Repetitive Cycloaddition Reactions

In ////-prepared l-adamantyl-2-diphenyl-phosphaalkene dimerizes spontaneously and forms an 1,2-diadamantyl-lA3,2A3-diphosphetane <2006IC5225>. Repetitive cycloaddition reactions between trimethylsilylphosphaalkyne 44f and cyclopentadiene, in a molar ratio of 3 2, lead to formation of the polycyclic triphospha compound 91 <1999EJ0363>. [WICOATHF] triggers [2+2] cycloaddition reactions of Mes -arsaalkyne 92 by rr-complexation... 

A more recent, and particularly successful, variant of the multifunctional coupling process to ladder polymers is the use of repetitive cycloaddition reactions that start from bifunctional monomers, for instance dienes and dienophiles (Diels-Alder-type cycloadditions) [2-4]. The fact, that both chains are generated simultaneously in a concerted process constitutes the important progress associated with such a route. 

Carbonyl O-oxides 144, generated in situ by the ozonolysis of vinyl ethers at low temperature (—70 °C), readily undergo [3+2] cycloaddition with imines 145 added to the reaction mixture to afford the corresponding 1,2,4-dioxazolidines 15-18 in 14-97% isolated yields (Scheme 41) <1994J(P1)2449, 1995J(P1)41>. The repetition of these reactions in MeOH results in the quantitative recovery of the initial imines 145. The steric bulk of the a-substituents did not appear to diminish the imines 145 reactivity, because the isolated yields of 1,2,4-dioxazolidines... 

Scheme 18. If the repetitive Diels-Alder reaction adducts reported were being formed under thermodynamic control, heating the hexadecadeutero 2 1 adduct 68 under reflux in toluene in the presence of a 10 molar equivalent of the bisdiene 37 should give rise to a mixture of compounds comprised of 68, 69, and 41. A retro Diels-Alder reaction of 68, followed by scrambling of the labeled bisdiene 66 with the excess of the bisdiene 37 should favor preferential cycloaddition with 37 to afford 69. The excess of 37 should ultimately drive the equilibration process toward the unlabeled 2 1 adduct 41. No evidence for such an equilibration process has been found... 

Stoddart and cowotkers have extensively exploited the principle of repetitive arene and bis-arene cycloaddition to furans for the design and synthesis of novel macropolycyclic stractures [22,23]. For example, bis-dienophile 21 obtained from ortho-xylene 20 with butyllithium followed by trapping of the resulting bis-arene equivalent 22 with 2 equiv of furan represents a suitable building block for the construction of the macropolycylic belt-like molecule 23. Diels-Alder reaction of 21 with an appropriate bis-diene under alternate thermal and high-pressure conditions afforded macrocycle 23 in good yield, together with an acyclic isomer (Scheme 13.8) [22,23]. 

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