Oxazolidinones enolates

With titanium enolates it was found that use of excess (3 equiv.) of the titanium reagent reversed facial selectivity of oxazolidinone enolates.140 This was attributed to generation of a chelated TS in the presence of the excess Lewis acid. The chelation rotates the oxazolidinone ring and reverses the facial preference, while retaining the Z-configuration syn diastereoselectivity. 

Evans succeeded in oxidizing A-acyl oxazolidinone enolate 143 or 145 using oxaziridine 141 as the oxidant (Scheme 4-55).110 Representative results are summarized in Table 4-19. 

Recently, a stereoselective synthesis of carbon-linked analogues of a- and 0-ga-lactoserine glycoconjugates has been reported using asymmetric enolate methodology [19]. The key step involved the electrophilic animation of a chiral oxazolidinone enolate with DTBAD. 

Hydroxylation of face-differentiated oxazolidinone enolates (vide supra) has also been enacted using dibenzoyi peroxydicarbonate as the eiectrophiiic spouse.Initial production of the a-benzyl carbonate... 

Chiral azo amide 68 reacts with an achiral oxazolidinone enolate to give a single product with the configuration indicated in Eq. 134, but the hydrazino amide could not be hydrolyzed.219 A remote chiral group attached to an achiral N-acyloxazolidinone directs a diastereoselctive animation as shown in Eq. 135.459... 

Table 14 Condensations of A/-(a-Bromoacetyl)oxazolidinone Enolates and Aldehydes ...

Figure 3.8. Syntheses using (in part) asymmetric alkylation of oxazolidinone enolates Prelog-Djerassi lactone [86] and ionomycin [87]. Stereocenters created by alkylation are indicated ( ).

Addition of chiral oxazolidinone enolates to two molybdenum-diene cations 66 and 67 was investigated, and the results were more satisfactory for the cyclohexadiene system than for its seven-membered counterpart. 

Yamazaki et al. employed the Evans oxazolidinone enolate in diastereoselective Michael additions to /I-CF3 acrylates to afford intermediate allyl silyl ketene acetals [8]. The products were isolated as ca. 2 1 mixtures of pentenoic acids and Michael addition adducts (Scheme 4.59). The rearrangement of the silyl ketene acetal was catalyzed by PdCl2(CH3CN)2. The rearrangement apparently occurred via the Z-silyl ketene acetal and exhibited high 1,2-asymmetric induction. Aspects of stereochemical control and Pd catalysis have been discussed previously (cf Scheme 4.25). 

Baran and coworkers developed the intermolecular heterocoupling of lithium enolates and elaborated oxidation systems that did not only avoid the (usually undesired) homocoupling but also do not require a large excess of one component [242]. For asymmetric versions, Evans lithium enolates were used and coupled with the lithium enolates of achiral ketones and esters. Two oxidants were studied in detail, Cu(II) and Fe(III), and the choice of the oxidant was found to have a distinct impact on the stereochemical outcome. This is illustrated for cfs-lithium enolate 507 derived from AT-phenacyl oxazolidinone and cfs-enolate 504 of propiophenone. In the Fe(acac)g oxidation system, the formation of 0 tf-coupling product 509 occurs predominantly, whereas syn-513 prevails if Cu(2-ethylhexanoate)2 was used. In both cases, however, diastereomeric mixtures were obtained with typical anti-syn and syn-anti ratios of about 2 1. The proposed mechanism is outlined in a simplified manner in Scheme 4.107 the lithium enolate 504 of the ketone is transmetallated to the iron(III) enolate 505 that might be considered an oxallyl radical 506, wherein the polarity is altered the species 506 features an electrophilic a-carbonyl atom that becomes susceptible to an attack of the nucleophilic AT-acyl oxazolidinone enolate 507. As a result, the radical 508 forms that is finally oxidized to the product 509. The authors... 

There are numerous noteworthy structural aspects of N-acyl oxazolidinones that give them a central role as auxiliaries for a large array of asymmetric transformations. Although the enolization reaction of esters and ketones can lead to mixtures of cis- and trans-enolates, the oxazolidinone imi-des exclusively form the corresponding cis-enolates. This observation has been attributed to the pronounced destabilization of the trans-enolate and the transition state structure leading to its formation as a consequence of A, 3 steric interactions (cf 124, Scheme 3.19) [15]. A second important feature of the oxazolidinone enolates relates to the ability of the auxiliary carbonyl functionality to form a chelate with coordinatively unsaturated metal centers (cf 118,121, or 125). This organizing feature provides rigidity to the en-... 

Evans has disclosed that oxazolidinone enolates can also be generated under mild conditions through the use of titanium tetrachloride and tertiary amine bases (Equation 11) [84]. These conditions have been referred to as "soft enolization and permit the use of electrophiles in enolate alkylations that are predisposed for Sml-type reactions [85]. As a demonstration of the synthetic utility of such conditions, the titanium enolate of oxazolidinone... 

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