Fèlix Urpí

A fast procedure for the reduction of azides and nitro compounds based on the reducing ability of Sn(SR)3-species

Abstract Tin(II) complexes prepared by treatment of SnCl 2 or Sn(SR) 2 with appropriate amounts of RSH and Et 3 N appear to be the best reducing agents for azides (to amines) reported so far. These tin(II) complexes also reduce primary and secondary aliphatic nitro compounds to oximes, usually within minutes at r.t. or hours in cold, and tertiary aliphatic as well as aromatic nitro compounds to afford the corresponding hydroxylamines. In general, azides react more rapidly than nitro substituents, whereas carbonyl groups, sulphoxides, sulphones, nitriles, and esters are practically unreactive under the same conditions. Some mechanistic details of the reaction of Sn(SPh) 3 - with azides and nitro compounds have also been elucidated.

New synthetic tricks. Triphenylphosphine-mediated amide formation from carboxylic acids and azides

Abstract Equimolar amounts of carboxylic acids, aryl or alkyl azides, and Ph3P in refluxing benzene (hexane, toluene) afford amides in good yields. Insolubility of zwitterions Ph3P+-NH(CH2)nCOO-, arising from μ-azido acids and Ph3P, limits the utilization of the method for lactame formation.

Asymmetric acetate aldol reactions in connection with an enantioselective total synthesis of macrolactin A

Abstract Asymmetric aldol-like reactions of cinnamaldehyde, dienal 3 (fragment C7-C12 of macrolactin A), and dienal 4 (fragment C15-C24) with (i) chiral acetylthiazolidinethione-derived enolates, (ii) chiral boron enolates, and (iii) silyl enolates in the presence of chiral titanium-2,2′-dinaphthol complexes are compared. Use of the thiazolidinethione auxiliary and TiCl 4 shows practical advantages; e.g., C5-C12 fragment 7 has been isolated enantiomerically pure in 74% yield.

New Synthetic “tricks”. [Et3NH][Sn(SPh3)] and Bu2SnH2, two useful reagents for the reduction of azides to amines

Treatment of Sn(SPh)2 with PhSH and Et3N affords a tin(II) complex, soluble in organic solvents, which is the best reducing agent for azides reported so far. Bu2SnH2, although not so reactive, also shows several advantages with regard to the standard reducing agents for azides, such as its solubility in most solvents or its scarce reactivity with water.

One-pot conversion of azides to Boc-protected amines with trimethylphosphine and Boc-ON

Abstract Reaction of azides with trimethylphosphine followed by addition of 2-(tert-butoxycarbonyloxyimino)-2-phenyl-acetonitrile (Boc-ON) at −20 °C and stirring at r.t. for 5 h affords the desired Boc-amines in 87–100% yields. Similar yields are obtained by mixing all components together (azide, Me3P, and Boc-ON) in toluene or THF.

Alternative procedures for the macrolactamisation of ω-Azido Acids

ω-Azido acids, after activation of the carboxyl groups as mixed anhydrides, N3(CH2)nCOOCOAr (Ar = 3,5-dinitrophenyl or 2,4,6-trichlorophenyl), can be converted to macrolactams in good yields by treatment with Bu3P under high-dilution conditions; the presence of 4-dimethylaminopyridine improves slightly the yields. Amides and peptides can be readily obtained by this procedure.

New synthetic ‘tricks’. Advantages of using triethylphosphine in some phosphorus-based reactions

Abstract It is shown that Et 3 P can advantageously replace Ph 3 P, Bu 3 P, and other P(III) reagents in phosphazene reactions (amide and phthalimide formation) and disulphide-cleavage-based reactions (reduction of disulphides, thioester formation, and oxime hydrolysis).

Unconventional biradical character of titanium enolates.

Experimental and theoretical studies evidence an unconventional biradical character for the titanium enolates derived from α-alkoxy ketones and TiCl4. EPR experiments and ab initio calculations suggest that these titanium enolates have an open shell electronic ground state.

A practical procedure for the preparation of carbamates from azides

Abstract A practical procedure for the direct conversion of azides to carbamates has been found. High yields are obtained when primary and secondary aliphatic azides, as well as α-azido esters, are treated in THF with Me 3 P and several chloroformates (ClCOOBn, ClCOOMe, ClCOOEt, ClCOOCH 2 CCl 3 , ClCOOCH 2 CHCH 2 ).

β3-Amino acids by nucleophilic ring-opening of N-nosyl aziridines

Abstract N -Nosyl aziridines can be easily prepared from 1,2-amino alcohols derived from α-amino acids. Nucleophilic ring-opening of N -nosyl aziridines with cyanide ions followed by hydrolysis of the corresponding nitriles lead to N -nosyl β 3 -amino acids, which can be readily converted into a variety of derivatives bearing adequate functionality for peptide synthesis. The proposed methodology is simple, efficient, and amenable to large-scale preparations.