Generating acylated

The photochemical cyclisation of p.y-unsaturated ketoximes to 2-isoxazolines, e.g., 16—>17, has been reported <95RTC514>. 2-Isoxazolines are obtained from alkenes and primary nitroalkanes in the presence of ammonium cerium nitrate and formic acid <95MI399>. Treatment of certain 1,3-diketones with a nitrating mixture generates acyl nitrile oxides, which can be trapped in situ as dipolar cycloadducts (see Scheme 3) <96SC3401>. 

The simplest and most direct manner to generate acyl anion equivalents is through reaction of an NHC with an aldehyde, generating an enamine species 8, commonly referred to as a Breslow intermediate . Subsequent reaction with an electrophile, classically using aldehydes or enones, generates the benzoin and Stetter products 10 and 11 respectively (Scheme 12.1). 

Carboxylic acids can be converted to acyl chlorides and bromides by a combination of triphenylphosphine and a halogen source. Triphenylphosphine and carbon tetrachloride convert acids to the corresponding acyl chloride.100 Similarly, carboxylic acids react with the triphenyl phosphine-bromine adduct to give acyl bromides.101 Triphenviphosphine-iV-hromosuccinimide also generates acyl bromide in situ.102 All these reactions involve acyloxyphosphonium ions and are mechanistically analogous to the alcohol-to-halide conversions that are discussed in Section 3.1.2. 

Halogenations are strongly catalyzed by mercuric acetate or trifluoroacetate. These conditions generate acyl hypohalites, which are the active halogenating agents. The trifluoroacetyl hypohalites are very reactive reagents. Even nitrobenzene, for example, is readily brominated by trifluoroacetyl hypobromite.19... 

Acyl nitroso compounds react with 1, 3-dienes as N-O heterodienophiles to produce cycloadducts, which have found use in the total synthesis of a number of nitrogen-containing natural products [21]. The cycloadducts of acyl nitroso compounds and 9,10-dimethylanthracene (4, Scheme 7.3) undergo thermal decomposition through retro-Diels-Alder reactions to produce acyl nitroso compounds under non-oxidative conditions and at relatively mild temperatures (40-100°C) [11-14]. Decomposition of these compounds provides a particularly clean method for the formation of acyl nitroso compounds. Photolysis or thermolysis of 3, 5-diphenyl-l, 2, 4-oxadiazole-4-oxide (5) generates the aromatic acyl nitroso compound (6) and ben-zonitrile (Scheme 7.3) [22, 23]. Other reactions that generate acyl nitroso compounds include the treatment of 5 with a nitrile oxide [24], the addition of N-methyl morpholine N-oxide to nitrile oxides and the decomposition of N, O-diacylated or alkylated N-hydroxyarylsulfonamides [25-29]. 

Scheme 30 Nucleophilic addition to an electrochemically generated acyl imminium ion.

In addition to the direct nucleophilic alkylation of carbonyl complexes, the acylation of metallates with, e.g., carboxylic acid chlorides [73,100,102] or anhydrides [79] is a practical way of generating acyl complexes (Figure 2.4). Illustrative examples are given in Table 2.3. 

There are other activation procedures which generate acyl halides in situ in the presence of the nucleophile. Refluxing a carboxylic acid, triphenylphosphine, bromotri-chloromethane, and an amine gives rise to the corresponding amide103. 

The introduction of substituents into position 7 of a 2,4-disubstituted pteridine can be effected very cleanly by the use of acyl radicals typically and has been known for many years. Treatment of aldehydes with /-butyl hydroperoxide and iron(ll) generates acyl radicals which add selectively to the 7-position. A recent exploitation of this chemistry has provided a large number of new examples including both aryl and alkyl acyl radicals as reagents <2004PTR129> pA , data have been compiled (Section 10.18.4) and many nucleophilic substitution reactions of the 7-acylated pteridines and functional group modifications have been described (Section 10.18.7.2). 

Scheme 2. Assembly of benzodiazepines using in situ-generated acyl fluorides.

Acylphosphonates generate acyl anion equivalents with cyanide via phosphonate-phosphate rearrangement. These anions react with aldehydes to provide cross-benzoin... 

Acyl radicals. Tributyltin hydride in combination with AIBN can generate acyl radicals from phenylselenol esters, which are a better source than acyl chlorides or phenylthiol esters for acyl radicals for intramolecular cyclization. 

In addition to alkyl, aryl and vinylcobalt/radical species, acid chlorides may be converted into acylcobalt species by reaction with the cobalt(I) salens. These generate acyl radicals under photolysis, which participate in similar reactions to the alkyl radicals in most cases (equations 184 and 185). Acylcobalt(III) species bearing an a-aryl or vinyl substituent, on the other hand, undergo concomitant decarbonylation to afford a benzylic or allyl radical, which then may undergo a number of bond-forming processes, including homocoupling361. 

Finally, nucleophilic halide ions can enhance the reactivity by promoting the removal of generated acyl moieties from the metal centers ... 

And there now is permeation, and a new transacylation Generating acyl CoA in the matrix space. 

The enantiopure 1-oxa-1,3-diene 278 is prepared by acylation of benzyl vinyl ether with oxalyl chloride. This generates acyl chloride 276, which acylates the lithium salt of 2-oxazolidinone 277. In the presence of Me2AlCl, diene 278 adds to (Z)-l-acetoxy-2-ethoxyethene giving mostly adduct 279, whereas, when using Me3SiOTf as promoter of the hetero Diels-Alder addition, diastereomer 280 is the major adduct (Scheme 13.79). Adducts 279 and 280 have been converted into ethyl P-d-mannopyranoside and ethyl-P-L-mannopyranoside, respectively [149]. 

Narasaka and Sakurai found that chromium carbene complexes, when exposed to a copper(II) reagent, generate acyl radicals by a one-electron oxidation, and these then undergo addition to electron-deficient alkenes (Scheme 4-27) [50J. The resulting copper(I) species reduces the resulting radical to an anion, and subsequent protonation leads to the addition product. This redox type acyl radical transfer reaction works particularly well for aromatic acyl radical systems, for which decarbonylation is not a problem. Related work has also recently appeared [51]. 

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