Cycloisomerization photochemical

Scheme 19. Photochemical A-D cycloisomerization presumed reaction path.

According to the Woodward-Hoffmann rule [6, 7], conjugate polyenes with 4n and 4n+2 n electrons undergo cychzations in conrotatory and disrotatory fashions under the thermal conditions, respectively. Recently, novel cycloisomerizations were found to be catalyzed by Lewis acid and to afford bicychc products [39] as photochemical reactions do [40]. The new finding supports the mechanistic spectrum of chemical reactions. 

If photochemical apparatus is not available, the cycloisomerization reaction can be conducted using trimethylamine N-oxide to promote oxidative decarbonylation of molybdenum hexacarbonyl in a mixture of EtjN and EtgO, followed by addition of 1-phenyl-3-butyn-1-ol (1). In the submitters hands, this procedure required somewhat higher loading of molybdenum hexacarbonyl, and purification of the 2-phenyl-2,3-dihydrofuran (2) product required silica gel chromatography. 

Copper(I) catalysis is very well established to promote intramolecular [2+2] photocycloaddition reactions of l,n-dienes (review [351]). The methodology recently enjoyed a number of applications [352-354], It is assumed that CuOTf, which is commonly applied as the catalyst, coordinates the diene and in this way mediates a preorganization. The Ghosh group recently reported a number of CuOTf-catalyzed photochemical [2+2] cycloaddition reactions, in which an organocopper radical complex was proposed as a cyclization intermediate (which should, however, have a formal Cu(II) oxidation state) (selected references [355-357]). A radical complex must, however, not be invoked, since the process may either proceed by a [2+2] photocycloaddition in the coordination sphere of copper without changing the oxidation state or according to a cycloisomerization/reductive elimination process. 

An impressive example of the possibilities of metal complex photochemistry is the substantiation of the critical step in corrin synthesis reported by Eschenmoser 145> Ring closure is brought about by antara-facial cycloisomerization of a secocorrinoidic palladium complex by photochemical 1.16-hydrogen shift. 

Cycloisomerization. Homopropargyl alcohols give, after mild oxidation, dihy-drofurans. The (EtjNlMofCO) complex, obtained by photochemically induced ligand exchange, promotes the isomerization of epoxyalkynes to furans. ... 

Krautler B, Pfaltz A, Nordmann R, Hodgson KO, Dunitz JD, Eschaunoser A (1976) Experiments on a simulation of the photochemical A/D-secocerrin to corrin cycloisomerization by redox prex esses. Electrochemical oxidation of nickel(II)-l-methylidene-2,2,7,7,12,12-hexamethyl-15-cyano-l,19-secexr)rrinate perchlorate. Helv Chim Ac4a 59 924—937... 

---