Photochemical Key Steps

This textbook contains a collection of multistep experiments that all feature one or two photochemical key steps. More than 40 researchers active in the field of organic photochemistry have contributed their favorite experiments to this unusual and modern textbook,... 

In summary, our own synthesis of kelsoene comprised 18 steps starting from (-)- -citronellene (50) and 14 steps starting from the previously described malonate rac-49. Despite its powerful photochemical key step rac-65 rac-66 the subsequent epimerization at carbon atom C-6 is a major drawback. An alternative enantioselective access to key intermediate 65lent-65 was not pursued. 

Both target compounds discussed in this review, kelsoene (1) and preussin (2), provide a fascinating playground for synthetic organic chemists. The construction of the cyclobutane in kelsoene limits the number of methods and invites the application of photochemical reactions as key steps. Indeed, three out of five completed syntheses are based on an intermolecular enone [2+2]-photocycloaddition and one—our own—is based on an intramolecular Cu-catalyzed [2+2]-photocycloaddition. A unique approach is based on a homo-Favorskii rearrangement as the key step. Contrary to that, the pyrrolidine core of preussin offers a plentitude of synthetic alternatives which is reflected by the large number of syntheses completed to date. The photochemical pathway to preussin has remained unique as it is the only route which does not retrosynthetically disconnect the five-membered heterocycle. The photochemical key step is employed for a stereo- and regioselective carbo-hydroxylation of a dihydropyrrole precursor. 

Mattay J, Griesbeck A. Photochemical Key Steps in Organic Synthesis. Weinheim, New York, Basel, Cambridge, Tokyo YCH, 1994. 

Numerous applications of PET cyclization reactions in organic synthesis appeared over the last years [65], and we have tried to summarize important target compounds in the following Table 1. Since most preparations follow multi-step procedures, we have additionally included the photochemical key-step. The interested reader may refer to the original article for further details. 

A number of additional examples can be find. In Mattay J, Griesbeck AG, eds. Photochemical Key Steps in Organic Chemistry. Weinheim VCH, 1994. 

Fenestranes represent particular targets. Like steroids, they are con-formationally rigid, strained, and chemically robust molecules. These properties make them interesting for application in various fields [78]. Fenestranes 103 [79], 104 [80], and 105 [78] (Sch. 20) were synthesized via met a photocycloadditions as photochemical key step. 

Propellane derivatives are available via intermolecular meta cycloaddition. Compounds 106 [81,82] and modhephene 107 [81] were obtained using this type of photocycloaddition (Sch. 21). Isoiridomyrmecin 108 [83] and decarboxyquadrone 109 [82] can be synthesized via the same photochemical key step. 

Dopp D, Bredehorn J, Memarian H-R, Miihlbacher B, Weber J. re/-(lR,4R)-1-Acetyl-1,4-dihydro-l,4-ethano-naphthalene-9-one. In Mattay J, Griesbeck AG, eds. Photochemical Key Steps in Organic Synthesis. Weinheim VCH, 1994 186-188. 

J. Mattay and A. Griesbeck (Eds.), Photochemical Key Steps in Organic Photochemistry, (WUey, Weinheim, 1994). 

Photochemical Key Steps in Organic Synthesis Edited by Jochen Mattay and Axel G. Griesbeck copyright VCH Verlagsgesellschaft mbH, 1994... 

Many syntheses in this collection represent multistep procedures starting with readily available compounds which are transformed using non-photochemical methods into the necessary substrates for the photochemical "key-step". A number of sequences, however, already require substrates which must be prepared using procedures from the literature. In each case, you should consult the literature which is given by the authors and find out about a) the source for all substrates and reagents used in the synthesis, b) the photochemical set-up which was used by the authors, c) the purification of starting materials and solvents as well as the characterization of the products, d) the mechanism of the reaction. 

A menthol-based route to grandisol is described by H.-D. Scharf using the [2+2] cycloaddition of ethylene with an enantiomerically pure menthyl-substituted furanone as the photochemical key step. The asymmetric induction of this reaction is relatively poor, the diastereomeric photoproducts, however, can be easily separated. Both enantiomers of grandisol can be synthesized using this methodology. 

Finally, in order to overcome the problems of short wavelength excitation, energy wasting by Z,E-isomerisation and unfavorable entropy effects, the intramolecular [2+2] cycloaddition of 1,6-dienes can be efficiently catalyzed by means of copper(I)triflate. J. Mattay made use of this procedure for the synthesis of the pheromone grandisol (cf. chapter 1.2). The photochemical key step is the excitation of the preformed copper (I) complex with the 1,6-hexadiene acting as a bidentate ligand. 

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