Ozonation, acyl groups

The 1,2-azole ring systems are relatively stable to oxidative conditions, even allowing snbstitnent alkyl- or, more efficiently, acyl-groups to be oxidised up to carboxylic acid Ozone cleaves the isoxazole ring. ... 

These reaction pathways are in parallel with those for alkanes mentioned in Sect, 7.2.2, and the reactions (7.7, 7.8 and 7.9) after CH3 radicals are formed in reaction (7.43) are the same as those in the oxidation processes of methane described in Sect. 7.1. The specific feature of oxidation reactions of aldehydes is the formation of a metastable peroxy acyl nitrates from the reaction of peroxy acyl radicals with NO2 by reaction (7.41). In the case of acetaldehyde, peroxy acetyl nitrate, CH3C(0)00N02, is formed. This compound is called PAN (Peroxy Acetyl Nitrate), and is known to have much stronger toxicity to plants than ozone. A group of peroxy acyl nitrates are collectively called PANs. 

Carbacephalosporins The ketene-imine cyclization described above has been extended to a synthesis of a chiral carbacepham (4). This synthesis uses a dihy-droanisole group as the equivalent of a p-keto ester. Thus the azetidinone 1, obtained in 80% yield by the above route, was reduced and acylated in situ to provide 2. Ozonization followed by a rhodium-catalyzed cyclization of an a-diazo-P-keto ester provides 3, which is a useful intermediate to various substituted car-bacephams such as 4. 

One approach to unnatural amino acids is to use a readily available amino acid, such as L-phenyl-alanine, as the starting material. The Birch reduction of L-phenylalanine (1) was carried out with lithium in ammonia, followed by acylation of the amino group to produce compound 2, which was further esterihed to produce the cyclohexa-l,4-dienyl-L-alanine derivative 3 (Scheme 11.1). The ozonolysis step of the reaction was carried out at -78°C in a dichloromethane solution presaturated with ozone to reduce the extent of oxidation of the diene 3 to produce 4. Cyclization was then carried out by the introduction of either hydroxylamine hydrochloride to produce the isoxazol-5-ylalanine derivative 5 or phenylhydrazine to give a 1 1 mixture of (l-phenylpyrazol-3-yl)alanine derivative 6 and the (l-phenylpyrazol-2-yl)alanine derivative 7.4,5... 

Cmde lecithin contains a number of functional groups that can be successfully hydrolyzed, hydrogenated, hydroxylated, ethoxylated, halogenated, sulfonated, acylated, succinylated, ozonized, and phosphorylated, to name just a few possibilities (1). The only chemically modified food-grade products produced in significant commercial quantities at the present time are the ones obtained by hydroxylation, acetylation, and enzymatic hydrolysis (58). Hydroxylated or acylated lecithins represent chemical modifications to improve the functionality in water-based systems. 

Two disadvantages are associated with the use of 5-acetyl or 5-benzoyl derivatives in peptide syntheses (a) Base-catalyzed hydrolysis of 5-acetyl- and 5-benzoylcysteine occurs with P-elimination to give olefinic side products, CH2=C-(NHPG)CO- (b) the yields of peptides formed by coupling an unprotected amino group in an 5-acyl-cysteine are low because of prior S-N acyl migration. An 5-acetyl group is stable to oxidation of a double bond by ozone (.20°C, 5.5 h, 73% yield). ... 

The formation of a dinitrone - in contrast to the ozonation of DOPPD - is inhibited most probably just by the stabilizing effect of the N-phenyl group. The same structural moiety stabilizes aromatic nitro and nitroso compounds formed via the amine oxide pathway no Bandrowski bases were detected in the ozonation product of HPPD. The authors (40) favor the N-alkyl side reactivity in HPPD also in the interpretation of the formation of N-phenyl-N -acyl-l,4-PD, i.e. in the condensation with aliphatic aldehydes. 

In the case of poly(chloroprene), the ozonized mixtures were treated with piperidine before the FAB-MS analysis to transform the reactive acyl chloride groups, formed by cleavage of the double bounds along the main chain, to stable amide end-groups. 

N-acylated esters of (cyclohexa-l,4-dienyl)-L-alanine are ozonized aiming at the synthesis of novel uimatural amino acids. The combined reduction and ozonolysis followed by condensation with a suitable nucleophile results in transformation of the aromatic ring of L-Phe to isooxasolyl, N-phenylpyrazolyl and to bicyclic pyrazolo[l,5-a]pyrimidine groups. The preparation of heterocyclic alanine derivatives is reported [89]. 

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