2- Bromo-2-cyclopentenone

Bromo-2-cyclopentenone ethylene ketal was prepared according to a published procedure. The compound is quite unstable and should be purified b distillation before use to remove impurities. The submitters stored the... 

The preparation of (S)-(-)-menthyl p-toluenesulfinate described in Part A is based upon the procedure reported by Solladie. 2-Bromo-2-cyclopentenone ethylene ketal is available from 2-cyclopentenone by the procedure of Smith and co-workers. The present procedure has been used by the submitters to prepared analogous chiral ci-sulf1nyl a,B-enones (Table I).11 The utility of these chiral synthons is enhanced by their stability, the facility of their... 

Bromo-2-cyclopentenone ethylene ketal l,4-Dioxaspiro[4.4]non-6-ene, 6-bromo- (9) (68241-78-1)... 

B. 2-Bromocyclopentenone ethylene ketal. A solution of 22.00 g (136.7 mmol) of freshly distilled 2-bromo-2-cyclopentenone, 21.80 g (351.2 mmol) of ethylene glycol, 1.5 L of benzene (Note 4), and 60 mg of p-toluenesulfonic acid monohydrate is refluxed for 64 hr (Note 5), with azeotropic removal of water, in a 3-L, round-bottomed flask, equipped with a Dean-Stark trap, condenser, and Drierite drying tube. The solution is cooled to room temperature, dried with potassium carbonate, and filtered by vacuum through 15 g of Celite. The filter cake is washed with 150 mL of benzene. Removal of the solvent under reduced pressure yields a mobile yellow oil. Distillation (65-67°C, 0.7 mm) affords 22.4 g (109.0 mmol, 80%) (Note 6) of the ketal (Note 7). 

A. 2-Bromo-2-cyclopentenone. In a well-ventillated hood, a solution of 18.98 g (231.2 mmol) of 2-cyclopentenone (Note 1) in 150 mL of carbon tetrachloride is added to a 1-L, three-necked, round-bottomed flask fitted with a mechanical stirrer, thermometer, and an addition funnel. The solution is chilled to 0°C with an ice bath and a solution of 40.5 g (253.4 mmol, 13.0 mL) of bromine in 150 mL of carbon tetrachloride is added dropwise during 1 hr. Then a solution of 35.1 g (346.8 mmol, 48.3 mL) of triethylamine in 150 mL of carbon tetrachloride is added dropwise over 1 hr with vigorous stirring while the reaction is held at 0°C. Stirring is continued for an additional 2 hr at room temperature the resulting dark suspension is filtered with suction and the filter cake washed with carbon tetrachloride. The filtrate and washings are combined and washed with two 100-mL portions of 2 N hydrochloride acid, one 100-mL portion of saturated sodium bicarbonate solution, one 100-mL portion of water, and one 100-mL portion of saturated sodium chloride solution. The resultant solution is dried over anhydrous magnesium sulfate, filtered, and the solvent removed under reduced pressure. Distillation of the resultant oil (69-78°C, 1.0 mm) afforded 23.7 g (147.2 mmol, 64%) (Note 2) of a white crystalline solid (mp 36-37°C, lit.2 mp 39-39.5°C) (Note 3). 

Pure 2-bromo-2-cyclopentenone ethylene ketal displayed the following spectroscopic properties IR (CC14) cm 2975 (s), 2950 (s), 2880... 

Extension to the natural (+) series was accomplished by transfer of chirality from sulfur to carbon via (+ )-2-tolylsulfinyl-2-cyclopentenone, 7 (Scheme 2). The latter was prepared in optically pure form from the ethylene ketal of 2-bromo-2-cyclopentenone 5 by lithiation and treatment with (- )-menthyl p-to-luenesulflnate to yield ( + ) 6, followed by deketalization. Conjugate addition of... 

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