Photochemische reaktionen—XXV : Photochemische synthese von ring A-annelierten gibberellinen der GA1-reihe

The photochemical 2 + 2]-cycloaddition of ethylene and tetramethylethylene to 3-dehydro gibberellin A₃ (1) under n → π^*-excitation conditions has been investigated. The reaction leads to a 3 : 1 ratio of the cis-fused α- and β-cyclobutane annelated epimers 3 and 5 as well as 18 and 20 in 70 and 86% yield, respectively, besides small amounts of the phenolic acid 22 and dimeric material. NaBH₄-reduction of 3 gives stereospecifically the 3α-hydroxy compound 7 whereas the 1β,2β-epimer 5 yields the corresponding ring A annelated gibberellin A₁ analogues 12 and 16 in a 6:1 ratio. The constitution and stereochemistry of the products are deduced by physical methods and some mechanistical aspects discussed on the basis of qualitative quenching experiments.