Cyclopentenone esters

Cycloalkenones generally perform poorly as dienophiles in Diels-Alder reactions but their reactivity can be enhanced by Lewis acids [105]. SnCU is effective in promoting the Diels-Alder reaction between simple 1,3-butadienes, for example isoprene and piperylene, and cyclopentenone esters (Eq. 67) [106], Cycloaddition does not, however, occur in the presence of SnCU when the diene contains an oxygen-bearing substituent such as an alkoxy or siloxy group. For such compounds, as is generally true for the Diels-Alder reactions of cycloalkenones, other Lewis acids such as zinc chloride are more effective. 

Nazarov cyclization. a, a -Dicnonc esters 1, prepared by acylation/Knocvenagel condensation of various esters, undergo Nazarov cyclization in the presence of trimethylsilyl triflatc to form cyclopentenone esters 2. 

Trimethylsilyl triflate (5 cquiv.) also converts 3-ethoxycarbonyltctrahydro-y-pyroncs (3) into cyclopentenone esters (4) by ring opening followed by Nazarov cyclization. 

In the presence of a double bond at a suitable position, the CO insertion is followed by alkene insertion. In the intramolecular reaction of 552, different products, 553 and 554, are obtained by the use of diflerent catalytic spe-cies[408,409]. Pd(dba)2 in the absence of Ph,P affords 554. PdCl2(Ph3P)3 affords the spiro p-keto ester 553. The carbonylation of o-methallylbenzyl chloride (555) produced the benzoannulated enol lactone 556 by CO, alkene. and CO insertions. In addition, the cyclobutanone derivative 558 was obtained as a byproduct via the cycloaddition of the ketene intermediate 557[4I0]. Another type of intramolecular enone formation is used for the formation of the heterocyclic compounds 559[4l I]. The carbonylation of the I-iodo-1,4-diene 560 produces the cyclopentenone 561 by CO. alkene. and CO insertions[409,4l2]. 

Decenoic acid (143), easily prepared from the same telomer 142, was cyclized via acid chloride using A1C13 to give 2-pentyl-2-cyclopentenone (144). Michael addition of methyl malonate followed by removal of one ester group produced methyl dihydrojasmonate (145) (129) ... 

Cyclopentenones. 1,3-Dicarbonyl compounds add to enol ethers or esters (terminal) in the presence of Mn30(OAc)7 (excess) to form l-alkoxy-l,2-dihydro-furans. These can be converted to a 1,4-diketone, which undergoes aldol cyclization to fused (or spiro) cyclopentenones.1... 

The Ir(lll) complex also funchoned as a catalyst in a tandem Nazarov cyclization-Michael addition. The reaction of monocyclic a-alkylidene-P-keto-y.b-unsaturated ester with nitroalkene gave bicyclic cyclopentenones which possessed an alkyl side chain, with high yield and diastereoselectivity (Scheme 11.36) [47]. 

Pentyl-5 ( 2 ) -carbomethoxyoctyl-2-cyclopentenone and 2(5)-Hexyl-5 (2)-carbomethoxyheptyl-2-cyclopentenone (4). These cyclic ketones constitute about 35% (estimated by GLC) of the neutral diethyl ether extract of the salts and amount to 4.6% of the final reaction mixture of carboxylated linoleate. Analyses by GLC (JXR column) show one peak of retention time of 1.06 relative to that of 1 TLC (silica gel G, diethyl ether-petroleum ether, 1 3) Rf 3.74 compared with 5.04 for 2 IR (neat) 1740 (ester C=0), 1700 and 1630 cm"1 (C=C—C=0) UV max... 

Carbonylation of epoxides.4 The epoxide of a 1-alkene undergoes carbony-lation under catalysis with Co2(CO)8 in the presence of K2C03 (1 equiv.) in ethanol at moderate temperatures to afford (3-hydroxy esters. A by-product is the ketone formed by rearrangement of the epoxide. This reaction provides an essential step in the synthesis of the cyclopentenone 3 from the epoxide (1) of ethyl 10-unde-cenoate. 

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