Exo ring conformation

Fig. 9. Possible conformations of fused-ring compounds (e.g. (58)). Nitrogen atoms and ring substituents are omitted. The first two letters for each structure refer to the ring conformations (b = boat, c = chair) and the second two letters refer to the phosphorus-exo-substituent (a = axial, b = bowsprit, e = equatorial, f = flagpole)...

According to the results of ab initio calculations <2003PCA7574>, the two O-envelope conformers of the bicyclic primary ozonide of benzene are nearly equally stable. The product of ozone addition in the immediate vicinity of the hydroxyl group of phenol prefers an exo-O-conformation. The carbon rings in the primary ozonides of benzene and phenol are found to retain their planarity. 

In addition, Sg ring conformations of lower symmetry than 04a are hkely candidates in liquid sulfur. For example, the exo-endo isomer 3 (Fig. 4) is by only AG°298=27.8 kJ mol" less stable than the ground state conformation and therefore its relative concentration in liquid sulfur at the boiling point will be also ca. 1% of all Sg species [109]. 

All known solid state structures shown in Fig. 1 contain sulfur homocycles. While the 84 cation is a regular square, the 8g cation adopts an approximately Cs symmetric exo-endo conformation with three transannular contacts and 819 consists of two seven membered rings linked by a penta-sulfur chain and incorporates two tricoordinate sulfur atoms. An analysis of the soHd state anion contacts of the 84 and 8g cations showed that the positive charge is delocalized almost equally over the entire cations [11, 23]. 

Fig. 9.12. Scattergram of the scores on the first two factors from a principal component analysis of the torsion angles of 181 furanose rings. The two factors include essentially 100% of the variance of the ring torsion angles. The numbers show the position of some selected ring conformations. 1 envelope (e) with mirror plane through C(2 ) endo 2 e C(3 ) endo 3 e C(3 ) exo 4 e C(2 ) exo 5 twist with Ci axis through C(T) 6 e 0(T) endo...

Packing forces alone can change the ring puckering considerably. There are several examples of crystal structures with two symmetry independent molecules that show different ring conformations. For example, in oxanosine (13) [36] one molecule has the C(2 ) exo and the other the C(3 ) exo conformation, a phase change of about 150°. 

The active compounds studied (7e, 7f, 7n, 109a, 109d and 109h) have a preference for a sugar ring conformation which is centered around the Cy-exo... 

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