2- Allyl-3-oxazoline-5-ones

P,y-Unsaturated ketones. The last step in a synthesis of /J.y-enones involves hydrolysis of a 2-allyl-3-oxazoline-5-one, previously conducted with Ba(OH)2 in variable yield. Recent investigations reveal that treatment with CrfOAclj-HjPOj or reduction with NaBIl4 in TlIE -CIljOH followed by a treatment with citric acid is superior. 

The initial stage of this scheme, i.e. the formation of the 2-allyl-3-oxazolin 5-one, proceeds via intermediacy of the 4-allyl-2-oxazolin-5-one (399) which is converted to (400) by a... 

The synthesis is straightforward. The oxazoline 66 directs the orf/to-lithiation and then the lithium derivative is reacted with an allyl bromide. One thing we did not consider in the analysis was the synthesis of the oxazoline itself and perhaps we should. One way to make oxazolines (and there are many) is to react an acid chloride with an amino alcohol 74 followed by thionyl chloride. 

It is evident that steric repulsion occurs between one of the allylic substituents and the syn-disposed oxazoline substituent, causing a significant lengthening of the corresponding Pd-C bond and a slight rotation (ca. 15 °) of the allyl moiety. Comparison of the -intermediate 26 with the absolute configuration of the product indicated that nucleophilic... 

Tris(oxazoline) complexes have also been investigated as ligands in the allylic oxidation reaction. Katsuki and co-workers (116) observed that Cu(OTf)2 com-plexed to the tris(oxazoline) 160 is a more selective catalyst than one derived from CuOTf, Eq. 99, in direct contrast to results observed with bis(oxazohnes) or pyridyl bis(oxazohnes) as ligands (cf. Section III.A.3). When the reaction is conducted at -20°C, the cyclopentenyl benzoate is delivered in 88% ee albeit in only 11% yield after 111 h. Larger cycloalkenes are less selective (cyclohexene 56% ee, cyclohep-tene 14% ee, cyclooctene 54% ee). 

Similar results were reported by the Barret group by using stoichiometric amounts of an enantiopure 2-(2-pyridinyl)-2-oxazoline [46], hi 1996, Iseki and Kobayashi achieved a catalytic version of the asymmetric allylation [47], They applied proline-based chiral HMPA derivatives for the allylation. The catalyst 21 proved to be the best one regarding catalyst loading down to 1 mol% (Scheme 16) [48],... 

Cyclization of an allyl A-acyl-1 -phenylglycinate (398) with triphenylphosphine, hexa-chloroethane and triethylamine affords a 2-allyl-3-oxazolin-4-one (400) (81AG(E)395, 77AG(E)394). Reductive cleavage of the oxazolinone with chromium(II) acetate and aqueous hypophosphorous acid furnishes a j3,y-unsaturated ketone (Scheme 88). 

More recent efforts investigating the kinetics and mechanism of cationic polymerization of oxazolines are aimed at the preparation of various types of functional polymers as described in a recent review [181] (cf., Section IV.D). One of the examples is the development of efficient di- and tetrafunctional initiators of oxazolines polymerization, allylic or benzylic dihalides and allylic tetrahalides [182], e.g. ... 

The N,P phosphine-oxazoline chelate (59) is chiral, and complexes can act as homogeneous catalysts for asymmetric synthesis the Ir(l) and Pd(II) complexes promote enantioselective olefin hydrogenation and allylic substitution respectively. An N,P analog of the N,N didentate ligand 2,2 -bipyridine is (60), the soft P donor helping to stabilize low-valent metals. Further, 2,2-bipyridine derivatives such as (61) can bind metals such as Ir and Ru as N,C chelates with one pyridine nitrogen rotated to the opposite side, away from the metal ion. 

Reactions between substituted allylic Grignard reagents and cyclic iminothioethers or iminoethers have been tried with 2-methylthio-2-thiazolines or 2-oxazolines. Interestingly, only one addition-elimination reaction is observed [77 Eq. (38)] ... 

One of the exciting developments of the Wacker-type oxidation is the asymmetric synthesis of the reaction. For instance, using a new chiral bis(oxazoline) ligand L = 3,3 -Disubstituted 2,2 -bis(oxazolyl)-l,r-binaphthyls (boxax), a catalytic asymmetric Wacker -type cyclization converted allyl-phenol 55 to dihydrofuran 74 with 67% ee.ss... 

These excellent results, supported by an X-ray structure analysis of complex 15, could be explained convincingly [18] one benzyl group in the ligand and one phenyl substituent at the allylic terminus repel each other considerably. This steric hindrance is, for example, illustrated by a clear twisting of the oxazoline ring in question and from the lengthening of the corresponding Pd-C bonds. The nucleophile attacks at the allylic terminus which is sterically strained, since in this way, in combination with the rupture of the Pd-C bond, the steric strain can be reduced. 

Reactions of Oxazoles. 2-Amino-4-methyloxazole (359) forms the rearranged hydro-peroxide (360) by the action of hydrogen peroxide. Thermolysis of 2- (3,3-di-methylallyloxy)-4,5-diphenyloxazole yields the oxazolin-2-one (361) with inversion of the allyl group by [ , ] sigmatropic shift irradiation, on the other... 

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