Oxazolidinones alkenic

Scheme 18 Radical allylation to chiral oxazolidinones alkene substrates in the presence of Lewis acids.

The TiX2-TADD0Late-catalyzed 1,3-dipolar q cloaddition reactions were extended to include an acrylate derivative [66]. In the absence of a catalyst, the reaction between nitrones 1 and acryloyl oxazolidinone 19b proceeded to give a mixture all eight regio-and stereoisomers (Scheme 6.23). However, application of in this case only 10 mol% of Ti(OTs)2-TADDOLate 23d as catalyst for the reaction of various nitrones 1 with alkene 19b, led to complete regioselectivity and high endo selectivity in the reaction and the endo products 21 were obtained with 48-70% ee (Scheme 6.23) [66]. 

No single examples have been reported so far for the catalyzed asymmetric diazoalkane cydoadditions. Based on the kinetic data on the relative reaction rates observed by Huisgen in the competitive diazomethane cydoadditions between 1-alkene and acrylic ester (Scheme 7.32), it is found that diazomethane is most nu-deophilic of all the 1,3-dipoles examined (kaciyiate/fci-aikene = 250000) [78]. Accordingly, the cydoadditions of diazoalkanes to electron-defident alkenes must be most efficient when catalyzed by a Lewis acid catalyst. The author s group has become aware of this possibility and started to study the catalyzed enantioselective diazoalkane cydoadditions of 3-(2-alkenoyl)-2-oxazolidinones. 

The syntheses in Schemes 13.45 and 13.46 illustrate the use of oxazolidinone chiral auxiliaries in enantioselective synthesis. Step A in Scheme 13.45 established the configuration at the carbon that becomes C(4) in the product. This is an enolate alkylation in which the steric effect of the oxazolidinone chiral auxiliary directs the approach of the alkylating group. Step C also used the oxazolidinone structure. In this case, the enol borinate is formed and condensed with an aldehyde intermediate. This stereoselective aldol addition established the configuration at C(2) and C(3). The configuration at the final stereocenter at C(6) was established by the hydroboration in Step D. The selectivity for the desired stereoisomer was 85 15. Stereoselectivity in the same sense has been observed for a number of other 2-methylalkenes in which the remainder of the alkene constitutes a relatively bulky group.28 A TS such as 45-A can rationalize this result. 

Chiral active pharmaceutical ingredients, 18 725-726. See also Enantio- entries Chiral additives, 6 75—79 Chiral alcohols, synthesis of, 13 667-668 P-Chiral alcohols, synthesis of, 13 669 Chiral alkanes, synthesis of, 13 668-669 Chiral alkenes, synthesis of, 13 668—669 Chiral alkoxides, 26 929 Chiral alkynes, synthesis of, 13 668-669 Chiral ammonium ions, enantiomer recognition properties for, 16 790 Chiral ansa-metallocenes, 16 90 Chiral auxiliaries, in oxazolidinone formation, 17 738—739... 

Due to the presence of an electron-withdrawing group on the dipolarophile, these processes are classified as type 1 reactions. The process involves the transference of charge from the dipole to the dipolarophile. When catalyzed by metallic compounds, coordination of the dipolarophile is highly desired. Usually, coordination of a nitrone to the Lewis acid is more feasible than coordination of a carbonyl compound. For this reason, alkenes that enable a bidentate coordination to the Lewis acid, such as 3-alkenoyl-oxazolidinones (Scheme 5), have been frequently employed as a model system to smdy the metal-catalyzed 1,3-dipolar cycloaddition... 

One of the problems related to the LA induced activation of a,p-unsaturated carbonyl compounds for the reaction with a nitrone is the competitive coordination of the nitrone and the a,(3-unsaturated carbonyl compound to the Lewis acid (Scheme 12.65). Calculations have shown that coordination of the nitrone to the LA is more feasible than a monodentate coordination of a carbonyl compound. However, this problem can be circumvented by the application of alkenes such as 3-alkenoyl-oxazolidinones, enabling abidentate coordination to the LA, which is favored over the monodentate coordination to the nitrone. 

Furukawa and co-workers (368,369) succeeded in applying the softer dicationic Pd-BINAP 260 as a catalyst for the 1,3-dipolar cycloaddition between 225 and 241a (Scheme 12.82). In most cases, mixtures of endo-243 and exo-243 were obtained, however, enantioselectives of up to 93% ee were observed for reactions of some derivatives of 225. A transition state structure has been proposed to account for the high selectivities obtained for some of the substrates (368). In the structure shown in Scheme 12.82, the two phosphorous atoms of the Tol-BINAP ligand and the two carbonyl oxygens of the crotonoyl oxazolidinone are arranged in a square-planar fashion around the palladium center. This leaves the ii-face of the alkene available for the cycloaddition reaction, while the re-face is shielded by one of the Tol-BINAP tolyl groups. 

Stereosectivity is a broad term. The stereoselectivity in cyclopropanation which has been discussed in the above subsection, in fact, can also be referred to as diastereoselectivity. In this section, for convenience, the description of diastereoselectivity will be reserved for selectivity in cyclopropanation of diazo compounds or alkenes that are bound to a chiral auxiliary. Chiral diazoesters or chiral Ar-(diazoacetyl)oxazolidinone have been applied in metal catalysed cyclopropanation. However, these chiral diazo precursors and styrene yield cyclopropane products whose diastereomeric excess are less than 15% (equation 129)183,184. The use of several a-hydroxy esters as chiral auxiliaries for asymmetric inter-molecular cyclopropanation with rhodium(II)-stabilized vinylcarbenoids have been reported by Davies and coworkers. With (R)-pantolactone as the chiral auxiliary, cyclopropanation of diazoester 144 with a range of alkenes provided c yield with diastereomeric excess at levels of 90% (equation 130)1... 

The cyclization of carbamate derivatives of unsaturated amines has proven synthetically useful. Cyclizations of carbamates of allylamines containing a terminal vinyl group give oxazolidinone products (equation 60 and Table 17, entries 1 and 2).99,161 Bromocyclizations of systems with a di- or tri-sub-stituted alkene often give mixtures of oxazolidinones and tetrahydrooxazinones,163 while cyclization of an A -cinnamyl carbamate with phenylsulfenyl chloride gave only the oxazolidinone product.163b,163c The stereochemistry of the cyclization of primary carbamates of either allylic or homoallylic amines is low... 

In three-component allyltin-mediated processes, if the alkenes contain a chiral auxiliary, the allylation step proceeds with a high degree of stereocontrol [35]. In an example in Scheme 6.20, an acrylated oxazolidinone having a chiral substituent in the ring is employed as the alkene portion. Magnesium bromide is used as a Lewis acid to fix the acrylate moiety [36]. Allylation takes place diastereoselectively so as to avoid the face in which the bulky diphenylmethyl group is located. 

Diels-Alder reactions of (Z)-A -substituted-4-methylene-5-propylidene-2-oxazoli-dinone dienes with methyl vinyl ketone, methyl propiolate, and captodative alkenes yield the highest regio- and stereo-selectivities in mixtures of H20 and MeOH or under BF3.Et20 catalysis.198 The asymmetric Diels-Alder reaction of cyclopentadiene and 3-acryloyl-2-oxazolidinone is catalysed by a new Cu(II) catalyst containing a chiral sterically congested roofed (2-diphenylphosphino)phenylthiazoline ligand (169).199... 

This coupling procedure with the thioesters proved sensitive to the substitution pattern of both the amino acid and alkene. In contrast, coupling reactions with the M-acyl oxazolidinone derivatives such as 22 proved to be much more effective (Scheme 14) [20]. Mechanistic studies suggested that an alternative pathway was operating in these cases, where reduction of the al-... 

By treatment of an alkene with phenyltellurinyl trifluoroacetate in combination with ethyl carbamate and boron trifluoride in refluxing chloroform, the addition product 5 was obtained in good yield and with complete diastereoselectivity. The stereochemistry of the product was confirmed on comparison with an authentic sample and is in agreement with the proposed mechanism. Further reduction with hydrazine gave the phenyltelluro derivative 6 in high yield, whereas heating provided the corresponding ru-oxazolidinone 725-28. 

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