Photochemistry of the Phthalimide System, 44. Photoinduced Reactions, 122. Regioselective Remote Photocyclization: Examples of a Photochemical Macrocyclic Synthesis with Sulfide‐Containing Phthalimides

S-Substituted N-(thioalkyl)phthalimides 2 underwent photocyclization to give azathiacyclols, up to 16-membered ring compounds, in good yields. The cyclization occurred between an imide carbonyl group and a terminal sulfide group located in a remote position from the carbonyl group. The quantum yield of cyclization was determined as 0.042 (acetonitrile) for N-(5-methylthiopentyl)phthalimide (2a) and the efficiency of remote cyclization was not significantly affected by the chain length examined. Quenching studies of 2a using isoprene or 1,3-cyclohexadiene showed linear Stern-Volmer plots with the slopes of kqτ = 340 M−1 or 630 M−1, respectively. The effective concentration (Ceff) of photocyclization was estimated within the limits of 0.01 M < Ceff < 0.1 M for 2a from a competition experiment with the intermolecular photoaddition of dimethyl sulfide. The efficient cyclophilic nature of remote photocyclization can be rationalized by an enhanced proton transfer from the sulfide group which is possibly located close to the carbonyl group to form a radical ion pair complex by an electron transfer process.