Oxyallyl cation, Chiral

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). 

A number of different furan-based approaches to the synthesis of 7-membered ring systems were reported in 2003. In the novel example shown below, the furan participated in an intramolecular [4+3] cycloaddition with nitrogen-stabilized chiral oxyallyl cation to form polycyclic structures <03JA12694>. Attempts to construct the [4.4.1] bicychc BC ring system of ingenol via a type-II intramolecular [4+3] cycloaddition between furan and an oxyallyl cation produced the desired product but only in 14% yield <03JOC7899>. 

The [4-1-3] cycloaddition between 2-aminofuran and oxyallyl cations, followed by base-induced elimination of the resulting adducts, was used in the synthesis of 3-aminotropones <05TL8475>. A chiral Lewis acid-catalyzed [4t-3] cycloaddition between furans and nitrogen-stabilized oxyallyl cations derived from allenamides was developed. As depicted below, the Cj-symmetric salen-based ligand is the most effective in promoting this asymmetric cycloaddition reaction <05JA50>. 

Recently, Lautens, Aspiotis and Colucci extended the [4+3] cycloaddition methodology to include the diastereoselective intermolecular cycloaddition between an oxyallyl cation and a chiral furan [45]. The best results were obtained employing furan 26 bearing a free hydroxyl group in the 2-position, reacting with excess 1,3-dibromopentanone in the presence of diethyl zinc. Under the optimized conditions, up to 80% yield of the crystalline oxabicyclo[3.2.1]octene 27 was obtained with a diastereoselectivity of a 19 1. The other product was the minor diastereomer 28, Eq. 17. 

The chemistry of intramolecular [4+3J cycloaddition of nitrogen-stabilized oxyallyl cations derived from chiral allenamides, originally reported in 2003, was extended to the use of a furan tethered to either the a- or p-position of the allene. As demonstrated below, polycyclic compounds were synthesized in good yields and with a high diastereomeric ratio (d.r.)<04AG(E)615>. 

Hoffmann and coworkers further extended the scope of this reaction to chiral acetals to achieve asymmetric [4-1-3] cycloadditions [30]. Under similar Lewis acid catalysis, the homoacetal derivatives of 105 prepared from 2 equiv of chiral alcohols proceeded to cycloadditions, but this approach suffered from the difficulty of cleaving the chiral auxiliary from the cycloadduct, as well as being wasteful of 1 equiv of the chiral alcohol, which was eliminated. Hoffmann and coworkers then devised the use of mixed chiral acetals such as 117 as chiral oxyallyl cation precursors (Scheme 18.25). They demonstrated that under silyl triflate... 

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