Allenes, cycloadducts stability

Epoxidations of chiral allenamides lead to chiral nitrogen-stabilized oxyallyl catioins that undergo highly stereoselective (4 + 3) cycloaddition reactions with electron-rich dienes.6 These are the first examples of epoxidations of allenes, and the first examples of chiral nitrogen-stabilized oxyallyl cations. Further elaboration of the cycloadducts leads to interesting chiral amino alcohols that can be useful as ligands in asymmetric catalysis (Scheme 2). 

The reaction with silyl enol ethers 3f and 3g gave only the [3 + 2] cycloadducts in comparison with effective formation of acyclic adduct 15 in the reaction with ketene silyl acetals 3a and 3e at lower reaction temperature. This can be explained by the reactivity of cationic intermediates 19 the intermediates from 3f and 3g are more reactive owing to lower stabilization by the oxy group than those from 3a and 3e, and react with the internal allene more efficiently to give the cycloadduct(s). Cyclic product 17a could be obtained at higher temperature via the reaction of 3a (entry 2). 

Allene carboxylate 144a afforded the product 145 through an intramolecular [4+2]-cycloaddition [123], On the other hand, allene carboxylate 144b gave the [2 + 2]-cycloadduct 146 exclusively. The switching of the reaction pathway is ascribed to the conformational stability of the transition state. 

The reaction between isocyanic acid and allenes was also studied and showed different regiochemistries depending on the substitution pattern (Scheme 35). Thus, in the simplest case (not studied experimentally), the formation of the 4-substituted cycloadduct (141) is predicted to be strongly favored, mainly because of the lower distortion of the allenic moiety at the transition structure (140). In contrast, substituted allenes such as 1,3-dimethylallene form the 3-alkylidene derivative (143) via a more stabilized transition structure like (142), in good agreement with the experimentally observed regiochemistry[113-115]. 

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). ... 

---