Envelope conformation, and

P212121 Z = 4 Dx = 1.21 R = 0.05 for 1,605 intensities. The carbon chain has a sickle conformation, with C-1-C-2-C-3-C-4, C-2-C-3-C-4-C-5 torsion angles of —60°, -50°, respectively. The dioxolanes have a twist and an envelope conformation, and are related by an approximate, two-fold axis along C-3-C-4. The double-bond stereochemistry is Z. 

Most furanoses prefer the envelope conformation and it appears that a quasi-equatorial exocyclic side chain and a quasi-axial C(l)—0(1) bond (anomeric effect) are equally important stabilising factors (Figure 1.8). 

Relatively few such heterocyclic systems have been studied by microwave spectroscopy some data are included in Table 7. In 1,3-dioxolane the twist conformation (see Table 41) is more stable than the envelope conformation, and pseudorotation occurs. In 1,2,4-trioxolane the equilibrium conformation is the twist form, in which the peroxide bond straddles the plane of the other three atoms, and there is a barrier of 6.3 kj mol 1 opposing pseudorotation <1974PM H (6)53>. Preference for the twist conformation is also observed in the crystal structures of some 1,2,4-trioxolane derivatives (see Table 41, Section 2.4.4.4) . 

The pyrrolidine ring in 79 has an envelope conformation and the morpholino groups in 77 and 83 as well as the piperidino groups in 78 possess a chair conformation. The cyclohexene moieties in 77 to 82 exist in half-chair conformations. 

For the camphor-derived isoselenazoline 5, the bond lengths and angles are normal with Se-N and Se-C distances of 1.865 and 1.943 A, respectively, and an N-Se-C angle of 87.1°. The mean value of the Csps-C ps bond lengths in the camphor moiety is 1.542 A. The five-membered ring has an N-envelope conformation and the molecules are hydrogen-bonded, forming chains with O- O and O-H- O separations of 2.769 and 1.840 A, respectively. 

Pure dried tylophorine is a yellow powder, and crystallization from common solvents is difficult. Spectral data, including MS, H-NMR, C-IMMR and chiroptical (ORD and CD), of phenanthroindolizidine alkaloids have been well established [18, 42, 44-45]. The three-dimensional crystal structure of tylophorine (1) was first determined by Wang et al [46]. The structure of tylophorine B, as the benzene solvate, has conjoined phenanthrene and indolizidine moieties. The aromatic rings lie almost in the same plane with dihedral angles of 1.7° (A/B), 2.8° (B/C), 2.2° (A/C), and 7.3° (B/D). The E ring adopts an envelope conformation and makes a dihedral angle of 6.7(3)° with the D ring [46]. 

LUMO. These two effects reinforce and favour dienophile attack from the face anti to the better electron donor substituent, in this case the C-Me. In summary, the most electron-donating rr-bond (i.e., that which possesses the higher cr-orbital energy and/or the cr -orbital nearest in energy to the diene orbital) will block attack from that face, because as the reaction occurs the cyclopentadiene develops an envelope conformation and the electron-donating group (i.e. C-Me in the case discussed) is pseudo-axial. The HOMO/LUMO energy difference is less than for attack from the other face anti to C-X. 

The structures of several related naphtho-fused ring systems have been determined by X-ray crystallography. Thus, the cyclic silazane 112 (R = Ph) is planar at the nitrogen atom and all the rings lie almost coplanar, while the related compound 112 (R = Bu but with an anisyl group attached to one of the silicon atoms) has the silicon atoms out of the plane of the tricyclic system, and the ylide 113 has the heterocyclic ring in an envelope conformation and the ylidic carbon is pyramidal and not planar <1996CB495> (see Section 9.18.4.3.2). 

The addition of nucleophiles to cyclic acetals and hemiacetals is an efficient method to access substituted THFs in high diastereoselectivity which is, in general, predictable and can be explained by stereoelectronic effects (1999JA12208). In this transformation, the intermediate is an oxocarbe-nium, which preferentially adopts an envelope conformation, and the nucleophile attacks on the inside face of the envelope. In the presence of a methyl at C3, the conformer possessing pseudoequatorial substituents is favored and the, 2>-trans product is obtained as the major product (63 64) (Scheme 33). It is worthy of note that when an alkoxy group is present at C3, the 1,3-ds product, resulting from an inside attack on the diaxial conformer, is favored (65 66) (Scheme 33). 

In elaborating this hypothesis it was assumed, and subsequently supported by calculations [23-25], that the intermediate oxocarbenium ion adopts a " //3 halfchair or E envelope conformation and that attack on this electrophile takes place... 

Molecule 113B (Fig. 4) [119,123] has a similar structure the cycles F, T , and T have the envelope conformation, and the six-membered cycle takes the conformation of twisted sofa . 

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