1.3- Dienes cyclopent-3-enones

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... 

The cyclopent-2-enone required for the photodimerisation is prepared by the hydrolysis and oxidation of 3-chlorocyclopentene, which is obtained by the low temperature addition of hydrogen chloride to cyclopentadiene. The latter is obtained by heating dicyclopentadiene. This depolymerisation is an example of a reverse (or retro) Diels-Alder cycloaddition reaction the diene readily reforms the dicyclopentadiene on standing at room temperature. 

Reactions of 4-amino-l-metalla-l,3-dienes, generated from (1-alkynyl) carbene complexes 1 by addition of secondary amines (Scheme 32), with alkynes may lead to production of different compounds, depending on the substitution pattern as well as on the reaction conditions employed.3r Whereas successive addition of a secondary amine and one molecule of a terminal alkyne to a (l-alkynyl)carbene complex lj affords cyclopentadi-enes (Scheme 35), addition of two molecules of a terminal alkyne to complex lj yields cyclopenta[h]pyrans (Scheme 36). In an extension of this reaction sequence, (methylene)cyclopent-2-enones 103 are obtained from (1-alky-nyl)carbene chromium compounds 1 (R = rc-Pr, Ph, CH2CH2CH2OTBDS) by addition of secondary amines (NR) = NMe2, morpholino) and disubsti-tuted alkynes (Scheme 37).9e... 

The reaction of 1,3-dithienum tetrafluoroborate with 1,3-dienes was among the first to be studied with this reagent and was found to follow a path which initially produced a bicyclic sulfonium salt (eq 9). Deprotonation of this salt with / butyllithium led to a vinylcyclopropane which rearranged under thermal conditions to a spirodithiane. Hydrolysis of the dithiane moiety gave a cyclopent-3-enone in an overall sequence that is formally equivalent to the addition of carbon monoxide across the terminal carbons of a 1,3-diene. 

This type of reaction can also be catalyzed by Pd complexes. Thus, e. g. hepta-1,6-diene afforded 5-ethyl-2-methyl-cyclopent-2-enone and 2-ethyl-5-methyl-cyclopent-2-enone in 16% yield when reacted with CO at 200 °C and 1000 atm in the presence of a 2% solution of diiodo-bis-(tributylphos-phine) palladium (II) [818]. 

Several syntheses of aldehydes and ketones based on the insertion of carbon monoxide have been reported. Reaction of norbornadiene with monosubstituted acetylenes in the presence of octacarbonyldicobalt gives the fused cyclopen-tenone (35) in high yield conjugate addition of a cuprate and vacuum pyrolysis completes a simple synthesis of 2,3-disubstituted cyclopent-4-enones (36) (Scheme 30)." Conjugated dienes with methyl iodide in the presence of the same cobalt reagent yield conjugated dienones (37) [equation (8)]." In a review of carbonyl insertion reactions, the same reagent is reported to catalyse the reaction... 

The coupling of allylic chlorides with trimethylsilylalkynes in the presence of [PdCl2(PhCN)a] also exhibits good regio- and stereo-selective control to give 4-trimethylsilyl-5-chloro-l,4( )-dienes. Furthermore, upon treatment with fluoride, these produce the corresponding 1,4-enynes. Dicyclopentadienylzirconium diene complexes are readily carbonylated, and the adducts, after hydrolysis with acid, afford cyclopent-3-enones, sometimes in synthetically useful yields. ... 

Grubb s ruthenium carbene catalyst to afford medium-sized carbocycles (Equation (10))." Iron tricarbonyl systems containing diazoketones tethered to the terminal position of the acyclic coordinated diene have been utilized to effect ring formation, via inter- and intramolecular carbene reactions, affording new cyclohexa-2,4-dienone and cyclopent-2-enone tricarbonyl iron complexes. ... 

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