42-Oxazoline ring closure

N-Acylaziridine-2-carboxylates readily rearrange to oxazolines under thennal, acidic, or nucleophilic conditions [91, 123-127]. Treatment of trans-aziridine-2-car-boxylate 176 (Scheme 3.63) with Nal in acetonitrile, for example, resulted in ring-expansion product 177 through the so-called Heine reaction. The reaction involves initial opening of the aziridine ring by iodide and subsequent oxazoline ring-closure by Sn2 displacement of the resultant iodide intermediate [127]. 

Thus attack of the TosMlC anion 9 on a carbonyl carbon is followed (or accompanied) by ring closure of the carbonyl oxygen to the electrophilic isocyano carbon to form an oxazoline (12). Loss of p-tolylsulfinic acid provides the 5-substituted oxazole 13. ... 

The behavior of unsaturated azlactones with organometallic reagents has been studied in detail. Arylmagnesium halides and phenyllithium attack 4-arylidene-5-oxazolones at the carbonyl carbon to give ring-opened amido tertiary alcohols (26) and oxazolines (27) (by ring closure), usually as mixtures [Eq. (17)]. The nature of the... 

The overall pathway for the conversion of the unsaturated azido ether 281 to 2,5-dihydrooxazoles 282 involves first formation of the dipolar cycloaddition product 287, which thermolyzes to oxazoline 282 or is converted by silica gel to oxazolinoaziridine 288. While thermolysis or acid-catalyzed decomposition of triazolines to a mixture of imine and aziridine is well-documented [71,73], this chemoselective decomposition, depending on whether thermolysis or exposure to silica gel is used, is unprecedented. It is postulated that acidic surface sites on silica catalyze the triazoline decomposition via an intermediate resembling 289, which prefers to close to an aziridine 288. On the other hand, thermolysis of 287 may proceed via 290 (or the corresponding diradical) in which hydrogen migration is favored over ring closure. 

In this aforementioned Heine reaction the initial ring opening takes place by iodide ions. Subsequent ring closure by S 2 displacement of iodide by reaction with the negative oxygen center then leads to the products. This process proceeds with double inversion at the same carbon atom, thus with net retention. Hydrolysis of these oxazolines gives j9-hydroxy-a-amino acids (Scheme 31) [1,38]. The stereochemical course of ring expansion is the same as that observed in Scheme 29. 

Optically active, a-branched lactams 30 have been built by means of Meyers chiral auxiliaries [ 10]. The key step included the diastereoselective a-alkylations of the initially formed co-i -sulfonamido oxazolines 26. The R or S configuration in the product 27 was obtained reacting the appropriately configured intermediate aza enolates with alkyl halides, high diastereoselectivities have been reported. Several attempts to achieve a complete ring closure to the lactams 30 (via 29) by an acidic cleavage of the oxazolines 27 failed. Varying mixtures of... 

For secondary (3-hydroxy amides, the ring closure occurs via an Sn2 mechanism with complete inversion at carbon bearing the hydroxyl group. Thus, both cis- and trans-4,5-disubstituted oxazolines can usually be obtained reliably. Representative examples are shown in Table... 

An unusual example of oxazoline formation is illustrated in the following example in which the hydroxyl moiety is masked as a tetrahydrofuran ring. Depending on reaction conditions, regioselective ring closure to one of the two oxazolines can be realized. Thus, addition of methanesulfonyl chloride to a mixture of substrate and EtsN resulted in the expected oxazoline 46. On the other hand, addition of < 1 equiv of triethylamine to a mixture of substrate and methanesulfonyl chloride, followed by acid catalysis produced oxazoline 47. Intermediate 47, obtained in 72% overall yield from 45, was susequently converted to the human immunodeficiency virus (HlV)-protease inhibitor Nelfinavir 48 (Scheme 8.18). 

Allyl alcohols readily react with trichloroacetonitrile to give the corresponding trichloroacetimidates 145. Activation of the double bond with electrophilic reagents results in ring closure to yield oxazolines 146. The most commonly employed electrophiles include iodine, iodine monochloride, phenylselenyl chloride, and mercuric trifluoroacetate. Other nitriles including cyanogen bromide and N,N-dimethylcyanamide can also be used. Since oxazolines readily hydrolyze to amides, the net effect of this reaction sequence is to produce p-amino alcohols 147 from an allyl alcohol. This strategy has been employed in numerous total syntheses of natural products. Examples are listed in Table 8.18 (Fig. 8.7 Scheme 8.43). ° ... 

Epoxytrichloroacetimidates 153 also undergo oxazoline ring formation in the presence of a catalytic amount of Lewis acids.Diethylaluminum chloride was found to be superior to boron trifluoride, which tends to further hydrolyze the oxazoline 154 to the trichloroacetamide. Generally, formation of the six-membered ring oxazine 155 is not favored, but it can be a serious side reaction if the epoxide contains substituents that stabilize the incipient cation generated prior to ring closure. Examples from this study are summarized in Table 8 19 (Fig 8 8 Scheme 8.45). 

Oxazoline formation from 5-vinyloxazolidinones promoted by palladium (0) is also known. Oxidative insertion of palladium with loss of CO2 results in a pair of equihbrating 71-allyl palladium complexes. The stereochemistry of the vinyl group is therefore not important. Ring closure from the thermodynamically more stable transition state accounts for the trans-isomer as the major product. Depending on the exact substitution, diastereoselectivities ranging from 2.5 1 to 16 1 can be obtained (Scheme 8.68). 

An interesting extension to this finding resulted when 2-aminoethanol was reacted with propionitrile. Initial formation of amidine 9 occurs, with subsequent ring-closure giving the final product, 2-ethyl-2-oxazoline 10. 

Ring closure of (3-hydroxy-a-amino acids with sulfuryl chloride/triethylamine 68 is accompanied by formation of (3-chloroalanine,16 1 whereas cyclization of urethane-protected serine and threonine by the Mitsunobu reaction 54 69 70 leads to oxazoline and dehydroalanine formation as side products. 47,71 Formation of dehydroalanine can be prevented by bulky carboxy protecting groups such as tert-butyl esters. 69 ... 

Ring closure of a 2-methoxyphenyl derivative of propanol to a chroman has also been achieved. Treatment of the aryloxazoline (259) with sodium hydride yielded the chroman (260) (81JOC783). The intramolecular nucleophilic displacement of the o-methoxy group is promoted through oxazoline activation and proceeds through an addition-elimination sequence. The initial attack involves coordination of the metal alkoxide to both the oxazoline moiety and the methoxy group, and aromatization follows with displacement of methoxide ion (Scheme 67). Hydrolysis of the oxazoline moiety to a carboxyl group has been accomplished. 

The use of co-(2-bromophenyl)alkyl-2-oxazolines (13) in base-promoted ring-closure reactions provides a good access to 1-phenyl-indane and 1-phenyl-tetralin derivatives (14) that contain easily manipulated oxazoline moieties. Satisfactory isolated yields of cyclized products 14 are obtained with lithium diisopropylamide (LDA) in THF at room temperature under irradiation with UV light, even when quaternary centers are formed (Scheme 10.39) [55]. The yield of 14 (n= 1, R= Ph) increases to 75% when the reaction proceeds under laboratory light for 48 h at room temperature. 

The diastereoselective ring-closure reaction of 2-bromophenylpropyl precursors containing chiral oxazolines yields diastereoisomeric indanes. However, the maximum selectivity achieved with a 4-isopropyl substituent in the oxazoline (48% d.r.) is modest [55]. 

It is clear, nonetheless, that successful dearomatising cyclisations are not confined to systems which can undergo electrocyclic ring closure 456 for example cyclises to 457, which may be protonated or alkylated stereoselectively.201 Without the oxazoline substituent, similar reactions lead to rearomatisation. 

What is the structure of the intermediate formed in step d Suggest a reason for the closure of the oxazoline ring on the B-face (formation of 4) and not on the a-face. 

Now an oxazoline thiazoline conversion is achieved by nucleophilic opening of the tris(oxazolines) to the 2-methylcysteine peptide 79 with thioacetic acid. For the ring closure Wipf uses titanium tetrachloride according to Heathcocks protocol. The side chain of tris(thiazoline) 80 is oxidized with benzenesele-nic acid to provide the desired natural product thiangazole 4. 

Their cyclizations would then lead to the observed oxazolines of mixed stereochemistry. The relative proportion of II and III should reflect the equilibrium composition of VII and VIII because the ring closure of cations IX, and X to II and III is assumed to be faster than the C-C bond rotation that would furnish cis-trans isomerization. 

Imidazoles can also be made by heating 4-tosyloxazolines in saturated methanolic ammonia or monoalkylamines. These reactions proceed through intermolecular condensation of a-aminoketones and amidines and intramolecular cyclization of a-ainidinoketones, respectively [49] (see Section 4.2). When A -unsubstituted 4-oxazolin-2-ones are added to isocyanates, the 2-oxo-4-oxazoline-3-carboxamide products cleave under the influence of strong acids and heat. Subsequent ring closure gives 4-imidazolin-... 

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