and C. Alternatively, epoxidation of protected 3-(dihydropyran-2-yl)-3-arylpropanoic acids results in spirolactonisation to generate ent-sawaranospirolide C; a related acid-mediated spirocyclisation gave access to ent-sawaranospirolide D.

Unprecedented bicyclic methylene aziridines are prepared by rhodium(II)-catalyzed allene aziridination of buta-2,3-dienyl carbamates. Aspects of their NMR and X-ray data are described and a preliminary reactivity profile is given, including overall SNV-mode ring-opening with organometallic reagents.

Silatropic carbonyl ene cyclisations of β-(allylsilyloxy)- and β-(crotylsilyloxy)butyraldehydes are shown to proceed with high stereoselectivity but at a much reduced rate in comparison to the cyclisation of analogous α-substrates. In the second section, olefin cross-metathesis is explored as a route to substituted α-(allylsilyloxy)aldehydes and the method applied to the synthesis of diastereomeric 2-deoxy- and 2-deoxy-2-C-phenyl hexose derivatives from butanediacetal-protected D-glyceraldehyde.

The synthesis is described of a range of 3-alkylidene-2-oxabicyclo[2.2.1]hept-5-ene and 3-alkylidene-2-oxabicyclo[2.2.2]oct-5-ene derivatives; Claisen rearrangement of these substrates either thermally or in the presence of an added Lewis acid results in the formation of bicyclic cyclobutanones with generally good conversions. These reactions may be performed in hydroxylic solvents, supporting a largely non-dissociative pathway for the rearrangement.

The products obtained from the reaction of diphenylketene with a variety of isocyanides are shown to depend heavily on the concentration of diphenylketene; a high concentration results in the precedented dioxolane derivatives, at much lower concentrations the reactions follow an alternative course and polycyclic β-lactams are generated by a cascade of formal pericyclic reactions.

This review highlights synthetically useful transformations of organic free-radicals in which a key step is relocation of the radical site by intramolecular abstraction either of a hydrogen atom or a group (e.g. phenyl, cyano, trialkylsilyl). The material is organised around the nature of the radical that initiates translocation and coverage is largely confined to reports that have appeared within the past decade, reference to earlier work only being made in order to establish a context for more recent results.

Aziridines formed upon treatment of allylic carbamates and homoallylic sulfamates with Rh(II) carboxylate catalysts under oxidative conditions are trapped by suitably-disposed hydroxyl groups to give functionalised tetrahydrofurans.

Silatropic carbonyl ene cyclisations of β-(allylsilyloxy)- and β-(crotylsilyloxy)butyraldehydes are shown to proceed with high stereoselectivity but at a much reduced rate in comparison to the cyclisation of analogous α-substrates. In the second section, olefin cross-metathesis is explored as a route to substituted α-(allylsilyloxy)aldehydes and the method applied to the synthesis of diastereomeric 2-deoxy- and 2-deoxy-2-C-phenyl hexose derivatives from butanediacetal-protected D-glyceraldehyde.

Two routes are described for the synthesis of the sawaranospirolides, stereoisomeric spirolactone ascorbigenins isolated from Chamaecyparis pisifera. Trapping of the keto enal formed by oxidation of a functionalised 2-(4-hydroxybutyl)furan affords a potential butenolide spiroacetal precursor to sawaranospirolides A and C. Alternatively, epoxidation of protected 3-(dihydropyran-2-yl)-3-arylpropanoic acids results in spirolactonisation to generate ent-sawaranospirolide C; a related acid-mediated spirocyclisation gave access to ent-sawaranospirolide D.

We describe the synthesis of a series of oxy-substituted butenolide spiroacetals and spiro-N,O-acetals by oxidative spirocyclisation of 2-[(4-hydroxy or 4-sulfonamido)butyl]furans. The axial–equatorial preference of each oxy-substituent is investigated (NMR) by an acid-catalysed thermodynamic relay of configuration between the spiro- and oxy-centres. The axial site is preferred for most oxy-substituents at synthetically useful levels. The potential origins of this preference are discussed in terms of a stabilising gauche effect combined with the influence of solvation. These results have relevance to the synthesis of bis(acetylenic)enol ether spiroacetals including AL-1 and related compounds.

Unprecedented bicyclic methylene aziridines are prepared by rhodium(II)-catalyzed allene aziridination of buta-2,3-dienyl carbamates. Aspects of their NMR and X-ray data are described and a preliminary reactivity profile is given, including overall SNV-mode ring-opening with organometallic reagents.

Rhodium-bound nitrenoids are trapped by tethered allenes generating acyloxy-enamines, aminocyclopropanes, and methylene aziridines. The aminocyclopropanes undergo substitution of the acetoxy group by a variety of nucleophiles.

We describe a concise synthesis of (+)-isoaltholactonevia a Au-catalysed cyclisation of a monoallylic diol to form the tetrahydrofuranyl ring. Analogous cyclisations show that the stereochemical outcome is dictated by the stereochemistry of the diol substrate.