Envelope-boat conformation

Compounds 93 (72), 94 (73), and 95 (74) are examples of those having chair-envelope conformations in the crystalline state. The envelope part of the skeleton is rather rigid. It should be mentioned that the endo-1 substituent does not destabilize the chair part of the bicyclic system enough to convert it into the envelope-boat conformation. Both in solution ( H-NMR data) and in the solid state (X-ray data) 95 exists in the chair-envelope conformation (74). 

Interestingly, for compounds 269 and 270 (211), the envelope-chair and envelope-boat conformations were found, respectively, in the solid state though both compounds contain exo-6 and exo-1 acetoxy groups as well as an endo-1 substituents. 

The six-membered heterocycle central to 5 does not adopt the boat conformation shown by the related anthracenyl derivatives discussed earlier (see Section III,B,2) rather the GeC moiety is coplanar, with the phosphorus atom lying 0.33 A out of this plane in an envelope arrangement reminiscent of (CH3)-1GeCr)H5 (173). The structure of HCB10H ioCGe(CH3J3 has also been determined. 

The envelope conformation is, however, only a particular case of out-of-plane distortions that are able to generate conical intersections in aromatic and heteroaromatic rings. We have already seen in the previous sections examples of conical intersections with screw-boat and boat conformations arising in aminopyrimidine and pyridone. In all these cases, the relaxation on the excited-state surface is... 

In cyclopentene systems with fixed envelope conformation, the torsional interactions induce a high preference for the epoxide with boat conformation of the peripheral six-mem-bered ring (including the transferred oxygen, see Table 1), as, for example, epoxidation of norbomene (1) to give exo-2. This is shown mechanistically from the projection along the C-C bound adjacent to the reacting C-C double bond. 

Cyclobutane derivatives are usually, but not always, nonplanar. Cy-clobutane itself exists as two butterflylike conformers that are easily interconvertible by inversion. Cyclopentane is a cyclic five-membered ring structure that can exist as a set of half-chair (twist-boat) forms (C2) and a set of envelope (C5) conformations. These are low-energy conformations and are readily interconverted by twists about bonds without any bond angle changes, only changes in torsion angles. These interconversions are called pseudorotations. 

In accordance with the characteristics of conformer B, the thiazine ring in 92a assumes a skew-boat shape shifted somewhat toward the twist-boat form, whereas in 92b it has an almost perfect skew-boat conformation. The Cremer-Pople puckering parameters (< > = 89-%° and 0 = 69-77°) and the asymmetry factors for these conformations indicate a less (92a) and a more (92b) perfect symmetry axis bisecting the C-4—S-5 bond. Thus, the cw-fused triazoline ring with its N-2—C-3 double bond has an envelope conformation in 92a with the C-10 on the flap, but a halfchair conformation in 92b with a twofold axis bisecting C-3. Together,... 

Compounds 271 (212) and 272 (213) were found to exist in the solid state in a conformation close to the envelope-chair form. Hexaaza compound 273 adopts the chair boat conformation (214) and diimide 274 (215) exists in the envelope-envelope form according to the X-ray data. Interesting results were... 

Fig. 24 shows as an example the construction of the bicyclo[3.2.1]octane skeleton, which consists of one five-membered and one six-membered ring system (SSSR). In the first step, the two low-energy conformations, the envelope conformation of the five-membered and the chair conformation of the six-membered ring, are combined. Also, the more strained boat conformation of the six-membered ring can be fused to the envelope form of five-membered ring. 

It is known that the 2-cyclohexenone system exists, principally, as two rapidly exchanging envelope (also called sofa) conformations (93, 94). Conjugate addition of a nucleophile can occur to either face of the 2-cyclohexenone. Parallel or anti-parallel (with respect to the axial substituent at C4) attack is possible in principle, however, a nucleophile must approach from an axial direction for satisfying the requirement of the stereoelectronic effect. Anti-parallel attack leads to a favorable chair-like intermediate, whereas parallel attack leads to an unfavorable boat-like intermediate in each case. In an anti-parallel attack, the newly introduced nucleophile forms a frarcs-diaxial arrangement found in a chair conformation. Conversely, parallel attack leads to a syn-diaxial arrangement found in a boat conformation. Therefore, anti-parallel attack is favored as this leads to a lower energy intermediate. 

The structure was solved by the heavy-atom method, and the absolute configuration determined by examination of Friedel pairs. Rings c and e were shown to exist in envelope conformations, rings A and D were in boat conformations, and rings b and F exist in chair conformations. 

Artemisinin 17, 1,2,4-trioxane 18 itself, and some ring-fused, artemisinin-analogous 1,2,4-trioxanes have been studied theoretically and reported previously in CHEC-II(1996). While the nonsubstituted 1,2,4-trioxane ring prefers an envelope conformation, stmcturally related analogs favor a twist boat conformation. l,2,4-Trioxane-5-one 19 and a number of derivatives, also investigated theoretically, prefer half-chair conformations (cf Figure 2). Molecular electrostatic potentials (MEPs) obtained by theoretical studies (CHEC-II(1996)) of certain derivatives of 17 and 18 were included in structure/antimalarial activity correlations however, distinct conclusions between MEPs and antimalarial activities were not drawn. 

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