Photocyclization retro

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). 

Hesse s excellent general introduction to alkaloid chemistry naturally contains numerous references to indole alkaloids, and in particular contains a detailed discussion of the structure elucidation of villalstonine, by way of illustration of modern experimental methods used in alkaloid chemistry.1 Indole alkaloids also receive mention in a brief review of hypotensive principles from plants,20 in a review of synthesis via photocyclization of enamides,2 and in another on the use of lactones in alkaloid synthesis.21 Kametani has also reviewed (unfortunately, in Japanese) the synthesis, by his group, of alkaloids by retro mass spectral synthesis .2d The proceedings of the 1979 meeting of the Phytochemical Society of Europe, which were entirely devoted to indole alkaloids, are now available.3... 

Reduction of bacteriorhodopsin during illumination in the presence of borohydride produces a fluorescent species which absorbs at 360 nm [195,196]. The fine structure of the absorption band of this form is like that of retro-retinal, but the participation of such an isomer in the photocycle of bacteriorhodopsin [215,216] is unlikely. Rather, planarization of the chromophore by constraints of the retinal binding site might be responsible for the fine structure [110]. 

Allenes have been added to many cyclic a,/8-unsaturated ketones to give methylenecyclobutane derivatives. The stereochemistry of such additions has been rationalized by Wiesner, who has used the method extensively in the synthesis of diterpenoid alkaloids. For example, the synthesis (Scheme 10) of talatisamine (39) involved the cycloaddition of allene to the enone (35) to give the photoadduct (36) which was converted into the masked aldol (37). Retro-aldol reaction followed by another aldol cyclization gave (38) which was converted into talatisamine (39). Intramolecular photocyclizations of appropriate allenic enones can also be expected to be synthetically useful. The formation of (41) from (40) in high yield is a promising reaction. A closely related process is the cyclization of (42) to (43) (Scheme 11). ... 

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