Dienophiles cyclopentenone

More substituted and unactivated dienophiles such as 3-acetoxy-2-methyl-cyclopentenone, did not react with 112 even in the presence of BF3 etherate. 4-Acetylspiro[2.1.2.3]decen-9-one (128) could also be isolated under these con-... 

Cyclopentenones. One limitation to Jung s cyclopentenone synthesis (8, 178) is that telrasubstituted alkcncs react slowly, if at all, with this diene (I). An expedient in the use of a dienophile such as isobutenyl acetate (2), which does react, even if slowly, to give 3 as the major adduct. The adduct can be converted in several steps lo a cyclopcntcnonc such as 4. The cyclopentenone 4 (R = p-tolyl) has been converted to /(-cuparcnonc (5).2... 

Diels-Alder reactionsThis reagent is more reactive in Dicls-Alder reactions than cyclohcxcnone. Treatment of the /i-nitro ketone with DBN effects elimination of HNO, to form the a,/ -enone.2 Thus the final products correspond to adducts of a "cyclohcxynone, but with reversed regioselectivity. 3-Nitro-2-cyclopentenone, m.p. 69- 7T, is also a useful dienophile (second example). 

The domino carbonylation and Diels-Alder reaction proceed only as an intramolecular version. Attempted carbonylation and intermolecular Diels-Alder reaction of conjugated 2-yne-4-enyl carbonates 101 in the presence of various alkenes as dienophiles give entirely different carbocyclization products without undergoing the intermolecular Diels-Alder reaction. The 5-alkylidene-2-cyclopenten-4-onecarboxy-lates 102 were obtained unexpectedly by the incorporation of two molecules of CO in 82% yield from 101 at 50 °C under 1 atm [25], The use of bidentate ligands such as DPPP or DPPE is important. The following mechanism of the carbocyclization of 103 has been proposed. The formation of palladacyclopentene 105 from 104 (oxidative cyclization) is proposed as an intermediate of 108. Then CO insertion to the palladacycle 105 generates acylpalladium 106. Subsequent reductive elimination affords the cyclopentenone 107, which isomerizes to the cyclopentenone 108 as the final product. 

Cyclopentenones. l-Ethynyl-2-propenyl acetates (ly in the presence of this Pd(ll) salt and acetic acid (1 equiv.) cyclize to cyclopentenones (2) in 50-90% yield. The cyclopentadiene a has been identified as an intermediate by trapping with a dienophile. 

Numerous examples of the preparation of versatile and valuable starting materials are also evident. Current theoretical interest in adamantane is reflected by a convenient procedure for its preparation (p. 8). 2-Cyclopentenone (p. 38) and 2-cyclopenten-1,4-dione (p. 36) are not only useful dienophiles for Diels Alder reactions but also attractive starting materials for a variety of compounds of theoretical interest. Similarly, the preparations of allene (p. 12), tetrolic acid (p. 97), ethylene sulfide (p. 59), and 2-norbornanone (p. 79) provide convenient access to exceptionally useful compounds. 

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. 

The use of nickel catalysts allows the homo-Diels-Alder reaction to occur between norbornadiene and a-unsaturated ketones and sulfoxides to give the products 8 often with excellent yield and stereoselectivity. Whereas the exo preference, usual in the catalytic homo-Diels-Alder reactions, is here evident for the open-chain dienophiles, the endo selectivity observed with cyclopentenone could be due to its s-trans conformation." ... 

The simplest mechanism consistent with the experimental facts is depicted for 2-cyclopentenone in equation 12. While initial attack by the phosphorus reagent at the carbonyl (eqs. 13 and 14) is conceivable, subsequent rearrangement of phosphorus from oxygen to carbon via a phosphorane intermediate is not possible, due to the restricted geometry of the intermediate in equation 13. Since only the 7-ketocyclopentyl-phosphonate ester (72%) and none of the enol phosphate ester (eq. 13) or hydroxyphosphonate ester (eq. 14) were detected in the product, attack by phosphorus is exclusively at the terminal carbon atom of the conjugated system. It is reasonable to assume that a similar mechanism holds for the majority of dienophiles. 

Arene cis-diols can undergo cycloaddition reactions, acting as both diene and dienophile to produce homoadducts (12). Heteroadducts have also been synthesized, for example recent work by Banwell and coworkers exploited the reaction of toluene cw-diol with cyclopentenone as the initial step in their synthesis of complicatic acid and related triquinanes (20). We found that upon prolonged standing at room temperature in concentrated form, the acetorddes 5a and 5b were slowly converted to the Diels-Alder adducts depicted in Figure 2. 

The use of oxazole-alkene Diels-Alder cycloadditions to form biologically relevant molecules has recently been applied to the synthesis of isoindoles, useful intermediates for the preparation of substance P antagonists. Thus 5-ethoxy-4-methyloxazole 8 reacted with 4,4-dimethyl-2-cyclopentenone 78 in refluxing benzene in the presence of catalytic zinc bromide to give the (l//)-cyclopenta(c)-pyrrole 80 as a separable 3 1 mixture of cis and trans isomers in 53% yield (Fig. 3.22). The reaction is presumed to proceed via the intermediacy of cycloadduct 79. When cyclohexenone 81 was used as the dienophile, the product was the hydro-(l//)-isoindole 82, obtained via dehydroformylation of the cycloadduct, in 85% yield after only 30 min in refluxing benzene. 

Diels-Alder Reactions.—Cyclopentenone is a notoriously poor dienophile, but its a-phenylthio-derivative (140) reacts cleanly with excess butadiene (catalysed or uncatalysed) to give (141) or (142) after appropriate desulphurizations. " Thus, in the latter case, (140) acts as a cyclopentynone equivalent. 

Ishibashi, M., Takeuchi, S., and Kobayashi, J. (1993b) Untenone A, a new cyclopentenone from the Okinawan marine sponge Plakortis sp. corresponding to the dienophile in the biosynthesis of manzamenones. Tetrahedron Lett., 34, 3749-3750. 

---