Bowsprit hydrogen

C), (D) or (E). It was emphasised that the result contrasts sharply with the known preference of 1,3,2-dioxaphosphorinanc rings attached in apical-equatorial positions to occupy boat/twist conformations and the ease of population of boat/twist forms by 1,3,2-dioxaphosphorinanes containing three or four-coordinate phosphorus. The results were rationalised in terms of severe repulsive interactions between the apical P-O4 bond and the bowsprit hydrogen at C5 in... 

The distance between their centres at normal tetrahedral angles should have been only about 1.8 A, but the sum of the van der Waals radii of the two hydrogen atoms is 2.4A. This is sometimes called bowsprit-flagpole interaction and this too makes its contribution to the increased energy of the boat form. 

As a result of these unfavourable interactions, i.e., opposition to bond strain i.e., between the pair of hydrogens shown at the bottom) and also due to bowsprit interaction, the potential energy of the boat form becomes high and this is why the boat conformation is not the preferred one. Hassel in 1947 established by means of electron diffraction studies that cyclohexane exists predominantly in the chair form. This has also been confirmed by electron diffraction studies and results obtained from Raman and I.R. spectra. Calculations made on the basis of entropy show that only about one molecule in a thousand will be in the boat form. 

The cycloheptenone ring in carolenalone, a sesquiterpene lactone, adopts a flattened twist-chair conformation in which the C=C—C=0 system is distinctly non-planar. The dibenzocycloheptene rings in butaclamol and dexaclamol hydrobromides adopt conformations such that the hydrogen on C-13 experiences a flagpole-bowsprit interaction with the hydrogen on C-9. ... 

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