Current Organic Chemistry - Volume 6, Issue 13, 2002

Acetylene is a multi-thousand ton chemical feedstock now mainly produced by pyrolysis of hydrocarbons. In a view of the rapid depletion of hydrocarbon resources, acetylene and its derivatives, which can alternatively be manufactured from coal and other carbon-containing materials, including wastes, are expected to acquire increasingly more important role as universal chemical intermediates and building blocks. The high and flexible reactivity of these compounds makes the syntheses with their participation easier and less energyconsuming than those with alkenes.Nucleophilic attack at the acetylenic moiety leads to diverse vinyl compounds, acetylenic alcohols, allenes, 1,3- dienes and heterocycles.In the review, the recent advances both in modernization of classic reactions of acetylene and its derivatives with nucleophiles (superbase-catalyzed vinylation of alcohols, polyols, thiols, ethynylation of carbonyl compounds, prototropic rearrangements) are analyzed from modern positions. The new reactions: direct vinylation of sulfide, selenide and telluride ions, elemental sulfur, selenium, tellurium, phosphorus, oximes and amidoximes, synthesis of pyrroles from ketoximes, direct synthesis of 2-vinyloxy-1,3-alkadienes by superbase-catalyzed hydrative trimerization of acetylene are also considered within the same framework.A special emphasis is made on the activation of acetylenes and nucleophiles in superbase media such as MOH / DMSO, MOH / NMP (M = Na, K, Rb, Cs) and the like.Quantum chemical calculations of the complexing of acetylene with MOH and its role in the nucleophilic reactions of acetylene and its derivatives are revisited.Most recent syntheses of functionalized pyrroles, thiophenes, dihydropyridines, quinolines and other fundamental heterocycles by the reaction of carbanions of propargyl and allene derivatives with isothiocyanates are also briefly reviewed.

Recent developments in ruthenium-catalyzed chemo-, regio-, and stereoselective C-C bond forming reactions as well as a C-C bond cleaving reaction via an η3-allylruthenium intermediate are disclosed, in which novel ambiphilic reactivity of η3-allylruthenium complexes is discussed. In addition, a new route to cyclopentenones via ruthenium-catalyzed carbonylative cyclization of allylic carbonates with alkenes, which would be the complement of the catalytic intermolecular Pauson-Khand reaction, is also presented.

Heterocycles present the core of many biologically or pharmaceutically interesting compounds. A new concept that addresses crucial issues of heterocycle synthesis such as selectivity of the actual ring closure step and tolerance of other functional groups, employs the cumulated ylide keteneylidenetriphenylphosphorane Ph3P=C=C=O. Featuring a unique combination of ylidic and ketene properties in a dipolar electronic structure, it reacts with derivatives of carboxylic acids bearing additional OH, NH-, SH- or CH-acidic groups. This reaction proceeds by a domino addition-Wittig olefination giving rise to the formation of the corresponding heterocycles with five- to seven-membered rings. Further pericyclic steps such as Diels-Alder cycloadditions, Claisen rearrangements, ene reactions, and various combinations thereof may ensue with the newly installed C=C bond. In some cases, the outcome of these processes is fully controllable by merely adjusting the external conditions. For example, allyl and cinnamyl α-hydroxycycloalkanoates could be selectively converted either to Claisen rearranged 3-allyltetronic acids or to Claisen-Conia rearranged 3-(spirocyclopropyl)-dihydrofuran-2,4-diones. The former furnished anti-HIV-active 5-spiro-3-(α-cyclopropylbenzyl)-tetronic acids upon Simmons-Smith reaction and the latter could be stereoselectively ring-opened with alcohols, amines and water to give herbicidal 3-(β-synalkoxy / amino)tetronic acids. Congenerous esters with di- or trialkyl substituted allyl residues undergo a formal [2,3]-sigmatropic rearrangement also proceeding via 3-(spirocyclopropyl)dihydrofuran-2,4-diones but leading to 3- exo-alkylidene-5-spirodihydrofuran-2,4-diones which in turn rapidly autooxidize to give potentially antimalarial spirotricyclic hemiketal endoperoxide lactones as products of an overall seven-step cascade.This review takes stock of these and other recent developments in the field of domino Wittig-pericyclic synthesis of bioactive heterocycles.

Paprika, Capsicum annuum is one of the oldest, most important and widely used carotenoid food colorants. The composition of the carotenoid pigments produced by paprika has been investigated for a long time. The red carotenoids in paprika are mainly capsanthin, capsorubin, and capsanthin 5,6-epoxide, possessing a 3- hydroxy κ-end group. At the same time, the fruits are also rich in yellow xanthophylls such as β-cryptoxanthin, zeaxanthin, and antheraxanthin, as well as β-carotene. In the past fifteen years many other carotenoids with interesting structures, especially those with the 5-hydroxy-5,6-dihydro-3,6-epoxy-β (oxabicyclo)-end group (cycloviolaxanthin, cucurbitaxanthin A and B, capsanthin 3,6-epoxide, cucurbitachromes), the 3,5,6-trihydroxy-5,6- dihydro-β-end group (5,6-diepi-karpoxanthin, 6-epikarpoxanthin, 5,6-diepilatoxanthin, 5,6-diepicapsokarpoxanthin) and the 6-hydroxy-γ-end group (nigroxanthin, prenigroxanthin), have been isolated and characterized. Capsanthone and capsanthone 3,6-epoxide, possessing a 3-oxo-κ-end group, were also isolated from red paprika.This review gives a summary of the analyses of different kinds of paprika and the isolation and structure clarification of the above mentioned carotenoids, paying special attention to the stereochemistry of the 3,5,6- trihydroxy-5,6-dihydro-β-end groups. The occurrence of these carotenoids in other natural sources is also demonstrated. The formation of these carotenoids in red paprika may be interrelated with the biosynthesis of carotenoids containing the κ-end group. The biosynthesis of paprika carotenoids is discussed, focusing on the possible biosynthetic route in the red paprika, in comparison with that of yellow paprika.