An unusual acyliminium cyclization and other drawbacks during an attempted synthesis of a chiral primary α-phosphinoalkanamine

Studies towards the synthesis of a chiral primary alpha-phosphinoalkanamine la are reported. O-Activated, N-carbamate-protected phenylalaninol 3a did not undergo S-N reaction with KPPh2: instead, after N-deprotonation, intramolecular substitution led to formation of the aziridine derivative 5a (Scheme 2). N-Phthalimido-protected, O-activated phenylalaninol 3b also underwent an intramolecular process on treatment with KPPh2, i.e., an unusual aryl-acyliminium cyclization furnishing the (epoxymethano)isoindolo[1,2-a]isoquinolinone 7 (Scheme 3). In a reaction with KPPh2, the N,N-dibenzyl-protected and activated phenylalaninol 3d finally yielded the intermolecular S-N reaction product 2a (Scheme 4). However, debenzylation by catalytic hydrogenation turned out to be impossible.