Puckered ring

Dioxolane also pseudorotates essentially freely in the vapor phase. 2,2 -Bi-l,3-dioxolane (128) has been shown by X-ray crystallography to have a conformation midway between the half-chair and envelope forms. The related compound 2-oxo-l 3-dioxolane (129) shows a half-chair conformation. This result is confirmed by microwave spectroscopy and by NMR data. Analysis of the AA BB NMR spectra of the ring hydrogen atoms in some 1,3-dioxolane lerivatives is in agreement with a puckered ring. Some 2-alkoxy-l,3-dioxolanes (130) display anti and gauche forms about the exocyclic C(2)—O bond. 

The electron configuration in the valence orbitals of the sulfur atom (3s 3p4) suggests that it will form two covalent bonds by making use of two half-filled 3p orbitals. This is, in fact, observed in the molecule S8, which is present in the common forms of solid sulfur. The S8 molecules assume the form of a puckered ring, as shown in Figure 20-3. As with the phosphorus, the stability of this crystalline form of sulfur is due to van der Waals forces between discrete molecules. 

Under normal conditions, sulfur forms yellow crystals. The ciystals consist of individual Sg molecules, with the eight sulfur atoms of each molecule arranged in a puckered ring. 

Although S42+, Se42+, andTe42+ ions have square planar structures, the ions S82+, Se82+, Te82+ exist as puckered rings ... 

Casey and co-workers visualized the stereochemistry of 2-pentene metathesis in terms of the relative stabilities of the various substituted metallocycle derivatives (14). As in Katz s scheme, a puckered ring was assumed, but Casey emphasized only that reaction pathways were favored which provided conformations possessing the fewest axial substit-... 

Therefore the concept of puckered rings was introduced and the existence of boat and chair forms in cyclohexane. 

N4P4Cl3(NMe2)5 2-cis-4-cis-6) Earlier assignment corrected irregular puckered ring, mean P—N(exo) 1.633 A 209... 

FIGURE 47. Three-dimensional structure of the cyclohexene ring of retinal in bR as determined by 2H NMR, relative to the membrane surface in the x-y plane. Analysis of the orientations of the three deuterium labeled methyl groups on the puckered ring (skew around C1-C6) indicates that the chromophore has a 6s-trans conformation around the C6-C7 bond. Reprinted with permission from Reference 60. Copyright (1997) American Chemical Society... 

Fig. 44. Hexagon structure of Ice Ih (a) reference plane of puckered rings perpendicular to "c (b) hexagon channels viewed parallel to "e (c) fore-shortened channel in "a" direction (from Ref. 74>)...

Fig. 47. Formation of Ice III (a) Second neighbor connections across puckered rings and boat raings positions of discarded H2O molecules (b) pentagon rings and channel in "a" direction (c) O j and Ojj lattice points sharing one and two 0—0 bonds respectively with three pentagons (d) Oj columns defining fourfold screw axis (from Ref., 4>)...

Five- and six-membered rings formed by coordination of diamines with a metal ion have the stereochemical characteristics of cyclopentane and cyclohexane. The ethylenediamine complexes have puckered rings and the trimethylenediamine complexes have chair conformations. The methylene protons are nonequivalent in these nonplanar conformations, taking on the character of equatorial and axial substituents. They are made equivalent as the result of rapid conformational inversion at room temperature, just as in the alicyclic compounds (Fig. 7.1). This has been observed in nmr studies of planar and octahedral complexes of ethylenediamine-type ligands with a number of metals. 

Fig. 7.1 A simple representation of the two conformations of the five membered puckered ring in metal complexes of ethylenediamine and derivatives. Rings are viewed along the plane containing the two nitrogens and the metal. The C-C bond is nearer the observer than the two nitrogens. It is skewed down to the right for a 5 conformation and to the left for a X conformation.

X-Ray diffraction analyses of vincristine methiodide (39) and vinzoli-dine 1-naphthalene sulfonate (40) provide atomic coordinates of compounds that are either modified in the velbanamine portion (alkylation at N-6, Fig. 3) or in the vindoline portion (a spiro-fused oxazolidinedione at C-3 and C-23, Fig. 4). These structures show a chair-boat conformation for ring C with C-8 exhibiting an endo pucker. Ring D is clearly in a chair conformation, but, in contrast to the conclusions drawn from C-NMR studies, the N-6 -C-7 bond is axial relative to the piperidine ring. 

Fig. 8 The puckered eight-membered Asg ring in the MAsg" derivatives. This puckered ring arises from severe flattening of a square antiprism...

Exact molecular parameters of oxathietanes and thiazetidines have not been assessed, although the nonplanar puckered-ring structure appears to be the most favorable orientation. ... 

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