The assignment of secondary alcohols can be carried out by using one of several CDAs [13–15]. The most used and most reliable ones are MPA, 9-AMA, and MTPA [35–40]. Figure 3.1 shows their structures, the correlation models, and a summary of the experimental conditions. MPA and 9-AMA esters share the same conformational composition [37, 39] and only differ in the intensity of their shieldings; therefore both auxiliaries present the same correlation between sign distribution and stereochemistry. MTPA has a different conformational composition and correlation model [38]. As shown in Chapter 1, MPA esters of secondary alcohols and other AMAA esters (e.g., 9-AMA esters) are composed of two sp/ap conformers in fast equilibrium [37, 39]. The sp conformer is more stable than the ap conformer, and this allows the NMR spectrum of an AMAA ester to be interpreted as if it had originated from just the sp form: carbonyl, Cα, and methoxy groups in the auxiliary part and a methine group (Cα′-H) at the alcohol moiety are in the same plane. When we consider this conformation in the (R)- and the (S)-AMAA esters, the L1 group is located under the shielding cone of the aryl in the (R)-AMAA ester, while the L2 is shielded in the (S)-AMAA ester (we strongly recommended that the reader builds Dreiding, or similar, models to assist in visualizing this spatial array). A subtraction defined as the chemical shift in the (R)-AMAA ester minus that in the (S)-AMAA ester results in a negative value for L1 and a positive one for L2 (i.e., negative ΔδRS for L1 and positive ΔδRS for L2). Therefore, for any secondary alcohol derivatized as an AMAA ester, the protons showing a positive ΔδRS sign are located in the tetrahedron around the asymmetric carbon (Cα′) as L2 (at the back) while the protons resulting in a negative ΔδRS take the position of L1.
OXIDATION OF BENZYLIC SECONDARY ALCOHOL WITH PYRIDINIUM CHLOROCHROMATE-ALUMINA (PCC-Al2O3)