Helicenes, review

That chiral molecules can be produced in a CPL field, either from achiral precursors by photo-activated synthesis or by preferential chiral photodestruction of a racemic mixture, is now well demonstrated and has been reviewed. [46] In all cases currently known, however, such processes have proved very inefficient. For example, asymmetric photochemical ring-closures of achiral helicene precursors induced by CPL have produced only about 0.2% e.e. in the products. Likewise, the CPL-induced photolysis of racemic camphor produced about 20 % e.e., but only after 99% photodestruction, and photolysis of D.L-glutamic acid produced only 0.22 % e.e. after 52 % photodecomposition. [71]... 

The photochemical synthesis of helicenes by irradiation of 1,2-diarylethylenes in adilute solution and in the presence of an oxidizing agent is based on the well known photocyclodehydrogenation of stilbene into phenanthrene. There is an overwhelming amount of literature on this type of photoreaction. Details about scope and limitation can be found in previous reviews 7,8). Therefore, only a short survey will be given of the mechanism of the reaction. 

Reviews of compounds having a helical structure, others than helicenes, have been given by Mislow 48) and by Voegtle 49). 

In the period under review the science and art of crystallography has made an extremely rapid progress in instrumentation and available software and as a consequence in insight and rate of analysis. This is well illustrated with examples from the X-ray analysis of helicenes. 

A review of helicenes together with a compilation of optical rotations may be found in... 

The reader is referred to the Number of Benzenoids and Poly hex Hydrocarbons subsection for the definition and classification of benzenoids and polyhex hydrocarbons, as well as for additional references for this class of compounds, which is only partially reviewed because of space limitations. Let us recall that the direct counting approach has difficulties with molecules that cannot be represented by tree-like structures, such as pericondensed polyhexes. Furthermore, the counting approach caimot separate nonplanar polyhexes (helicenes) from planar benzenoids. Consequently, for benzenoids and polyhexes, enumeration is not only a valuable tool that provides a concise description of the structures being enumerated, but also enumeration computes isomer numbers that cannot be derived otherwise. 

Electrocyclization - helicenes. Brief cover reviews of the synthesis of helicenes were published.In the formation of helicenes and substituted phenanthrenes via the photochemical electrocyclization of diaryl-ethenes in the presence of iodine as an oxidant, the addition of THF... 

The material in this chapter has been organised into ring-type and functional-group order so that required information may be rapidly obtained. Monocyclic systems and their alkyl derivatives are reviewed first, followed by related compounds, in order of their functional groups. This is followed by polycyclic systems (non-fused and fused). Derivatives of these are covered under the appropriate parent compound heading, appearing in the same order as that in the earlier sections. Later sections include more specialised systems such as helicenes and cyclophanes. Synthesis and reactivity are discussed collectively under the appropriate system heading. 

In principle, the double photocycHzation of 1,4-distyrylbenzenes offers a convenient route to dibenz[a,h]anthracenes and [5]helicenes dependent on the sites of reaction. In the first report of the photochemistry of 1,4-phenylene bisjphenylmaleic anhydride) 27, the product was identified as the dibenz[a,h]anthracene 28," but more recently it has been shown that a mixture of isomers is formed (overall yield of 80%), in which 28 comprises less than 5%, with the (5]helicene 29 as the major product. This approach towards the synthesis of polynuclear arenas, including helicenes, is complemented by the photocyclizations of styryl-naphthalenes, -phenanthrenes, etc. and of 1,2-dipolyarylethenes. Examples of these processes pubhshed during the review period are now considered. 

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