Ephedrine Diels-Alder reactions

Other Applications. Chiral oxazaborolidines derived from ephedrine have also been used in asymmetric hydroborations, and as reagents to determine the enantiomeric purity of secondary alcohols. Chiral l,3,2-oxazaborolidin-5-ones derived from amino acids have been used as asymmetric catalysts for the Diels-Alder reaction,and the aldol reaction. ... 

Resolution of 42 was accomplished by traditional methods. Conversion of 42 to hydroxy acid 53 was followed by resolution as its ephedrine salt. Variants of the Diels-Alder reaction were developed. For example, use of thallium cyclopentadienide (rather than sodium cyclopentadienide) provided a more stable, less basic nucleophile for use in generation of 37. 

Reports on general chiral auxiliaries for the diastereoselective hetero-Diels-Alder reaction have been more sporadic than those of chiral auxiliaries developed for the parent Diels-Alder reaction (Section 17.4). Tietze demonstrated that an (-)-ephedrine-derived auxiliary functioned as an excellent controller group in the Et2AlCl-catalyzed inverse-electron-demand cydoaddition reaction of enone 216 (Scheme 17.30) [112]. After the formation of 217 (dr >98 2), the auxiliary could be readily excised by hydrolysis to give chiral lactone 218. 

Schreiber utilized oxazolidine 220, derived from ephedrine, as an auxiliary for asymmetric hetero-Diels-Alder reactions (Scheme 17.31) [113]. In an experiment that anticipates subsequent developments in organic catalysis, condensation of 220 with aldehyde 219 led to covalent attachment of the auxiliary to the substrate, generating enamine 221. The stage was thus set for intramolecular cycloaddition, which afforded 222 as a 17 1 mixture of diaster-eomers. The dihydropyran 222 served as a key intermediate in the synthesis of the macrolide antibiotic brefeldin C (223). 

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