Conformation of some sparteine N-16 oxides revisited

The observed and calculated 1H and 13C chemical shifts of tertiary amines and their N‐oxides

“…In the literature, there are several examples of the combination of experimental NMR data with calculated NMR parameters (shielding nuclear constants and scalar couplings) addressing various stereochemical problems, e.g. the conformation of sparteine N ‐oxides,14, 15 the conformation and configuration of tertiary amines via GIAO‐derived 13 C chemical shifts and a multiple independent variable regression analysis,16 the conformation of N ‐substituted piperidines and pyrrolidines17 or the relevance of the calculated 1 H, 13 C and 15 N shieldings in amines in conformational analysis 18. GIAO NMR calculations combined with probability analysis were also used for the assignment of stereochemistry when the NMR data were available only for one diastereoisomer 19…”

Indolizidine and quinolizidine alkaloids

“…As part of an ongoing series of investigations into the structure and conformation of sparteine derivatives in solution, Polish investigators have shown by means of NMR spectroscopy and computational methods that sparteine N-16 oxide 298 (R = H), prepared by oxidising (−)-sparteine with aqueous hydrogen peroxide, exists mainly in a previously unknown chair-chair-boat-chair conformation of the four rings with cis-fusion of rings C and D, rather than in the expected all-chair conformation. 106 The same applies to the synthetic 2substituted derivatives 298 (R = Me, Ph). By contrast, the 13␣hydroxy-N-16 oxides of sparteine and lupanine (299 and 300 respectively) retain the boat conformation of ring C, but have a trans-ring junction between rings C and D, as do lupanine N-16-oxide 301 and the corresponding sparteine epi-N-oxide 302, prepared by reducing 301 with sodium borohydride.…”

NMR Spectra of Sparteine N1-oxide and α-Isosparteine N-oxide

“…In the reactions of sparteine with H 2 O 2 a 1:3 mixture of the two sparteine N-oxides: sparteine N 1 -oxide and sparteine N16-oxide is obtained [ 19 , 20 ]. Sparteine N1-oxide ( 1 ) occurs in a chair-boat type equilibrium involving inversion of the lone pair on the N16 atom [ 20 ]; the N16-oxide of sparteine ( 2 ), previously thought to adopt the all chair conformation, has been found recently to have ring C in a boat conformation and a cis C/D ring junction [ 21 ]. Sparteine epi-N-oxide ( 3 ) has the C ring boat conformation and must be obtained by NaBH 4 reduction of lupanine N-oxide [ 20 ].…”