A Lewis acid dependent asymmetric Diels–Alder process in the cyclization of new chiral acrylamides with dienes

Abstract

Diels–Alder cycloadditions of chiral acrylamides with cyclopentadiene proceed with high diastereofacial selectivity, giving either endo-R to endo-S products depending of the Lewis acid used.

References

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7. In a typical experimental, a Lewis acid (1 mmol) was added to a solution of 1(0.5 mmol) in CH₂Cl₂(5 ml) under N₂. After stirring 10 min, 4(5 mmol) was added. The reaction mixture was stirred while following the reaction by TLC, quenched with 1 M HCl solution, and then extracted with CH₂Cl₂ three times. The organic layer was dried over anhydrous MgSO₄ and concentrated in vacuo. The endo configurations were determined by the known iodolactonizations of 6a–8a with I₂ in DMF [8a lactone: [α]_D— 110.6 (c 1.0, CHCl₃)][ref. 5(b)]. The ratio of endo-R and endo-S was determined by HPLC analysis using a chiral column (Daicel OD, PrⁱOH–n-hexane 1 : 9).
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9. (a) The ¹³C NMR spectrum of the mixture of 6a and SnCl₄(1 : 1) was taken to show the significant chemical shift changes of the acrylamide carbonyl carbon (δ 163.8) and ester carbonyl carbon (δ 171.7) to δ 169.6 and 175.0, respectively, which support formation of a seven-membered ring complex between 6a and SnCl₄. (b) In the case of 6a–Et₂AlCl (1 : 2) no significant chemical shift changes for the two carbonyl carbons could be observed.