Cyclopropa-aromatics, synthesis

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

While the synthesis of the requisite methano[10]annulenes is not always easy, the [ 2 + 2 + 2] cycloreversion has provided the essential structural variations of cyclopropa fusion into an aromatic moiety that complement other methodologies (Section II.E). Moreover, the sequence lends itself to further exploitation. Thus Vogel s group50 has obtained the dicyanoacetylene adduct 40 which provides phenylacetylene on fvp (equation 9). It is likely that methylenecyclopropabenzene (9) is initially formed as a molecule too... 

The cyclobutanone ring in the ether 7, derived by ring expansion of an oxaspiropentane, intramolecularly alkylates the activated aromatic system on treatment with PTSA, leading to 2W-cyclobuta[c]chroman-4-ols 8 (X = OH). Subsequent fission of the four-membered ring yields 3,4-disubstituted 2//-chromenes <02SL796>. A variation of this approach allows the synthesis of the 4P-phenylsulfanyl derivative of 8 (X = SPh), oxidation of which affords the cyclopropa[c]chroman entity <02OL2565>. 

Synthesis and subsequent thermal decomposition of aryldiazomethanes under standard Bam-ford-Stevens conditions has also been utilized to prepare aryl-substituted cyclopropanes. When benzaldehyde tosylhydrazone is mixed with solid sodium methoxide in naphthalene, phenan-threne, and anthracene and heated above 130°C, phenylcarbene is generated. All the three aromatic hydrocarbons are attacked at the site of highest double-bond character to give, albeit in low yield, norcaradienes. The best results were obtained with naphthalene, which gave 1-phenyl-la,7b-dihydro-l//-cyclopropa[a]naphthalene (7) in 9% yield. ... 

The dihalocyclopropane route is the method of choice for the synthesis of benzocyclopropenes and linearly fused cyclopropanaphthalenes, but is unsuccessful for angular cyclopropa[u]naph-thalene. Treatment of l,l-dichloro-la,2,3,7b-tetrahydro-l/f-cyclopropa[a]naphthalene(5) with potassium /ert-butoxide gave a mixture of 1-chloromethylnaphthalene, 1-terr-butoxymethyl-naphthalene and 2-terf-butoxy-l-chloromethylene-l,2,3,4-tetrahydronaphthalene, but none of the expected cyclopropa[n]naphthalene. The failure of the reaction has been attributed to difficulties in the aromatization. Isomerization of the initially produced cyclopropenyl double bond into the cyclohexane ring would require disruption of the aromatic character of the adjacent benzene nngT t-r>> 62 Similarly, attempted aromatization of l-chloro-l,2,3,7b-tetra-hydro-l//-cyclopropa[a]naphthalene (6) with 2,3-dichloro-5,6-dicyanobenzoquinone or via N-bromosuccinimide bromination followed by reaction with base afforded 4,5-benzotropone instead of cyclopropa[fl]naphthalenc. ... 

This method has been utilized for the synthesis of 2,7-dimethyl-l//-cyclopropa[g]isoquinoline (7b) aromatization of the heterocyclic precursor 6b gave the substituted cyclopropaisoquinoline 7b in only 7% yield, and the reaction was completely unsuccessful when applied to the parent compound, i.e. 6a 7a. ... 

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