Acid dimers, orientations

Acceptor species concentrations, equations, 400-401 Acentric materials biomimetic design, 454-455 synthesis approaches, 446 Ar-(2-Acetamido-4-nitrophenyl)pyrrolidene control of crystal polymorphism with assistance of auxiliary, 480-482 packing arrangements, 480,481-482/ Acetylenes, second- and third-order optical nonlinearities, 605-606 N-Acetyltyrosine, phase-matching loci for doubling, 355,356/, t Acid dimers, orientations, 454 Active polymer waveguides, applications, 111... 

There are many kinds of capsules but few have the capacity to position co-guests in predictable orientations. We used the ability of 1.24.1 to do so, and applied it to evaluate hydrogen-bonding interactions between boronic acids, carboxylic acids, and primary amides [52], The phenyl boronic acids are useful as components of covalently self-assembled systems [53], and we found that the p-methyl, methoxy, ethyl, and isopropyl derivatives all fit as symmetrical dimers inside I.24.I. The structure of the boronic acid dimer has been debated but a recent theoretical study found the doubly hydrogen-bonded exo/endo conformer (Fig. 10) to be lowest in... 

Fig. 1 The two tautomers of the benzoic acid dimer. For each species the orientation of the vector connecting the two protons relative to the direction of the applied magnetic field (B0) is different [14]...

In the majority of examples in this review the hydrogen bond donors are XH bonds and the acceptors are the lone pairs on electronegative atoms, though other sources of electron density such as Ti-bonds can also act as hydrogen bond acceptors. The interactions between molecules or ions can be described in terms of the number and orientation of hydrogen bond donors (D) and acceptors (A) present. For example the carboxylic acid dimer in Fig. la can be denoted DA-AD (or DA AD, or DA=AD) whereas the fragment in Fig. lc can be denoted ADA-DAD (or ADA=DAD). 

FIGURE 12.4 (a) Diagram illustrating the concerted double proton transfer in a carboxylic acid dimer and the corresponding double-well potential curve. Computed 0 (b) quadrupolar coupling (QC) and (c) CS tensor orientations in salicylic acid and aspirin. for and for Oj are perpendicnlar to the Oj-C-O plane. For both O, and O, 5 is perpendicular to the O-C-Oj plane. Adapted with permission fom Ref. [46]. Copyright 2013, American Chemical Society. 

Unusual modes of dimerization of cinnamic acids have been found in the solid state photodimerization of the octadecyl esters of these acids. Bolt and co-workers have crystallized this ester of cinnamic acid in three different forms, one of which cyclized to an a-truxilic acid type of dimer, one of which was photo-inert, and one of which gave the 6-truxinic acid dimer. Two crystal forms of the ester of trans-p-chloro-cinnamic acid were isolated, one of which cyclized to an a-truxillic acid, the other being photo-inert. The long-chain ester group presumably modifies the crystal structure, and use of orienting groups of this type may allow the formation of other dimers not usually obtained in solid state dimerizations. 

The Lewis acid-catalysed orientation reversal in the reaction between substituted cyclohexa-1,3-dienes and 2,6-dimethyl-l,4-benzoquinone ° has been employed in an interesting synthesis of quassin (218). ° Thus, reaction at room temperature of the diene (215) with the above quinone in the presence of an equivalent quantity of Bp3,OEt2 gave the adduct (216) which was converted by several subsequent steps into (218). In the absence of the catalyst the alternative adduct (217) was obtained. Periodic acid oxidation of substituted o-cresols ° and of 2-methoxyphenols in methanol solution affords intermediate o-quinol methyl ethers or o-quinone dimethyl ketals which dimerize to give dienediones with structures related to those of (216) and (217). Another report concerns the formation of a Diels-Alder dimer upon hypochlorite oxidation of 2,2 -methylenebis(4-methyl-6-t-butyl)phenol. ... 

Figure 12. X-ray diffraction pattern of an oriented sample of the 2,3,4-trihexyloxycinnamic acid dimer (compound 3, R=n-C5H]3) in its nematic phase. According to the scattering maxima, the periods of the density waves in angstroms are as follows (a) 4.4, (b) 19.5, and (c) 14.0 (from [23]).

The phenyl ether oxygen atoms allow the two borazaphenanthrene rings to pivot with respect to each other, therefore this dimeric boronic acid anhydride can potentially exist in two isomeric forms, either face-to-face or helical (Fig. 18). In the face-to-face form the boron atoms of the bis(borazaphe-nanthrene) moieties have syn-orientation, while they have approximate anti-orientation in the helical form. Compound 68 has been characterized by X-ray crystallography in the helical form [109]. The dimensions of the cavity can be described by the transannular C C contacts between the carbon atoms in 2-position of the phenyl ether units, which have values of 5.12 and 6.21 A. 

A similar pre-orientation involving unsaturated carbon chains was operative on generating twelve-membered enediyne 23 and arenediyne lactams 24 [7]. The seco methylesters 21 and 22 were cleaved with LiOH, the corresponding carboxylic acids underwent cyclizations after activation with 2-fluoro-pyridinium tosylate 25 [8]. Dimerization products were found as by-products (<10%). It should be pointed out, that the lactamization succeeded in a single step in about 75% yield by treating the seco-methylesters 21 and 22 with Me3Al in refluxing methylene chloride. Obviously, the latter route was more convenient (Scheme 5). 

P2j Z = 2 DX = 1.43 R = 0.067 for 1269 intensities. The uracil residue is in the anti (63.4°) disposition. The conformation of the D-ribosyl group is 2T3 (176.8°, 37.5°). The orientation about the exocyclic, C-4 -C-5 bond is t (—174.2°). The phenyl and uracil ringsofthe same molecule lie in almost parallel planes, 120 pm apart. The phenyl group is disordered. The uracil ring is sandwiched by the phenyl rings, and vice versa. The 0-1 and N-a atoms of the peptide backbone are hydrogen-bonded to 0-4 and N-3 of atranslationally related uracil to form cyclic dimers. Such interactions serve as models for nucleic acid-protein interactions. [Coordinate errors H(02 ) x should be —1574, instead of —1474 H(Na)2 z should be —145 instead of— 645.]... 

In another recent example, Hashimoto reported photoaffinity experiments on retinoic acid receptors (RAR). Retinoic acid plays a critical role in cell proliferation and differentiation. RARs belong to the superfamily of nuclear/ thyroid hormone receptors. They consist of six transmembrane domains (A-F) which is a general feature of these receptors. The A/B domains have an autonomous transactivation function while the C-domain contains the Zn-finger, which binds to DNA. The large E-domain participates in ligand binding, dimerization, and ligand dependent transactivation. Finally, D- and F-domains help the orientation and stabilization of the E-domain. 

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