Cyclopentadiene, asymmetric intramolecular

The chiral dialuminum Lewis acid 14, which is effective as an asymmetric Diels-Alder catalyst, has been prepared from DIBAH and BINOL derivatives (Scheme 12.12). " The catalytic activity of 14 is significantly greater than that of monoaluminum reagents. The catalyst achieves high reactivity and selectivity by an intramolecular interaction of two aluminum Lewis acids. Similarly, the chiral trialuminum Lewis acid 15 is quantitatively formed from optically pure 3-(2,4,6-triisopropylphenyl)binaphthol (2 equiv) and MeaAl (3 equiv) in CH2CI2 at room temperature (Scheme 12.12). " The novel structure of 15 has been ascertained by NMR spectroscopic analysis and measurement of the methane gas evolved. Trinuclear aluminum catalyst 15 is effective for the Diels-Alder reaction of methacrolein with cyclopentadiene. Diels-Alder adducts have been obtained in 99% yield with 92% exo selectivity. Under optimum reaction conditions, the... 

Applications of chirally modified titanium Lewis acids have been reported most cases use various acetal diols derived from tartrate as the chiral auxiliary26 33,31 90. Various methods of catalyst preparation are known, as well as the use of different types of dienes (open-chained, cyclopentadiene) and dienophiles (acroleins, acrylates, crotonates, fumarates and amides derived from oxazolidinone), including intramolecular cycloaddition30. Addition of 4 A molecular sieves can improve asymmetric induction31,34 (as observed with the Sharpless epoxidation, loc. cit 31 in ref 6) and shows remarkable solvent effects on enantioselectivity. This method has been applied to the asymmetric Diels-Alder cycloaddition of cyclopentadiene and open-chain dienes to acrylamides28, 35. 

The use of amides and imides as chiral auxiliaries in [4 + 2]-cycloadditions has been the subject of numerous investigations. Oppolzer reported the use of camphorsultam 112 in asymmetric Diels-Alder reactions [26, 67. 68], The camphor sultam is readily available in both enantiomeric forms from cam-phorsulfonyl chloride. Acrylate 113 was found to react with cyclopentadiene in the presence of EtAlCl2 to give 115 with high selectivity (endo/exo = 99.5 0.5, dr = 97.5 2.5, Equation 10) [67]. Oppolzer also reported that triene 116 participated in an intramolecular Diels-Alder cycloaddition reaction to afford endo product 117 in 71 % yield and 96.5 3.5 dr (Scheme 17.19) [69]. This bicyclic product was subsequently converted into the pyridine alkaloid (-)-pulo upone (118). 

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