Cyclopropane, Allenyl

Direct photolysis of l,l-dicyano-3,3-dimethyl-l,4,5-hexatriene 29 gives an allenyl-cyclopropane 30 via DPM rearrangement in 57% yield, while acetone-sensitization yields a hausan 31 via an intramolecular [2 + 2] cycloaddition (Scheme 4.15) [20]. 

Since cyclopropane rings possess electronic properties similar to those of double bonds and are capable of stabilizing an adjacent positive charge, systems such as 247 are related to allenyl substrates. Therefore, solvolysis of such... 

The cyclopropyl effect has controlled the regioselectivity of the cross-coupling reactions of propargylic/allenylic metallic species with electrophiles afford alkynic cyclopropanes (Scheme 9).70 Cyclopropyl ring strain, which makes the formation of vinylidenecyclopropanes unfavourable, is believed to control the regioselectivity. 

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. 

For chirality transfer via a furan ring transfer reaction of tetrahydrobenzofuranyl allenyl ethers and Claiscn rearrangement see refs 675 and 676. The synthesis of cyclopropanes via Claisen rearrangements of propargylic alcohol derivatives is reported in ref 677 and for acetylene Claisen rearrangements of thio derivatives see refs 678 and 679. 

Tandem coupling and cyclization. Functionalized allenes are converted to a-arylvinylated cyclopropanes and heterocycles including epoxides, tetrahydrofurans, and oxazolidinones. Furan derivatives are formed from allenyl ketones." ... 

Intramolecular domino reactions proceed smoothly to afford cyclic compounds [2]. Domino 5-exo cyclization of the carbonate 22 generated the allenyl intermediate 23, and 3-exo cyclization of 23 gave the cyclopropane 24, which was converted to a dimeric product 25 [3]. 

The copper-catalysed 3 + 2-cycloaddition reactions of nitrones with alkynes leading to / -lactams have been extensively reviewed. The 3+2-cycloaddition reactions of dialkyl-substituted 2-benzylidenecyclopropane-l,l-dicarboxylates (54) and C-carbamoyl nitrones (53) produced simple isomeric spiro[cyclopropane-l,4-isoxazolidine] cycloadducts (55), which are readily transformed into isoxazolidine-fused / -lactams (56) in high yields (Scheme 15). BINOL-derived chiral phosphorami-date Au(l) catalysts have been used to catalyse the 3+2-cycloaddition of A(-allenyl amides with nitrones to produce chiral 4-alkylidenyl isoxazolidines in high yields and excellent enantioselectivity (up to 99% cc). The 3+2-cycloaddition of a-phenylnitroethene and (Z)-CA -diphenylnitrone in polar media (nitromethane and water) yielded 3,4-fra 5 -2,3,5-triphenyl-4-nitroisoxazolidine via a zwitterionic, two-step mechanism. ... 

Selenium and tellurium ylides other than sulfonium have also been reported to be useful for the cyclopropanation. Selenium ylides were smdied by Kataoka and coworkers, and cyclopropane formation was observed. Diphenyl vinyl or allenyl selenium triflate serves as an electron-deficient alkene, and selenium ylides can be generated by the conjugate addition. The resulting yhdes underwent cyclopropane formation by an MIRC reaction (Scheme 1.31) [54]. 

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