Cyclopropane-substituted allenes

The reaction of cyclopropane-substituted allenes with tetracyanoethylene or bis-trifluoromethyldicyanoethylene proceeds similarly to give the [2-1-2] cycloadducts 200 . 

Transition metals 172 a-bonded to cyclopropanes, substituted on the a-carbon with a halogen atom, are interesting intermediates for cyclopropylidene complexes 173 or allene ones 174 [88]. The former complexes are also supposed to be precursors of the above-mentioned nickel enolates. (Scheme 65)... 

In a single case, thermal treatment of enone 36 induced a 1,3-sigmatropic silyl shift which led to the formation of the rearranged product 37 (Eq. 8.3) [40], So far, only one example of a ring-opening reaction of a cyclopropane 38 has been reported which furnished tetramethoxy-substituted allene 39 (Eq. 8.4) [41],... 

Scheme 7. Cyclopropanation of allenes as a synthetic approach to substituted bicyclopropyl-idenes [48-50,52,53]...

Although the examples are less compared than for alkenes, cyclopropanes and allenes are also oxidized by Tl(lll) salts via oxythallation (Schemes 9.18 [22], 9.19 [23], and 9.20 [24]). Alkynes react with TTN to give a variety of oxidation products such as diketones and carboxylic acids via oxythallation, the products depending on the substitution pattern on the alkynes (Schemes 9.21-9.23 [25]). 

The same method is used in asymmetric cyclopropanations of substituted allenes, giving optically active derivatives of methylenecyclopropane and spiropentane39. 

Some remarks concerning the scope of the cobalt chelate catalysts 207 seem appropriate. Terminal double bonds in conjugation with vinyl, aryl and alkoxy-carbonyl groups are cyclopropanated selectively. No such reaction occurs with alkyl-substituted and cyclic olefins, cyclic and sterically hindered acyclic 1,3-dienes, vinyl ethers, allenes and phenylacetylene95). The cyclopropanation of electron-poor alkenes such as acrylonitrile and ethyl acrylate (optical yield in the presence of 207a r 33%) with ethyl diazoacetate deserve notice, as these components usually... 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

Reaction of Me3GeCl with a substituted cyclopropene in the presence of lithium diisopropylamide (LDA) yields different products depending on the order of addition of the reagents.97 Addition of LDA to a mixture of the reactants gives the dimetallated cyclopropene (Equation (76)). Dilithiation of the cyclopropane followed by addition of Me3GeCl gives the allene (Equation (77)). 

In 1998, Saigo et al. [150] discovered the Rh(I)-catalyzed allenic version of the vinylcyclopropane rearrangement. With a subsituent on the cyclopropane ring two different isomers can be formed depending on which of the different allylic C-C bonds in the three-membered ring is broken. Depending on R2, different regioselec-tivites were obtained with 250. Alkyl-substituted substrates lead to 251 and aryl-substituted substrates to 252 (Scheme 15.79) [150]. 

Byproducts of this rearrangement are cyclobutenes, cyclopropane derivatives and allenic alcohols. The ratio of these products depends on the substitution of the substrate and on the reaction conditions. For example, 3-methyl-5-tosyloxypenta-l,2-diene (3) gives 75% of 1-methyl-2-methylenecyclobutanol (4) upon hydrolysis with water and calcium carbonate at 100 °C, while acetolysis with acetic acid/sodium acetate at 80 °C, and subsequent treatment with lithium aluminum hydride, provides only 37% of the cyclobutanol.12... 

The first nonintroductory section of the text starts with the observation made early in this century that cyclopropanes have significant olefinic character. That is, corresponding ethylene and cyclopropane derivatives have significant similarities. There are literature comparisons of the thermochemistry of direct counterparts such as the parent species (1 and 2, X = H), propene and methylcyclopropane with X = Me, and of methyl acrylate and methyl cyclopropanecarboxylate 8 with X = COOMe. But the chemistry of substituted eth-ylenes is far richer than just that of vinyl compounds. One can retrieve enthalpy of formation data for cumulenes ( cumulenated olefins) such as allene (3) and both cis- and trans-2,3,4-hexatriene (4a and 4b), and for conjugated olefins such as 1,3-butadiene (5) and both (Z)- and (E)-1,3,5-hexatriene (6a and 6b). For the cyclopropane chemist it is natural to... 

As demonstrated below, a Lewis acid-mediated reaction was utilized in the synthesis of dihydro[b furan-based chromen-2-one derivatives from l-cyclopropyl-2-arylethanones and allenic esters <070L4017>. The TiCh-catalyzed anti-Markovnikov hydration of alkynes, followed by a copper-catalyzed O-arylation was applied to the synthesis of 2-substituted benzo[6]furan <07JOC6149>. In addition, benzo[6]furan-based heterocycles could be made from chloromethylcoumarins <07SL1951>, substituted cyclopropanes <07AGE1726>, as well as benzyne and styrene oxide <07SL1308>. On the other hand, DBU-mediated dehydroiodination of 2-iodomethyl-2,3-dihydrobenzo[6]furans was also useful in the synthesis of 2-methylbenzo[Z>]furans <07TL6628>. 

Similar behaviour has been observed by Kohler and Allen and by Smith and Showell on treating the tertiary nitrocyclopropyl ketone 198 with a base. Base catalyzed stereoisomerizations of ketones like 198 had also been observed by Kohler and Smith and by Smith and ShowelP and were in agreement with the deprotonation of such cyclopropanes to the corresponding cyclopropyl anions. It thus did not seem as if the deprotonation of C(0)R-substituted cyclopropanes would cause any problems. 

Besides the T-cyclopropanes, T-butenes and T-pentenes, several highly unsaturated tritium-labeled products, including acetylene, allene, propyne and 1,3-butadiene were observed in the hot T reaction with EC and DMC. Except for T-acetylene, whose yield is about 10 % of the substituted parent molecule, the yields of the other products are very low. 

The synthesis of novel 4a-substituted sterols was undertaken in the laboratory of C.H. Robinson. These compounds are potential inhibitors of sterol 4-demethylation. To prepare the desired 4-allenyl-5a-cholestan-3p-ol, the exocyclic olefin precursor was first reacted with bromoform/potassium f-butoxide to afford the geminal dibromo-substituted cyclopropane derivative. Next, methyllithium was used to bring about the rearrangement to afford the allene, and finally acidic conditions were applied for the removal of the THP protecting group. 

Highly substituted alkylidenecyclopropanes 130, 131 and 133, 134 were made accessible by a regioselective hydroxy-directed cyclopropanation of a-allenic alcohols 129 and 132 using sa-marium/diiodomethane. ... 

In the same way, phosphoryl-substituted carbenes reacted selectively with one double bond of an allene to give cyclopropane 1 no yield was reported. ... 

Other competing reactions include intramolecular alkylations of the intermediate meta-lated cyclopropanes when the leaving group is substituted at the cyclopropylmethyl carbon, e. g. 36 and 38. In this case no allene is produced and the major products are bicyclobutanes. However, when the leaving group is substituted at a site further removed from the cyclopropane ring, e. g. 41 and 43, allenes are produced in reasonable yield. 

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