Hetero-Diels-Alder reaction 2.3- dimethyl-1,3-butadiene

The hetero-Diels-Alder reaction of chiral sulfines 24, bearing the relatively cheap (S)-proline as chiral auxiliary, with 2,3-dimethyl-l,3-butadiene (17) proceeds with low to moderate diastereoselectivity66. 

The diastereoselective hetero-Diels Alder reaction of imines leads to enantiomerically pure 2-substituted piperidines, important synthons for the synthesis of nitrogen-containing natural products. 2,3,4,6-Tetra-0-pivaloyl-/ -D-galactopyranosylimines 1, easily synthesized from the respective 1-galactosamine. react with isoprene (2a), 2,3-dimethyl-l,3-butadiene (2b) and ( )-l-methoxy-3-trimethylsiloxy-l,3-butadiene (4) under zinc chloride etherate catalysis71. Adducts 3 are obtained with moderate to good diastereoselectivities, while adduct 5 is produced with a diastereomeric ratio of greater than 95 5. Adduct 5 can then be converted into the alkaloid (S)-anabasin. 

H. Junge, G. Oehme, Hetero Diels-Alder reaction between 2,3-dimethyl-l,3-butadiene and perfluorooctanonitrile under high pressure. Tetrahedron 54 (1998) 11027-11032. 

Using a diene, the reaction can proceed through the ene pathway as above, or through the hetero-Diels-Alder pathway. For example, the condensation of 2,3-dimethyl-1,3-butadiene 103 with glyoxylate esters 99, 104, and 105 can proceed to form either a hetero Diels-Alder cycloadduct 106 or an ene product 107 (Fig. 9.35). 

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