Allenes cyclopropyl

The allene 149 gave by reaction with maleic anhydride (entry 1) and N-phenylmaleimide (entry 2) the [2 + 2] adducts 155a, b as mixtures of two diastereoisomers [36], Nevertheless, their chemical yield was very low and competitive reactions, mostly [4 + 2] cycloadditions on a rearranged al-lylidenecyclopropane and on a primary 1 1 adduct derived from an ene reaction (see Sect. 2.1.2), prevailed. Allenes 149 and 563 cycloadded to tetracyano- and l,l-bistrifluoromethyl-2,2-dicyanoethylene (Table 45, entries 3-6) also selectively at the cyclopropyl substituted double bond in order to remove most of the ring strain [149a],... 

Finally, there has also been research into the ozonolysis of allenes. Thus sterically hindered allenes react by transfer of one oxygen atom, forming a mixture of reaction products214. Recently, the ozonolysis of a cyclopropylallene has been shown to yield a diastereomeric mixture of cyclopropyl esters215. 

The discovery of carbene and carbenoid additions to olefins was the major breakthrough that initiated the tapping of this structural resource for synthetic purposes. Even so, designed applications of cyclopropane chemistry in total syntheses remain limited. Most revolve around electrophilic type reactions such as acid induced ring opening or solvolysis of cyclopropyl carbinyl alcohol derivatives. One notable application apart from these electrophilic reactions is the excellent synthesis of allenes from dibromocyclopropanes 2). 

Under suitable conditions, carbon atoms react with olefines to form allenes 46 presumably via intermediate cyclopropyl carbenes viz. 

Evidently a large part of the energy liberated in the approach of the carbon atom to ethylene will go into this normal mode — which is the one required for conversion of 30 to 31. Unless the interconversion of vibrational energy is incredibly efficient, one would then expect the initially formed 30 to be converted to 31 even at the lowest temperatures. The fact that allene is formed at -190° is not therefore surprising. On the other hand the existence of a large barrier between 30 and 31 would prohibit rearrangement of 30 if formed under milder conditions free cyclopropyl carbenes do not rearrange to allenes if formed by conventional methods in solution 49). 

A richer structural variety is realized when the propargyl carbonates 90 are treated with cyclopropyl(cyano)thienyl cuprates [56] such as 141, allowing the preparation of allenes with up to four different substituents (Scheme 5.19) [57]. 

The cydopropanated version of 3, l,3-di(cyclopropyl)allene (18), has been used as a coupling partner in Heck-type reactions. For example, with iodobenzene the conjugated cyclopropylhexatriene 330 is obtained whereas repetition of the experiment in the presence of dimethyl maleate yields the Diels-Alder adduct 331 [58]. 

Addition of PhPdl to the allene triggers cyclopropyl ring opening to generate a cr-palladium species, which readily leads to a 1,3,5-triene through /3-elimination. From the observed diastereoselectivities, the reaction seemingly proceeds stepwise via the well-stabilized zwitterionic intermediate. 

Carbonylative [5 + l]-cycloaddition of allenylcydopropanes was successfully achieved by use of an Ir(I) catalyst to yield (2-alkylidene)cyclohexenones in good yields (Scheme 16.43) [43]. No carbonylative [5 + l]-cycloaddition was observed in the case of an allenylcyclopropane lacking substituents at the allenic terminus. It can be deduced that the metal is too distant to open the cyclopropane ring, probably owing to the preferred -coordination at the allenic Jt-bond distal to the cyclopropyl group. 

Both acridone and dibenzo[6,/]azepine produce unexpected products (Scheme 7.39) when reacted with dimethylvinylidene carbene (7.1.18.A). Acridone reacts initially at the nitrogen atom to produce the 10-(3,3-dimethylallenyl) derivative (13%) and a pyrroloacridone (10%) which, if the structure is correct, could be derived from the allene by sigmatropic shifts [16]. The dibenzoazepine reacts as expected to produce a cyclopropyl derivative but, under the reaction conditions, the adduct rearranges spontaneously to yield a 1,6-methanodibenzo[b,/]cyclo-prop [J]azepine, the structure of which was confirmed by X-ray crystallography [17]. 

Another feature of carbenoid-type reactivity is the cyclopropanation (reaction c). Again, this reaction does not only take place in vinylidene but also in alkyl carbenoids . On the other hand, the intramolecular shift of a /3-aryl, cyclopropyl or hydrogen substituent, known as the Fritsch-Buttenberg-Wiechell rearrangement, is a typical reaction of a-lithiated vinyl halides (reaction d) . A particular carbenoid-like reaction occurring in a-halo-a-lithiocyclopropanes is the formation of allenes and simultaneous liberation of the corresponding lithium halide (equation 3). ... 

Allen estimated that conjugation of a cyclopropane with an attached substituent was about 70% as effective as conjugation with a double bond45. This estimate is consistent with the 60% figure derived by Pete on the basis of an analysis of the UV spectra of cyclopropyl-containing systems53. 

Scheme 5. Formation of dihydroquinones from cyclopropyl allenes 13.

Cyclopropyl-substituted allenes open the door to yet another reaction mode. When treated with aryl iodides in the presence of a typical Heck-catalyst system and a dienophile, cyclohexene derivatives 77 were obtained (Scheme 11) [53,54]. Thus, the initially formed arylpalladium iodide car-bopalladates 72 to form a a-allylpalladium intermediate 73. It swiftly undergoes the cyclopropylcarbinyl to homoallyl rearrangement yielding the ho-moallylpalladium species 74 which finally suffers /1-hydride elimination. The thus formed 2-aryl-1,3,5-hexatrienes 75 are prone to undergo polymerization, but can be efficiently trapped by an appropriate dienophile at the least steri-... 

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>. 

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