The many faces of (R∗)-4-[(S∗)-1-methanesulfonyloxypropyl]-3-methoxy-1,3-oxazinane

Abstract

The ¹H NMR spectrum of (R∗)-4-[(S∗)-1-methanesulfonyloxypropyl]-3-methoxy-1,3-oxazinane (3) is incomprehensible at room temperature due to line broadening from the dynamic exchange of many conformations. VT-NMR studies show two equal populations at low temperatures. As the temperature increases, they coalesce into a single population with a free energy of activation for this process of 14.0–14.6 kcal/mol. DFT studies indicate that each of the low-temperature populations is a collection of rapidly interconverting species, and the critical barrier to interconversion is an inversion at nitrogen in compound 3 with calculated barriers of 14.3–16.0 kcal/mol, depending on the computational method employed.