afforded the desired products in good yields and a-stereocontrol e.g. 6a-6c (44-87% a only) and 5a-5e, 5g-5i (36-77%, 3:2 a:b to only a), respectively. Next, we evaluated the reactivity of the catalytic system in the synthesis of an 1,1,-⍺,⍺' linkage, which are often more challenging targets due to the assembly of two anomeric centers in one step in the presence of other competing pathways. Reactions of either lactol 2a or 2b with a range of hemiacetal acceptors of differing reactivity (2a, 2b, 6e, 6e and 6d) proceeded smoothly to give the desired products 7a-7e in yields of 54-75% and with complete ⍺,⍺'-stereocontrol (Scheme 4). Preliminary 1 H-NMR spectroscopy studies in CD2Cl2 of mixtures of hemiacetal 2a and selenonium 1e showed broadening of the protons at the phenyl region of the catalyst protons (d 8.1-7.9 and 7.75-7.55 ppm) and formation of the 2a-a,a'-dimer (d 5.26 ppm) as the concentration of 2a increases (Figure ), suggesting an interaction between the hemiacetal and the catalyst. NMR titrations of OH acceptor 3a and 1e also showed the catalyst phenyl protons broadening. The interaction between the catalyst and 3a is further supported by IR analysis of the mixture showing suppression of the IR alcohol stretch frequencies (3479-3413 cm - 1 ) (Figure ).