STEREOSPECIFIC REARRANGEMENT OF OPTICALLY-ACTIVE TERTIARY ALLYLIC EPOXIDES TO GIVE OPTICALLY-ACTIVE QUATERNARY ALDEHYDES - SYNTHESIS OF ALPHA-ALKYL AMINO ALDEHYDES AND ACIDS

2-Methyl-2-vinyl-3-alkyloxiranes, readily obtained from Sharpless-Katsuki asymmetric epoxidation of allylic alcohols, undergo facile 1,2-alkyl migration with inversion of configuration leading to 2-methyl-2-vinylalkanals, thereby establishing an acyclic quaternary carbon in high yield and optical purity. The reaction conditions necessary for rearrangement are generally quite mild, e.g., BF3 . OEt(2) at -78 degrees C for 2 min, 5 M LiClO4 in refluxing ether, anhydrous Zn(OTf)(2) or ZnCl2, EtAlCl(2), silica gel, and sonication. As an application of this methodology, and to prove the stereochemical course of the process, the synthesis of (S)-(-)-alpha-methylphenylalanine [(+)-16a] is described. This methodology also permits access to optically active N-protected amino aldehydes [i.e., (-)-15a and (-)-15b], compounds which are difficult to make by other routes. The key step in each case is the rearrangement of (-)-8a or (-)-8b to give the quaternary aldehydes (+)-9a or (+)-9b in good yield and optical purity.