4.5- disubstituted cyclopentenones

There are two possible modes (a and b) for opening of the cyclopropane ring however only mode a, which gives rise to 4,5-disubstituted cyclopentenones, occurred. ... 

An early use of the norbornadiene reaction in synthesis employed conjugated addition followed by retro-Diels-Alder to deliver 4,5-disubstituted cyclopentenones [Eq. (42)] [102]. More recently, this approach was adopted with considerable methodological improvement (including use of dimethylsulfoxide (DMSO) as a promoter/solvent) to a preparation of substituted cyclopentadienyl ligands for use in transition metal chemistry [Eq. (43)] [103]. 

The generality of these observations is supported by the data in Scheme 8. In the case of the reactions of monosubstituted l-(l-alkynyl)cyclopropanols 15, 5-substituted cyclopentenones 17 are obtained with complete selectivity when trans-15 are used as substrates. On the other hand, 4-substituted cyclopentenones 16 are obtained with good to high regioselectivity by employing ferf-butyl-dimethylsilyl ethers of cis-l5 as substrates. Furthermore, 5,5-disubstituted cyclopentenones 20 including a spirocyclic derivative are obtained selectively starting from 2,2-disubstituted l-(l-alkynyl)cyclopropanols 18. 

Cyclocarbonylation cyclopentenones.1 The adducts of allyl halides and al-kynes,3 particularly those with ds-configuration, can be converted into mono- or disubstituted cyclopentenones by reaction with Ni(CO)4 in CH3CN containing CH3OH and N(C2H5)3. 

The Woodward-Hoffman rules also predict that in a given cyclization mode a permutation of alkene geometry must be reflected in the configuration of the products. This test is precluded under the normal reaction conditions (acid, light) which would isomerize the dienone double bonds. However, Corey recently reported the formation of a c -disubstituted cyclopentenone from a (Z, )-precursor, derived fiom an allene oxide, which cyclizes via the 2-oxido pentadienylic cation (Section 6.3.8). ... 

Another characteristic general photochemical reaction of a,j3-epoxyketones is the transformation of disubstituted cyclopentenone oxides to 2-pyrone derivatives (Eq. 

This reaction was first reported by Hunsdiecker in 1942. It is the synthesis of 2,3-disubstituted cyclopentenones via an intramolecular condensation of 1,4-diketone (or y-dicarbonyl compounds) by means of base treatment. When 2-acetyl ketone is treated with a base, two possible cyclopentenones—2-alkyl-3-methyl cyclopentenone and 3-alkyl cyclopentenone might form. However, only former has been reported as the isolated product because the intramolecular condensation occurs via a reversible and the tran-... 

The hydroacylation of alkynals was also reported by Tanaka and Fu, who found that the Rh(I)-Tol-BlNAP system was the catalyst of choice for the hydroacylation-desymmetrization of 3-6is-alkynals 194 to give 4,4-disubstituted cyclopentenones 195 in excellent yields and high enantioselectivities. Cyclopentenones are important intermediates in the synthesis of natural products such as prostaglandins. This catalyst system was also found to be extremely effective in the kinetic resolution of racemic 3-disubstituted alkynals 196 giving the unreacted aldehyde 197a with near perfect enantioselection. 

The diastereoselective formation of C(4),C(5)-disubstituted cyclopentenones has been reported involving formation of a pentadienyl cation via diastereoselective oxidation of a vinyl alkoxyallene followed by conrotatory electrocyclization (Scheme 96). ... 

Oxidation-initiated Nazarov cyclization of vinyl alkoxyallenes provides a mild method for the diastereoselective formation of C4, C5-disubstituted cyclopentenones (Scheme 6.34) [38],... 

Acyclic y-epoxy-sulphones have been converted into cyclobutenones (Scheme 28)while cyclic /8-epoxy-sulphones have been used for the synthesis of a,/8-disubstituted cyclopentenones (Scheme 29). 

With a similar strategy of combined diarylprolinol silyl ether and N-heterocyclic carbene catalysts, j0rgensen and coworkers [21] examined the cascade reaction of easily accessible i-keto heteroaryl-sulfones as nucleophiles with a,P-unsaturated aldehydes (Scheme 43.11). In this cascade reaction, following the initial iminium ion-catalyzed Michael reaction of nucleophiles to a,(i-unsaturated aldehydes, the subsequent step was then promoted by carbene catalyst 57 to afford 2,4-disubstituted cyclopentenones 55 via an intramolecular benzoin condensation initiated Smiles rearrangement. The superiority of combinational use of two catalysts in the similar Michael/benzoin cascade reaction was also independently demonstrated by Enders et al. (Scheme 43.12) [22]. 

Laronze, J.Y., Boukili, R.E., Cartier, D., Laronze, J, and Levy, J. (1989) The Sheradsky rearrangement of a,a-disubstituted cyclopentenone aryloximes a synthesis of the sesquiterpenes ( )-aplysin and ( )-filiformin. Tetrahedron Lett., 30, 2229-2232,... 

The sequence of chiral 1,4-reduction of a fi-substituted cyclopentenone followed by electrophilic trapping of the intermediate enolate provides an efficient route to chiral 2,3-disubstituted cyclopentanones that generates two chiral centers in the process (Eq. 352)459... 

In early 1994, Padwa et al. (119) synthesized both mono- and bicyclic core skeletons of the illudins and ptaqualosins. By utilizing a 1,1-disubstituted cyclopropane as the core of the dipolar cycloaddition, Padwa was able to add a number of dipolarophiles including cyclopentenone and cyclohexenone to produce cycloadduct 233 amenable to subsequent transformations to form the illudins and ptaqualosins. The cycloaddition forms the bicyclic constrained ether in... 

Some selected examples of such a pinacol rearrangement applied in the preparation of cy-clopentanones are shown in Table 2 further examples can be found in refs 35-39. A number of cyclopentenones, 2,2-disubstituted cyclopentane-1,3-diones and 2,2-disubstituted 3-methylenecyclopentanones were synthesized by this method. 

Vinylcyclobutanones may be converted to cyclohex-2-enones 1 on treatment with Eatons reagent (methanesulfonic acid/phosphorus pentoxide).69 The yields are usually only moderate due to a side reaction leading to cyclopentenones. This side reaction is especially important for 2,2-disubstituted cyclobutanones (R3 4 H).69 In all cases the carbonyl group migrates to the /(-position of the double bond. 

Dlsubstituted 2-cycbpentenones. Cross-conjugated dienones, or the corresponding ethylene kelals, are cyclized almost entirely to 2,3-disubstituted 2-cyclo-pentenones by H3P04-HCOOH (1 1) or HBr-CHjCOOH (1 3). 3,4-Disubstiluted 2-cyclopentenones are formed, if at all, in low yield. 

Chiral 4,4-dialkyl-l-cyclopentenones.1 The chiral bicyclic lactam 2, derived from levulinic acid and 1, on monoalkylation exhibits slight if any selectivity regardless of the electrophile. However, a second alkylation exhibits high endo-selectivity. This product (3), after reductive cleavage, furnishes a keto aldehyde that is cyclized by base to a chiral 4,4-disubstituted-2-cyclopentenone (4). Either antipode of 4 can be prepared by the sequence of alkylation. 

Acylation of alkynes, cyclopentenones. Acylation of alkynes with a,a-disubsti-llllotE/f, y-unsaturated acid chlorides, catalyzed with A1C13> results in 5,5-disubstituted-2-cyclopentenones by an unexpected intramolecular cyclization-rearrangement.2 Example ... 

Three-component reactions involving zirconocyclopentadienes have been also employed in cyclopentenone synthesis. The method combines disubstituted alkynes, isocyanates, and arylidene or alkylidene malononitriles to assemble polysubstituted cyclopentenones 171 (Scheme 8.77) [160],... 

The 3(2H)-furanones are - as the other 4-hetero-2-cyclopentenones - normally 2,2-disubstituted to avoid enolization to the respective 3-hydroxyfuran. If one of the substituents is an alkenyl side chain, then intramolecular [2 + 2]-photocydoaddition reactions are possible with the regioselectivity being dependent on the chain length (Scheme 6.20). The allyl-substituted substrate SO (n— 1) gave predominantly the formal straight product SI [62], while the butenyl-substituted substrate 50 (n = 2) resulted in formation of the crossed product 52 [63]. 

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