Samarium Reformatski reaction

Analogous asymmetric, samarium Reformatsky reactions of chiral 3-bro-moacetyl-2-oxazolidinones have been described by Fukuzawa.140 For example, reduction of 124 with Sml2 generates a samarium enolate that then reacts with pivalaldehyde to give the a-unbranched (I-hydroxycarboximide 125 in 87% yield and in high diastereoisomeric excess (Scheme 5.89). The reaction is synthetically noteworthy as highly diastereoselective acetate aldol processes are difficult to achieve. Sm(III) ions are likely to play an important role in the... 

A Rh-catalyzed Reformatsky reaction of chiral imine (24) led to the stereoselective preparation of the a,a-difluoro-jS-amino acid (25). 25 was converted to difluor-oalkene (26), and subsequently L-Val-i/r[(Z)CF=CH]Gly derivative (23) in greater than 82% for both steps. The samarium diiodide-mediated reductive transformation of the y,y-difluoro-a, S-enoates proceeded via successive two-electron transfers to form a dienolate species which upon kinetically controlled trapping with fert-BuOH formed 23 (Scheme 6). 

A substoichiometric protocol for Reformatsky-type addition of a-halo esters, a-halo ketones, a-halonitriles, and a-halophosphonates to carbonyl compounds has been developed via samarium-mediated reaction.150 p-Hydroxy esters and P-hydroxynitriles have been obtained in good to excellent yields. 

The ability of Sml2 to reduce alkyl halides has been exploited in a number of carbon carbon bond-forming reactions. Radicals generated from the reduction of alkyl halides can be trapped by alkenes in cyclisation reactions to form carbocyclic and heterocyclic rings (see Chapter 5, Section 5.3), and the alkyl-samarium intermediates can be used in intermolecular and intramolecular Barbier and Grignard reactions (see Chapter 5, Section 5.4). The reduction of ot-halocarbonyl compounds with Sml2 gives rise to Sm(III) enolates that can be exploited in Reformatsky reactions (Chapter 5, Section 5.5) and are discussed in Section 4.5. 

More recently, Concellon has reported a stereoselective method for the formation of ( )-a,p-unsaturated esters that exploits a Sml2 Reformatsky reaction followed by an elimination.141 For example, ethyl dibromoacetate reacts with benzaldehyde in the presence of Sml2 to form samarium alkoxide 126, which is reduced further to give a second Sm(III) enolate 127. Elimination then affords ( )-a,p-unsaturated ester 128 in good yield (Scheme 5.90).141... 

In the last 10 years, the Barbier reaction method has been utilized more with other metals than with magnesium. The number of papers published on the lithium-Barbier reaction is three to four times that of magnesium [149-157]. Reactions can be carried out at lower temperatures than possible with Mg-Barbier reactions and yields are often very high with fewer by-products. Other metals of note are samarium, in the form of its diiodide [158-165], and zine [166-169], the metal of choice for the pre-Barbier one-step reaction, and the classic Reformatsky reaction. In situ reactions have been reported for A1 [170], Ce [171], Cd [172,173], Ga [174], Hg [175], In [176], Mn [177], Pb [178-180], Sb [181-183], Sn [184,185], and Ti [186]. An excellent paper by Li reviews the Barbier reactions of many metals in aqueous media [187]. 

The total synthesis of pederin, a potent insect toxin was achieved by T. Takemura and co-workers. One of the key steps of the synthesis was an intramolecular samarium diiodide induced Reformatsky reaction to construct the lactone subunit of the molecule. The transformation was carried out in tetrahydrofuran at 0 °C without the use of additives or catalysts. 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Zinc versus Samarium Mediated Reformatsky Reactions... 

The more than 125-year-old Reformatsky reaction of aldehydes or ketones with a-haloesters is a valuable alternative to the aldol reaction leading also to P-hydroxycarboxylic esters [158]. In its classic version, the reaction occurs through zinc enolates, but not only zinc can serve as a mediator but also various other metals in low oxidation stage like titanium, indium, and samarium. In... 

Guided by the success of the Evans and related auxiliaries, several attempts were made to use enantiomerically pure a-bromoacyl oxazolidinones for stereoselective Reformatsky reactions. Fukuzawa and coworkers developed the reaction of various bromoacetyl oxazolidinones 323 as an alternative to an asymmetric acetate aldol addition. The conversion was mediated by samarium iodide and yielded P-hydroxy carbonyl compounds 325 with high diastereoselectivity in optimal combinations of auxiliary group and aldehyde. Among the different auxiliaries, the geminal dimethyl- and diphenyl-substituted ones performed better than the original Evans oxazolidinones. The stereochemical outcome was rationalized by assuming that an O-bound samarium(III) enolate reacts via a chair-like... 

Very limited are applications of classical Reformatsky protocols in macrocyclisation reactions, where samarium(II) is the most widely used metal, as shown in Table 1. Vedejs succeeded in cyclizing 45 by using Rieke zinc unfortunately, the intermediate adduct underwent spontaneous dehydration and the isolated product was 46 (equation 31)107. 

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