Letter pubs.acs.org/OrgLett

Efficient Synthesis of 9‑Aryldihydrophenanthrenes by a Cascade Reaction Involving Arynes and Styrenes Sachin Suresh Bhojgude,† Anup Bhunia,† Rajesh G. Gonnade,‡ and Akkattu T. Biju*,† †

Organic Chemistry Division, and ‡Center for Materials Characterization, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India S Supporting Information *

ABSTRACT: A mild, general, and transition-metal-free protocol for the synthesis of 9,10-dihydrophenanthrenes is reported. The aryne generated by the fluoride-induced 1,2-elimination of 2(trimethylsilyl)aryl triflates undergoes an efficient cascade reaction initiated by the Diels−Alder reaction with the differently substituted styrenes leading to the formation of 9-aryl-9,10dihydrophenanthrene derivatives in moderate to good yields.

Table 1. Optimization of the Reaction Conditionsa

9,10-Dihydrophenanthrene derivatives are common structural units in various biologically active natural products.1 For example, the natural product juncusol,2 isolated from the plant Juncus roemerianus, has been reported to possess anticancer and antimicrobial activity, lusianthridin3 isolated from the orchid Lusia indiuisa has been found to show α,α-diphenyl-2piorylhydrasyl (DPPH) free-radical scavenging activity, and orchinol4 isolated from Orchis militaris shows antifungal activity (Figure 1). Due to their diverse biological activity, 9,10dihydrophenanthrenes are interesting synthetic targets. Consequently, the search for efficient and flexible synthetic methods toward this structural motif has attracted much attention in synthetic organic chemistry.5 The synthesis of 9,10-dihydrophenanthrenes has been known to utilize transition-metal-catalyzed cyclization reactions involving arynes.6,7 Based on our interest in the transition-metal-free carbon−carbon and carbon-heteroatom bond-forming reactions using arynes,8 we envisioned that the [4 + 2] cycloaddition reaction between arynes generated by the fluoride induced 1,2elimination of 2-(trimethylsilyl)aryl triflates9 with styrenes could result in a straightforward access to 9,10-dihydrophenanthrenes. This will be interesting as the utility of styrenes as the 4π-component in Diels−Alder reactions utilizing a carbon− carbon double bond, which is involved in aromaticity,10 can also meet with problems involving polymerizations11 as side reactions. Notably, the reaction of styrene with aryne generated from 2-bromofluorobenzene leading to the formation of 9phenyl 9,10-dihydrophenanthrene was reported by Dilling as early as 1966.12 However, this reaction is limited to only one example. Subsequently, the reaction of aryne with α-methyl

F− source

solvent

temp (°C)

time (h)

yield of 3a (%)b

yield of 3a′ (%)b

1 2c 3 4 5 6d

CsF KF TBAF CsF CsF CsF

CH3CN THF THF CH3CN CH3CN CH3CN

30 30 30 60 30 30

12 12 12 12 6 12

77 (73) 59

Efficient synthesis of 9-aryldihydrophenanthrenes by a cascade reaction involving arynes and styrenes.

A mild, general, and transition-metal-free protocol for the synthesis of 9,10-dihydrophenanthrenes is reported. The aryne generated by the fluoride-in...
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