Electrophilically induced cyclic acetal formation of the O-benzyl dipeptide esters Fmoc-NMeIle-Thr-OBn (1) and of Fmoc-Pro-Thr-OBn (6) has been observed to lead predominantly to the (R) diastereomers 2b and 8b at the 2-C position of the resulting substituted Id-oxazolidine (Psi Pro) unit, while upon acetalization of the corresponding O-methyl ester 4 the 2-C(S) epimer 5a is predominantly formed under the same proton catalyzed cyclization conditions. With boron trifluoride etherate as Lewis acid the reaction is particularly fast and leads selectively to the prolyl threonine derived 2-C(R) dipeptide building block 8b, which could conveniently be assembled into a nonamer with a virtually solvent independent CD-spectrum of the polyproline type I (cis amide bonds).