Synthesis and trans–cis isomerization of azobenzene dendrimers incorporating 1,2-isopropylidenefuranose rings Ankur Ray, a Sudeshna Bhattacharya, b Subir Ghorai, a,  Tapan Ganguly b and Anup Bhattacharjya a, * a Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Kolkata 700 032, India b Department of Spectroscopy, Indian Association for the Cultivation of Science, 2A&B, Raja S. C. Mullick Road, Kolkata 700 032, India Received 25 April 2007; revised 21 August 2007; accepted 30 August 2007 Available online 5 September 2007 Abstract—Azobenzene dendrimer 2 was synthesized from a known dendritic azo-tetracarboxylic acid and a dendritic amine incor- porating 1,2-isopropylidenefuranose rings, and its trans–cis isomerization was studied by UV–vis absorption spectroscopy. Ó 2007 Elsevier Ltd. All rights reserved. The area of dendrimers has attained great significance due to the unusual architectural features of the dendri- mer structure coupled with their interesting properties. 1 The importance of azobenzene dendrimers stems from the facile cis–trans isomerization of the azobenzene moi- ety, a property which may be useful for the construction of photoswitchable devices. 2 Introduction of diversity in dendrimer structures is an important task, because future applications of dendrimers will also be dependent on the structural characteristics of the dendrimers. We recently introduced chiral dendrimers incorporating furanose ring substituted phenyls as branching units. 3 Apart from introducing chirality, these branching units would allow future functionalization of the exterior and the interior of the dendrimer. We report herein the synthesis and the study of the trans–cis isomeriza- tion of dendritic molecules 1 and 2 (Fig. 1) based on an azobenzene core and 1,2-isopropylidenefuranose branching units. The azobenzene derivative 1 has recently been used in the synthesis of dendritic azobenzene hydrogelator 3. 4 Amine 4 proved to be a useful branching unit for the synthesis of novel carbohydrate derived dendrimers. 3a It appeared to be a worthwhile exercise to attempt a coupling of the tetraacid 3 and with the amine 4 in order to prepare the novel azodendrimer 2. Toward this end, treatment of 3 and 4, both prepared according to the lit- erature procedures, 3a,4 in the presence of HATU and diisopropylethylamine at 25 °C for 96 h followed by purification by HPLC led to the formation of the desired dendrimer 2 in 73% yield (Scheme 1). 5 The structure of 2 was established on the basis of NMR and mass spectral analyses. Single sets of peaks due to the symmetry related protons and carbon atoms in the 1 H and 13 C NMR spectra clearly indicated the C 2 -symmetric nature of 2. The four-proton doublet at d 6.09 due to the symmetry related interior furanoside protons served as a reference for the introduction of the desired number of branches. The appearance of integrations of one of the two sets of core aromatic protons (2H singlet at d 7.37), interior furanoside anomeric protons (4H doublet at d 6.09), peripheral furanoside anomeric protons (8H doublet at d 5.87), and one of the two sets of peripheral aromatic protons (8H singlet at d 7.52) in the ratio of 1:2:4:4 lent ample support to the structure of 2. This rapid check for the introduction of the correct number of branch units was a useful feature of the characteriza- tion process. The ESI mass spectrum of 2 exhibited the molecular ion peak at m/2z 1784.82 corresponding to the doubly charged species (M+2Na) as expected for the structure of the dendrimer. 5 The UV–vis absorption spectrum of 1 in acetonitrile (MeCN) exhibited two p–p * bands at 325 nm due to the trans isomer and at 237 nm due to the cis-isomer 0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.tetlet.2007.08.115 * Corresponding author. Tel.: +91 33 24728697; fax: +91 33 24735197; e-mail: anupbhattacharjya@iicb.res.in   Present address: Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106, USA. Tetrahedron Letters 48 (2007) 8078–8082