Summary6-Benzylbicyclo [4.4.0]dec-1-en-3-one (9) and the 2-methyl homologue (10) underwent a (y+u)-1,3-benzyl shift to the P,y-unsaturated ketones 21 and 22, respectively, when excited in the n+n* absorption band. The quantum yield was ca. 0.1 at 254 nm for the formation of both products in alkane solvents. These reactions occur specifically from the S2 ( n , n*) state in competition with its decay to the S1 (n, n*) and T states. The triplet reaction of 9, initiated by n + n* irradiation and by sensitization, was a double-bond shift to 20, whereas no identifiable product was observed from 10 under these conditions. Direct and acetone-sensitized irradiations of 21 and 22 resulted in oxadi-n-methane rearrangements to mixtures of syn-and anti-30 and syn-and anti-31, respectively.Over the last years, organic photoreactions from upper excited states in solution have been reported in a number of examples2). Among these, appropriately 4-substituted 2-cyclohexenones, on n --f n* excitation, have been found to undergo such reactions, presumably from the Sz (n, n*) state. In the (2)-propenyl enone 1 an intramolecular hydrogen transfer from the allylic methyl group to the enone a-carbon atom and cyclization to product 2 occurred with a quantum yield @* = 0.29 at 245 nm [l]. A similar process ( 3 4 4 ) together with an overall ( y -+ u ) -l , 3-shift of the substituent (3 + 5 ) via a cleavage and alternate recombination sequence, was observed with the dimethoxymethyl enone 3 [3] (Scheme 1). We now report that a 1,3-benzyl shift (9+ 21 and 10+ 22) was the exclusive reaction of the benzyl enones 9 and 10, in which the hydrogen transferlcyclization process is barred by geometric and energetic constraints.
Ausgehend von den Komponenten (I) und (II) werden über die Stufen (III) die Titelverbindungen (IV) sowie durch H‐D‐Austausch das deuterierte (IVa)‐Analoge (V) dargestellt.
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