Sickle conformation

Using the smaller van der Waals cutoff radius (3.0 2, the calculations gave three conformations Sickles l and 3, and Extended) totaling almost 93% of the total population (Table II). As expected, Sickle-1 and Sickle-3 were the dominant conformers, but the Extended conformer was also calculated to be present in... 

Among the hexitols, n.m.r. and X-ray crystallographic data " showed that galactitol and D-mannitol exist as planar zigzag conformers. Solutions of the others contain appreciable proportions of sickle and other bent carbon-carbon chain conformers. The low sweetness of the hexitols could possibly be due to increased hydrogen-bonding due to increased chain-length. 

P212121 Z = 4 D = 1.25 R = 0.069 number of intensities not reported. This analysis was to confirm a configuration derived from n.m.r. data. The pyranoid rings are slightly distorted 4Q, due to fusion to the trioxacyclooctane ring, which has an almost ideal, boat-chair conformation. The overall molecule has a convex, sickle conformation. There is an error in the atomic coordinates reported, which do not correspond to the structure given in the paper. 

Pi Z = 2 Dx = 1.452 R = 0.066 for 2,097 intensities. The carbon chain has a sickle conformation, being cis with respect to the double bond. The carbon and oxygen atoms lie in two planes, with C-2-C-3-C-4-C-5 = +71°. One of the carboxyl groups is eclipsed to the C-C bonds, with 0-l=C-l-C-2—C-3 = + 1°, and the other is so inclined that 0-5=C-5-C-4-C-3 = +22°. The carboxyl groups are hydrogen bonded to afford dimers, with one of the hydrogen atoms disordered. 

P212i21 Z = 4 Dx = 1.298 R = 0.056 for 1,045 intensities. The D-ara-binitol residue has a sickle conformation, with a C-1-C-2-C-3-C-4 torsion-angle of —55°. The C-6 atom of the phenyl group forms a planar, zigzag chain with C-2 to C-5. The terminal hydroxyl group is -I- sc to the carbon chain. The plane of the phenyl ring is almost per-... 

P212121 Z = 4 Dx = 1.21 R = 0.05 for 1,605 intensities. The carbon chain has a sickle conformation, with C-1-C-2-C-3-C-4, C-2-C-3-C-4-C-5 torsion angles of —60°, -50°, respectively. The dioxolanes have a twist and an envelope conformation, and are related by an approximate, two-fold axis along C-3-C-4. The double-bond stereochemistry is Z. 

Angyal and co-workers (8) concluded that the conformational distribution of xylitol pentaacetate in CDCl is between two sickle conformations with the extended zig-zag conformation being unimportant. The 3 3 4 average value of 5.2 Hz is from a large coupling from AntipArallel (or H2-H2) and a small... 

When a van der Waals cutoff radius of 3 A was used to calculate the conformational populations of JV,J -dihexylxYlaramide (2), similar results were obtained (Table III). Sickles-1 and 3 and the Extended conformation accounted for 94% of the total conformational population. Again, good agreement was obtained for calcd (3.66 Hz) and J2 3 xpt (3.32 Hz). 

As has already been pointed out (see p. 59), the carbon chain in ribitol17 is nonplanar, and the molecule adopts a sickle conformation.18 This conformation is just one of the conformational changes that can be directly attributed to interaction (repulsion) between nonbonded atoms, Were ribitol planar, the contact between 0-2 and 0-4 would be close, as shown in Fig. 3. The C-O bonds would be... 

The conformations in solution of various acylated l,l-bis(acyl-amido)-l-deoxypentitols have been examined by p.m.r. spectroscopy.793 In line with general behavior observed78 with other acyclic-sugar derivatives, it was found that the arabino and lyxo derivatives adopt extended, zigzag conformations, whereas the ribo and xylo derivatives favor sickle conformations that result by rotation about one carbon-carbon bond of the backbone chain to alleviate a destabilizing 1,3-interaction of acyloxy substituents that would be present in the extended, zigzag conformation. 

This work has shown that in acyclic sugar systems the planar zigzag arrangement is the favored conformation in solution unless such an arrangement would lead to parallel 1,3-interaction between substituent groups, in which case the molecule adopts a sickle conformation to alleviate this interaction. 

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