159299-39-5Relevant academic research and scientific papers
Stereodivergent Hetero-Diels-Alder Reactions of Chiral 1-Oxa-1,3-butadienes through a Conformational Switch induced by Lewis Acids
Tietze, Lutz F.,Schneider, Christoph,Grote, Andrea
, p. 139 - 148 (2007/10/03)
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 Me2AlCl gives predominatly 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 1a in thepresence of Me2AlCl 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 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 ethers 1b-g were used, some of which afforded excellent induction.Mechanistic considerations are used to explin the results. - Keywords: asymmetric syntheses; Diels-Alder reactions; dihydropyrans; Lewis acids; oxabutadienes
