2-Aminoalcohols oxazolines

Scheme 15 Parallel synthesis of 2-oxazolines from carboxylic acids and aminoalcohols using PS-Mukaiyama reagent...

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

Since 1980 the interest in this reaction increased because enantiospecificity was introduced and much more valuable products could be made. A wide variety of ligands was tested, such as chiral dipyridines, phenanthrolines, diphosphines, aminoalcohols, bis-oxazolines, bis-oxazolines with a third donor atom in the centre, bis-thioureas, diamines, etc [33], In 1981 the highest e.e. reported was still only 20%. For many years the best results were obtained with chiral diimines and phenanthrolines but e.e. s were below 70% [34], Pfaltz introduced bis-oxazolines for this reaction and obtained e.e. s as high as 91% [35] in 1991. 

Rosen, A.A. Synthesis of Oxazoline-2-thiol from 2-Aminoalcohols. J. Am. 

It should be noted that the 2-oxazolines are connected directly with enamides, being their products of cyclization on the one hand ", as well as being the heterocyclic precursors of W-acylenamines on the other . However, it is no less important that the 2-oxazolines are cyclic derivatives (and precursors also) of the 1,2-aminoalcohols. In essence, the reactions 117 122 (equation 41) and 125- 128 (equation 43) are the transformations of ketones to 1,2-aminoalcohols, i.e. they are the reversal of one of the pinacol rearrangement variants, i.e. the Tiffeneau and MacKenzie reactions . These transformations which proceed via the iV-acyliminium intermediates 118, 126 are examples of a real retropinacol rearrangement according to structural, functional and redox features (see Section III.B.3). 

Scheme 11.8 Synthesis reaction and chiral N-donor ligands (1 P-aminoalcohol, 2-4 monooxazolines and 5-8 bis(oxazolines) used for the preparation of Ru nanocatalysts. Adapted from Ref [44].

Chaudret and coworkers have studied the activity of chiral aminoalcohol- and oxazoline-stabilized Ru colloids in the hydrogen transfer reaction (Scheme 11.8) [44]. The reduction of acetophenone has been studied using isopropanol as hydrogen source and preformed nanocatalysts under basic conditions and at room temperature (Table 11.10). 

Among the polymer-supported ligands, derived-aminoalcohol ones have been largely used in this reaction but few are derived either from A-sulfonamideaminoalcohols, or oxazoline or salen. 

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