Bowsprit position

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). 

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... 

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)). 

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

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 ). 

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