No abstract

Regio-and Stereoselective Photocycloaddition Reactions of Aromatic Aldehydes to Furan and 2.3-DihydrofuranThe Paterno-Biichi reaction between benzaldehyde (la), tolualdehyde (1 b), mesitylaldehyde (1 c), and 2,4-di-tert-butyl-4-methylbenzaldehyde (1 d) and the electron-rich alkenes furan and 2,3-dihydrofuran has been investigated. The reaction with furan leads exclusively to exo-6-aryl-2,7-dioxabicyclo[3.2.0]-hept-3-enes 2a to 2d in yields of 78% to 97% (2a, 2b

Flash vacuum thermolysis (FVT) of suitably substituted isoxazol-5(4H)-ones 7-9 leads to three different types of ketenimines, namely, the isoxazolonoketenimines 2, the novel bisiminopropadienes RN=C=C=C=NR (9, and the C-cyanoketenimines 14, all characterized by a combination of FVT/matrix isolation/IR spectroscopy and FVT/MS. An unusual, linear C=C=N-C backbone in ketenimines 2g and 2h is revealed by their exceptional spectroscopic properties as well as an X-ray crystal structure of 2g. and confirmed by density functional calculations (B3LYP/6-31 G*); these compounds are best described as resonance hybrids of ketenimines and isonitrile

ω‐Hydroxy‐ortho‐quinodimethane (2), generated by photoenolization of 2‐methylbenzaldehyde (1) in benzene, is quantitatively trapped with trifluoroacetone to give the bicyclic hemiacetal 6 and the hydroxy aldehyde 7. A less effective trapping reaction is observed with benzaldehyde with the formation of the bicyclic hemiacetal 5. The formation of benzocyclobutenol 4 is probable due to a two‐photon process involving electronically excited 2.

Stereoselectivity in Photocycloaddition Reactions, 5[1]. – Photoreaction of 2‐Cyclopenten‐1‐one with Enol Ethers The regio‐ and stereoselectivity of [2 + 2] photocycloaddition reactions between 2‐cyclopenten‐1‐one (1) and the electron‐rich alkenes: ethyl, isopropyl, and phenyl vinyl ether (2–4) as well as 2,3‐dihydrofuran (10) were examined. In all cases the annulated cyclobutene derivatives 5–7 and 11 were isolated in yields between 70 and 99%. The regioisomeric HT/HH products were formed in a ratio of 3:1 to 4:1. For the determination of the product stereochemistry, INADEQUATE and 1H,1‐decoupling experiments were performed. Computer‐assisted investigations on the molecular geometry and 1H,1H‐coupling constants were performed by using semiempirical methods (AM1, FINITE). The HT regioisomers were formed with moderate endo diastereoselectivity, whereas in the case of the HH regioisomers both diastereomers (exo/endo) were formed in equal amounts. Photoreaction with 2,3‐dihydrofuran (10) led to almost exclusive formation of exo‐diastereomeric HH and HT regioisomers 11. The observed stereoselectivity is rationalized by the optimal geometry of the intermediate triplet 1,4‐biradicals during intersystem crossing.