Aldol reaction chiral auxiliaries, camphor

Acyliron complexes with central chirality at the metal are obtained by substitution of a carbon monoxide with a phosphine ligand. Kinetic resolution of the racemic acyliron complex can be achieved by aldol reaction with (1 R)-( I (-camphor (Scheme 1.14) [41], Along with the enantiopure (R, c)-acyliron complex, the (Spe)-acyliron-camphor adduct is formed, which on treatment with base (NaH or NaOMe) is converted to the initial (SFe)-acyliron complex. Enantiopure acyliron complexes represent excellent chiral auxiliaries, which by reaction of the acyliron enolates with electrophiles provide high asymmetric inductions due to the proximity of the chiral metal center. Finally, demetallation releases the enantiopure organic products. 

As with most camphor-based chiral auxiliaries, the size and steric congestion at the C-8 position of the camphor moiety can determine its efficiency. Through the formation of a connection between C-8 and C-2, a novel camphor-based oxazinone auxiliary 29, which can be prepared from camphor in 3 steps,52 becomes highly effective in directing stereoselective aldol reactions (Scheme 5.11).53... 

Aldol reactions using chiral auxiliaries are popular as the stereochemical outcome is usually highly predictable and, as such, they provide a reliable method for the incorporation of adjacent stereocenters. The oxazolidinone-based imides 36 and (ent)-36 are the most commonly employed, and these lead to syn aldol products with high levels of stereocontrol [20]. The reaction can be extended to include a variety of a-heteroatom functionality as in 37 (Scheme 9-13) [21]. Numerous examples of the use of these auxiliaries in the synthesis of polypropionate natural products have been reported. Many related auxiliaries are also available and the camphor-based sultam 38 is notable [22]. 

In the middle of the 198O s some silyl enolates derived from homochiral esters were reported to enable highly enantioselective synthesis of aldols [106]. Later, Oppolzer et al. disclosed the utility of camphor sultam as a chiral auxiliary for asymmetric aldol reactions [107]. Braun et al. have recently achieved high levels of asymmetric induction in the aldol reaction of ketones with homochiral silyl enolate 43 (Scheme 10.38) [108]. 

Asymmetric aldolization of a-isocyanoacetamide and fluorinated benzaldehydes has been realized with a gold(I) salt and a ferrocenyl amine-phosphine ligand. (Salen)-Ti complexes serve well in catalyzing the condensation of diketene with aldehydes. " A camphor lactam is an adequate chiral auxiliary as its derived imide undergoes asymmetric aldol reactions. 

Yan and covrorkers developed titanium enolate acetate aldol reactions as an extension of their boron acetate enolate methodology [13, 14]. Good yields and diastereoselectivity vere reported vhen using camphor-derived N-acyloxazolidinethione 19 (Scheme 2.3, Table 2.4, entries 1-7, and Figure 2.1). The high selectivities vere attributed to additional chelation afforded by the thiocarbonyl of the chiral auxiliary in transition state assembly 20, sho vn in Scheme 2.3. The corresponding camphor-derived oxazolidinone acetate imide provided no stereocontrol, supporting the chelation control hypothesis. 

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