Retro aldol fission

An ingenious new method for the preparation of Woodward s intermediate (97) by Pearlman70 constitutes another formal synthesis of reserpine. The essence of this new approach is an adaptation of the de Mayo reaction which allows the introduction of vicinal aldehyde and acetic ester functions on to a double-bond (Scheme 13). An internal [2tt + 27r] photocyclization of the diene (98) gave the tetracyclic cyclobutane derivative (99), which was converted into the ester (100) by standard procedures. Methanolysis of the acetate function with concomitant retro-aldol fission completed the introduction of the vicinal aldehyde and acetic ester functions obvious manipulation then gave the desired intermediate (97). 

Irradiation at 254 nm of the imidate (Scheme 2) in aqueous solutions results in the formation of 4-cyanophenol and A -isopropylbenzamide. The photochemical step involved in the formation of the products has been shown to be C-O bond fission with the production of the phenoxide and the benzonitrilium ion. Mariano and co-workers have reported a detailed study of the electron-transfer photochemistry of a-anilino carboxylates, p-anilinoalcohols and a-an-ilinosilanes. This study has measured the rates of decarboxylation of aniliniumcarboxylate radicals. The base induced retro-Aldol fragmentations of the radical-cations formed from the p-anilinoalcohols and the influence of substituents on the nitrogen on the desilylation of the a-anilinosilanes was also investigated. In addition, the synthetic potential of some of the electron-transfer photochemistry of the carboxylate salts (181) and (182) has been examined. In these examples irradiation, using DCA in methanol or acetoni-... 

The acid-catalyzed fission of macralstonidine can then be explained by the reversal of these processes, i.e., as a combination of retro Michael and retro aldol cleavages on the phenolic hydroxyketone LVII (35). 

A novel variant on this theme is contained in Coates synthesis of (13), which contains the ring skeleton of the sesterterpene ophiobolin. In this case reductive fission of the cyclobutane ring [(12) - (13)] was used instead of the more usual retro-Aldol cleavage. 

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