Allenyl groups

Very recently, Cook and Danishefsky [24] reported an interesting regioselectivity of intramolecular Diels-Alder reactions reversed by the change in the dienophihc moieties from vinyl to allenyl group (Scheme 19). For R = 2-propenyl group, C is bonded to the methyl substituted carbon Cj of the cyclohexadienone ring. For R = 2,3-butadienyl, C is bonded to Cy... 

The observed /runfavorable interaction between the trimethyl-silyl group and the allenyl group (Scheme 72). 

Allenylcobaloximes, e.g. 26, react with bromotrichloromethane, carbon tetrachloride, trichloroacetonitrile, methyl trichloroacetate and bromoform to afford functionalized terminal alkynes in synthetically useful yields (Scheme 11.10). The nature of the products formed in this transformation points to a y-specific attack of polyhaloethyl radicals to the allenyl group, with either a concerted or a stepwise formation of coba-loxime(II) 27 and the substituted alkyne [62, 63]. Cobalt(II) radical 27 abstracts a bromine atom (from BrCCl3) or a chlorine atom (e.g. from C13CCN), which leads to a regeneration of the chain-carrying radical. It is worth mentioning that the reverse reaction, i.e. the addition of alkyl radicals to stannylmethyl-substituted alkynes, has been applied in the synthesis of, e.g., allenyl-substituted thymidine derivatives [64],... 

The effect of methyl and phenyl substituents in the allyl and allenyl group on the rate of acidolysis is evidently complex. 

As already discussed above several C-nucleophiles also add smoothly to y-lactols (Eq. 85, Table 9). BF3 is the promotor of choice allowing efficient and highly diastereo-selective reactions with allyl and cyanotrimethylsilane or with bis(trimethylsilyl)ace-tylene which introduce the corresponding substituents at C-5 of the methyl tetra-hydrofuran-3-carboxylates 204, In the case of propargyltrimethylsilane an allenyl group is transferred to the heterocyclic 93). 

Intramolecular carbometallation of active methylene compounds having allenyl group proceeds in a complete regioselectivity in the presence of SnCl4 and Et3N to give a cyclohexene derivative (Equation (109)).278... 

This is readily done by isopropylidene removal, diol cleavage, and formation of the anomeric p-nitrobenzoate 315. Axial introduction of the allenyl moiety yields stereoselectively, as predicted, the a-C-manno derivative 316 after chair inversion. Degradation of the allenyl group to... 

D. Carbon-Hydrogen Activation as Source of Allenyl Groups.. .. 73... 

In summary, there is a wide range of chemical shift values for Ca, C, and of the allenyl ligands. However, the C chemical shifts of individual carbon atoms of the allenyl group are sensitive to whether they are (i) coordinated or uncoordinated (e.g., 17 versus 17 ) (ii) the specific bonding... 

C. Reactions of Cluster-Bound Allenyl Groups I. Adduct Formation... 

Kitagawa. O., Suzuki, T., Fujiwara, H., Fujita, M., and Taguchi, T., Alkaline metallic reagent-catalyzed hydrocarbocyclization reaction of various active methine compounds having an unactivated 4-aIkynyl or allenyl group. Tetrahedron Lett., 40, 4585, 1999. 

Pauson-Khand reaction of alkynyl ketones in which an allenyl group is extended further from the a -position is intriguing. It has been found that one of the double bonds of the allene unit can be selected to participate by using certain transition metal catalysts besides modification of the substrates. ... 

In some situations, bicyclization of polyenynes leads to hydrocarbon products and arenes containing an allenyl group in a sidechain are subject to cycloisomerization. 

Hydroamination. Formation of the A-Boc derivative of 3-vinyl-5-phenylisoxazoli-dine from an O-alkylhydroylamine that contain a p-allenyl group paves the way toward elaboration of sedamine. The intramolecular hydroamination is a high-yielding reaction. 

Reductive cyclization. Epoxides substituted with a carbon chain terminated (at a proper length) in an allenyl group are converted into cyclic products containing vicinal hydroxymethyl and vinyl substituents. 

CycUzations. Activation of an allenyl group with SnCLt and linkage to a C-nucleophile constitute a ring formation process. The reaction of Al-arylaldimines with allylsilanes (or allylgermanes) in the presence of SnCl, leads to 2,4-disubstimted 1,2,3,4-tetrahydro-quinolines due to rapid interception of the carbocationic intermediates. 

Allen-l-ols also undergo arylative fragmentation (Eq. 31) [53]. It has been proposed that the insertion of the allenyl group into an arylpalladium species... 

We have also observed that under the catalysis of Pd(PPh3)4, the reaction of 3,4-allenyl toluenesulfonamides with aromatic halides afforded 2,3-dihydro-pyrroles 41 or a mixture of vinylic azetidines and 1,2,3,6-tetrahydropyridines, depending on the substituent at the 3 position of the 3,4-allenyl group (Scheme 18)... 

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