Comparative Infrared,Raman, and Natural‐Bond‐Orbital Analyses of King's Sultam

By means of ¹H‐NOESY‐ and Raman‐spectroscopic analyses, we experimentally demonstrated the presence of the equatorial N? Me conformer of King's sultam 4b in solution, resulting from a rapid equilibrium. As a consequence, the value of the N lone‐pair anomeric stabilization should be revised to 1.5–1.6 kcal/mol. Independently from the N tilting, natural bond orbital (NBO)‐comparative analyses suggest that the S d* orbitals do not appear as primordial and stereospecific acceptors for the N lone pair. Second, the five‐membered‐ring sultams do not seem to be particularly well‐stabilized by the S? C σ* orbital in the N‐substituted pseudo‐axial conformation, as opposed to an idealized anti‐periplanar situation for the six‐membered‐ring analogues. In this latter case, the other anti‐periplanar C? C σ* and C(1′)? H/C(2′) σ*orbitals are as important, if not more, when compared to the S? C σ* participation. In the pseudo‐equatorial conformation, γ‐sultams particularly benefit from the N lone‐pair hyperconjugation with the anti‐periplanar S? O₁ σ* and C(2)? H/C or C(1′)? H/C σ* orbitals. This is also the case for δ‐sultams when the steric requirement of the N‐substituent exceeds 1.6 kcal/mol. When both axial and equatorial conformations are sterically too exacting, the N‐atom is prone to sp² hybridization or/and conformational changes (i.e., 12c ). In that case also, the mode of stereoelectronic stabilization differs from γ‐ to δ‐sultams.