O-acyl hydroxamate

Treatment of O-acylated hydroxamic acids with base provides isocyanates. 

Cyclocondensation of malonyl derived O-acyl hydroxamic acid derivatives 6, in the presence of phosphazene super base P2-r-Bu 7, gave rise to isoxazolone carboxylic esters 8 <03TL7763>. 

P-Lactams.1 A biomimetic synthesis of /3-lactams from chiral amino acids such as L-serine has been developed by Mattingly and co-workers. The protected amino acid (1) is first converted into the O-alkyl or O-acyl hydroxamate (2), which undergoes cyclization to derivatives of l-hydroxy-2-azetidinones on treatment with triphenylphosphine-carbon tetrachloride. This cyclization is also possible with triphenylphosphine-diethyl azodicarboxylate.2 The final step involves reduction of the N—OH group with TiClj.3 The advantage of this method over that of Wasserman (9,428), which involves cyclization of /3-haloamides, is that a strong base such as NaH is not required. 

Rearrangement of O-acyl hydroxamic acid derivatives with base or heat to amines or urea derivatives (via isocyanates), or rearrangement of carboxylic acids via their hydroxamic acids to amines (see 1st edition). 

Clark, A.J., Al-Faiyz, Y.S.S., Patel, D. and Broadhurst, M.J. (2001) Rearrangement of unactivated A-alkyl-O-benzoyl hydroxamic acid derivatives with phosphazene bases. Tetrahedron Letters, 42, 2007-2009 Clark, A.J., Patel, D. and Broadhurst, M.J. (2003) Base-mediated reaction of A-alkyl-O-acyl hydroxamic acids synthesis of 3-oxo-2,3-dihydro-4-isoxazole carboxylic ester derivatives. Tetrahedron Letters, 44, 7763-7765. 

The biological activity of a drug can also depend on its rate of release from the carrier. Molz compared the cytotoxic effect of polymers in which the N,N-bis(2-chloroethyl)amino group was linked to the polymer either via an urethane or O-acyl-hydroxamic bond. The latter polymeric drug was much more effective due to faster... 

Hence, the implication of combinatorial chemistry for high throughput generation of structurally diverse hydroxamic acids is self-evident. Several solid-phase approaches for their syntheses have been reported,1 7-11 the majority of which are based on the anchoring of iV-hydroxyphthalimide onto an appropriate solid support. After hydrazine-mediated /V-dcprotcction, /V-acylation of the resin-bound hydroxylamine would yield the desired O-anchored hydroxamic acid, which is typically released by acidolysis. 

Preparation of hydroxamic acids through acylation of hydroxylamines is a common and straightforward reaction. However, acylation of iV-alkylhydroxylamines is known to proceed on both the oxygen and nitrogen atoms and can result in mixtures of N- and 0-acylation products . In iV-alkylhydroxylamines possessing a bulky " or an electron-poor substituent on nitrogen atom, O-acylation is predominant. 

Intramolecular addition of hydroxylamines and hydroxamic acids to the non-activated double bonds is possible through oxidative cyclization. Reaction of O-Acyl fi,y-unsaturated hydroxamates (e.g. 56, equation 38) with bromine provides 3,4-substituted iV-hydroxy -lactams such as 57 with high diastereoselectivity. Analogous reaction of O-benzyl hydroxylamine 58 (equation 39) with iodine results in five-membered cyclization with 2 1 ratio of diastereomers. ... 

The O-acyl derivatives of hydroxamic acids225 give isocyanates when treated with bases or sometimes even just on heating, in a reaction known as the Lossen rearrangement. The-mechanism is similar to that of 8-14 and 8-15 ... 

Hydroxylamines and hydrazines can be acylated on insoluble supports using the same type of acylating agent as is used for the acylation of amines [146-149]. Because of their higher nucleophilicity, hydroxylamines or hydrazines can be acylated more readily than amines, and unreactive acylating agents such as carboxylic esters can sometimes be successfully employed (Table 13.10). Polystyrene-bound O-alkyl hydroxamic acids can be N-alkylated by treatment with reactive alkyl halides and bases such as DBU (Entry 5, Table 13.10). 

The lactim/lactam tautomerism of hydroxamic acids and their O-alkyl and O-acyl derivatives has also been studied [146], Hydroxamic acids exist in the solid state and in polar solvents as the lactam tautomer only, whereas in nonpolar solvents the hydroximic tautomer is also present. Further analogous solvent-dependent lactim/lactam equilibria have been observed for certain Schiff bases (prepared from anilines and 2(4)-hydroxybenzaldehyde [256] or 2-hydroxynaphthaldehyde [257]), for 3-hydroxypyrazole [258], and for 3-methyl-l-phenylpyrazolin-5-one [259]. 

The fourth related rearrangement reaction is the Lossen reaction, which generally occurs by base treatment of 0-substituted hydroxamic acids which possess electron-withdrawing functions at the oxygen atom (e.g. O-acylhydroxamic acids), giving amines via isocyanates (equation 6). Preliminary 0-activation (e.g. O-acylation) of hydroxamic acids is essential for a smooth rearrangement, otherwise it will not occur, ilie Lossen reaction is not as useful as the other three rearrangements since hydroxamic acids are not readily available. 

A further example is the Lossen rearrangement, in which an O-acyl derivative of hydroxamic acid, RCONHOCOR, gives an isocyanate on treatment with hydroxide ion, which in turn may be hydrolysed to the amine. Illustrate this reaction pathway. 

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