Ring conformations half-chair

Takao and co-workers (60) have examined a number of hydro-benzo[c]phenanthridine alkaloids and their derivatives by a variety of physical methods in order to determine the conformation of the B/C rings (Fig. 18). The 13C chemical shifts (Table XVII) were particularly sensitive to the influence of substituents on the conformation of the B and C rings. Chelidonine (94) for example, with a cis ring junction, has both the B and C rings as half-chairs [see Fig. 1 in (60)]. Acetylation of the hydroxy l group to form 95 shielded C-6, C-12, and C-13 relative to 94. The interpretation of these observations was that ring C in 94 adopted a twist half-chair conformation which increased the number of gauche interactions for these carbons. 

While palladium(II) and copper(II) build solely 1,2-diolato complexes with erythritol which leads to five-membered chelate rings, boron(III) is involved in the six-membered rings as in the bis(phenylboronic acid ester) 13 (O Fig. 2). Erythritol adopts a zigzag conformation similar to that in 9 but the oxygen atoms are grouped alternatively on opposite sites of the C4-chain. The six-membered chelate rings in half-chair conformation provide a suitable bond pattern for a boron center involved in the delocalized jr-hond system of the phenyl suhstituent. 

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 1,2,4-trioxolane ring prefers a half-chair conformation (131) the C—O—C portion of the ring forms the reference plane, and alkyl substituents prefer the equatorial positions. 

Dithiolane (132) derivatives also possess non-planar skeletons the most important conformation is probably of symmetry C2 (half-chair). The dithiolane ring may be quite flexible and a minimum energy. conformation is only well defined if there is a bulky substituent at the 2-position. 

The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffiaction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. ... 

Examine the geometry of norcarane. What is the conformation of the cyclohexane ring Choose a name (chair, twist boat, half-chair, etc. see Chapter 5, Problem 4) that accurately describes its shape. The bridgehead hydrogens in norcarane are cis. Do you think a trans stereoisomer is possible Explain. 

Bond orders, charges on the atoms in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one and its protonated form were calculated by quantum chemical calculations by the semiempirical AMI method. According to the results, the equilibrium conformation of the ring in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one is planar, while l//-pyrimido[l,2-u]quinolin-1-one adopts a conformation close to a half-chair due to the unfavorable interactions between the oxygen atom of the carbonyl group and the ring C-10 atom in the pen-position (97MI22). 

The annulation of 4//-thiopyran and cyclohexane rings in 50a results in the planarity of the heterocycle and a half-chair conformation of the carbocycle (81KGS1342). On the other hand, a boat conformation of the 2//-thiopyran ring was found in the crystal of224b [91JCS(P2)2061], Other geometrical parameters were within the limits of the expected values (Fig. 2). 

The [Pt(Se4)2]2 complex (133) has been prepared by the reduction of the platinum(IV) species [Pt(Se4)3]2 with excess borohydride.328 The X-ray structure of the complex shows that each of the five-membered rings adopts a half-chair conformation, with the platinum coordination sphere exhibiting slight distortion from a square-planar geometry.329... 

Bisquinolizidine alkaloids have also been widely studied by this technique. For instance, the crystal structure of (—)-A16(17 ,-dehydrolupaninium perchlorate 23 was obtained from sealed-tube and synchroton X-ray diffraction data, and showed that the A, B, C, and D rings assume distorted half-chair, chair, distorted sofa, and chair conformations, respectively it was also used to determine the most precise dimensions so far known for the iminium group <1999JST245>. The crystal structure of quinolizinium hexafluorophosphate has also been studied <2001CSC174>. 

The positions of maximum energy are conformations called half-chair conformations, in which the carbon atoms of one end of the ring have become coplanar. 

X-Ray elucidation provided a final proof for the structure elucidation of 8-(4-chlorophenyl)-8-hydroxy-5-methyl-8/f-[l,4]thiazino[3,4-H[l,2,4]oxadiazol-3-one 49, which was obtained as a product of ring-transformation reaction <1997J(P2)2407>. This analysis revealed that the oxadiazole ring is planar, whereas the thiazine ring is in a distorted half-chair conformation with a displacement asymmetry parameter AC2 (S—G(3)) = 0.031. The structure analysis represents the first X-ray elucidation of a [l,4]thiazine ring fused to a [l,2,4]triazole moiety. 

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