Photochemical elimination of leaving groups from zwitterionic intermediate generated via electrocyclic ring closure of anilides
Abstract
Photochemical cleavage reactions have found widespread use in biological applications that require intracellular photochemical release of biologically active substrate or as photoremovable protecting groups and photolinker for the synthesis of biooligomers. In buffer pH 7, methacrylanilides, ArN(CH 3 )COC(CH2 LG)=CH2 , bearing allylic leaving groups (LG- = BocAla, PhCOO- , PhCH2 COO - , PhO- ) undergo photochemical elimination of the leaving group to produce an α-methylene lactam, which is accompanied by a minor lactam that retains the leaving group in a ratio of 2-∞:1. The expulsion of the leaving groups also occurs in nonpolar solvent, with the exception of LG- = PhO- , which gives lactam with retained LG- . The proposed mechanism involves excited state electrocyclization to produce a ground state zwitterionic intermediate which eliminates the LG- or undergoes a 1,5-H shift to give the minor lactam. In protic solvent the minor lactam is also formed via protonation of the enolate moiety of the zwitterionic intermediate. The photoelectrocyclization occurs in the singlet excited state rather than the triplet excited state according to quenching studies combined with laser flash photolysis experiments. In aqueous media, quantum yields for total product of paracarbomethoxy-substituted anilides are Φ tot = 0.042-0.054 at 310 nm, whereas for the parabenzoyl-substituted anilides, Φtot = 0.075-0.095 and, (Φisc = 0.15 at 365nm.
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