Allenes, cycloadducts isomerization

Nitrile oxides also undergo [3 + 2]-cydoaddition with allenes. As shown below, a variety of products were formed depending on the substituents of both nitrile oxide and allene [72b, 79]. The cycloadduct 80 was slowly converted to the isomeric isoxazole 81 in the presence of ZnCl2 via a sigmatropic rearrangement [79a]. 

In a similar manner, Brummond et al. demonstrated the first total synthesis of 15-deoxy-A12,14-prostaglandin J2 (162) that was completed using a silicon-tethered allenic Pauson-Khand reaction to obtain the highly unsaturated cyclopentenone substructure [36]. Treatment of alkynylallene 160 with molybdenum hexacarbonyl and dimethyl sulfoxide affords the desired cycloadduct 161 in 43% yield (Scheme 19.30). Trienone 161 was obtained as a 2 1 Z E mixture of isomers in which the Z-isomer could be isomerized to the desired E-isomer. The silicon tether was cleaved and the resulting product converted to 15-deoxy-A12,14-prostaglandin J2 (162). 

Another reactive ketene which undergoes efficient cycloaddition with allenes is tert-butyl-cyanoketene (TBCK). Again mixtures of isomeric cycloadducts are obtained when unsymmet-rically substituted or 1,3-disubstituted allenes are employed.10-12... 

The same cyclopropanone gives a cycloadduct with furans—this must surely be a reaction of the oxyallyl cation and we can conclude that the three isomeric reactive intermediates (allene oxide, cyclopropanone, and oxyallyl cation) are all in equilibrium and give whichever product is appropriate for the conditions. 

Kiefer and Okamura 73> demonstrated stereoselectivity with respect to the olefinic component in an allene-olefin cycloaddition. 1,1-Dimethyl-allene with dimethyl fumarate at 160—180 °C gave two cycloadducts, both retaining the trans disposition of ester functions with better than 99% stereoselectivity dimethyl maleate gave the isomeric cis adducts with... 

The IMDA-Grob fragmentation strategy that starts with an allene cycloaddition reaction is illustrated in Scheme 43. The allene-furan cycloadduct was formed together with the other exoadduct which had the opposite configuration at the i-propyl group. Partial isomerization of the other product into the thermodynamic product shown and conversion into the 10-membered ketone constituted a formal synthesis of periplanone-B <88TL650l> since that ketone had been used in an... 

A one-step Lewis acid-catalysed intermolecular 4- -3-cycloaddition of aromatic a,)3-unsaturated aldehyde and ketones (105) with epoxides (106) formed seven-membered oxacycles (107) under mild conditions (Scheme 34).The effect of oxygen-, sulfur-, and halogen-substituents on the reactivity of nitrogen-stabilized oxyallyl cations in 4- -3-cycloaddition reactions has been extensively investigated. Aza-oxyallyl cationic intermediates react with cyclopentadiene and furan via an aza-4 -I- 3-cycloaddition reaction to form bicyclic cycloadducts in moderate yields. The intramolecular formal 4- -4-cycloaddition of conjugated enynes with an e-deflcient cyclobutene (108) yielded a strained six-membered cyclic allene (109) that isomerized to a 1,3-cyclohexadiene (110). This intermediate underwent a thermal or acid-promoted six-electron electrocyclic ring opening to yield a 2,4,6-cyclooctatrienone (111) (Scheme 35). ... 

Heating of the cycloadduct (R = CeHn) in pyridine causes formation of an isomeric cyloadduct resulting from addition across the C=N bond. A small amount of this isomer is also formed in the reaction with of the titana allene with t-butyl isocyanate. 

The 2,3-dihydro-4-/7-pyran-4-ones 189 react with allene stereospecifically to give two isomeric cycloadducts 190 and 191. ... 

Using styrene to trap the strained allene 217 produces a 10 1 mixture of exo and endo-8-phenyl-3-oxabicyclo[4.2.0]oct-5-ene in 53 % yield. The cycloadduct of 217 and fiiran is obtained in 21 % yield. An isomeric strained heterocyclic allene is also trapped with... 

When malononitrile is reacted with allenes in the presence of palladium catalysts, insertion into the C-H bond is observed with formation of a linear adduct Also, phosphine catalysts are used in an asymmetric [3+2] cycloaddition of carbo-ethoxyallene 302 with enones to give the isomeric cycloadducts 303 and 304 in 54-87 % yield (% ee in 303, the major product, 87-89. ... 

Intramolecular palladium-catalyzed [3+2] cycloaddition reactions of the allenes 327 across the cyclopropyl substituted double bonds afford two isomeric cycloadducts 328 and 329 in high combined yields . ... 

In the reaction of allenes with dienes, such as cyclopentadiene, [4+2] cycloadducts are obtained. From allene at 200-230 °C only a 49 % yield of the [4+2] cycloadduct is obtained, while fluoroaUene reacts at 0 °C for 4 days to give a 99 % yield of a mixture of two isomeric cycloadducts in a ratio of 1 1. In contrast, difluoroallene reacts even faster with cyclopentadiene (1 min at -20 °C) to give the [4+2] cycloadduct in 99 % yield. In contrast, 1,3-butadiene, on reaction with difluoroallene at 110°C, affords mixtures of the [4+2]... 

In vinylallenes with a pendant phenyl or vinyl substituent eight-membered ring carbo-cyclic byproducts are formed by a head-to-head [4+4] cycloaddition From 1,2,4-cyclohexatriene and 2,5-dimethylfuran or fiiran, [4+2] cycloadducts are also obtained. Similarly, trapping of the strained allene 366 with diphenylbenzofuran affords the two isomeric [4+2] cycloadducts 367 and 368 in low yields... 

Craft and Gung developed a paUadium-catalyzed transannular [4+3] cycloaddition route in which all of the rings of cortistatins are prepared in one step from a single macrocyclic precursor (Scheme 19.50) [114]. Exposure of macrocyclic allene 233 to a catalytic amount of palladium (II) acetate in the presence of excess lithium bromide resulted in the formation of 238 as a single isomer in 37% yield. This is the first report of a transannular [4+3] cycloaddition. The proposed mechanism is shown in Scheme 19.50. The formation of allene-palladium complex 234 affords a a-allylpalladium intermediate, which rapidly isomerizes to the 7i-allylpalladium intermediate 235. This can then undergo intramolecular cycloaddilion via an endo (compact) transition strucmre 236 to give bromonium ion 237. The loss of a proton results in the formation of the observed product 238. Cycloadduct 238 was readily converted into the tetracyclic core skeleton of cortistatins 239 by selective reduction of the olefin formed by cycloaddition with furan, followed by reductive debromination. 

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