Boat conformation energetics

In conclusion, in the case of 1,4-conjugated additions to a,B-unsaturated ketones, some substrates, in order to avoid steric interaction, react through a boat conformation to give an equatorially substituted product, but when there is no steric interaction, the axial attack through a chair conformation is energetically favored. Both processes are however stereoelectroni-cally controlled. 

Figure 2.3. Boat and chair forms of P-D-glucuronic acid, (a) Cj chair conformation (left) and planar projection (right) (b) Boat conformation and (c) 1C chair conformation of P-D-glucuronic acid.The latter two conformations are energetically less stable than the Ci chair conformation.

The piperdine (D) ring is in the energetically favored chair conformation, but the C ring of morphine is in a boat conformation, which places the 6a-hydroxyl in the equatorial position (see Ref 298). (+)-Morphine, the enantiomer of the naturally occurring (-)-mor-phine, has been synthesized (see Refs. 283, 299) and is devoid of analgesic activity. 

The two 5,6,8,9-tetraaza[3.3]paracyclophanes 6a,b exist as chair and boat conformers, the boat form being energetically favored.27 The chair conformer with the propylene bridge arranged anti to the methoxy substituents predominates over the chair conformation with syn arrangement of the bridge. 

Limited rotation around the carbon-carbon single bonds of cycloalkanes also results in a variety of conformations of cycloalkanes. In cyclohexane the chair conformation is the most energetically favored. Another conformation is the boat conformation. 

In the chair conformation the hydrogen atoms, and thus the electron pairs of the C—H bonds, are farther from one another. As a result, there is less electron repulsion and the structure is more stable (more energetically favored). In the boat conformation, the electron pairs are more crowded. This causes greater electron repulsion, producing a less stable, less energetically favored conformation. 

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