With a-Hydroxy Ketones

2-Substituted quinoxalines can be also prepared by a one-pot process commencing from hydroxy ketones using a Mn02 (Raw et al. 2004), FeCla/morphoIine (Song et al. 2012), and silica gel (Jeena and Robinson 2014)-mediated tandem oxidation process (Table 2.1). 

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With a-hydroxy ketones and their related tosyloxy derivatives. The imidazo [2,T ]thiazole 364 was prepared by acetic acid-catalyzed cyclocondensation of 2-hydroxy-l,2-diphenyl-ethanone with thiophenyl-substituted 2-aminothiazole 363 (Equation 163) <2002MI110>. Under MW irradiation and in the presence of montmorillonite K-10 clay, a mixture of a-tosyloxyketones 365 and 2-imidazolidinethione led to the substituted 5,6-dihydro-imidazo[2,l- ]thiazoles 366 (Equation 164) <1998J(P1)4093>. When using a-tosyloxyacetophenone, prepared by reaction of acetophenone with [hydroxyl(tosyloxy)iodo]benzene (HTIB), 5-aminopyrazole 367 could be converted to imidazo[l,2- ]pyrazole 368 in basic medium (Equation 165) <2005JHC209>. 

Allylation of a-hydroxy ketones.1 An allyltrifluorosilane and triethylamine (1 1) react with a-hydroxy ketones in refluxing THF to form tertiary homoallyl... 

The reaction of urea (and thiourea) derivatives with a-hydroxy-ketones yields imidazoles. Thus, condensation of benzoin with N-pheny 1 thiou rea in hexanol in the presence of catalytic quantities of HC1 (or dry HC1) gives l,4,5-triphenylimidazole-2-thione (21) in 50-60% yield.100... 

When urea (or thiourea) reacts with a-hydroxy ketones or a-diketones the products are imidazolin-2-ones (or -thiones) (70AHC(12)103,66RCR122). The reaction is limited to the preparation of 4,5-alkyl(or aryl)- or l,4,5-trialkyl(or triaryl)-imidazoles since an oxygen or sulfur function appears at C-2. Benzoin condenses with iV-phenylthiourea in hexanol in the presence of catalytic quantities of HCl to give l,4,5-triphenylimidazoline-2-thione (131) in 50-60% yield (Scheme 69). While 1-methylurea can also take part in the reaction. 

The crucial role of chelation in terms of increasing rigidity and reactivity of chiral dienophiles was first demonstrated (1983) with a-hydroxy ketone derivatives (Scheme 83, Table 18). Conjugated a-hy-droxy ketones (340) and their enantiomers were prepared via separation of acid (338) from its enantiomer, followed by successive treatment with Bu"Li and the appropriate vinyllithium reagent (339). 

A significant improvement was the introduction of zinc borohydride, which has become the reagent of choice for a variety of chelation-controlled reductions. With a-hydroxy ketones as substrates (Table 3)15,16 the zinc-based reagent is reliably superior to lithium aluminum hydride, presumably because of the increased tendency of zinc(II) ions, compared with lithium ions, to form chelated complexes. The results arc not uniformly excellent, but in many cases the selectivity is highly satisfactory. The method can give useful results with relatively complex substrates, e.g., the reduction of. sv w-3-hydroxy-4-mcthyl-5-triphenylmethoxy-2-pentanone. 

Most 2,4-dinitrophenylhydrazones crystallize well and have sharp melting points. However, they tend to form mixed crystals, which may lead to errors in identification of the carbonyl compound. They are most suitable for separation and identification of the simple ketones. Their use with a-hydroxy ketones is not recommended.909... 

The starting hemiacetal equilibrates with a hydroxy ketone. 

Reaction of butadiene with a-hydroxyketones, R C(0)CH(0H)R , or a-acylamino ketones, R C(0)CH(NHCOR )R, gives predominantly (24), Z=RiC(0)C(OH)R orZ=RiC(0)(NHCOR )R, respectively, when catalysed by PdCl8+p-MeCeH4S0aNa.4H80. In contrast, a [Pd(PPh3)4]-catalyst alone gives a mixture of (24) and (25) with a-hydroxy ketones, whereas a-acylamino ketones normally fail to react. 

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