1.3- Cyclopentadiene, Diels-Alder

The chiral dialuminum Lewis acid 14, which is effective as an asymmetric Diels-Alder catalyst, has been prepared from DIBAH and BINOL derivatives (Scheme 12.12). " The catalytic activity of 14 is significantly greater than that of monoaluminum reagents. The catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids. Similarly, the chiral trialuminum Lewis acid 15 is quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and MeaAl (3 equiv) in CH2CI2 at room temperature (Scheme 12.12). " The novel structure of 15 has been ascertained by NMR spectroscopic analysis and measurement of the methane gas evolved. Trinuclear aluminum catalyst 15 is effective for the Diels-Alder reaction of methacrolein with cyclopentadiene. Diels-Alder adducts have been obtained in 99% yield with 92% exo selectivity. Under optimum reaction conditions, the... 

Cyclopentadiene Diels-Alder adducLs such as 11 are highly strained. Huw M.L. Davies of the State University of New York, Buffalo, has established (J. Am. Chem. Soc. 2004,126,2692) an important strategic connection between Diels-Alder adducts such as 13 and the enantiomerically-enriched cycloheptane derivative 14. Oxidative cleavage of the alkene of 14 would lead to a cyclohexane derivative that would be difficult to access by other means. The same adduct 14 (albeit in racemic form) is also available directly by AIC1,-mediated addition of mcthacrolein 15 to cyclopentadiene. 

We characterized and further studied this basic mechanism of covalent affinity labeling using spectroelectrochemical techniques. The kinetics and stability of quinone oxidation products at high dilution and low pH were consistent with the proposed mechanism, as was the concentration dependence of rapid labeling reactions of the more reactive catechol with the receptor.1215 Spectroelectrochemical and direct cyclic voltametric determination of the half-potentials of the hydroxylated quinones were further consistent with their intermediacy in the labeling reactions of TMC.15 The quinone oxidation products of 4- and 5-HTMC were characterized in part as cyclopentadiene Diels-Alder adducts.15 The instantaneous reactions of these hydroxy TMCs with receptor were consistent with their intermediacy in the TMC reactions. From the concentration dependence of the half-of-sites labeling reactions we could deduce Kd for each isomer fC,(4-HTMC) = 224 98 pM, K/5-HTMC) = 39 17 juM. 

Retro-Diels-Alder reactions giving thiocarbonyl compounds are favored when simultaneously a comparatively stable diene is formed1. This is the case with anthracene and cyclopentadiene Diels-Alder adducts 81 and 82 which, upon heating, afford a wide array of thioaldehydes and thioketones (equation 84). These adducts are stable at room temperature and have become a convenient way of storing very reactive thiocarbonyl compounds. Cyclopentadiene is the cheapest and most reactive diene for use in Diels-Alder reactions. Also, strain in the bridged cycloadducts facilitates retro-Diels-Alder cleavage224. 

The 1,4-benzoquinone-cyclopentadiene Diels-Alder adduct 2 is well known.5 The procedure described here is adapted for large-scale preparation. For the zinc reduction, aqueous acetic acid has also been used.4 The present procedure allows recovery of most of the acetic acid. The entire procedure can be completed in 3-4 days. 

Keck and Krishnamurthy have shown that the Diels-Alder reaction of cy-clopentadiene and bromoacrolein is facilitated by a Lewis acid derived from titanium tetraisopropoxide and S-BINOL (59) (Scheme 47) [126]. The cycloaddition may be conducted with isoprene at slightly lower levels of enantioselectivi-ty methacrolein-cyclopentadiene Diels-Alder reactions are only moderately selective. 

A stable diaquo titanocene catalyzes aldehyde-cyclopentadiene Diels-Alder reactions in moderate to good enantioselectivities Odenkirk W, Bosnich B (1995) J Chem Soc Chem Commim 1181... 

A C2-symmetric bridged ferrocene catalyzes the methacrolein-cyclopentadiene Diels-Alder reaction in 10% ee Gibis K-L, Helmchen G, Huttner, G, Zsolnai L (1993) J Organomet Chem 445 181... 

Chlordane is prepared by reacting hexachlorocyclopentadiene (40) formed as the main product in the chlorination of cyclopentadiene (42) with hypochlorite or in the high temperature chlorination (470°C) of pentane, neopentane or cyclopentane with cyclopentadiene (Diels-Alder reaction), and chlorinating the 4,5,6,7,8,8-hexa-chloro-4,7-methano-3a,4,7,7a-tetrahydroindene (chlordene, 43) yielded by this reaction until two additional chlorine atoms are absorbed, obtaining 4,7-methano-... 

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