Cycloaddition repetitive

Reacting 126 with dienophiles 283 produces Diels-Alder adducts 284, which, as cydobutenes, can thermally be ring opened to the conjugated dienes 285, ready for a repetition of the cycloaddition step to provide the 2 1 adducts 286. Hence the whole sequence represents a diene-transmissive Diels-Alder protocol [118]. 

The 1,3-dipolar cycloaddition ofp-nitrobenzenesulfonyl azide (219) onto di-spirocyclopropanated bicyclopropylidene 56 has been used as a key step in a general repetitive procedure for the preparation of [n]rotanes 222 (Scheme 49) [140]. 

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... 

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... 

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... 

Triynes 24 derived from l-iodo-2-naphthol by repetitive Sonogashira couplings afford the helically chiral heptacyclic derivatives 25 through a [2+2+2] cycloaddition catalysed by Rh(II) complexes both yields and ee are good. In some instances, the alternative ladder-type... 

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. 

A spiro-type ladder polymer is produced when l,4,5,8-tetrahydro-l,4,5,8-diepoxyanthracene (23), as the bisdienophile component, is reacted with the bicyclic silaspirodiene 33 [111]. The products formed (34) in the repetitive cycloaddition (under high pressure conditions) possess number average molecular weights of up to 11000 (Dp = 13). Polymers 34 are promising candidates for... 

In many of their contributions to the DTDA chemistry of [3]dendralenes, they illustrated the ease with which multicyclic frameworks can be generated via repetitive cycloadditions. In one of their most striking examples, four rings are generated in a one-pot operation through the combination of five molecules. 

Scheme 11 Synthesis of kinked GNRs by repetitive Diels-Alder cycloaddition (AA+BB route)...

The following case created a breakthrough. Instead of the above AA+BB combination or the AB2-type tetrapenylcyclopentadienone 33 for dendrimer synthesis, we made the AB-type species 33b, which was additionally equipped with long linear or branched aUcyl tails. One readily envisages that a polyphenylene 33c with a rod-like shape is formed in a repetitive Diels-Alder cycloaddition (Scheme 12). 

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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