Helical geometry

Bau, R. (1998) Crystal structure of the antiarthritic drug gold thiomalate (Myochrysine) A double-helical geometry in the solid state. Journal of the American Chemical Society, 120, 9380-9381. 

Such a transition state is likely to be highly strained, however, and no such 1,3-antarafacial shifts have actually been observed. A 1,7-thermal antarafacial shift in (36, x = 2), where the T.S. is likely to be much less strained (i.e. able to adopt the required helical geometry) has, however, been observed in the vitamin D series. 

The purpose of this analysis is to design an industrial size version of the pilot process, which achieves the same dispersive mixing without overheating the polymer melt. In order to simplify the solution we lay the helical geometry flat, a common way of analyzing single screw extruders. 

Note The helix types are H for a helix and G for 310 helix. The helical geometry is followed by a measure of the deviation from an ideal helix (this value should be 0 for a perfect helix). For the interacting pairs, the interaction type describes where in each of the helices the distance of closest approach occurs (C, beyond the C terminus of the helix N, beyond the N terminus of the helix I, internal to the helix). 

Another interesting group of chiral compounds results when molecules are forced to adopt a helical geometry. Like the turn of a screw, the turn of the helix can be either right-handed or left-handed. One example is the compound known as hexahelicene, which has six aromatic rings fused together. The molecule is forced to adopt a helical... 

There are abundant examples in the literature which reveal that the thermal electrocyclization of 8e systems to form eight-membered rings proceeds with lower activation energies than for the lower vinyl-og, (Z)-hexatrienes. The helical geometry of the transition state leads to less steric congestion about Ae reacting termini of the octatetraene and this likely accounts for the facility of the thermal 8e conrotatory process. Minor structural differences can induce cycloreversion of the process. ... 

X-Ray structures of the corresponding cobalt(II), nickel(II) and zinc(ll) derivatives of 61 confirm that each of these complexes adopts a related helical geometry in which the co-ordination sphere is filled by six nitrogen donors from two essentially planar (meridionally co-ordinated) pyridine-2,6-diyldiimine units belonging to the macrocycle. 

If we have a longer conjugated system, as in the triene (149), then a [1,7] shift takes place on heating.134 In Fig. 4-14 we can see that this is favoured only if the hydrogen leaves the top surface and arrives on the lower surface, in other words, in an antarafacial manner. Unlike the case of the [1,3] shift, there is a helical shape, shown in the figure, in which continuous overlap can be maintained. The known examples of this kind of reaction take place in open-chain trienes, in which helical geometry, like that shown, is possible. There is no proof that the shift is in fact antarafacial, but it is known that in cyclic trienes, like the... 

Single-stranded regions consist of unpaired nucleotides between helices in the secondary structure and at the 5 and 3 ends. The single-stranded regions may be ordered by base stacking in a helical geometry unless they are involved in tertiary structures. 

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