The irradiation of ( Z )-2-phenyl-4-aryliden-5(4 H )-thiazolones 2 with blue light (465 nm) in CH 2 Cl 2 solution promotes [2 + 2]-photocycloaddition of the exocyclic C=C bonds and the formation of the dispirocyclobutanes 3 . This reaction takes place with high stereoselectivity, given that the ε-isomer (1,3 head-to-tail syn coupling) is formed in more than 90% yield in most of the cases. However, irradiation of 5(4 H )-thiazolones 2 with blue light (456 nm) in dry MeOH in the presence of BF 3 ·OEt 2 leads to the monospirocyclobutanes 4 with full stereoselectivity, also affording the ε-isomer. A ring-opening reaction of only one of the thiazolone rings appears to have taken place in 4 upon methanolysis, leading to the corresponding ester and thioamide groups. The treatment of free 4-aryliden-5(4 H )-thiazolones 2 with a base in alcohol (NaOR/ROH) also produces a ring-opening reaction of the heterocycle by methanolysis, although, under these reaction conditions, further intramolecular S-attack at the exocyclic C(H)=C bond and cyclization is observed, forming the dihydrothiazoles 5 or 6 as mixtures of cis ( RS / SR )- and trans ( RR / SS )-isomers with high diastereomeric excess. trans -( RR/SS )-Dihydrothiazoles 6 can be isolated as pure diastereoisomers by column chromatography. Surprisingly, dihydrothiazoles 5 can also be obtained by the treatment of 4-aryliden-5(4 H )-thiazolones 2 with BF 3 ·OEt 2 in methanol in the absence of a base.

The irradiation of (Z)-4-arylidene-5(4H)-oxazolones 1a-1u with blue light (465 nm) in presence of the photosensitizer [Ru(bpy)3](BF4)2 (2.5% mol) and of the Lewis acid BF3·OEt2 (2 equiv.) in deoxygenated methanol at...

The irradiation of ( Z )-2-phenyl-4-aryliden-5(4 H )-oxazolones 1 in deoxygenated CH 2 Cl 2 at 25 °C with blue light (465 nm) in the presence of [Ru(bpy) 3 ](BF 4 ) 2 (5% mole ratio) as a triplet photocatalyst promotes the [2+2] photocycloaddition of the C=C bonds of the 4-arylidene moiety, thus allowing the completely regio- and stereoselective formation of cyclobutane-bis(oxazolone)s 2 as single stereoisomers. Cyclobutanes 2 have been unambiguously characterized as the μ-isomers and contain two E -oxazolones coupled in an anti -head-to-head form. The use of continuous-flow techniques in microreactors allows the synthesis of cyclobutanes 2 in only 60 min, compared with the 24–48 h required in batch mode. Ring opening of the oxazolone heterocycle in 2 with a base affords the corresponding 1,2-diaminotruxinic bis-amino esters 3 , which are also obtained selectively as μ-isomers. The ruthenium complex behaves as a triplet photocatalyst, generating the reactive excited state of the oxazolone via an energy-transfer process. This reactive excited state has been characterized as a triplet diradical 3 ( E / Z )- 1* by laser flash photolysis (transient absorption spectroscopy). This technique also shows that this excited state is the same when starting from either ( Z )- or ( E )-oxazolones. Density functional theory calculations show that the first step of the [2+2] cycloaddition between 3 ( E/Z )- 1* and ( Z )- 1 is formation of the C(H)–C(H) bond and that ( Z ) to ( E ) isomerization takes place at the 1,4-diradical thus formed.