Sodium borohydride oxazolines

Acyl-2-phenyloxazole derivatives undergo a reductive photocyclization in the presence of sodium borohydride to generate a bicychc oxazoline with a cis-fused pyridinone ring 307. The stereochemistry of the product is consistent with hydride attack from the less hindered surface of the cyclic intermediate 306. The oxazoline containing pyridinone is a key intermediate used for the synthesis of pseudodisto-mins 308 (Scheme 8.85, p. 415). ° ... 

Tetroses and Pentoses - 4-0- -Butyldimethylsilyl-2,3-0-isopropylidene-L-threose (1) has been prepared in seven efficient steps from o-xylose. 3,4-0-Isopropylidene-D-eythrulose (4) has been synthesized from the known tetritol derivative 2 by primary protection as the silyl ether 3, followed by Dess-Martin oxidation and desilylation. Compound 2 was derived from D-isoascorbic acid (see Vol. 22, p. 178, refs. 9,10). In a similar reaction sequence, the enantiomer 5 has been obtained from L-ascorbic acid. The dehomologation of several di-0-isopropylidenehexofuranoses e.g., 6- 7) has been carried out in two steps without intermediate purification, by successive treatment with periodic acid in ethyl acetate, followed by sodium borohydride in ethanol. Selective reduction of 3-deoxy-D-g/jcero-pentos-2-ulose (8) to 3-deoxy-D-g/> cero-pent-2-ose (9) has been achieved enzymically with aldose reductase and NADPH." 4-Isopropyl-2-oxazolin-5-one (10) is a masked formaldehyde equivalent that is easily converted to an anion and demasked by mild acid hydrolysis. One of the three examples of its use in the synthesis of monosaccharides is shown in Scheme 1. ... 

Fragmentations of heterocycles have played an important role in the preparation of amide derivatives. Moderate to good yields of a-hydroxy-amides were obtained on reaction of a-hydroxy-acid aceto-nides with primary amines.N-Alkyl-2-methyl-2-oxazolinium salts (obtained by mixing alkyl halides and 2-methyl-2-oxazoline in dichloromethane) were found to react with sodium benzeneselenolate to yield -(2-phenylselenoethyl)-t -alkylacetamides, which after oxidation to ] -vinyl analogues with sodium metaperiodate in methanol, gave secondary amides on sequential treatment with mercuric acetate in aqueous tetrahydrofuran and sodium borohydride/3M... 

A number of alkylation products from MeOTf and heterocycles have been advocated as useful intermediates. Thus treatment of 2-substituted thiazoles with MeOTf in acetonitrile, followed by reduction of the salt formed with sodium borohydride/CuO in CH2CI2, leads to aldehydes. l-(Benzenesulfonyl)-3-methylimidazolium and l-(/>-toluenesulfonyl)-3-inethylimidaz-olium triflates have been proposed as efficient reagents for the preparation of aryl sulfonamides and aryl sulfonates. MeOTf alkylates 2,5-oxazoles to give salts which can be reduced by PhSiHs/CsP to give 4-oxazolines, and these provide a route to stabilized azomethine ylides. ... 

Quaternization of the nitrogen atom in a heterocyclic system can activate the ring towards addition reactions. An oxazoline moiety was converted to an aldehyde in 97% overall yield by alkylation of oxazoline with MeOTf, reduction with sodium boro-hydride, and hydrolysis of the resulting aminal (eq 10). Alternatively, treatment of an TV-methylated oxazolium triflate with a Grignard reagent followed by aqueous acid produced a ketone. Reaction of alkenyl oxazoles with MeOTf induced spontaneous intramolecular [4 -i- 2] cycloaddition at room temperature leading to a hydroindole or a hydroisoquinoline after reduction by sodium borohydride (eq 11). ... 

There are relatively many stereoselective synthetic methodologies for synthesizing these compounds. The one reported by Bundle and co-workers takes advantage of the synthetic utility of functionalized 1,2-oxazolines (Scheme 9). The synthetic sequence requires a stereoselective opening of 1,2-oxazoline with the formation of methyl 2-aminoglucoside in the pyranose form. The C-3 mesylated derivative after deprotection of 5,6-diisopropylidene functionality can be easily converted to epoxide, which can also be stereoselectively opened with an azide anion to form C-4 axial azide. C-6 mesylation of azide, followed by its reduction with sodium borohydride/nickel chloride produces methyl 2-amino-4-acetamidogulopyranoside. 

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