Scope and Nomenclature

9 tt Stahly, G. R, Diversity in single- and multiple-component crystals. The search for and prevalence of polymorphs and cocrystals , Cryst. Growth Des. 2007, 7, 1007-1026. 

A solid solution is a solid mixture of one or more compounds within a solvent of another solid crystalline compound. For the mixture to be considered a solid solution the crystal structure of the solvent must remain essentially unchanged upon the addition of the solute(s) and the whole mixture must remain as a single homogeneous phase. We can distinguish two types of inclusion within the solvent crystal substitutional in which the solute molecule replaces a solvent molecule in the lattice, or interstitial in which the solute molecule fits into a space in between solvent particles. The properties of the solvent material are affected by the formation of solid solutions because the process results in distortions in the crystal lattice and disrupts the overall homogeneity of the host material. 

N-Hydroxyamino acids should, at present, be treated as a new, separate, characteristic group of amino acids. This is necessitated by the particular biological action of these compounds and their derivatives. Although a-N-hydroxyamino acids were first discovered by Miller and Plochl 1) as early as 1893, they have aroused particular interest mainly in the last twenty years. 

N-Hydroxyamino acids are similar in structure to amino acids but with the hydroxyamine group replacing the amine group. Therefore a-N-hydroxy amino acids can be represented by formula (1) and co-N-hydroxyamino acids by formula (2). 

A great number of important publications on the synthesis of the title compounds, their substrates and derivatives, methods of incorporating such residues into complex organic molecules as a part of the total synthesis of the latter, as well as papers on the synthesis of N-hydroxydiketopiperazine and N-hydroxypeptides have appeared in recent years. Several syntheses of analogues of naturally occurring compounds have also been described. 

The present review will be devoted mainly to these synthetic aspects. 

A description of physical and chemical properties of N-hydroxyamino acids and methods of their analysis will be included and brief mention will be made of the more important natural products containing N-hydroxyamino acid residues as well as the main aspects of their biogenesis. 

Although this review deals with the pyrimidine nucleosides, it will be necessary to refer occasionally to their purine counterparts. The authors take into account the previous reviews in this series by Tipson,10 Jeanloz and Fletcher,11 and Barker,12 which have dealt in greater or lesser degree with this subject. Some recounting of earlier work in the field of nucleoside chemistry will be necessary, in order to place newer developments in proper historical context. 

We are chiefly concerned with LC structures wherein mesogenic entities are present as substituents connected (generally via a spacer) to a flexible polymer backbone that is not itself inherently mesogenic, although combined or double systems (Table 1.1 in Chapter 1) will be mentioned. The mesogens confer the property of mesophase formation (i.e. between conventional solid and liquid phases) upon the composite structures under thermal control (hence thermotropic ). 

Referring to the nomenclature defined in Table 1.1, we shall encounter mainly one-comb, double-comb, parallel (and biparallel) and network LCPs that exhibit calamitic phases (such as smectic and nematic). Calamitic phases result from mesogens that are lath-shaped. We shall 

Thermal data quoted in this chapter present transition temperatures between phases that are abbreviated as follows  

N = nematic mesophase (RN being a re-entrant nematic phase) 

Ch = cholesteric mesophase (chiral nematic) i = isotropic liquid phase, fully realized above the clearing temperature Tci. 

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