Oxocane, conformation

The conformational properties of such eight-membered ring molecules have been reviewed fairly recently (74MI51900). Cyclooctane is the archetypical molecule in this class, and the heterocyclic analogs, such as the azocanes, oxocanes and thiocanes, as well as carbocyclic derivatives, such as cyclooctanone, all have closely related conformational features and a brief overview of their conformations will now be given. 

The above discussion provides a background for a discussion of the conformations of various azocanes, oxocanes, thiocanes and related compounds which lack strong ring torsional constraints. 

The X-ray structure of the substituted oxocane laurencienyne, (429), shows that the ring has the BC-3 conformation (actually, the oxygen is at the mirror image 7 position of the boat-chair) and one chlorine atom is in an axial-like position. The other substituents are in unhindered positions on the ring (80TL2299,80CSC777). 

Since oxygen is much smaller than a methylene group, the same kind of situation occurs in XVII as was discussed in the previous section. The barrier to methyl rotation in dimethyl ether is 2.7 kcal/mole >, only slightly lower than in propane, where the beirrier is 3.4 kcal/mole. Oxocane should therefore have the BC-1 conformation, as in methylenecyclooctaue rather than the BC-3 and BC-7 conformations. The presence of only a single process in the proton spectrum of XVII is immediately consistent with the BC-1 conformation, but requires rapid pseudorotation between the BC-3 and BC-7 forms at —170 °C if the latter two forms are the correct conformations. The pseudorotation barrier in XVII should be higher than in cyclooctane, and probably comparable to that in cyclooctanone (6.3 kcal/mole). Thus, pseudorotation of the BC-3 form should not be rapid at —170 °C, and further support for this h5q)othesis is provided by 1,3-dioxocane (see below). It is therefore probable that oxocane has the BC-1 conformation. 

The presence of a heteroatom, a C-C unit, or a carbonyl influences the conformation of medium-size rings, but the changes are often subtle. The calculated low-energy conformation of cyclooctane (174) is compared with those of cii-cyclooctene (175), the ether oxocane (176) and cyclooctanone (177). Although 175 is somewhat flattened, the conformations of the other three eight-membered rings are rather similar. ... 

Pakes PW, Rounds TC, Strauss HL (1981) Vibratianal spectra and potential functions of cydooctane and some related oxocanes. J. Phys. Chem. 85 2476 Conformations of cyclooctane and some related oxocanes. ibid. 24 ... 

A molecular mechanics (MM2) study has been reported for oxocane <92JOC29l>. As in a previous force field study <80ZN(B)1479>, the BC-3 (8) conformation is calculated to be the lowest energy conformer, favored by 1.1 kcal mol over the BC-1 conformer. 

Barriers for interconversion between BC-3 and its enantiomer, BC-7, have been calculated. Passage through either a BC-5 or a BC-1 conformation, are calculated to be 5.0 kcal mol and 6.7 kcal mol , respectively. The small barriers allow for a rapid pseudorotation between BC-3 and BC-7, even at low temperatures, and account for the symmetric spectra. NMR results on oxocane-2,2,7,7-d4 have allowed a definitive assignment for the H and C chemical shifts. Lanthanide-induced shift reagents provide compelling proof, that the lowest energy conformations are BC-3 and BC-7. 

The tub conformation for 9,1 l-diphenyl[5](2,6)-pyranophan-10-one has been determined by x-ray diffraction methods <90CB1587>. The structure of a novel dibenzo[c/]oxocane spiro compound, derived from the reaction of two benzocyclopropene and diphenylcyclopropenone, has also been reported <90ZN(B)1577>. 

Mono-substitution in all but the 5-positioh in oxocane results in a chiral center, accounting for the optical activity in many natural products. (Substitution at the 5-position for some conformations such as the crown or chair-chair would not give rise to a chiral center however, for some boat-chair and other less symmetric conformations, a chiral center could be generated. This issue appears not to have been specifically addressed in the literature.) The base-catalyzed isomerization of some 2,8-disubstituted oxocane derivatives has been studied. It has been found that the c -geometry is preferred over the trans-geometry <88TL4333). 

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