Asymmetric Diels-Alder Reactions

Asymmetric Diels-Alder Reactions - CH 3 O Ph H 3 C H 3 C O...

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Unformatted text preview: CH 3 O Ph H 3 C H 3 C O O O I OBn HO BnO CH 3 O H 3 C H 3 C O AlCl 3 BnO O OR O OR O RO Et 2 AlCl CH 3 CH 3 CH 3 i-Bu 2 AlCl i-Bu 2 AlCl Et 2 AlCl AlCl 3 O O O O CH 3 H 3 C CH 3 CH 3 H 3 C CH 3 CO 2 R CO 2 R Chem 215 Asymmetric Diels-Alder Reactions Myers Reviews: Oppolzer, W. In Comprehensive Organic Synthesis ; Trost, B.M. and Fleming, I. Eds.; Pergamon: Oxford, 1991, Vol. 5, pp. 315-399. Kagan, H. B.; Riant, O. Chem. Rev. 1992 , 92 , 1007-1019. Reilly, M.; Oh, T. Org. Prep. Proceed. Int. 1994 , 26 , 131-158. Evans, D. A.; Johnson, J. S. In Comprehensive Asymmetric Catalysis ; Jacobsen, E. N.; Pfaltz, A.; Maruoka, K.; Yamamoto, H. In Catalytic Asymmetric Synthesis , Ojima, I., Ed.; Wiley-VCH: New York, 1993, Chapter 9, pp. 413-440. Kent Barbay Chiral Auxiliaries – Dienophiles: Helmchen, G.; Karge, R.; Weetman, J. In Modern Synthetic Methods ; Scheffold, R., Ed.; Springer- Verlag: Berlin, Heidelberg, 1986, Vol 4, pp. 262-306. (–)-8-Phenylmenthol: • Endo-selective cycloaddition is proposed to occur from the unblocked β-face of the s-trans acrylate-Lewis acid complex. • (–)-8-phenylmenthol, derived from (–)-pulegone, is commercially available. Recovery of the auxiliary was accomplished in 94% yield following reductive removal. • A favorable π-stacking interaction is proposed to enhance the stereoselectivity of this process. (Acrylates derived from menthol afford lower diastereoselectivity – ca. 40%). Corey, E. J.; Ensley, H. E. J. Am. Chem. Soc. 1975 , 97 , 6908-6909. Ensley, H. E.; Parnell, C. A.; Corey, E. J. J. Am. Chem. Soc. 1978 , 43 , 1610-1612. Dimenthyl Fumarate: From: Furuta, K.; Iwanaga, K. Yamamoto, H. Tetrahedron Lett. 1986 , 27 , 4507-4510. • The menthyl auxiliaries exhibit cooperative asymmetric induction in the case of the fumarate ester, resulting in excellent selectivity for cycloaddition from the back face. Diene Lewis Acid Temperature (°C) Yield de (%) –40 –20 –20 25 56 94 70 92 95 95 96 99 AlCl 3 , CH 2 Cl 2 –55 °C 89%, 97% de (Intermediate in prostaglandin synthesis) toluene –78 °C 100%, 99% de • Lewis Acid(s?) Yamamoto, H., Eds.; Springer: New York, 1999; Vol III, pp. 1177-1235. • The stereochemical model for chiral induction by the 8-phenylmenthol controller has recently been applied in the design of a practical auxiliary for asymmetric Diels-Alder reactions: Corey, E. J.; Sarakinos, G. Org. Lett. 1999 , 1 , 1741-1744. S O O O O H O OR , BCl 3 PhCH 3 , –78 °C 99% yield, 98.5 : 1.5 endo : exo, 97% de H O OR H OR O CH 3 H OR O CH 3 diene product endo : exo yield (%) de (%) 94 > 99 : 1 97 97-- 94 98-- 94 73 88 65 70 95 : 5 97 : 3 3 : 97 6 : 94 93 : 7 95 : 5 5 : 95 95 : 5 97 : 3 2 : 98 93 : 7 >99 : 1 97 : 3 <1 : 99 >99 : 1 99 : 1 <1 : 99 >99 : 1 N O O R' O R COX C R' N O O R O Bn CH 3 CH 3 O X C H H O X C X C N O O Bn N O O C 6 H 11 N O O H 3 C O R Al Et Et Et 2 AlCl 2 N O O H 3 C O H 3 C Al Et Et Ph Et 2 AlCl 2 N O O R' O H 3 C Ph COX C R' R H H CH 3 CH 3 H 3 C R X P O CH 3 R X P O H H O...
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Asymmetric Diels-Alder Reactions - CH 3 O Ph H 3 C H 3 C O...

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