Cholesteryl derivatives

When the mesogenic compounds are chiral (or when chiral molecules are added as dopants) chiral mesophases can be produced, characterized by helical ordering of the constituent molecules in the mesophase. The chiral nematic phase is also called cholesteric, taken from its first observation in a cholesteryl derivative more than one century ago. These chiral structures have reduced symmetry, which can lead to a variety of interesting physical properties such as thermocromism, ferroelectricity, and so on. 

Several mixtures of hexanethiol capped gold nanopartides and triphenylene based discotic LCs have been studied. These mixtures display liquid crystal behavior (columnar mesophases) and an enhancement in the DC conductivity, due to the inclusion of gold nanoparticies into the matrix of the organic LC [70]. Other studies of mixtures of gold nanoparticies with mesogens indude a series of cholesteryl phenoxy alkanoates. The inclusion of the nanopartides does not change the inherent liquid crystal properties of the cholesteryl derivative but the mesophases are thermally stabilized [71]. 

Further studies on the cholesteryl/i-cholesteryl system were reported by Dodson and Reigel4 and particularly by Winstein and his coworkers5-6. This early work showed that the interconversion of the two cholesteryl derivatives, one of which has an open or homoal-lyF form 1 and the other a cyclopropylcarbinyl form 3, could be understood in terms of the intermediate 2 (equation 1). Electron delocalization in 2 was suggested to occur across the intervening carbon atom rather than between adjacent carbon atoms as in normal conjugated systems. 

Actually, these considerations are confirmed by experiments 82). The systems investigated are shown in Table 8, No. 4, which are induced cholesteric polymer systems. The nematogenic host molecules of benzoic acid phenyl esters are linked via spacers of different length m (m = 3, 4, 5, 6) to the polymer backbone. The polymers are converted to polymers having a cholesteric phase by the chiral cholesteryl derivative, which is also linked to the polymer backbone (copolymer). 

Carbon-7 is located on a plane that bisects the 2,3-bond and is in fact homoallylic to both sides of the double bond. Therefore a developing p orbital on it is in a position to overlap equally with each of the p orbitals of the double bond as is shown in Figure 6.7. No attack of solvent at C2 or C3 occurs in this system to give a three-membered ring analogous to t-cholesteryl derivatives and to 49 because the resulting carbon skeleton would be too strained. The fact that the product is 100 percent awh -acetate is a result of the hack side of Ct being hindered bv the three-center bond that is fully formed in the intermediate carbonium ion. 

A systematic analysis to determine the biophysical characteristics of some cationic lipid/DNA complexes from this series was undertaken (Eastman et al., 1997). Product 1 (Figure 15.7) is currently being used in clinical trials as a carrier of CFTR gene in cystic fibrosis. The synthesis of these cholesteryl derivatives is similar to that of DC-Chol cholesteryl chloroformate is reacted with spermine or spermidine as polycation (Lee et al., 1996). 

R = 26 distinct fatty tails from 4 to 18 carbon atoms long R = 4 cholesteryl derivatives... 

Jin, Y., and Ai, P. (2004), Cholesteryl derivatives of nucleoside analogues, China Patent CN1566130. 

The initial suggestion that a remote double bond could be involved in a displacement reaction was made by Shoppee in 1946 in order to account for the stereochemical results of the transformations of 3-cholesteryl derivatives Building on the considerable work then underway on neighbouring group participation in ionization reactions Shoppee suggested that the double bond in the cholesteryl system, 1, could function in a similar manner to Lewis base containing groups such as acetate or alkoxy, etc. 

Burger, A., Hetru, C., and Luu, B., Stereoselective synthesis of cholesteryl derivatives bearing a chiral allenic group in the side chain. Synthesis, 93, 1989. 

Kunitake has also studied the hydrolysis of -nitrophenyl esters in the presence of aqueous bilayer vesicles. The rate of inter-vesicle S2 hydrolysis by a cholesteryl derivative of 4(5)-imidazole carboxylic acid was much slower than the intra-vesicle counterpart.(32) Rate-limiting substrate transfer was responsible for the lower rates and was influenced by the crystalline-liquid crystalline transition.(33)... 

These sterols constitute about 0.2% of the tobacco weight. Table XXV-7 illustrates the relative proportions of these sterols in tobacco. These data indicate that cholesterol and cholesteryl derivatives are the least plentiful of the free and bound sterols in tobacco. These levels are similar to those of the standard and reference cigarettes in the National Cancer Institute (NCI) Less Hazardous Program (1329,1330,1332 1333, 2683). 

The chiral nematic state, first observed in cholesteryl derivatives, was later detected in other chiral mesogens, and can also he induced hy adding small chiral molecules to a host nematic TCP. Chiral nematic LCPs have thus heen synthesized as side-chain polymers hy introducing a chiral unit in the tail moiety of mesogens or hy copolymerizing cholesterol-containing monomers with another potential mesogenic monomer. Examples [4] of these types are shown as structures (V) and (VI) ... 

Copolymers. The second part of our model is concerned with the copolymerization of monomers with different spacers. In this case a statistical distribution of the centers of gravity of the mesogenic chain might be preferred even more. This prediction was tested with cholesteryl derivatives ... 

Table IV. Phase Transitions of Homo- and Copolymers of Cholesteryl Derivatives 2 ...

Polymerization of self-assembled LMOG molecules within the fibers of a molecular gel locks the scaffold-like structures and can make new stable materials [99,100]. Photopolymerization in the gels of diacetylenic cholesteryl derivatives, such as 63X-n [88,89], has been successfully employed to access organic nanowires with potential electrical conductivity, which could be used in molecular electronics. This strategy was based on the known propensity of diacetylenes to undergo topochemical soUd-state 1,4-addition reactions leading to conjugated polymers [101]. 

Unfortunately, a large degree of anchimeric assistance does not automatically lead to a greatly reduced isotope effect. The cholesteryl derivative (45), the cholestanyl derivative (46), and the 7-norbornenyl derivative (47) show the same cc-d effect, yet (45) and (47) react with strong anchimeric assistance from the n electrons. Furthermore, (45) and (47) have greatly... 

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