Cyclopentadienone Diethyl Acetal1

(R = Et)

[2931-32-0]  · C9H14O2  · Cyclopentadienone Diethyl Acetal  · (MW 154.21) (R = Me)

[2931-31-9]  · C7H10O2  · Cyclopentadienone Diethyl Acetal  · (MW 126.16)

(isolable cyclopentadienone synthon(s) for use in Diels-Alder and related cycloadditions2)

Physical Data: thermally unstable liquid which rapidly dimerizes in solution.

Solubility: sol pentane, pyridine, hexanes, ether, ethyl acetate; insol water.

Preparative Methods: cyclopentanone acetal (ethyl or methyl) is brominated with Pyridinium Hydrobromide Perbromide. The 2,5-dibromo acetal is double dehydrobrominated using Potassium t-Butoxide-Dimethyl Sulfoxide, the product extracted into cold pentane and used immediately.2

Analysis of Reagent Purity: analysis of the dimeric product.

Handling, Storage, and Precautions: pentane solutions should be used immediately.

Diels-Alder Cycloadditions.

Early attempts to utilize cyclopentadienone in Diels-Alder cycloadditions proved fruitless because spontaneous dimerization occurs.3-5 The same was true of cyclopentadienone ethylene acetal;3-5 thus attention turned to n-alkyl acetals. The diethyl acetal can be prepared in good yield from dehydrobromination of 2,5-dibromocyclopentanone diethyl acetal, but care must be exercised during isolation.2 A solution in pentane left to stand for 24 h produces the dimeric Diels-Alder adduct in 70-95% yield.2 Immediate utilization of the diene allows a versatile range of cycloaddition products to be accessed. A number of phthalimide derivatives have been studied, giving good yields of cycloadducts (eq 1).2,6-9 Elaboration of this strategy involves unmasking of the ketone function, best achieved using Boron Trifluoride Etherate (eq 2),6 allowing full realization of the diene's potential as a cyclopentadienone equivalent. A range of electron deficient dienophiles have been examined in cycloaddition reactions with this diene (eq 3).2,10,11 Cycloaddition with diethyl maleate has been used to prepare an important intermediate en route to norborna-2,5-dien-7-one.10 In the cycloaddition with nitroethylene the [4 + 2] adduct is obtained with an endo:exo ratio of 2.5:1.11 A similar ratio was observed for the product of cycloaddition with acrolein.2

Cycloaddition to the cyclobutadiene iron carbonyl complex has been accomplished; the structurally interesting adduct was used to prepare a homocubyl species (eq 4).12 A [4 + 3] cycloaddition to cyclopentadienone diethyl acetal has been reported, the reactive dienophile involved being generated in situ using a Zinc/Copper Couple (eq 5).13 The cycloaddition mechanism to this allyl cation was elucidated, and determined to proceed firstly by an electrophilic and then by a nucleophilic step.

Reaction of the cyclopentadieneone diethyl acetal with 4-Phenyl-1,2,4-triazoline-3,5-dione (PTAD), followed by hydrogenation of the alkenic bond and liberation of the azo moiety, resulted in isolation of a novel and potentially versatile cycloadduct (eq 6).14

Cyclopentadienone dimethyl acetal, produced by an identical process, undergoes much of the chemistry as the analogous diethyl acetal.2 The rates of dimerization of these acetals have been studied in depth. From these studies it emerged that at 25 °C the dimethyl acetal dimerizes 1.7 times faster than the diethyl acetal (eq 7),2,15 but much slower than the ethylene acetal which proceeds approximately 1070 times faster. This translates into a 500-fold more rapid dimerization for the cyclopentadienone dimethyl acetal than cyclopentadiene, the short half-life explaining the lack of general utility of these dienones.16 Using electron deficient dienes, however, reasonable yields of cycloadducts can be obtained, a pertinent example being addition to an azo-maleimide (eq 8).11

1. Carruthers, W.; Cycloaddition Reactions in Organic Synthesis; Pergamon: Oxford, 1990. Fieser, M.; Fieser, L. FF 1969, 2, 94.
2. Eaton, P. E.; Hudson, R. A. JACS 1965, 87, 2769.
3. DePuy, C. H.; Ponder, B. W.; Fitzpatrick, J. D. AC 1962, 74, 489.
4. Vogel, E.; Wyes, E. G. AC 1962, 74, 489.
5. DePuy, C. H.; Ponder, B. W.; Fitzpatrick, J. D. JOC 1964, 29, 3508.
6. Fuchs, B.; Scharf, G. JOC 1979, 44, 604.
7. Garratt, P. J.; Hollowood, F. JOC 1982, 47, 68.
8. Camps, P.; Castane, J.; Feliz, M.; Figueredo, M. T 1984, 40, 5235.
9. Baasov, T.; Fuchs, B. TL 1982, 23, 1373.
10. Birney, D. M.; Berson, J. A. JACS 1985, 107, 4553.
11. Hoffmann, R. W.; Csomor, J. CB 1976, 109, 1577.
12. Barborak, J. C.; Pettit, R. JACS 1967, 89, 3080.
13. Rawson, D. I.; Carpenter, B. K.; Hoffmann, H. M. R. JACS 1979, 101, 1786.
14. Buchwalter, S. L.; Closs, G. L. JACS 1979, 101, 4688.
15. Abramson, S.; Zizuashvili, J.; Fuchs, B. TL 1980, 21, 2351.
16. Fuchs, B.; Zizuashvili, J.; Abramson, S. JOC 1982, 47, 3474.

Graham B. Jones & Brant J. Chapman

Clemson University, SC, USA

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