Aminoalcohol benzyl-protected

Petrini and co-workers used the bis(methoxymethyl)-protected nitrone 150, also derived frern L-tartrate, as an electrophile rather tfaiui as a 1,3-dipole (Scheme 21, top line) (89). In their key step, reaction with 4-ben loxybutylmagnesium bromide gave the cyclic hydroxylamine 151 in 82% yield (de 90%). Transfer hydrogenation with ammonium formate and a palladium catalyst cleaved both the hydroxylamine and the benzyl ether, affording the aminoalcohol 152. Cyclization via the corresponding primary chloride created the protected indolizidine 153, acidic hydrolysis of t ch completed this short synthesis of ( + )-132 in 16%... 

The progress has been much slower with acyclic systems. However, the trans-metallation of N-benzyl-N-Cbz- [350], N-Boc-N-methyl- [351], and N-Boc-N-tertbu-tylthiomethyl- [352] protected aminotins provided the corresponding amino-lithium derivatives that may be trapped with electrophiles with complete retention of configuration. This chemistry was applied to the preparation of /3-aminoalco-hols, N-methyl-/3-aminoalcohols and N-methyl-/3-amino acids. It can be noted that the preparation of enantio-enriched a- or /3-amino carbanions can be achieved starting from linear racemic precursors when the tin-to-lithium exchange is done in the presence of (-)-sparteine [353]. 

---