Stereodivergent hetero-Diels-Alder reactions of chiral 1-oxa-1,3-butadienes through a conformational switch induced by Lewis acids

The stereodivergent asymmetric hetero-Diels-Alder reaction of achiral and chiral 1-oxa-1,3-butadienes carrying an oxazolidine moiety with various enol ethers in the presence of different Lewis acids is described as a highly stereoselective and efficient approach to dihydropyrans, which can be used for the synthesis of carbohydrates. In the cycloaddition of the achiral oxabutadiene very good endo/ exo selectivity was possible, and with the chiral oxabutadienes excellent 1,6-asymmetric induction was additionally observed. In the processes a reversal of facial selectivity occurs by changing the Lewis acid, allowing the synthesis of both enantiomers of the dihydropyrans with the same auxiliary. Thus, cycloaddition of 1 to 2 in the presence of Me(2)AlCl gives predominantly the endo product 3 (3:4 = 10:1), whereas with SnCl4 the exo product 4 is obtained (3:4 = 1:15). The reaction of 7 and la in the presence of Me(2)AlCl as promoter nearly exclusively yields the endo-I adduct 16a (16a+17a:18a+19a = > 50:1;16a:17a = 60:1), whereas with TMS-OTf the endo-II-product 17a was obtained as the main component (16a+-17a:18a+19a = > 50:1; 16a:17a = 1:7.9). The use of SnCI, leads to a mixture of endo and exe, again, however, with excellent induced selectivity. A similarly good induction was obtained with the oxabutadiene 9 containing the new auxiliary 8. Also, other enol ethers 1b-g were used, some of which afforded excellent induction. Mechanistic considerations are used to explain the results.