1.6- Methano annulene reactions

A triple scission of a six-membered ring on the surface of C60 leading to one of the most remarkable transformations in metal-fullerene chemistry has been observed in 219. This reaction occurred via addition of CpCo(CO)2 on the bis-methano annulene 218 (Equation (36)). 

Heteroannulenes electrophilic reactions, 7, 726 tr-excessive synthesis, 7, 727 nucleophilic reactions, 7, 727 Hetero[l IJannulenes structure, 7, 715 Hetero[ 12]annulenes pyridine-like methano-bridged, 7, 715 Hetero[ 13]annulenes unrestricted structure, 7, 716... 

A similar tandem Dotz-Mitsunobu reaction has been reported starting from a l,6-methano[10]annulene carbene complex, but no conclusion could be reached on the influence of the chiral information regarding the stereoselective course of the reaction since the chromium fragment could not be kept coordinated to the benzannulation product [47]. 

When 2-methoxy-l,6-methano[10]annulene 32 was subjected to a cyclopropana-tion with diazomethane and cuprous chloride as catalyst reaction occurred preferentially at the 5,6-, 6,7- and/or 1,10-bonds and the adducts spontaneously underwent disrotatory opening yielding the corresponding methoxybicyclo[5.4.1]dodecapen-taenes. Hydride abstraction with triphenylmethyl fluoroborate was performed on the mixture and the ions 33 and 34 so produced were treated with dilute aqueous potassium hydroxide. The annulenones 13 and 14 were then separated by chromatography. 

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

Alder-Rickert cleavage was also examined as a potential route for 100 and 102, but the required precursors could not be synthesized. Bis-methano[14]annulene 105 added dicyanoacetylene in the wrong sense, and did not yield 106, which could have served as precursor of 102. Similarly, the attempted Diels-Alder reaction of the bridged cyclopropafl OJannulene 107 with dlmethoxycarbonylacetylene, which should furnish 108, produced only decomposed material. This is consistent with the observation of Halton and Russel that 107 does not add to 4-phenyl-1,2,4-tria-zoline-3,5-dione. In order to circumvent the low reactivity of 107 in cycloaddi-... 

Other approaches to alkylidenecycloproparenes have been attempted without success. Aromatization of appropriate alkylidenecyclopropanes or their precursors could not be realized, and flash vacuum pyrolysis of methylene phthalide and 3-methylene-2-coumaranone afforded rearrangement products rather than alkylidenecycloproparenes via extrusion of 002. The photochemical or thermal decomposition of the sodium salt of benzocyclobutenone p-toluenesulfonyl hydrazone led to products derived from dimerization of the intermediate benzocy-clobutenylidene, or from its reaction with the solvent, but no ring-contracted products were observed. When the adduct of methylene-l,6-methano[10]annulene to dicyanoacetylene (249) was subjected to Alder-Rickert cleavage, phenylacety-lene (250) was formed, which derives reasonably from the parent 234. ... 

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). ... 

Of particular interest is the syn-l,6-imino-8,13-methano[14]annulene (59) which represents the first authenticated example of a stable 1H-azepine with a free NH group (80AG(E)1015). The annulene with aluminum oxide undergoes a remarkable isomerization to the anti isomer (61). Investigation shows that the isomerization is not a thermal reaction but involves alumina-catalyzed proto tropic shifts via the 3/7-azepine tautomer (60). This system is unique in that it is the first example of a 3H -> 1H azepine tautomerism, and is a consequence of the high degree of strain in the anti-Bredt 3iT-azepine (60). 

Prior to the work of Cheng et al. Rubin et al. reported a similar [4 + 4] pho-tocyclization reaction [299]. Rubin has shown that a cyclohexadiene derivative underwent a very facile photochemically promoted rearrangement to the stable bridged bisfulleroid 198 (Scheme 76). This process occurs via the initial [4 + 4] photoadduct (not observed), which undergoes a thermally allowed [2 + 2 + 2] cycloreversion to afford a bis-methano[12]annulene structure 198. Alternatively, compound 198 can be obtained by photolysis of the allylic alcohol 197 under reflux and acidic conditions in good yields. The allylic alcohol is obtained by acidic... 

Compound 36, X = Cl, leads to l,6-methano-[10]annulene (37), 4-methylazulene (38) and 39 on dehydrohalogenation with strong base, in the same way, 36, X = Br, leads to 37, a reduced amount of 38 and the monobromide 40". Each of the products arises by a distinct reaction pathway, though a 1,4-elimination to leading 41 is thought to be involved in the case of 37. 

The benzannulation technique is also applicable to the benzene homologation and functionalization of annulenes, as well to a quadruple arene modification of dendritic cores. The reaction of chromium carbene functionalized l,6-methano[10]annulene 82 with 3-hexyne under standard conditions afforded a fair yield of the benzannulation product 83 after protection and oxidative work-up (Scheme 32) [75]. The chromium complex 84 evidently partly survived the oxidation conditions using Feln a syn-stereochemistry with respect to the Cr(CO)3 fragment and the methano bridge was suggested on the basis of NMR data, which is in contrast to the preferred formation of anti-annulation products bearing cydophane skeletons [75b]. 

The diketothiocane 4-thia-l,7-methano[ll]annulene-2,6-dione 10 has been prepared according to Scheme 11 in a simple reaction sequence <2001H(54)159>. It is the starting material for further syntheses of enlarged diketothio-canes as /(-6,15-epithia-8,13-methanobenzo[e][14]annulene-7,14-dione 4 (Equation 6). Both diketothiocanes 10 and 4 have also been transformed into dihydro thiocines by reaction of keto functions (Section 14.03.7). 

Two unique ring enlargements which can also be classified as divinyl-cyclopropane-cycloheptadiene expansions are shown in equations 177 and 178 ". The last reaction has been employed for constructing the l,6-methano[10]annulene carbon skeleton required for the unusual furanosesquiterpene dihydrospiniferin-1. ... 

The third returns the reader to chemi-ionization of cyclopropane derivatives with excited state electron acceptors. In the particular, it has been observed that oxidation of l,6-dimethylbicyclo[4.1.0]hept-3-ene (46) and [4.4.l]propella-2,6-diene (47) result in 2,6-dimethylcycloheptatriene (48) and l,6-methano[10]annulene (49) (equations 28 and 29 respectively). The former compound is conceptually the deprotonation product of the dication wherein the C-C bond between the bridgehead carbons has been totally depleted of electrons. In this case, however, the dication is not expected to be as stable as in the above pyramidal or cyclobutadiene cases and indeed, the product is seemingly not formed via the dication route. The methanoannulene likewise does not arise from the tetracation of its precursor. Rather, proton and electron transfer reactions involving merely radical cations proceed to remove hydrogens sequentially. 

A piquant reaction is that of benzocyclopropene (32) which is formed via a retro-Diels-Alder reaction of the adduct of l,6-methano[10]annulene with acetylenedicarbo-xylic ester (34). This adduct is fragmented at 400° C and 1 Torr. Reaction of the retro-product with another dienophile gives the unstable adduct 35 which spontaneously loses CO2 to afford. . . l,6-methano[10]annulene (25) La Ronde ... 

Within the framework of reactions of various propellane substrates with very reactive dienophiles, N-methyl- and iV-phenyl-triazolinediones, l,6-methano[10]annulene itself and many bridge- and ring-substituted derivatives were subjected to such Diels-Alder reactions. All of the products are (more complex) [4.4.1]propellane derivatives. In general it is clear that the rates of reaction are very much slower than those of various tetraenic. 

In the reaction of l,6-methano[10]annulene with bis-triazolinediones the 2 1 adduct (lid) has the following structure ... 

This strategy has been of basic importance not only in the field of cascade molecules. In 1978, Vogel et al. synthesized methano-bridged annulene derivates by a repetitive synthetic strategy6 (Scheme 3). The repetitive steps (initial Wittig reaction followed by reduction of the resulting ester groups to the aldehyde) led him to a new class of aromatic compounds. 

Since with 3,8-methano-l //-cyclopropa[10]annulene the conjugate addition to C4-C7 preceded addition to the cyclopropene bond, obtaining the dibromo derivative derived from the latter directly is impossible. The compound 5 can, however, be obtained in 90% yield on reaction of the tetrabromide 4 with potassium iodide 5 opened to give the pentaene 6 upon refluxing in benzene. When the dibromo addition was carried out at — 70"C, however, the cyclopropane ring opened (see Section 3.3.2.). 

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. ... 

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