Bicyclic enones, carboxylation

Cyclic alkynyl carbonates undergo carbonylation in the presence of a palladium catalyst and carbon monoxide (5 MPa) in MeOH to give allenic carboxylates (Eq. 9.118) [92], Bu3P proved superior to Ph3P as the catalyst ligand. An enynyl cyclic carbonate underwent double vicinal carbonylation at 80 °C to produce a five-membered lactone product in 52% yield (Eq. 9.119). When the reaction was performed at 50 °C, the bicyclic enone lactone was produced in 75% yield along with 10% of the y-lactone. 

It has been demonstrated by Olah et al.420 that a,/3-unsaturated ketones are O-protonated in HF-SbF5 to form hydroxyallylic cations, which were directly observed by NMR spectroscopy. Jacquesy and Coustard have found indirect evidence for diprotonation of a,/3-unsaturated ketones (enones) by trapping the dication with CO.421 The resulting acylium ion centers are then quenched with methanol or benzene. An interesting synthetic method was therefore developed for carboxylation of bicyclic enones in superacid media at atmospheric pressure [Eqs. (5.158) and (5.159)]. 

An interesting method has been developed for the carboxylation of bicyclic enones in HF-SbFs [85]. It has been demonstrated that diprotonated a,/3-unsaturated ketones react with CO to form acylium ions. Quenching of these acylium ions with methanol leads to the corresponding carboxylic esters in good yields (Eq. 39). 

A final synthetic effort in the yohimbine alkaloid area concerns the studies reported by Loewenthal and his coworkers (Scheme 3.92) (143). The aim was to develop an efficient method to prepare the bicyclic enone 21, which serves as a key intermediate in the Woodward reserpine synthesis strategy (19). The route for preparation of 21 began with Friedel Crafts reaction of 2-methoxynaphthalene (523) with the oxalyl chloride equivalent 524, a process which afforded the acenaphthenoquinone 525. Oxidative-decarboxylation of 525 yielded the naphthalene-carboxylic acid 526 which was transformed by Birch reduction and esterification to the dihydro-derivative 527. Carboxyla-tion then provided geminal diester 528 which was epoxidized. Sequential lactonization and methylation afforded tricyclic lactone 529. Ester cleavage with subsequent decarboxylation gave lactone 530 which was demethylated to provide 531. While no further effort was given to the development of this... 

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