Cyclohexane ring system

Free radical reactions are used less frequently, to effect six- rather than five-membered ring closures, because carbohydrate-derived hept-6-enyl radicals are less available than are hex-5-enyl analogues, these being the species required for normal exo ring closure processes. Such reactions can, however, be used with good efficiency to produce cyclohexane ring systems bearing a carbon substituent (see Chap. 25). 

We will cover the conformations of cyclohexane in more detail than other cycloalkanes because cyclohexane ring systems are particularly common. Carbohydrates, steroids, plant products, pesticides, and many other important compounds contain cyclohexanelike rings whose conformations and stereochemistry are critically important to their reactivity. The abundance of cyclohexane rings in nature is probably due to both their stability and the selectivity offered by their predictable conformations. Nature probably forms more six-membered rings than all other ring sizes combined. 

NMR chemical shifts and their assignment for 129 are shown with the structure below. The 1 0 NMR signals for the two methoxy groups in this diterpene apparently overlap at 1.6 ppm. The chemical shift of 1.6 ppm is viewed by the authors as consistent with values of methoxy groups in axial positions of simple cyclohexane ring systems and the assignment was made on that basis[l 10]. 

J. Wang, M. Froeyen, C. Hendrix, G. Andrei, R. Snoeck, E. De Clereq, and P. Herdewijin, The cyclohexane ring system as a furanose mimic. Synthesis and antiviral activity of both enantiomers of cyclohexenyl guanine, J. Med. Chem., 43 (2000) 736-745. 

Radical reactions are not restricted to cyclizations onto alkenes or alkynes. Increasingly popular is the use of an imine or imine derivative, such as an oxime or hydrazone. ° Most examples involve 5- or 6-exo-trig cyclization to give cyclopentane or cyclohexane ring systems. Thus, treatment of the bromide 77 withtributyltin hydride gave the cyclopentane 78 (4.66). The stereoselectivity of the cyclization is in line with that expected on the basis of a chair-like transition state (compare with Scheme 4.53). 

Assemble a model of norbornane (bicyclo[2.2.1]heptane). Observe the two cyclopentane ring systems in the molecule. The structure may also be viewed as having a methylene (CH2) bridge between carbon atoms 1 and 4 of cyclohexane. Describe the conformation of the cyclohexane ring system in norbornane. How many eclipsing interactions are present ... 

Using a model of twistane, identify the cyclohexane ring systems held in twist conformations. In adamantane, find the chair conformation cyclohexane systems. How many are present Evaluate the torsional and angle strain in adamantane. Which of the three compounds in this exercise are chiral ... 

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