We developed a new and short synthetic route to 2,7-ditert-butyl-trans-15,16-dimethyldihydropyrene (DHP) via tetrahydroxy[2.2]metacyclophane in four reaction steps with a total yield of 37%. 2,7-Di-tert-butyl-trans-15,16-dimethyldihydropyrene functionalized by acetoxy groups at 4-, 5-, 9-, 10-positions was synthesized via 5, 13-di-tert-butyl-8,16-dimethyl-1,2,9,10-tetrahydroxy[2.2]MCP in five reaction steps with a yield of 24%, and its DHP structure was determined by 1 H NMR spectroscopy and X-ray crystal-structure analysis. Dihydropyrene (DHP) derivatives have been attracting considerable interests in various fields 2a-f due to their photochromic property between DHP and [2.2]metacyclophane-1,9-dienes ([2.2]MCP-diene) since 1967. 1The synthetic route to di-tert-butyldimethylDHP (3), which was the parent compound of DHP, was developed by Tashiro 3 and improved by Mitchell. 4 This route via dithia[3.3]MCP 2 required six reaction steps, and a total yield of 45% was achieved from 4-tert-butyltoluene 1 (Scheme 1). Each reaction yield of this route was over 76%, but the long reaction sequence and requirement of highly skilled techniques restricted the practical applications of DHP as advanced materials.Previously, we have reported facile and one-step synthesis of 5,13-di-tert-butyl-8,16-dimethyl-1,2,9,10-tetrahydroxy-[2.2]MCP (6) from the bezenedialdehyde derivative 5. 5a,b The MCP 6 has the potential to be the intermediate in DHP formation because it has two trans-diols at both its bridge positions, the reduction of which would afford a [2.2]MCP-diene, which is an equivalent of 3.However, the benzene-annulated DHP at the [e]-position showed a high quantum yield of photoisomerization 6 and possessed suitable properties to qualify as switching materials. 2b,d,e It is also expected that DHP substituted at the [e]-position or 4-, 5-, 9-, 10-positions could be prepared by using 6 as the intermediate in short reaction steps, and these DHP could be investigated for the various purposes. In this paper, we present a simple and short synthetic route to 3 and 9, which is substituted by acetoxy groups at 4-, 5-, 9-, 10-positions, via 6 as the intermediate. The synthesis of 3 was shown in Scheme 2. 2,6-Bis(bromomethyl)-4-tert-butyltoluene (4) and 2,6-di-formyl-4-tertbutyltoluene (5) were synthesized with yields of 90% and 70%, respectively. 4,5a Pinacol coupling of 5 in an ice bath afforded 6 in 79% yield, while previously reported methods afforded 6 only in 33% yield at room temperature. 5a,b The MCP 6 was reduced to 3 using imidazole, chlorodiphenylphosphine, iodine, and Zn powder. This method, which produced cis-olefin from trans-diol of carbohydrate, has been reported by Zhengchun. 7 A suspension of 6, imidazole, and chlorodiphenylphosphine in toluene was added to iodine at reflux temperature and stirred for 1 hour. Zinc powder was added, and the mixture was stirred for 8 hours to afford 3 and 7 in 75% and 7% yields, respectively. 8 This synthetic route afforded 3 with a total yield of 37%.The expected reaction mechanism f...
