Photochemistry of MTM- and MTE-esters of omega-phthalimido carboxylic acids: macrocyclization versus deprotection(1)

Abstract

The photochemistry of five linear methylthiomethyl (MTM)-esters of omega-phthalimido carboxylic acids Pht=N-(CH(2))(n)()COOCH(2)SCH(3) 1a-e (n = 1, 2, 3, 5, and 10), of the two methylthioethyl (MTE)-esters Pht=N-(CH(2))(n)()COOCH(2)CH(2)SCH(3) 2a,b (n = 1, 2), and of the two methyl-substituted MTM/MTE esters 3a and 3b was investigated. Two reaction channels were observed: (i) photocyclization to give medium-sized and macrocyclic rings, (ii) photochemical deprotection to give the free carboxylic acids. Photocyclization of 1b and 1c (n = 2, 3) resulted in 4b,c in excellent yields whereas the substrates 1a and 1d,e with shorter as well as longer spacer groups (n = 1, 5, 10) gave preferentially the deprotected products 5a,d,e. Subsequent photolysis afforded N-methylphthalimide (6) from 5a. The MTE-esters 2a and 2b gave the macrocyclic lactones 7 and E-8. Thus, the competition between cyclization and deprotection strongly depended on the chain length of the hydrocarbon linker between phthalimido chromophore and ester group. To examine the influence of the position of the ester group in the linker chain the model substrates 3a and 3b with identical number of atoms separating electron donor and acceptor group were investigated. The more flexible MTE-derivative 3b cyclized to give a 4:1 diastereoisomeric mixture of cis/trans-9b, whereas photolysis of the more reluctant MTM-ester 3a resulted in cis-9a only after prolonged irradiation. These results show that MTM can function as a photolabile protecting group whereas MTE cannot be removed photochemically. The distance dependence of the secondary reaction steps indicates that the primary electron transfer is not necessarily induced starting from close contact geometries.