Stereodivergent Hetero‐Diels‐Alder Reactions of Chiral 1‐Oxa‐1,3‐butadienes through a Conformational Switch Induced by Lewis Acids

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Abstract
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 Me2 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 1 a in the presence of Me2AlCl as promoter nearly exclusively yields the endo‐I adduct 16 a (16 a+17 a:18 a+19 a = > 50:1; 16 a:17 a = 60:1), whereas with TMS‐OTf the endo‐II‐product 17 a was obtained as the main component (16 a+17 a:18 a+19 a = > 50:1; 16 a:17 a = 1:7.9). The use of SnCl4 leads to a mixture of endo and exo, again, however, with excellent induced selectivity. A similarly good induction was obtained with the oxabutadiene 9 containing the new auxiliary 8. Also, other enol others 1 b–g were used, some of which afforded excellent induction. Mechanistic considerations are used to explain the results.

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