Simple and Induced Diastereoselectivity in Intramolecular Hetero-Diels-Alder Reactions of 1-Oxa-1,3-butadienes. Experimental Data and Calculations

Article abstract of DOI:10.1021/jo00080a030

The simple and induced diastereoselectivity of the intramolecular hetero-Diels-Alder reaction of the alkylidene 1,3-dicarbonyl compounds 7a-e, obtained in situ by Knoevenagel condensation of the aldehydes 5a-e and dimethylbarbituric acid 6 was investigated experimentally and theoretically.In all examples the selectivity is very high; thus the domino reaction of 5a and 6 gives nearly exclusively the trans-fused tricyclic dihydropyran 8 (trans/cis = 98.8:1.2).In the similar reactions of the chiral aldehydes 5b-e and 6 the trans cycloadducts 28, 33, 36, and 41 are the main products out of four possible diasteremers.The induced diastereoselectivity for the reaction of 5b and 5e with 6 is > 99:1 and for the reaction of 5c and 5d with 6, 95.2:3.6 and 94.1:4.7, respectively.Semiempirical AM1 calculations have been used to locate the transition structures of the intramolecular hetero-Diels-Alder reaction of 7a.The results were employed to create new parameters for the MM2-type force-field to determine the transition structures of 7b-e.The obtained data were compared with the experimental results of the cycloadditions and showed an excellent agreement.