Twist-boat conformation, coupling constants

The preferred conformation of the 1,3-oxazepinone derivative 13 was shown to be the twist-boat conformer 14 on the basis of NOE studies and the coupling constants for couplings involving H6. A nuclear Overhauser effect (NOE) was only seen between H6 and H5a and not between H6 and H5b other NOEs are indicated on 14 <1999H(51)365>. 

The solution-phase conformational analysis also reveals instances of boat and twist-boat conformations septanosides, examples of which are given in Figure 13.11. The coupling constants 73 4 = 7.61 Hz, 74,5 = 9.97 Hz, 75,6a = Hz, 75,5b = 10.66 Hz, and 7ba,6b = H-35 Hz ascertained a boat conformation for 128, whereas coupling constant 73,4 = 7.61 Hz indicated conformation [56]. 

Introduction of a-substituents into the 3- or 7-position of 12a leads to a shift of the prevailing conformation from chair-chair (cc) to boat-chair (be), and finally to twist double boat (16) as judged by H NMR spectra (121). The changes in the H H coupling constants in these derivatives can be interpreted with skewed cc conformations as well. MM calculations, however, ruled out the latter possibility (121). The results of acetolysis of exo- and endobicyclo[3.3.1]nonan-3-yl tosylates were also interpreted with the aid of MM calculations (122). 

The PM3 calculations showed that the twist-boat-chair conformation of dithiocin 307 is more stable by 5.9 kcal mol 1 than the corresponding chair-chair conformation. For the same compound, coupling constants for the AA XX system of the ethylene chain were simulated to be J(AA ) = 10.15 Hz,, /(XX ) OIIz, /(AX) =—14.0Hz, /(AX ) = 9.4Hz... 

The solution-phase conformation of 129 showed that it adopted a twist-boat TBj 4, (Figure 13.10), which was deduced from the observed coupling constants 76a,6b = 13.5 Hz, denoting an antiperiplanar arrangement of 05 with one of the geminal hydrogens of C6 [56]. 

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