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Timestamp: 2019-04-19 02:51:45+00:00

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Physical Data: (1) mp 87-89 °C; (2) mp 85-87 °C; (3) mp 71-72 °C; (4) mp 118-121 °C; (5) mp 118-121 °C; (6) mp 130-132 °C; (7) mp 130-132 °C; (8) mp 118-120 °C.
Solubility: sol most polar organic solvents.
Analysis of Reagent Purity: 99% purity attainable by GLC.
Handling, Storage, and Precautions: no special handling or storage precautions are necessary. There is no known toxicity. It may be harmful by inhalation, ingestion, or skin absorption and may cause skin or eye irritation.
Synthesis of the Chiral Oxazolidinone Auxiliaries.
(S)-4-Benzyl- (1), (R)-4-benzyl- (2), (S)-4-i-propyl- (3), (4R,5S)-4-methyl-5-phenyl- (4), (S)-4-t-butyl- (8), and (S)-4-phenyl-2-oxazolidinones (6) are commercially available. Typical procedures to form these chiral auxiliaries involve the reduction of a-amino acids to the corresponding amino alcohols or the purchase of amino alcohols, followed by formation of the cyclic carbamate (eq 1). A number of high-yielding methods of reduction have been employed for this transformation, including Boron Trifluoride Etherate/Borane-Dimethyl Sulfide,12 Lithium Aluminum Hydride,1,6,13,14 Sodium Borohydride/Iodine,15 and Lithium Borohydride/Chlorotrimethylsilane.8 Selection among these methods is largely based upon cost of reagents and ease of performance. Reagents for effecting the second transformation include Diethyl Carbonate/Potassium Carbonate12 or Phosgene,16 -18 with the former being preferable for large-scale production. Ureas,19,20 dioxolanones,21 chloroformates,22 trichloroacetate esters,22,23 N,N�-Carbonyldiimidazole,24 and Carbon Monoxide with catalytic elemental Sulfur25 or Selenium26,27 provide alternatives for the transformation of amino alcohols to the derived oxazolidinones.
Conversion of the appropriate a-amino acids to oxazolidinones may also be performed as a one-pot procedure, obviating the need to isolate the intermediate amino alcohols (eqs 2 and 3).28,29 Overall isolated yields for these procedures are 70-80%.
Carbamate-protected amino alcohols also yield oxazolidinones upon treatment with base (eq 4)30,31 or p-Toluenesulfonyl Chloride (eq 5).32 The latter reaction requires the protection of the amino group as the N-methylated carbamate for selective inversion of the hydroxyl-bearing center.
Lithiated oxazolidinones add to acid chlorides (eq 7)6,34 and mixed anhydrides (eq 8)35,36 in high yields to form the derived N-acyl imides. In the latter case the anhydride may be formed in situ with Trimethylacetyl Chloride, and then condensed with the lithiated oxazolidinone selectively at the less hindered carbonyl moiety.
Acryloyl adducts cannot be formed through traditional acylation techniques due to their tendency to polymerize. These adducts may be obtained through reaction of acryloyl chloride with the bromomagnesium salt of the oxazolidinone auxiliary9,37 or the N-trimethylsilyl derivative in the presence of Copper(II) Chloride and Copper powder.38 These methods yield products in the range of 50-70%.
The (Z)-enolate is also accessed exclusively using titanium enolization procedures.45,47 Irreversible complexation of Titanium(IV) Chloride with tertiary amine bases demands complexation of the substrate with the Lewis acid prior to treatment with either triethylamine or Hünig's base. Reactions using Hünig's base occasionally display higher diastereoselectivities, particularly in Michael additions.7,45 Of the alkoxy titanium species employed in imide enolization, only TiCl3(O-i-Pr) is capable of quantitative enolate formation. In these reactions, order of addition of reagents is not significant. These enolates demonstrate enhanced nucleophilicity, albeit with somewhat diminished diastereoselectivity.
For benzyloxymethyl electrophiles, titanium enolates are superior to the corresponding lithium enolates in both yield and alkylation diastereoselectivity (eq 11). Unfortunately, the analogous p-methoxybenzyl-protected b-hydroxy adducts cannot be obtained by this method. In other cases the titanium methodology complements the corresponding reactions of the lithium and sodium enolates for SN1-like electrophiles.47 It is noteworthy that imides may be selectively enolized under all of the preceding conditions in the presence of esters (eq 12).
Treatment of the silyl enol ethers of N-acyloxazolidinones with selected electrophiles that do not require Lewis acid activation similarly results in high induction of the same enolate face (eq 13).48 The facial bias of this conformationally mobile system improves with the steric bulk of the silyl group.
Enolate Alkylations with Transition Metal Coordinated Electrophiles.
Acylation of these enolates provides a direct route to b-dicarbonyl systems. Acylations generally proceed with >95% diastereoselection in 83-95% yields, with the valine-derived auxiliary providing slightly higher selectivity (eq 18).2 The sense of induction is consistent with reaction through the chelated lithium (Z)-enolate, and the newly generated stereocenter is retained through routine manipulations.
Titanium imide enolates are excellent nucleophiles in Michael reactions. Michael acceptors such as ethyl vinyl ketone, Methyl Acrylate, Acrylonitrile, and t-butyl acrylate react with excellent diastereoselection (eq 21).7,45 Enolate chirality transfer is predicted by inspection of the chelated (Z)-enolate. For the less reactive unsaturated esters and nitriles, enolates generated from TiCl3(O-i-Pr) afford superior yields, albeit with slightly lower selectivities. The scope of the reaction fails to encompass b-substituted, a,b-unsaturated ketones which demonstrate essentially no induction at the prochiral center. Furthermore, substituted unsaturated esters do not act as competent Michael acceptors at all under these conditions.
Treatment of the sodium enolates with the Davis oxaziridine reagent affords the hydroxylated products with the same sense of induction as the alkylation products (eq 23).5,35 Although high diastereoselectivity may be achieved with Oxodiperoxymolybdenum(pyridine)(hexamethylphosphoric triamide) (MoOPH), such reactions proceed in lower yields.
Amination likewise can be effected using Di-t-butyl Azodicarboxylate (DBAD).4,56 Despite the excellent yields and diastereoselectivity obtained using this methodology (eq 24), the harsh conditions required for further transformation of the resultant hydrazide adducts (Trifluoroacetic Acid and hydrogenation at 500 psi over Raney Nickel catalyst) limit its synthetic utility.
As a method for the synthesis of a-amino acids, the hydrazide methodology has now largely been supplanted by direct enolate azidation (eq 25).4,57 These adducts are susceptible to mild chemical modification to afford N-protected a-amino acid derivatives. Under optimal conditions, yields range from 74-91% and selectivities from 91:9 to >99:1. Imide enolization can be carried out selectively in the presence of an enolizable t-butyl ester and suitably protected amino groups.
The dibutyl boryl enolates of chiral acyloxazolidinones react to afford the syn-aldol adducts with virtually complete stereocontrol (eq 32).6,13,14,43,61-64 Notably, the sense of induction in these reactions is opposite to that predicted from the analogous alkylation reactions. This reaction is general for a wide range of aldehydes and imide enolates.36,65-69 Enolate control overrides induction inherent to the aldehyde reaction partner.
A second entry to dicarbonyl substrates utilizes the aldol reaction to establish the a-methyl center prior to oxidation of the b-hydroxyl moiety. Commonly, this oxidation is performed using the Sulfur Trioxide-Pyridine complex, which results in <1% epimerization of the methyl-bearing center (eq 34).2 Interestingly, this procedure procures the opposite methyl stereochemistry from that obtained through enolate acylation of the same enantiomer of oxazolidinone.
Boron enolates of the N-crotonyloxazolidinones have been shown to afford the expected syn-aldol adducts (eq 36).76,77 The propensity for self-condensation during the enolization process is minimized by the use of triethylamine over less kinetically basic amines.
In contrast to the reliably excellent selectivities of a-substituted dibutylboryl imide enolates, boron enolates derived from N-acetyloxazolidinones lead to a statistical mixture of aldol adducts under the same reaction conditions. Acetate enolate equivalents may be obtained from these enolates bearing a removable a-substituent. To this end, thiomethyl- or thioethylacetyloxazolidinones (eq 43)13 as well as haloacetyloxazolidinones can be submitted to highly selective boron-mediated aldol reactions. Products can be transformed to the acetate aldol products via desulfurization with either Raney Ni81 or Tri-n-butylstannane and Azobisisobutyronitrile,82 or via dehalogenation with Zinc-Acetic Acid (eq 44).81 This latter procedure provides several advantages over the sulfur methodology, including ease of imide preparation and improved overall yields.
The Reformatsky reaction of a-halooxazolidinones provides an alternative to the more conventional aldol reaction. Although the traditional zinc-mediated Reformatsky using valine-derived compounds proceeds nonselectively,87,88 the SnII modification with 2-bromo-2-methylpropionyloxazolidinone proceeds well (eq 46).89,90 In this particular case, however, the geminal dialkyl substituents favor the endocyclic carbonyl acyl transfer of the auxiliary by the aldolate oxygen.
(S)-N-benzoyloxazolidinones have been used as acyl transfer reagents to effect the kinetic resolution of racemic alcohols.10 The bromomagnesium alkoxides formed from phenyl n-alkyl alcohols selectively attack the exocyclic benzoyl moiety to afford recovered auxiliary and the derived (R)-benzoates in >90% ee and >90% yield (eq 47). The scope of this reaction seems to be limited to this class of substrates as selectivity drops with increasing the steric bulk of the alkyl group.
The N-arylsulfinyloxazolidinone methodology is readily extended to the formation of sulfinylacetates, sulfinates, and sulfinamides with >95% ee and high yields (eq 49).
Chiral a,b-unsaturated imides participate in Lewis acid-promoted Diels-Alder cycloaddition reactions to afford products in uniformly excellent endo/exo and endo diastereoselectivities (eqs 50 and 51).9,37,95,96 Unfortunately, this reaction does not extend to certain dienophiles, including methacryloyl imides, b,b-dimethylacryloyl imides, or alkynic imides. Cycloadditions also occur with less reactive acyclic dienes with high diastereoselectivity (eq 52). Of the auxiliaries surveyed, the phenylalanine-derived oxazolidinones provided the highest diastereoselectivities. This methodology has been recently extended to complex intramolecular processes (eq 53).68,95,97 In this case, use of the unsubstituted achiral oxazolidinone favored the undesired diastereomer.
Chiral oxazolidinones have been employed as the chiral control element in the Staudinger reaction as well as the ultimate source of the a-amino group in the formation of b-lactams.41 Cycloaddition of ketene derived from 4-(S)-phenyloxazolidylacetyl chloride with conjugated imines affords the corresponding b-lactams in 80-90% yields with excellent diastereoselectivity (eq 54). The auxiliary can then be reduced under Birch conditions to reveal the a-amino group.
The 4-phenyl-2-oxazolidinone auxiliary has also been employed in the TiCl4-mediated conjugate additions of allylsilanes (eq 58).103 Analogous reactions using the phenylalanine-derived auxiliary with dimethylaluminum chloride afforded lower selectivities.104 In these reactions the oxazolidinones perform better than the sultams.
Nucleophilic addition of thiophenol to chiral tiglic acid-derived imides proceeds in excellent yields and diastereoselectivities (eq 59).105 Complete turnover of both the a- and b-centers results from the use of the (Z) rather than (E) isomer. Poor b-induction was found with the imides derived from cinnamic acid.
Chiral imide enolates which contain g-halide substituents undergo intramolecular displacement to form cyclopropanes.106 Halogenation of g,d-unsaturated acyl imides occurs at the g-position in 85% yield with modest stereoinduction. The (Z) sodium enolates of these compounds then cyclize through an intramolecular double stereodifferentiating reaction (eq 61).
Removal of the Chiral Auxiliary.
In each of the following transformations, the oxazolidinone auxiliary is recovered in high yields (eq 62).
Hydroxide6,111 and peroxide112 conditions saponify acyl imides in excellent yields; however, with sterically hindered acyl groups endocyclic cleavage may predominate upon treatment with Lithium Hydroxide. Lithium Hydroperoxide, however, is highly selective for the exocyclic carbonyl moiety.
Reduction of acyl imides to their corresponding alcohols is effected by a number of reagents, including Lithium Aluminum Hydride,1 Lithium Borohydride,1 LiAlH4/H2/Lindlar's cat./TFA,113 LiBH4/H2O/Et2O,114 LiBH4/MeOH/THF,36 and Bu3B/HOAc/LiBH4.77 Although the sole use of LiAlH4 or LiBH4 affords product often in low yields, the addition of an equivalent of H2O or MeOH greatly enhances reaction efficiency. The MeOH/THF modification occasionally produces more consistent results. The last of the methods outlined above is effective in preventing retro-aldol cleavage in sensitive substrates such as crotyl or a-fluoro aldol adducts (eq 63).
This transformation is accomplished through a two-step procedure. One such variant requires reduction to the alcohol (e.g. LiAlH4, H2O) and subsequent oxidation (e.g. Swern conditions).36,85 Alternatively, Weinreb transamination78,115-117 followed by Diisobutylaluminum Hydride,78 or conversion to the thioester (see below) and subsequent Triethylsilane reduction,86 afford the desired aldehyde in excellent yields. Weinreb transamination proceeds with minimal endocyclic cleavage when there is a b-hydroxy moiety free for internal direction of the aluminum species.
The transformation of N-acyl imides into thioesters with lithium thiolate reagents proceeds with exceptional selectivity for the exo carbonyl moiety even in exceptionally hindered cases.121 A recent application of this reaction in a complex setting has been reported (eq 66).68,97 This transformation is significant in that the normally reliable peroxide hydrolysis procedure proved to be nonselective. The recently reported high yield reduction of thioesters to aldehydes86 enhances the utility of these thioester intermediates.
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