Contribution of the anomeric effect to the solution and crystal structure of [1S, 2S, 6S, 7S]-1,6-diaza-4,9-dioxa-2,7-dimethoxycarbonylbicyclo[4.4.1]undecane, a condensation product of L-serine methyl ester with formaldehyde

摘要

The reaction of L-serine methyl ester hydrochloride (I) with paraformaldehyde (2) in dichloromethane in the presence of triethylamine afforded a novel compound: [18,28,68,78]-1 ,6-diaza-4,9-dioxa-2, 7-dimethoxycarbonylbicy- clo[4.4.I]undecane (4) as a 2:3 adduct of I with 2. IH and 13C NMR spectroscopy were unable to discriminate between two possible symmetrical structures. The latter was unambiguously proved by X-ray crystallography. The crystal structure established: (i) the existence of two identical seven-membered rings each containing a N-C-O grouping; (ii) the existence of along C-O-C-N-C-N-C-O-C sequence in which each nitrogen belongs simulta- neously to a N--C-O (oxazolidine) and to a N-C-N (aminal) motifs; (iii) the existence of a chair-Iike conformation for both seven-membered rings; (iv) the antiperiplanar geometry of p N-C-O and consequently the manifestation of a strong anomeric effect in both N-C-O groupings, whereas anomeric effect was virtually absent in the N-C-N sequence, as corroborated by bond distances and bond angles. Chemical shifts, coupling constants and NOE effects confirm that the conformational features of 4 are preserved in solution.