Cyclopent-2-enone ring

Cyclopent-3-enone ring from a-cyanoketones Johnson-Stobbe condensation... 

Cyclopent-2-enone rings can be easily obtained under mild conditions from a,a-dibromoketones and enamines in the presence of iron carbonyls Spiro-annelation with S-cyclopropylids provides easy access to spiropentane derivatives and similar compounds . Ring expansion from 5- to 7-membered rings has been efficiently performed with cyclopropenes The transformation of nerolidol to the tricyclic a-cedrene has been achieved by a remarkable stepwise cyclization process... 

Methanesulfonic acid phosphorus pentoxide Cyclopent-2-enone ring from 2-acetylenel,4-diols... 

Cyclopent-2-enone ring. A mixture of startg. iodonium salt and K-/cr/-butoxide allowed to react in tetrahydrofuran at — 78° until reaction complete product. Y 61 %. The formation of the bicyclic compd. can be explained by regioselective 1,5-carbon-hydrogen insertion of the alkylidenecarbene intermediate. F.e.s. M. Ochiai et al., J Am. Chem. Soc. 110, 6565-6 (1988). 

Cyclopent-2-en-2-ol-l-one ring closure 21, 936 Cyclopent-2-enone ring... 

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... 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

Simple diastereoselectivities are observed in the reaction of cyclo-alkenones with cyclic alkenes, e.g., cyclopentene, wherein the transoid tricycle is formed preferentially. Whereas cyclopent-2-enone (7) affords 36 selectively, the more flexible cyclohex-2-enone (8) gives a 3 1 mixture of diastereoisomers 37 and 38 [7,61]. A stereogenic center in the cycloalkenone also has a strong impact on the product ratio as shown for the reactions of 4-alkylcyclohex-2-enones 39 and 40 with acyclic alkenes wherein the major diastereoisomer formed is the one in which the enone-alkyl group is trans to the new ring forming C-C bonds, i.e., 41t and 42t, respectively (Sch. 12) [62,63]. Cycloadducts 42 have been further converted to the pheromone periplanone-B. 

With ring-substituted methylenecyclopropanes, endojexo mixtures of [3-1-2] cycloadduct 9 or its isomer 10 are formed in the reaction with cyclopent-2-enone, depending on the actual type of substituents in the reactant. ... 

FIGURE 16 6 Polystyrene/divinylbenzene sulfmate [432] resin was used to prepare the indicated sulfone oxirane on a solid phase. This was subjected to epoxide ring-opening with Grignard and cuprate reagents. Subsequent oxidation of the secondary alcohol was accomplished with simultaneous release from the resin, affording substituted cyclopent-2-enones. 

When we extended the activated olefins to cyclohept-2-en-l-one and cyclooct-2-en-l-one we found that the reaction is very complicated, because the Lewis bases, solvents and the ring-size of the cyclic enone can all significantly affect the MBH reaction rate and even the reaction product. The reaction of cyclohept-2-en-l-one with A-arylidene-4-methylbenzenesulfonamides afforded the usual MBH adducts 203 along with abnormal adducts 204, whereas the corresponding reaction of cyclooct-2-en-l-one provided different aldol products (205-207) depending upon the Lewis base employed (no MBH adduct was formed in any of the cases) (Scheme 2.104). Moreover, the formation of aldol products 209 along with the usual MBH adducts 208 has been observed in the reaction between cyclopent-2-en-l-one and aromatic aldehydes in the presence of BU3P (Scheme 2.105). 

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