Synthesis of Strained Paracyclophanes

The substantial amount of strain in [6]paracyclophane (la) (Structures 1) and its lower homologues precluded ordinary synthetic methods for cyclophane synthesis. Viable alternative should therefore be based on transformation from high energy species such as strained molecules or reactive intermediates like 

2) by using this method [9]. The Dewar benzene precursor was prepared by [2 + 2] cycloaddition of DMAD with a cyclobutadiene-AlCla complex using the procedure developed by Hogeveen. 

We also developed a method based on the cleavage of the central bond of the [6.2.2]propellane skeleton by way of carbocation rearrangement [5b, 10], 

16 was prepared by (i) furan-acetylene [4 + 2] addition, (ii) intramolecular [2 + 2] addition, and (iii) thermal automerization of the oxaquadricyclane derivative. Bromination followed by reductive deoxygenation produced bis(ethoxycarbonyl)-substituted [6]paracyclophane (If) in good yield. Methyl ester Ig, 3-keto derivative Ih [14], and optically active carboxylic acid Id (Structures 2) [15] were also prepared by this procedure. Moreover, several hydroxymethyl based derivatives li-k [13b, 16] and anthraquinone derivative 

17 [17] were prepared from Ig (Structures 2). Extensive studies on the structure and reactivity were undertaken for these compounds. 

---