Conformational study of dibenzocyclo-octadiene systems related to the schizandrin-type lignans

Abstract

X-Ray and lH and 1% n.m.r. data have been obtained for the dibenzocyclo-octadiene lignan kadsurin and several of its synthetic precursors, in which di-ortho-substitution of the aryl rings leads to high energy barriers for rotation around the biaryl bond. The crystallographic and spectral information shows that for compounds in this series the eight-membered ring adopts either a twist-boat (TB) or twist-boat-chair (TBC) conformation. The relative stability of ring conformers and/or biaryl rotamers is determined by three factors : (i) a preference for the TBC over the TB form, (ii) stabilization of the TB conformation by conjugation of the benzylic carbonyl groups with the aryl rings, and (iii) the presence of benzylic hydroxy (or acetoxy) substituents i n endolexo orientations.

Conformational analysis and NMR assignments of dibenzoxocins from Sappan Lignum

“…While in 2008, 1 was reported as a mixture of protosappanin B and isoprotosappanin B again. [12] In our study, we noticed that many dibenzoxocin derivatives, including 1, exhibit two sets of signals in 1 H and 13 C NMR spectra. To explain the phenomena, two different points of views have been proposed.…”

“…2). [13][14][15] At the same time, the two benzene rings may exist in two possible rotamers because of the hindered biaryl rotation. Similar to the dibenzocyclooctadiene type lignans, such as schizandrin, the axis of chirality is defined as aS or aR (shown in Figs.…”

“…1 and 2). [16] These forms interconvert via the process as follows:(1) TBC-S → TB-S, (2) TB-S → TB-R and (3) TB-R → TBC-R. [13] For the conformational analysis of dibenzoxocin derivatives, it can be divided into two problems which biphenyl rotamer, aS or aR is preferred and which form, TBC or TB, is predominant for a particular biphenyl rotamer. Herein, the conformation of dibenzoxocin derivatives is discussed based on NMR data and CD results.…”

Lignans and Triterpenoids from the Stems of Kadsura induta

“…The circular dichroism (CD) spectrum showed negative and positive Cotton effects at 340 and 280 nm, respectively, indicating that 2 contains an axially chiral (aS)-1,1'biphenyl unit ((P)-helicity) [14]. Since the aromatic ring was conjugated with the ketone group, a twist-boat (TB) conformation is the only possible conformation for the dibenzocyclooctane ring system [15]. The result was corroborated by a ROESY experiment, exhibiting correlations of HÀC(11) with HÀC(9), Me (18) Kadindutic acid (3), obtained as a white powder, had the molecular formula C 30 H 46 O 3 , according to the HR-ESI-MS (m/z 454.3441 (M þ )).…”

Racemization Energy of 3,2'-Tetramethylene-2-phenyl-6-(pyrid-2''-yl)pyridine Estimated by Temperature Variation1H NMR Experiment

“…2 Gottlieb et al 4 suggests that 2,2'-tetramethylenebiphenyl (1d, 1,2,3,4-dibenzo-1,3-cyclooctadiene) can exist in solution in two conformations, a nonsymmetrical twist-boat (TB) and a twist-boat-chair (TBC) with C 2 symmetry, in nearly equal populations. These conformations interconvert via two processes: (A) I (IV) F II (III) and (B) II F III (Figure 1), of which process (A) has been suggested as a preferred one based on data from X-ray diffraction and 1 H and 13 C NMR spectra.…”