Bromocarbonyl compounds

A microwave-assisted, one-pot, two-step protocol was developed for the construction of polysubstituted 2-aminoimidazoles 101 via the sequential formation of imidazo[l,2-a]pyrimidinium salts from readily available 2-aminopyrimidines 99 and a-bromocarbonyl compounds 100, followed by opening of the pyrimidine ring with hydrazine <06OL5781>. A... 

Various cis-ot,g-epoxycarbonyl compounds are stereoselectively prepared via g-bromo-g-hydroxycarbony1 compounds by application of this Sn(ll) mediated aldol reaction to a-bromocarbonyl compounds (22). Also this reaction is employed for the stereoselective synthesis of 2-amino-2-deoxy-D-arabinitol. 

H-1,4-Benzothiazines can be obtained from 2-aminothiophenols, by their reactions with either a-bromocarbonyl compounds (70AC(R)383) or active methylene compounds (Scheme 34) (76JCS(P1)1146). 

There are several methods for the construction of 1,4-benzothiazines 2//-forms can be obtained by the cyclization of acetals (240 X = S) in the same manner as the corresponding oxazines (76T1407), and the 4H isomers are conveniently prepared by the cycloaddition of 2-aminothiophenol and disubstituted alkynes (80JHC793). Alternative syntheses also utilize 2-aminothiophenols, by their reactions either with a-bromocarbonyl compounds <70AC(R)383) or with active methylene compounds (Scheme 109) (76JCS(P1)1146). 

Condensation of selenocarboxamides with a-chloro- or a-bromocarbonyl compounds gives access to a wide variety of substituted 1,3-selenazoles by choosing suitable reaction partners and, in most cases, they are produced in good to excellent yields [3, 14, 174], For example, the phenacetyl bromide (100) and selenobenza-mide (101) gave 2,4-diphenyl-l,3-selenazole (102) (Scheme 30) [174],... 

Stephenson and coworkers applied reductive photoredox catalysis to trigger radical 6-exo cyclizations of co-pyrrole or co-indole-substituted a-bromocarbonyl compounds 124 [186] as well as radical 5-exo cyclizations of 2-bromo-2-(4-pentenyl)malonates 126 (Fig. 32) [187]. These cyclization processes provide bi- or tricyclic products 125 or cyclopentanecarboxylates 127 in moderate to excellent yields. The initial radical was formed with reduced ruthenium catalyst HOB generated similarly as above from 110 and a sacrificial amine... 

If the ketone is unsymmetrical, this reaction will occur on the more substituted side, for the same reason that acid-catalysed enol bromination gives the more substituted a-bromocarbonyl compound (see the box on p. 536). 

The reaction of dibenzoylacetylene and enol systems, such as acetylacetone, 5,5-dimethylcyclohexane-l,3-dione, 1-naphthol, 2-naphthol, 2,7-dihydroxynaphthalene, or 8-hydroxyquinoline in the presence of triphenylphosphine, leads to tetrasubstituted furans in 65-83% yield (Equation 115) <2002TL4503>. DABCO-catalyzed reaction of a-bromocarbonyl compounds with DMAD also yields highly substituted furans <2005JOC8204>. 

The anion of N-phenyltiiflamide is nucleophilic enough to react widi activated and unactivated halides under mild conditions. Base treatment of the adduct eliminates triflinate (trifluoromethanesulflnate, CF3SO2-) to give the anil, which is then hydrolyzed in acid to the aldehyde (Scheme 15). The method works quite well with a-bromocarbonyl compounds (Scheme 16). ... 

Various l-alkylimidazo[l,2-a]pyrazine 7-oxide quaternary salts have also been synthesized from imidazoles (71JCS(C)2748). Two approaches were used. Quaternization of the oximes (462) with a-bromocarbonyl compounds gave salts (463) which cyclized in strong acid to the 7-oxides (464), and quaternization of the acetals (465) with bromoacetaldehyde oxime afforded salts (466) which were treated with concentrated sulfuric acid. 

Reaction of 2-aminopyrimidines 1525 with a-bromocarbonyl compounds 1526 affords imidazo[l,2- ]pyrimidinium salts 1528, which undergo a ring-opening process in the presence of hydrazine to yield 2-aminoimidazoles 1529 in good yields (Scheme 395) <20060L5781>. 

In contrast to the anion of diethyl phosphoramidate or trifluoromethanesulfonamide, which cannot be cleanly monoalkylated, - the anion of trifluoroacetamide (100) was monoalkylated by alkyl halides or alkyl methanesulfonate. The resulting A -alkylamides (101) were converted into primary amines by alkaline hydrolysis or reduction (NuBHa Scheme 42). Various primary amines were prepared from (100) with primary alkyl iodides or methanesulfonate, benzyl and allyl halides, a-bromocarbonyl compounds and 2,4-dinitrochlorobenzene. However, competitive elimination is a serious side reaction for less reactive primary alkyl chlorides and secondary halides or methanesulfonate. The synthesis of secondary amines from (100) has also been reported. ... 

The Reformatsky reaction, in which an a-bromocarbonyl compound was treated with Zn to give an enolate, was for a very long time the only way of quantitatively making enolates of weakly acidic carbonyl compounds. Nowadays it has been superseded by strong nonnucleophilic bases like LDA and KHMDS. 

Leaving groups on the a-carbon of carbonyl compounds (e.g., Br and OR) are reduced away by one-electron reducing agents. a-Bromocarbonyl compounds are reduced to the corresponding enolates by Zn in the Reformatsky reaction (Chapter 2). After the initial electron transfer, several pathways are possible, but all lead to the enolate. The enolate is usually allowed to react immediately with an electrophile such as another carbonyl compound. Before the advent of strong, non-nucleophilic bases, the Reformatsky reaction was the only way to prepare enolates of simple carbonyl compounds quantitatively. 

Thiazolium salts also react with a-bromocarbonyl compounds to give... 

Thiazolium salts also react with a-bromocarbonyl compounds to give 5,6-dihydro-2//-1,4-thiazines.7 7 Thus thiamine hydrochloride (96) afforded... 

The reaction of benzoin with potassium isocyanate in DMSO or pyridine gives carbamates, which undergo cyclodehydration to form 4-oxazolin-2-ones. Instead of benzoin, a-bromocarbonyl compounds may be used147-149 Eq. (36)1. 

Synthesis of (257) including the tricyclic derivatives, involves a cyclocondensation reaction of l-amino-2-pyridinethiones with a-bromocarbonyl compounds at reflux temperature in benzene or ethanol (Equation (55)) <828645,88H(27)733>. 

Pyrrolo[l,2-a]benzimidazole 59a,b and pyrrolo[l,2-fl]quinoxaline 60a,b derivatives are obtained via one-pot, three-component reactions from 1-benzylbenzimidazoles, a-bromocarbonyl compounds, and unsymmetrical activated acetylenic dipolarophiles in propylene oxide as both the reaction medium and acid scavenger, at room temperature [127] (Scheme 91). 

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