Synthesis and study of the structural properties of oxa[5]helicene derivatives M. Shyam Sundar a , Sibaprasad Sahoo b , Ashutosh V. Bedekar a,⇑ a Department of Chemistry, Faculty of Science, M. S. University of Baroda, Vadodara 390 002, India b Sun Pharma Advanced Research Centre, Tandalja, Vadodara 390 020, India article info Article history: Received 25 May 2016 Accepted 28 June 2016 Available online xxxx abstract The preparation of the (À)-menthyl carbonate of 2-hydroxy-7-oxa[5]helicene and its structural charac- terization is reported. Single crystal X-ray diffraction analysis established the relatively flat structure of oxa[5]helicene unit thus indicating the absence of stable helical isomers. The unit cell of the crystal includes one molecule in the P-conformation and another in the M-isomer. However, its nitro derivative, 1-nitro-2-hydroxy-7-oxa[5]helicene, provides sufficient steric crowding to observe two helical isomers at low temperature, as confirmed by 1 H NMR at À20 °C. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction The chemistry of screw shaped helical molecules has grown into an important field of research due to the fascinating optical and electronic properties of the p-conjugated ortho-annelated aro- matic molecules. Such molecules have been widely studied in recent years due to the unique properties associated with their structure. The steric hindrance of the terminal rings and the repul- sion between the substituents present force the molecule to adopt a helical conformation. 1 These structures are known as helicenes, which present noteworthy left- or right-handed chiral helical structures. Primarily, there are two types of helical molecules: car- bahelicenes and heterohelicenes, with the former class being more studied initially. There are other molecules with a helical shape such as peptides, DNA and helical polymers. 2 However, recently due to more interesting properties and thus wider possible appli- cations, heterohelicene (i,e., oxa-, aza-, and thiahelicene) has become the subject of intense research. The introduction of hetero atoms in the helical framework have proven to be especially ben- eficial to the electronic, optical, and photorefractive properties of heterohelicene-based materials. 3 Also heterocyclic compounds have been known to facilitate interactions with the biological receptors and hence have opened up applications in pharmaceuti- cal sciences. Considerable attention has been devoted to thia- helicenes 2, 4 and aza-helicenes 1 5 but less on oxa-helicenes 3 (Fig. 1). Dinaphtho[2,1-b:1 0 ,2 0 -d]furan 3 can be classified as an oxy- gen-containing heteroaromatic system with a unique structure. Despite its helical structure, the molecule does not exhibit optical activity due to rapid racemization at ambient temperature. The molecule has received much attention recently due to its potential as a precursor for the synthesis of axially chiral binaphthyl deriva- tives, which are effective chiral building blocks in asymmetric reactions, in particular fused furans, are expected to provide rela- tively high HOMO levels 6 and are known to show interesting utility in electronic devices such as organic light-emitting diodes 7 or organic field-effect transistors. 8 The synthesis and study of oligo- naphthofurans has also recently been reported, 9 where the system- atic correlation of the number of naphthofuran units and the physical properties were evaluated. Compounds belonging to the general class of oxahelicenes have been synthesized, and their var- ious properties have been studied. 10 In addition to our work on the synthesis and study of helicene- like compounds, 11 and aza[n]helicenes, 12 recently we presented the synthesis and study of large oxygen-containing trioxa[11]he- licene from axial chiral molecules. 13 Herein we present our http://dx.doi.org/10.1016/j.tetasy.2016.06.020 0957-4166/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +91 0265 2795552. E-mail address: avbedekar@yahoo.co.in (A.V. Bedekar). NH O S 1 2 3 Figure 1. Examples of hetero[5]helicenes. Tetrahedron: Asymmetry xxx (2016) xxx–xxx Contents lists available at ScienceDirect Tetrahedron: Asymmetry journal homepage: www.elsevier.com/locate/tetasy Please cite this article in press as: Shyam Sundar, M.; et al. Tetrahedron: Asymmetry (2016), http://dx.doi.org/10.1016/j.tetasy.2016.06.020