Aldehydes nitrile oxide intermolecular cycloadditions

I.3.4.2. Intermolecular Cycloaddition at C=X or X=Y Bonds Cycloaddition reactions of nitrile oxides to double bonds containing heteroatoms are well documented. In particular, there are several reviews concerning problems both of general (289) and individual aspects. They cover reactions of nitrile oxides with cumulene structures (290), stereo- and regiocontrol of 1,3-dipolar cycloadditions of imines and nitrile oxides by metal ions (291), cycloaddition reactions of o-benzoquinones (292, 293) and aromatic seleno aldehydes as dipolarophiles in reactions with nitrile oxides (294). 

For intramolecular 1,3-dipolar cycloadditions, the application of nitrones and nitrile oxides is by far most common. However, in increasing frequency, cases intramolecular reactions of azomethine ylides (76,77,242-246) and azides (247-259) are being reported. The previously described intermolecular approach developed by Harwood and co-workers (76,77) has been extended to also include intramolecular reactions. The reaction of the chiral template 147 with the alkenyl aldehyde 148 led to the formation of the azomethine ylide 149, which underwent an intramolecular 1,3-dipolar cycloaddition to furnish 150 (Scheme 12.49). The reaction was found to proceed with high diastereoselectivity, as only one diaster-eomer of 150 was formed. By a reduction of 150, the proline derivative 151 was obtained. 

Modifying the aldehyde function in the pyrroles 1162 into 1,3-dipoles 1163 (nitrone) and 1167 (nitrile oxide) furnished tricyclic heterocycles 1164 and 1168 via intramolecular 1,3-dipolar cycloaddition reactions (Scheme 226) <2000T3013>. None of the isomeric bridged product 1165 is produced despite the preference for that regiochem-istry in the intermolecular reaction. Generated in situ (from the oximes 1166) nitrile oxides 1167 cyclized spontaneously to the dihydroisooxazole 1168 in quantitative yield at room temperature. 

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