198 Topics in Current Chemistry Editorial Board: A. de Meijere KN ...

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Supramolecular Synthons and Pattern Recognition 63 atom, the cluster approximates to a super-CBr 4 molecule. In retrosynthetic terminology, cluster 17 is easily understood then as the combination of two tetrahedral moities, the tetraphenylmethane molecular synthon 15 and the Br 4 supramolecular synthon 18. Now, if one interchanges the molecular and supramolecular synthons by replacing the Br 4-cluster 18 with covalently bonded CBr 4 and the C-Br covalent bonds with the Br◊◊◊Ph interaction 19, the result is complex 20. In 20, four molecules of 14 are linked to a CBr 4 molecule through synthon 19 (Br◊◊◊Ph centroid 3.67 Å) and the central carbon atoms of CBr 4 and tetraphenylmethane may be treated as spheres and joined to form a distorted diamondoid network. The crystal structures 17 and 20, derived from single- and two-components respectively, bear close and striking similarities at the supramolecular level and have similar diamondoid networks mediated by Br 4 and Br◊◊◊Ph supramolecular synthons. To continue, the crystal structure of tetrakis-(4-iodophenyl)methane (16) is isomorphous with 17 having an I◊◊◊I distance of 3.95 and 4.16 Å in the corresponding I 4-synthon. In this family of halo-substituted tetraphenylmethanes with S 4- or pseudo-S 4-symmetry, the molecules exhibit similar columnar packing in the solid state [14]. In summary, if a supramolecular structure is defined as a network and analysed retrosynthetically, general connectivity strategies can be systematically derived. 4 From Organic Synthesis to Crystal Engineering A smooth and logical progression from molecular synthesis to crystal engineering follows if functional groups are replaced with supramolecular synthons and target molecules with networks. The functional group approach for defining covalent bond connections is suited to the goals of molecular synthesis where carbon-carbon bonds are formed between atoms with unlike polarisations, as between nucleophiles (enolates, carbanions) and electrophiles (ketones, halides) [15, 16]. In traditional organic synthesis, the molecule is assembled in a stepwise and sequential manner such that carbon atoms in two functional groups react to form a covalent bond while the others are deliberately made to stand as innocuous spectators. In cases of potential competition between two or more functional groups, protection of the more reactive group is necessary. The development of mild and selective reagents and reaction conditions has been motivated by the need to minimise the number of steps and improve the stereoselectivity in covalent synthesis. This approach has served synthetic organic chemistry for close to five decades and has led to the total synthesis of a large number of molecules [17]. Complex natural products such as quinine, cortisone, strychnine, cedrol and reserpine were the landmark syntheses of the 1940s and 1950s. The period from 1960 to 1990 was one of intensive research encouraged by the advent of modern spectroscopic techniques and the ready availability of a large storehouse of starting chemicals. A number of natural [10,16–19] and non-natural [20, 21] target molecules were successfully tackled during this period. The total synthesis of vitamin B 12 completed by Woodward [22] and Eschenmoser and Winter
64 A. Nangia · G.R. Desiraju [23] in 1973 and 1977 represented the most complex natural product synthesised at that time. The project spanned two continents and occupied an army of researchers during a period of 12 years. The second “chemical Everest” of molecular synthesis was scaled in 1989 with the total synthesis of palytoxin 21 (Scheme 3) by the team of Kishi [24]. The total synthesis of this complex macromolecule took almost 10 years and involved more than 100 researchers. The rapid pace of research in the 1990s [25] was brought to the forefront with the publication in 1994 of two total syntheses of taxol from the laboratories of Holton [26] and Nicolaou [27] within a week of each other. The essential difference between organic synthesis and crystal engineering is that the stepwise and sequential covalent bond formation in the former is replaced by an organised self-assembly of the supermolecule involving orthogonal functionalities in a single step in the latter [28]. During crystallisation, all functional groups present in the molecule compete for the numerous possible combinations of intermolecular interactions even while it is understood that only some of these recognition events are eventually fruitful. Thus, if a molecule M containing functional groups F 1,F 2,F 3, ◊◊◊ F n approaches another molecule of M, then a matrix of intermolecular interactions, F i – F j is theoretically possible. Two or more molecules of M may now come together to form,in principle,several supramolecular synthons S 1,S 2,S 3, ◊◊◊ S n, some of which may be very close in energy. However, there is a simplifying feature here. Some combinations of F i – F j inherent in S 1,S 2,S 3, ◊◊◊ may exclude others with the result that the complex matrix of intermolecular interactions and supramolecular synthons converges rapidly to a free energy minimum (the crystal structure) without Scheme 3. Chemical structure of palytoxin. This giant molecule has 65 dissymmetric carbons and 10 21 possible isomers, weighs 2680 daltons and has dimensions of the order of 10–100 Å 21
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198 Topics in Current Chemistry Edi
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Design of Organic Solids Volume Edi
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Volume Editors Prof. Dr. Edwin Webe
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VIII preface tion at an amazing lev
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Contents of Volume 196 Carbon Rich
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2 J.P. Glusker 5 Interactions of Pl
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4 J.P. Glusker Perturbations to the
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160 Y. Aoyama 4. Hölderich W, Hess
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Springer and the environment At Spr
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