Oxazoline derivatives tagged with tosylated amino acids as recyclable organocatalysts for enantioselective allylation of aldehydes

Abstract

A series of amino acid-based oxazoline compounds have been prepared and successfully applied to the enantioselective allylation reaction of aldehydes. The fine-tuning of the structure of the oxazolines led to (S,S)-4 as an efficient organocatalyst which gave homoallyl alcohols in good yield (up to 90%) and excellent ee (up to 99%) for a wide range of substrates including aromatic, hetero-aromatic and a,bunsaturated aldehydes. The chiral organocatalyst was synthesized in three easy steps with an overall 88% yield and successfully recycled for up to three cycles. On the basis of the experimental observations and NMR studies, a probable mechanism was proposed for this reaction.Recently we have shown that tosylated phenylalanine-based amides are efficient organocatalysts for the enantioselective allylation of aldehydes. Therefore, instinctively we thought of combining tosylated phenylalanine and oxazoline and synthesising organocatalyst (S,S)-1 in three simple synthetic steps. The same strategy was used to prepare the remaining

Scalable Synthesis of N‐Acetylated α‐Amino Acid‐Derived Oxazoline Ligands

“…A short survey of the literature revealed that the amide coupling and the ring closure reactions could be carried out separately , and this seems to be more effective for bulk synthesis of oxazolines. We have chosen N ‐Boc‐protected α‐amino acids as a starting material, which is more compatible with the “atom economy” concept, given that the Boc protection should be removed prior closing an acid‐sensitive oxazoline ring.…”

Organocatalysts Derived from Unnatural α‐Amino Acids: Scope and Applications

“…l-tert-Leucine-oxazoline sulfonamideorganocatalyst 175 synthesized by Ghosh et al [85] showedg ood catalytic activity in the reactiono fv arious aldehydes with allyltrichlorosilane 176, and gave the final products in good yields and with good-toexcellent enantioselectivity under mild reaction conditions (Scheme 34). In addition, recycling experiments showed that 175 workedw ell for three cyclesa nd was easily recoverable from the reaction medium by precipitation.…”

“…l ‐ tert ‐Leucine‐oxazoline sulfonamide organocatalyst 175 synthesized by Ghosh et al . showed good catalytic activity in the reaction of various aldehydes with allyltrichlorosilane 176 , and gave the final products in good yields and with good‐to‐excellent enantioselectivity under mild reaction conditions (Scheme ).…”