Fixed conformation derivatives

As discussed in Chapter 2, most force fields are validated based primarily on comparisons to small molecule data and moreover most comparisons involve what might be called static properties, i.e., structural or spectral data for computed fixed conformations. There are a few noteworthy exceptions the OPLS and TraPPE force fields were, at least for molecular solvents, optimized to reproduce bulk solvent properties derived from simulations, e.g., density, boiling point, and dielectric constant. In most instances, however, one is left with the question of whether force fields optimized for small molecules or molecular fragments will perform with acceptable accuracy in large-scale simulations. 

All of the likely conformations of cellobiose, cellulose, and xylan are explored systematically assuming the ring conformations and IC-D-O-IC-4 ) angle for each pair of residues to be fixed and derivable from known crystal structures. The absolute van der Waals energies, but not the relative energies of different conformations, are sensitive to the choice of energy functions and atomic coordinates. The results lead to possible explanations of the known conformational stiffness of cellulose and Its solubility properties in alkali. The characteristics of xylan conformations are compared with cellulose. 

H, Me) in trifluoroacetic acid solution exhibit two-component equilibrium mixtures 89A 89B (92KGS1689). For 4-methyl derivatives (R4 = Me), two fixed conformers (C(2)--NHR4 bond rotamers) of the cyclic tautomer 89B (E and Z) were detected. The equilibrium 89A 89B is rather insensitive to the nature of the substituent X (R2 = XC6H4). On passing from the aromatic aldehyde to the acetophenone derivatives, the equilibrium amount of the cyclic tautomer 89B increases. The cyclic protonated form of 2-chloro-6-nitrobenzaldehyde thiosemicarbazone could not be detected at all. It would be of interest to test the thiosemicarbazones of other 2,6-disubstituted benzaldehydes. Recently, it has been found (94KGS119) that the protonated cinnamic aldehyde thiosemicarbazone 89A (R1 = PhCH = CH, R2 = R3 = R4 = H) likewise does not form the cyclic tautomer 89B in trifluoroacetic acid solution. 

Exposure of an allylstannane to halopyranoses in the presence of AIBN enables access to the corresponding allyl adducts [95] this procedure is useful for C-allyl glycosidation [96]. To enhance the practical utility of C-glycosidation, the effect of conformational restriction on a- and / -selectivity was studied in free-radical allylation at the anomeric positions of phenylselenoxylose derivatives of fixed conformation (Scheme 12.37) [97]. Treatment of a phenylthionocarbonate of 2 -deoxynucleo-side [98] and an iodopyrrolidine ]99] to allyltributyltin/AIBN resulted in substitution of the allyl moiety. 

There have been no systematic studies for systems (63)-(74). Derivatives of (68), containing the acyl guanidine moiety of fixed conformation have been prepared, and UV and IR have been used to examine the conformational and tautomeric preference of acylguanidines <85JCS(P2)1255, 85JCS(P2)1265>. 

B. Masci, D. Persian , P. Thuery, Complexation of quaternary ammonium ions by tetraester derivatives of [3.1.3.1]homooxacalixarene in mobile and in fixed conformation, J. Org. Chem., 2006, 71, 9784-9790. 

The Vhh couplings have been used by Abraham and Koniotou to determine the conformer populations of cis- and fr ms-cyclopentane-l,3-diol by the use of the Haasnoot, de Leeuw and Altona equation for the C-CH2-CH2-C fragment the following equation has been derived from five- and six-membered cyclic compounds in fixed conformations ... 

The photoreactivity of DTE derivatives in the crystalline state is of special interest because of their potential usefulness for holographic and three-dimensional memories. In crystals, molecules are regularly oriented and packed in fixed conformations. In many cases, free rotation is inhibited. Therefore, the photoreactivity in the crystalline phase is dependent on the space for free rotation of the thienyl rings and the conformation formed in the crystal lattice. Several reports on the synthesis of metal complexes of photochromic diarylethenes and their photo-reactivity in the single-crystalline phase or the photoswitching of the coordination structure have been reported. These studies demonstrate that complexation to metal ions does not prohibit the photochromic reactions of the diarylethene units in the single-crystalline phase. 

Interest in D-nor-steroids derives both from their biological activities, and also from their use as models for studies of carbonium ion rearrangements, since they possess cyclobutane rings which are held in fixed conformations by virtue of the fra s-fusion to ring c. Analyses of the stereoisomers (112a) and (112b) reveal that in both cases the cyclobutane rings are puckered. 

Similar to the triptycene-derived calixarenes, expanded oxacalixarene 10a wifli a fixed conformation could assemble into a tubular structure by virtue of C—Cl -Jt, C—H- ji and ji - jt interactions (Fig. 18.11) [16]. 10b could also stack into a tubular... 

Similar to other triptycene-derived macrocyclic arenes, triptycene-derived tetralactam macrocycles also had fixed conformations with large electron-rich cavities, which made them promising candidates as the host for some electron-deficient guests with comparatively large sizes. Squaraines [26] were a family of fluorescent dyes with specific near-IR photophysical properties, which had wide potential applications. However, their instability limited the utilization of them, and thus improving their chemical stability and the photophysical properties were the key to applications of squaraines [27]. Consequently, we [25, 28] found that macrocycles 35a-b could form a new kind of stable pseudorotaxane-type complexes with the squaraine in both solution and solid state. We further studied the chemical stability of squaraine in these complexes, and found that free guest 35b underwent hydrolytic decomposition to turn colorless in polar THF-water solvent in 4 days, but for squaraine 36b (Fig. 18.15) in complexation with 35a-b, its blue colors could be retained for several weeks. This observation revealed that the formation of complexes could efficiently protect the squaraine dyes from polar solvents. 

Show that the three partial derivatives obtained from PV = nRT with n fixed conform to the cycle rule,... 

Typically, a 3D structural database stores (I) the topology of the structures, including atom and bond types, and bond connectivity (see Connection Table Connectivity Table), (2) the topography of the structures, represented by the 3D coordinates of a given conformation, and (3) various types of data relevant to the conformation. This may include whole-molecule as well as per-atom or per-bond information. The database may also store various keys and indices to aid searching of structures and data. This information suffices to perform searches over fixed conformations (static 3D searches). For conformation-ally flexible structure searching (CFS), the database may also include information on upper/lower distance bounds between various types of atoms or it may include information derived from conformational analysis of the stored conformations, enabling reconstruction of various conformers at search time. ... 

The addition of large enolate synthons to cyclohexenone derivatives via Michael addition leads to equatorial substitution. If the cyclohexenone conformation is fixed, e.g. as in decalones or steroids, the addition is highly stereoselective. This is also the case with the S-addition to conjugated dienones (Y. Abe, 1956). Large substituents at C-4 of cyclic a -synthons direct incoming carbanions to the /rans-position at C-3 (A.R. Battersby, 1960). The thermodynamically most stable products are formed in these cases, because the addition of 1,3-dioxo compounds to activated double bonds is essentially reversible. 

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