Dienophile quinone-based

Within the cubane synthesis the initially produced cyclobutadiene moiety (see p. 329) is only stable as an iron(O) complex (M. Avram, 1964 G.F. Emerson, 1965 M.P. Cava, 1967). When this complex is destroyed by oxidation with cerium(lV) in the presence of a dienophilic quinone derivative, the cycloaddition takes place immediately. Irradiation leads to a further cyclobutane ring closure. The cubane synthesis also exemplifies another general approach to cyclobutane derivatives. This starts with cyclopentanone or cyclohexane-dione derivatives which are brominated and treated with strong base. A Favorskii rearrangement then leads to ring contraction (J.C. Barborak, 1966). 

Importantly, though. Nature does not have to rely on enzymatic assistance in order to utilize the full range of synthetic power afforded by the Diels—Alder reaction since she can also construct compounds whose unsaturated motifs are disposed to participate spontaneously in this pericyclic reaction on their own accord. For instance, in the previous chapter on longithorone A, we chronicled how a transannular Diels—Alder reaction was initiated under ambient conditions the instant that a quinone-based dienophile was unveiled in a macrocyclic ring bearing a neighboring diene system. Similarly, as described in Chapter 17 of Classics /, heating a small... 

Quinones, typical of unsaturated ketones, undergo base-catalysed epoxidation The epoxy ketone may then rearrange with the formation of a hydroxy quinone. Quinones may also act as dienophiles in the Diels-AIder reaction. This can be a useful way of constructing polycyclic ring systems (Scheme 3.54). 

Quinone methides are an interesting class of unstable dienes which may be generated from appropriate cyclobutenes and trapped in situ with dienophiles. Thus, heating the diene precursor 17 and the carbohydrate-based a,/ -unsaturated ester 16 stereoselectively produces a pair of adducts 18 (14%) and 19 (57%) d.r. [(l/ ,2/ ,3S)/(lS,2S,3/ )] 20 80 . 

We now present the chemistry of homophthalic anhydrides, mainly addressing the strong base-induced [4 + 2]-cycloaddition reaction with the C - C double bond, especially in quinone and dienophiles, and its application to the syntheses of anthracyclines and fredericamycin A. 

The Diels-Alder reaction ([4+2] cycloaddition) [8] provides one of the most powerful methods for the constmc-tion of PAHs because it affords one-step formation of the six-membered ring framework, the basic unit of these derivatives. This chapter, based on the results of our work, will highUght the synthesis of substituted oligoacenes including anthracene (1), tetracene (2), and pentacene (3) cores by Diels-Alder reactions using arynes and quinones as dienophile components and furans, thiophene-1,1-dioxides, and o-quinodimethane as diene components. The synthetic methodology will be accompanied by discussions on molecular and packing stmctures as weU as solid-state optical properties. 

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