Diels-Alder reactions heteroatom, enantioselectivity

In most of the successful Diels-Alder reactions reported, dienes containing no heteroatom have been employed, and enantioselective Diels-Alder reactions of multiply heteroatom-substituted dienes, e.g. Danishefsky s diene, are rare, despite their tremendous potential usefulness in complex molecular synthesis. Rawal and coworkers have reported that the Cr(III)-salen complex 15 is a suitable catalyst for the reaction of a-substituted a,/ -unsubstituted aldehydes with l-amino-3-siloxy dienes [21] (Scheme 1.28, Table 1.12). The counter-ion of the catalyst is important and good results are obtained in the reaction using the catalyst paired with the SbFg anion. 

Another example of primary bromide formation in which the halide addition occurs away from the heteroatom was reported in the work by Stoodley and co-workers.This work reported the functionalization of a precursor to 47t components for enantioselective Diels-Alder reactions. 

Utilization of heteroatom-substituted dienes in enantioselective Diels-Alder reactions was investigated by Rawal, who reported that both chromium(III) and cobalt(III) salen complexes are effective catalysts [103, 104]. The use of chromium(III) salen complex 197 [105] in the cycloaddition between azadiene 195 and aldehyde 196 is shown in Scheme 17.27. This provided endo product 198 in 96% ee as a single diastereomer. Subsequently, cycloadduct 198 was transformed into the Aspidosperma alkaloid (-i-)-taberso-nine (199) [104]. 

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