Annulenes cycloadditions

Scheme 6. Photochemical [2 + 2] cycloreversion of the dodecadehydro[18]annulene in furan as the solvent, followed by [4 + 2] Diels-Alder cycloaddition, gives the oxanorbornadiene adducts 38-40 [66]...

SCHEME 7. Competition between cycloadditions and retro- Diels -Alder reactions in the reaction of dicyanoacetylene and ll-methylene-l,6-methano[10]annulene... 

If the cycloaddition and cycloreversion steps occurred under the same conditions, an equilibrium would establish and a mixture of reactant and product olefins be obtained, which is a severe limitation to its synthetic use. In many cases, however, the two steps can very well be separated, with the cycloreversion under totally different conditions often showing pronounced regioselectivity, e.g. for thermodynamic reasons (product vs. reactant stability), and this type of olefin metathesis has been successfully applied to organic synthesis. In fact, this aspect of the synthetic application of four-membered ring compounds has recently aroused considerable attention, as it leads the way to their transformation into other useful intermediates. For example aza[18]annulene (371) could be synthesized utilizing a sequence of [2 + 2] cycloaddition and cycloreversion. (369), one of the dimers obtained from cyclooctatetraene upon heating to 100 °C, was transformed by carbethoxycarbene addition to two tetracyclic carboxylates, which subsequently lead to the isomeric azides (368) and (370). Upon direct photolysis of these, (371) was obtained in 25 and 28% yield, respectively 127). Aza[14]annulene could be synthesized in a similar fashion I28). 

The cycloaddition of 1 with 1,2,4-triazines (324), bearing one or more electron-accepting substituents, directly affords the 3,8-methanoaza[10]annulene 325 after loss of N2 from the cycloadduct. Similarly, the tetrazine-dicarboxylate 326 reacts with 1 via cycloaddition/cycloelimination to 327, which has norcaradiene struc-ture. a-Pyrones (328) and 1 undergo cycloaddition/C02 extrusion to 1,6-methano[10]annulenes (329). The sequence cycloaddition/C02 extrusion has also been reported for the reaction of cyclopropa[/]phenanthrene (142) with a-pyrone (328, R = H). ° Substituted 1,6-methano[ 10]annulenes 331 are obtained by analogy via cycloaddition/S02 extmsion of 1 with thiophene-1,1-dioxides (330). ... 

Ogawa et al. (12) used an intramolecular azide-alkene cycloaddition strategy to synthesize the oxygen-bridged aza[15]annulene 52 and the aza[15]annulene dicar-boxylate 55 (Scheme 9.12). 1,3-Dipolar cycloaddition of vinyl azide to the acrylate moiety followed by extrusion of nitrogen gave the aziridine 51. Rearrangement of 51 afforded the aza[15]annulene 52. The same approach was used to synthesize the aza[15]annulene 55. 

Maleic anhydride cycloaddition to furanothia[ll]annulene (28) led to the formation of the triply annulated parent (29) (see p. 715). 

Finally, thermal cycloaddition between benzocyclopropene and triazene (114) serves as a conveniently general means of constructing the bridged aza[10]annulene frame depicted in (115) (a direct relative of 23d), the preparation of the last two derivatives (115c, d) requiring the use of ultra-high pressure (1.5 kbar) (80TL7). 

The reactivity of zirconium imido complexes supported by a dibenzotetraaza[14]annulene (taa) macrocycle ligand toward a range of unsaturated substrates has been examined. Reaction with t-butyl isocyanate cleanly generated the cycloaddition product Zr N(2,6-( -Pr)2C6H3)C(0)N(t-Bu) (Me4taa) (equation 21). ... 

Very stable polycylic phosphiranes were obtained by an intramolecular cycloaddition reaction of aminodichloro-phosphines with the C-C double bond of 577-dibenzo[ , [7]annulene (dibenzocycloheptene) under the influence of Mg-turnings. Formally, these compounds are formed by an intramolecular [2-Fl] cycloaddition of an R2N-P phosphinidene unit to the C-C double bond of the central seven-membered ring. In analogy to the hydrocarbon barbalan the amino compound is named BARBAR-Phos <1999AGE1623>. In a comparable reaction sequence, dibenzophosphasemibullvalene was obtained (Equation 25) <2003AGE3955>. 

Methano[10]annulene has been prepared through the intermediacy of several [5.3.1]propellanes. The first of these, [5.3.1]propella-2,4,7-triene 23 was prepared by carbene addition to the trienone (22) . It is capable of undergoing [2-h2], [4-f2] and [6-h2] cycloaddition... 

The synthesis of highly strained linear and angular dicyclopropabenzenes has been attempted without success, since the precursors required for the cleavage could not be synthesized. Thus, iV77-l,6 7,12-diinethano[14]annulene 3 underwent addition to acetylenedicarbonitrile in the wrong sense and did not lead to the required norcaradiene. Cycloaddition of dimethyl acetylenedicarboxlate to cyclopropa-1,6-methano[10]annulene would provide a precursor 4 for the linear dicyclopropabenzene however, the desired cycloadduct was not obtained. In this context, cyclopropa-1,6-methano[10]annulene gave no cycloadduct with 4-phenyl-4//-1,2,4-triazol-3,5-dione. On the other hand, the addition product 5 of 1.6-dimethylenecyclohepta-triene with 1 -bromo-2-chlorocyclopropene reacted with the acetylene derivative, but the twofold dehydrochlorination did not lead to the required norcaradiene. ... 

Cycloaddition between cyclopentadienones and benzocyclopropene occurs at room temperature. The adducts are thermally unstable and decarbonylate to methanoannulenes (see Section 2.4.2.) except in the case of the adduct 3 of 2,5-diethyl-3,4-diphenylcyclopenta-2,4-dienone, which was isolated at 20 C. " The adduct 3 loses carbon monoxide on melting (107-109°C) or on heating in refluxing tetrahydrofuran. When the cycloaddition was carried out at 65 C the annulene 4 was formed in 87% yield. 

Under the conditions of the cycloaddition some adducts of cycloproparenes are thermally unstable and lose a fragment forming methanoannulenes. A typical example of this transformation is the reaction of benzocyclopropene with 2//-pyran-2-one, which gave 1,6-methano[l 0]an-nulene (la). Several substituted annulenes were accessible by this route, although the yields were generally below 35%. The same reaction has been reported for cyclopropa[/]phen-anthrene giving 2. ... 

Thiophene 1,1-dioxidcs bearing electron-withdrawing substituents undergo a cycloaddition reaction with benzocyclopropene in tetrahydrofuran followed by sulfur dioxide extrusion to give annulenes 3 in 5-35% yield. The unsubstituted thiophene 1,1-dioxide is, however, not sufficiently reactive and undergoes dimerization rather than cycloaddition. ... 

A surprisingly efficient intermolecular photochemical [2 + 2] dimerization was achieved in the synthesis of belt-shaped cyclophanes by irradiation of arenoannelated [18]annulenes like 176 [116]. The highly selective threefold [27t+27t] cycloaddition yields dimers 177 in up to 68%. The efficiency of this dimerization process even at high dilution can be explained with the perfect preorganization of the discotic mesogenes 176. 

Cycloaddition to a 1,2,4-triazine, followed by extrusion of nitrogen from the adduct, is a method of forming other aza-heterocycles the potential of this type of procedure is exemplified by the remarkably simple preparation of the 3,8-methanoaza[10]annulene (297), as depicted in Scheme 117. ... 

Scheme 1 Synthesis of a [6]annulene via its Dewar benzene isomer. The Dewar form is prepared via a [47H-27r] cycloaddition with [4]annulene and a substituted acetylene.

Table 1 Showing substrate and cycloadduct product from the intramolecular [Atz+Ik] cycloaddition involving the [4]annulene moiety...

Methano[10]annulene is resistant to pyrolysis up to 300 at higher temperatures it decomposes providing phenylcyclopentadicnes and naphthalene derivatives [161]. It undergoes cycloaddition... 

It was suggested that cis,cis,cis,trans,cis,trans-[l2]3xmu[QnQ was an intermediate in the formation of the tricyclo 6400 tetraene. Then this annulene could undergo two electrocyclizations to the a,o,o -tetraene (Scheme 13.4). Finally, extended pyrolysis at 30°C gave benzene in a forbidden retro 2 + 2 cycloaddition, no doubt, assisted by the stability of the biradical intermediates, relief of ring strain, and the product stability. The tricyclic tetraene has been synthesized by an alternative route and... 

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