Cycloaddition precursor cyclopentenone

In cycloadditions of enones to alkenes novel strategies have been adopted for ring expansion of the cycloadducts, either by the choice of appropriate alkenes, e.g. 2-(trimethylsiloxy)buta-1,3-diene,70 vmv-2-trimethylsiloxybuten-2-oales71 or 3,3-dimethylcyclopropene,72 or by using 3-oxo-l-cyeloalkene-l-carboxylates as enones.73 Asymmetric [2 + 2] photocycloaddition of cyclopent-2-enone to a (+ )-dihydrofuran acetonide constitutes the cornerstone of the synthetic strategy in the first total synthesis of the novel antitumor metabolite ( )-echinosporin.74 The cycloaddition product 25 from treatment of 2-(2-carbomethoxyethyl)-2-cyclopentenone (24) with ethene has been used as a precursor for the preparation of tricyclo[4.2.0.01,4]octane.75... 

Numerous synthetic applications of the inteimolecular Pauson-Khand reaction have been reported. Pauson has reported a number of very direct tqrplications of cycloadditions of ethylene in the synthesis of prostanoids and jasmone analogs (e.g. equations 15 and 16). - This is a reliable entry to 2-sub-stituted cyclopentenones. The suitability of cyclopentene and dihydrofuran as substrates has permitted the extension of this work to the preparation of still further varieties of prostaglandin analogs (e.g. equations 27 and 51). Simple 4,5-disubstituted 2-cyclopentenones are not as directly accessible, but may be prepared from the cycloaddition products of norbomadiene (equation 45). A sequence of conjugate addition followed by retro-Diels-Alder reaction affords the product (Scheme 5). Dihydrofuran cycloadditions have been used by Billington in the syntheses of the antibiotic methylenomycin B (Scheme 6), as well as cyclomethylenomycin A (synthetic precursor to the antibiotic methylenomycin A), cyclosarko-mycin (precursor to the antitumor agent sarkomycin) ° and the iridoid Jq>anese hop ether. ... 

Isoxazolidines are often used as synthetic precursors of different classes of compounds. Intramolecular cycloaddition of sugar-derived aUcene-nitrones was applied to the synthesis of some chiral cyclopentenone building blocks and natural compounds as exemplified by the synthesis of (-)-neplanocin A 94 starting from D-ribose <05JOC6884>. 

The Pauson-Khand reaction is a cobalt-mediated cycloaddition of an alkene, an alkyne, and carbon monoxide to yield cyclopentenones." When employing reactive precursors (acetylene or terminal akynes), the reaction proceeds in excellent yield however, chemoselectivity issues can contribute to decreased reaction yields and unwanted side products in cases where the alkyne component competitively undergoes cyclotrimerization... 

The use of arylamines in the Pd"-catalysed coupling with olefins has been reported. This modification of the classical Heck procedure further extends the utility of the method. A study of the regiochemistry of the cycloaddition of substituted trimethylenemethanepalladium complexes is necessary if they are to be generally adopted in synthesis. Reaction of cyclopentenone with a methyl-substituted trimethylenemethane species afforded a single stereoisomeric product, which was converted to a precursor of the natural product chrysomelidial (Scheme 24). Since previous work on the unsubstituted system suggested a... 

SCHEME 3 Synthesis of the cycloaddition reaction precursor from cyclopentenone. 

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