Bowsprit

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. 

For the planar DKP ring, the /3-carbon atom of an L-amino acid is located on the left-hand side, as shown in Figure 5(a). For the boat conformation, the /3-carbon atom of an L-amino acid is in the axial position (Figure 5(b)). For the bowsprit-boat form, the /3-carbon atom of an L-amino acid is in the equatorial position (Figure 5(c)). Finally, for the twist-boat form to exist, the /3-carbon atom of an L-amino acid must adopt a quasi-equatorial position (Figure 5(d)). 

Figure 5 In solution the DKP ring adopts four main conformations (a) planar, (b) boat, (c) bowsprit-boat, and (d) twist-boat forms.Adapted from N. Funasaki S. Hada S. Neya, Anal. Chem. 1993, 65, 1861-1867.

Fig. 9. Possible conformations of fused-ring compounds (e.g. (58)). Nitrogen atoms and ring substituents are omitted. The first two letters for each structure refer to the ring conformations (b = boat, c = chair) and the second two letters refer to the phosphorus-exo-substituent (a = axial, b = bowsprit, e = equatorial, f = flagpole)...

The 1,6-addition to a,s, y, a-dienones is also subject to stereoelectronic effects. Addition on the bottom face of dienone 137 leads to a chair-like intermediate while that on the top face leads to a boat-like intermediate 140 in order to maintain maximum orbital overlap. Also, in 140 the R group encounters an eclipsed 1,2-R/H interaction and more importantly, a 1.4-CH3/R steric interaction which resembles the bowsprit flagpole arrangement of a twist-boat form of cyclohexane. This analysis of Marshall and Roebke (48) predicts that the trans product 139 should prevail over the cis product 141. 

Position of bonds and substituents in the chair and boat conformations of cyclohexane a = axial, e = equatorial, b = bowsprit, and f = flagpole. 

Considering the factors favoring the CB conformation, it should be taken into account that the introduction of an endo-3 substituent (11) causes one of the CB forms (11a) to be destabilized by the resulting extra 1,3-diaxial interactions. Indeed, for each six-membered ring in the bicyclo[3.3.1]nonane skeleton, the second ring is equivalent to two axial substituents, the introduction of an endo-3 substituent being equivalent in this case to a third axial one. It leads not only to the absence in the equilibrium of the chair-chair conformation lib (which is mostly destabilized by the 3 - 7 repulsion) but also to the minor content of the chair-boat form 11a. The alternative boat-chair conformation 11c with the substituent in the bowsprit position dominates in this case (24,25). 

Bowsprit, Flagpole In the cyclohexane boat conformation the ligands on the two carbons that are out of the plane of the other four. Endocyclic ligands are flagpole, exocyclic ligands are bowsprit. 

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... 

Corey and co-workers also developed parameters to evaluate boat conformations, which are often important contributors to the conformational population of cyclohexane derivatives (see above). The energies for boat conformations with one or more substituents in the flagpole positions use a new term (b) (see 210 and 211) which is derived from the bowsprit-flagpole interactions (R-H in 210 or R -R in 211). Corey s examples include 212 and 213. In 212, the axial hydroxyl group receives a value (Aqh) of 0.8 kcal moT (3.35 kJ moT, see Table 1.5) and b] = 0 for bond a and b2 = 0.60 for bond c (see typical b values in Table 1.6. The conformational energy for 212 is (0.6 x 0.8) = 0.48 kcal moT (2.01 kJ moT ). 

To begin with, the conformation of the skeleton of cyclic dipeptide, i.e., the diketopiperazine (DKP) ring, will be considered. There are many possible conformations of the DKP ring 8) as shown in Fig. 14. For unsubstituted diketopiperazine, Cyclo-(Gly2) (R = R = H) a planar conformation (A) in crystalline state has been reported 95, 96). In solution, the planar conformation (A) or rapidly-inverting boat conformation (B) (C) may be supposed. This consideration was supported by NMR studies as described below. Monosubstituted diketopiperazines Cyclo-(Gly-X) (R or R = H) can assume two boat conformations, (B) and (C), as well as the planar conformation (A). The C -substituent R or R takes an axial position in (B) and an equatorial or bowsprit position in (C). Kopple and Ohnishi (97) jaroposed the possibility of a twist-boat conformation (D). Rotations around N—C and C —G... 

Katritzky and coworkers utilized X-ray aystaUography to analyze the structure consequences of VAE in systems with two endocyclic double bonds and an exocyclic benzotriazole substituent (Figure 6.80). The two non-equivalent exocyclic bonds of the sp - hybridized carbon atom occupy pseudoaxial (flagpole) and pseudoequatorial (bowsprit) positions. Importantly, the benzotriazole residue generally assumes the pseudoaxial position. The exocychc C-N bond is 0.02-0.03A longer than the C-N bond of the model compound without oxygen. 

Consider two twist boat conformations of methylcyclohexane—one in which the methyl group is in the "flagpole" position and one in which the methyl group is in the "bowsprit" position (page 124). A molecular mechanics calculation... 

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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