Synthesis of 2a,8b-Dihydrocyclobuta[a]naphthalene-3,4-diones.

On irradiation (λ = 350 nm) in neat hex-1-yne, naphthalene-1,2-dione monoacetals 1 afford mixtures of pentacyclic photodimers and up to 25% (isolated yield) of mixed photocycloadducts 2. Careful acidic hydrolysis of the acetal function of 2 gives the title compounds 3, the overall sequence representing a first approach to a (formal) [2 + 2] photocycloadduct of a 1,2-naphthoquinone to an alkyne.

Photocycloaddition of enynones (4-acylbut-1-en-3-ynes) to alkenes.

The title compounds undergo regio- and diastereoselective [2+2]-cycloaddition at the olefinic C-C bond on irradiation (350 nm) in the presence of either methacrylonitrile or 2,3-dimethylbuta-1,3-diene, affording hitherto unknown 1-cyclobutylalk-1-yn-3-ones or 1-bicyclo[4.2.0]octylalk-1-yn-3-ones, respectively.

Light-induced coumarin cyclopentannelation.

[reaction: see text] Whereas cyclopentenylcarbenes resulting from photocycloaddition of 4-alk-1-ynylcoumarins to 2,3-dimethylbut-2-ene undergo tandem cyclization to hitherto unknown tetracyclic (4-hetera)cyclopent[b,c]acenaphthylenes, the corresponding cyclopentenylnitrenes stemming from 4-cyanocoumarins and the same alkene are converted into tricyclic imines via H-abstraction.

Photocycloaddition of cyclohex-2-enones to alkylidenemalononitriles (1,1-dicyanoalkenes).

5,5-Dimethylcyclohex-2-enone (1a) photocycloadds to ethylidenemalononitrile (2a) to give 5-oxobicyclo[4.2.0]octane-7,7-dicarbonitriles with slight preference, in analogy to results already described for light induced reactions between cyclohexenones and acrylonitrile (AN).

Biradical to biradical rearrangement via 1,3-H atom transfer in photocycloisomerizations of 4-pent-4-enylcyclohex-2-enones.

An unprecedented 'carbonyl-O-assisted' 1,3-H atom transfer from a CH2 group to a primary alkyl radical center in 1,4-biradicals, formed in a crossed addition mode in the photocycloisomerization of 4-pent-4-enylcyclohex-2-enones, is discussed.

Synthesis and photochemistry of isothiocoumarins fused to an additional pyranone or thiopyranone ring.

Both 4H,7H-benzo[1,2-c:4,3-c']dithiin-4,7-dione (3) and 4H,7H-thiino[4,3-f][2]benzopyran- 4,7-dione (4) were synthesized from 1,2-bis(2-hydroxyethyl)benzene (7). On irradiation (lambda > 390 nm) in acetonitrile the symmetric tricycle 3 affords selectively the two-fold cis-cisoid-cis dimer 10 while the asymmetric tricycle 4 gives a 5:1 mixture of the cis-cisoid-cis dimers 11 and 12. On irradiation in benzene in the presence of 2,3-dimethylbut-2-ene 3 affords quantitatively a 3:2 mixture of the diastereomeric monocycloadducts 14c and 14t.