Cyclohexane ring conformation locked

Deuterium atom was neatly incorporated at the bridgehead position C-l in ketone 464, the only compound in which the cyclohexane ring is locked in a boat conformation. Examination of molecular models indicates that the cyclohexanone ring can easily adopt a boat form in 460 and 461. It appears to be more difficult with ketone 462 and almost impossible with ketone 463. 

Let us look more closely at trans-decalin, by far the more common stereoisomer of decalin. An important feature of this bicycloalkane is that each ring is locked into one chair conformation neither ring can invert to its alternative chair. This means, for example, that if an —OH group is equatorial in a decalinol (a decalin alcohol), it remains equatorial it cannot become axial because the cyclohexane ring is locked into this one conformation. Likewise, if an —OH group is axial, it remains axial. 

Norbornane has a conformationally locked boat cyclohexane ring (Section 4.5) in which carbons 1 and 4 are joined by an additional CH group. Note how, in drawing this structure, a break in the rear bond indicates that the vertical bond crosses in front of it. Making a molecular mode) is particularly helpful when trying to see the three-dimensionality of norbornane. 

Cyclohexane sometimes adopts a twist-boat conformation, but never a true boat structure, which represents an energy maximum, But boat structures are important in some bicyclic compounds where the compound simply doesn t have any choice in the conformation it adopts. The simplest compound locked into a boat structure is norbornane. The CH2 bridge has to be diaxial (otherwise it can t reach), which means that the cyclohexane ring part of the structure has no choice but to be a boat,... 

In contrast to the results for monocyclic compounds, in bicyclic compounds the rigidity of bridged structure frequently permits more substantial effects than in acyclic or monocyclic compounds. The bridging locks the cyclohexane ring into the boat and/or twist-bond conformation. For example, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have one and two boat cyclohexane rings respectively. The enthalpies of the boat and twist-boat conformers of cyclohexane are 27 and 23 kJ/mol higher than that of the chair conformer. Therefore, bicyclo [2.2.1] heptane and bicyclo [2.2.2] octane have ring strains of 64 and 54 kJ/mol, respectively, as shown in Table 4. 

Acid-catalyzed hydrolysis of a 1,2-epoxycyclobexane produces a trans-diaxial 1,2-diol. What product would you expect to obtain from acidic hydrolysis of cis-S-iert-butyl-1,2-epoxycyclohexane (Recall that the bulky teri-butyl group locks the cyclohexane ring into a specific conformation.)... 

There is an important difference between the cis- and /rani-decalin systems with respect to their conformational flexibility. Owing to the nature of its ring fusion, trans-decalin is incapable of chair-chair inversion c/x-decalin is conformationally mobile and undergoes ring inversion at a rate only slightly slower than cyclohexane (AG = 12.3-12.4 kcal/mol). The/ranx-decalin system is a conformationally locked system and can be used to compare properties and reactivity of groups in axial or equatorial environments. 

Cyclohexane 1,2-epoxide can be directly converted, in high yield, into the episulfide by the action of thiocyanate ion the initial product is free to shift its conformation to the diequatorial arrangement and thus to permit formation of the intermediate (91, n = 2). Pyranose epoxides which lack the conformation-locking, fused-acetal ring might also be expected to give episulfides readily by the direct route. 

This compound has a conformationally locked boat cyclohexane ring in which carbons 1 and 4 are connected by a CHj group (C-7). This compound has both angle strain and torsional strain. 

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