Diastereoselective 1,3-dipolar cycloadditions of chiral derivatives of 2-oxoethanenitrile oxide to noncyclic conjugated symmetrical alkenes

Asymmetric 1,3-dipolar cycloadditions of chiral derivatives of the nitrile oxides 3a-3c derived from (2R)-bornane-10,2-sultam, (2R)-10-(dicyclohexylsulfamoyl)isoborneol, and (1R)-8-phenylmenthol, to either (E)-stilbene 4 or dimethyl fumarate 5, leading to the corresponding 4,5-dihydroisoxazoles 6a-6c and 7a-7c in both moderate yields and diastereoselectivities, are presented. All cycloadducts were converted into the corresponding methyl esters 8 and 9. which were used for determination of their enantiomeric purities via chiral HPLC analyses. In the case of both stilbene cycloadducts 6a and 6b, their absolute configurations were determined by X-ray crystal-structure analyses. These [3+2] cycloadditions suggest the participation of the thermodynamically less stable SO2/CO syn-conformer in the pi(y) approach along the C=O bond of the linear nitrile oxide 3a.