2-BROMOCYCLOPENTENONE

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). 

There are a couple of examples of cascade processes starting by a Michael-type addition of a carbon nucleophile proceeding under phase-transfer eatalysis conditions which deserve to be mentioned at this point. The first one eonsists of an enantioselective cyclopropanation of 2-bromocyclopentenone by a cascade Michael/intramolecular nucleophilic displacement in which a variety of C-H acidic carbon pro-nucleophiles such as nitromethane, cyanomethylsulfone and benzyl cyanoacetate reacted with this Michael acceptor in the presence of a quinidinium salt of type 67 (Scheme 7.79). In addition, the conditions needed to be optimized for each Michael donor employed, requiring a different catalyst and inorganic base for each case. Under the best conditions, the final cyclopropanes were obtained in moderate yields and enantioselectivities, albeit as single diastereoisomers. 

Scheme 7.79 Asymmetric cyclopropanation of 2-bromocyclopentenone by cascade Michael/a-alkylation.

Careful examination of our retrosynthetic options suggested that bromocyclopentenone acetal 42 and bromocyclopentadiene 45 might serve our needs. To this end, we advanced to bromo enone 41 as a common link (Scheme 7).16 Despite the steric bulk offered by the... 

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