The reaction mixture ( ). Firstly, when -terpinine was employed as a substrate, ascaridole was formed in 63% yield as quantified by H NMR. Endoperoxide formation is a hallmark reporter for 1 O 2 via its Diels-Alder [4+2]-cycloaddition with dienes. Secondly, the presence of DABCO as an additive inhibited conversion in 1b's reaction. We confirmed this was not due to it competing for 1 DCAS* as a reductive SET quencher, since the rate constant (k q = 7.28 x 10 8 M -1 s -1 ) showed it was an even less efficient quencher than O 2 or 1a/1b. Rather, DABCO is a wellknown physical quencher of In a recent study of Rovis and co-workers' photocatalytic functionalizations of cyclic amines, 112 they proposed that a reversible and fast HAT is responsible for the endocyclic selectivity. In our case, we deem that the HAT between the singlet oxygen and amine substrate is irreversible, thus sterics govern the selectivity (i.e. at the N-methyl position). Further discussion on the selectivity of singlet oxygen reactions with related amines is ongoing. In the case of less-constrained trialkylamines, the 1 O 2 -bound exciplex can react promiscuously with endocyclic / non N-CH 3 positions and other functional groups of substrates (e.g. benzylic groups, free alcohols) leading overall to degradation.