Stereoselective addition of lithioethyl acetate to Boc-l-prolinal. A convenient chiral synthetic building block for the pyrrolizidine alkaloid ring system

Optically active N-protected .alpha.-amino aldehydes in organic synthesis

“…Aldol condensation of 53 with lithiated ethyl acetate produced a 4:1 mixture of diastereoisomeric alcohols syn-54 and anti-55 (Scheme 15). 49 This degree of diastereoselectivity is substantial when compared with the results of a similar aldol condensation with the acyclic aldehyde 33 (cf. Scheme 13).…”

Structure Revision and Syntheses of Epohelmins A and B

“…Kitahara obtained mixtures in the addition of lithium enolates to N -Cbz-phenylalaninal; the zinc enolate added stereoselectively . Addition of zinc and lithium enolates of ethyl acetate to N -Boc-prolinal afforded 2:1 to 4:1 mixtures of isomers favoring the Felkin−Anh product …”

Samarium Iodide-Mediated Reformatsky Reactions for the Stereoselective Preparation of β-Hydroxy-γ-amino Acids: Synthesis of Isostatine and Dolaisoleucine

“…However additions of achiral lithium-enolates to ( S )-prolinal derivatives have also been shown to be highly diastereoselective. 16 To determine whether the selectivities observed with the Sm II -mediated Reformatsky reactions to form products 13 and 15 are derived from the chirality of the Sm III -enolate (ie. auxiliary control) or from preferential approach of a Sm III -enolate to a low energy conformation of the aldehyde (ie.…”

“…auxiliary control) or from preferential approach of a Sm III -enolate to a low energy conformation of the aldehyde (ie. Felkin addition), 16 we coupled N -Boc-( S )-prolinal (6) to both α-chloroacetyloxazolidinones ( R )-12 and ( S )-12 using the above reaction conditions (Scheme 4). …”

“…It should be noted that chiral induction by α-amino aldehydes generally favors the erythro products. 16 This preference accounts for the difference in selectivity observed in the formation of threo product 17 (dr 5:1) vs. erythro product 19 (dr 10:1). Utilizing the appropriate chiral auxiliary can enhance this preference ( erythro : threo = 10:1 vs. 4:1 16a ) or reverse it ( erythro : threo = 1:5).…”