Heteroatom substituted diene

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. 

These results can be interpreted as being due to the reversible formation of the 3-substituted dehydrohydantoins by way of the 5-carbonium ions. Reaction of the dehydrohydantoins or the ions with dienes is slower than reaction with methanol but is irreversible. This system is useful for the preparation of certain cycloadducts of dehydrohydantoins. Its limitations result from the presence of methanol and acid, which preclude observation or isolation of the dehydrohydantoin and the utilization of most heteroatom-substituted dienes. 

Dienes, including cyclobutadiene and heteroatom-substituted dienes, e.g. vinyl ketones... 

Table 6 Influence of High Pressure on the Diels-Alder Addition of Heteroatom-substituted Dienes to Acrylates...

Ring-closing metathesis of heteroatom-substituted dienes 06H(70)705. 

Reactions of Achiral Carbonyl Dienophiles with Chiral Heteroatom-substituted Dienes... 

Azo compounds, such as A-phenyl- or iV-methyltriazoline-dione, have been applied as dienophiles in thermal asymmetric hetero Diels-Alder reactions with heteroatom-substituted dienes, leading to cycloadducts with excellent diastereoselectivities. 

Heteroatom-substituted dienes can be used to prepare amino acids. Azadiene 7.202120 reacted with 7.202, 21 for example, to give 7.204 [methyl 3-(N-Boc... 

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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