Determination of the Relative Configuration and Conformational Analysis of Five–membered N ‐Acyl‐Iminosugars by Relevant 3 J Coupling Constants.

We have analyzed the J values of 2,5‐disubstituted and 2‐monosubstituted N ‐acylpyrrolidines in all the eight d ‐aminohexitol and the four d ‐aminopentitol series. The N ‐acyl functionality trends to drive the aliphatic substituents in the 2‐ and 5‐positions into a preferred axial orientation, restricting the possible conformations of the pyrrolidine ring to only one or two sets giving rise to characteristic J (2,3) and J (4,5) couplings (5.4–8.7 Hz for a cis ‐relationship and usually below 3 Hz for trans ones). The 2,5‐disubstituted N ‐acyl‐pyrrolidines appear mainly in a 3 T 4 or E N conformation (in the d ‐series), and more generally as an equilibrium between 2 conformation sets at each side of these diaxial conformations. For the 2‐monosubstituted N‐acyl‐pyrrolidines, the above general rules are conserved for J (2,3). In addition, we have studied the conformational trend of the unsymmetrical 2,5‐disubstituted pyrrolidines with free N−H, which are known as potent glycosidase inhibitors. The lack of the N‐acyl substituent led to a clear conformational change with R 2 and/or R 5 in equatorial or quasi‐equatorial position.