Chair-boat conformation

P212121 Z = 4 D = 1.25 R = 0.069 number of intensities not reported. This analysis was to confirm a configuration derived from n.m.r. data. The pyranoid rings are slightly distorted 4Q, due to fusion to the trioxacyclooctane ring, which has an almost ideal, boat-chair conformation. The overall molecule has a convex, sickle conformation. There is an error in the atomic coordinates reported, which do not correspond to the structure given in the paper. 

A cascade of alkene addition reactions begin through a chair-boat-chair conformation transition state. 

MM calculations are in agreement with the boat-chair conformation of symmetry being slightly more stable than the crown conformation [5]. 

At this stage of the synthesis, it was necessary to resort to a second conformational control element (shifting the B ring from a chair-chair to a boat-chair conformation), which ensures the generation of the C(l)-C(2) enolate of a C(2) ketone and allows the epimerisation of the substituent at C(3)a (which will return the B ring to the chair-chair conformation). 

J. T. Groves and J. R. Olson, Inorg. Chem. 24, 2715 (1985) and previous references. Metal coordination forces the boat-chair conformation in the bicyclic azalactam portion of the tridentate ligand, alleviating severe steric interactions present in the chair-chair conformation of the free ligand. 

In the tetra-bridged phosphocavitands, the preorganized structure is imposed by the fixed boat-chair conformation of the four fused eight-mem-bered rings. Inwards (i) and outwards (o) configurations are defined relatively to the endo and exo orientations of the P=X bonds (X=0, S, electron pair), and six different stereoisomers arise from the equatorial or axial orientation of the substituents on the phosphorus atoms (Scheme 3). 

Among quinolizidine alkaloids, sparteine and its stereoisomers have been studied in detail by X-ray analysis (42-50). It was demonstrated that proper conformation was not reorganized in monohydrates (42), diperchlorates (43), or methyliodides of a-isosparteine (11) (53). Unlike in the case of a-isosparteine, in spareteine diperchlorate rings C/D appear to have a boat-chair conformation (44-46). On the basis of spectroscopy data a cis conformation for sparteine methyliodide (12) was proposed (57,52). However, radiographic examination (53) of this compound showed it to have the trans conformation (13). 

X-Ray diffraction results are available for salts of azocane, but not for the free base, which is a liquid. Unlike the chloride salt, which is disordered and gives little conformational information (B-68MI51902), the tosylate gives excellent data and shows that the cation has the boat-chair conformation (419) (77CC939). 

Dynamic 13C NMR measurements show that azocane exists at -112 °C as a 97 3 mixture of a boat-chair and a crown family conformation (AG° = 5 kJ mol-1, AG+44kJmol-1) (78JOC3021). From the NMR spectrum the barrier to ring inversion in the boat-chair is 31kJmol-1, but the barrier to pseudorotation is apparently too low for measurement. Likely candidates for the crown family and boat-chair conformations of azocane are the chair-chair (420), and the BC-1 (421), BC-3 (422) and BC-7 (423) conformations, respectively, but the precise position of the nitrogen atom is not known experimentally. JV-Methyl-and iV-chloro-azocane also have predominantly boat-chair conformations (75JOC369). 

Two distinct dynamic NMR effects occur in 1,3-dioxocane and its derivatives having a gem-dimethyl group at the 2- and/or 6-positions (chemical, not conformational numbering) (76JA2059). These molecules have unsymmetrical boat-chair conformations (e.g. 432) which minimize non-bonded and dipole-dipole repulsions... 

Several other sulfur eight-membered heterocycles also exist in boat-chair conformations. 5-Thiacanone (441) has a conformation similar to its oxygen analog (430) and shows barriers to pseudorotation and ring inversion of 28 and 34 kJ mol-1, respectively (80JOC1224). The... 

Finally, 1,2,5,6-tetrathiocanes seem to prefer the twist-boat-chair conformation (82TL3231,69AX(B)2il4). The ring torsional angles in the present compound are given in (443). 

The bridged biphenyl (472) (67ZN(B)1296> has a rigid twist-boat-chair conformation in the crystal (72CC985) and is an eight-membered heterocycle with two torsional constraints in a 1,3 relationship. It has been resolved by chromatography on cellulose acetate, and AG for racemization is 120 kJ mol-1 (67M1323). 

Several methods are available for the synthesis of cycloarsoxanes (a) hydrolysis of organoarsenic(III) dihalides RASX2 (X = Cl, Br, I), (b) reduction of arsonic acids RAsO(OH)2 and (c) oxidation of primary arsines RASH2 or cyclopolyarsines (RAs) . The tetramer cyclo-(MeAsO)4 exhibits the boat-chair conformation, whereas the mesityl derivative cyclo-(MesAsO)4 adopts the crown conformation (Figure 11.24). The As-O bond distances are in the range 1.77-1.82 A, typical for single bonds. 

The activation energy for interconversion of these two forms is about 10 kcal mole-1. The boat-chair conformation 9 is quite flexible and movement of its CH2 groups between the various possible positions occurs with an activation energy of only about 5 kcal mole-1. 

Figure 12-17 Twist boat-chair conformation of cyclononane (after Dale)...

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