Letters in Organic Chemistry - Volume 2, Issue 1, 2005

This short review focuses on the impressive examples of endeavors to make stoichiometric reactions catalytic. The strategies employed encompass the reuse of waste coproducts to achieve the catalytic cycles and to reduce the amounts of reagents of the transformations. Several methodologies for in situ regeneration of active agents are highlighted.

Reaction of 1 equivalent of N-“-acetyl-cis-4-azido-L-proline with 1 equivalent of an aldehyde and a catalytic amount of Pd-C under an H2 atmosphere cleanly and efficiently yields the corresponding N-“-acetylcis- 4-(N-γ-alkylamino)-L-proline derivative.

A new efficient one-pot protocol for the synthesis of tetrahydropyrrolo[2,3-d]azocines and tetrahydroazocino[4,5-b]indoles starting from ready-available tetrahydropyrrolo[3,2-c]pyridines and tetrahydropyrido[4,3-b]indoles correspondingly and dimethylacetylene dicarboxylate or ethyl propiolate is being offered.

The cycloadditions of a [4.3.0]-bicyclic α,β-unsaturated lactam, derived from 6-oxopipecolic acid, have been investigated. Although yields are modest, diastereoselectivity is high; reasons for the low reactivity are postulated, based on experimental observations and molecular modelling.

The reported synthesis of 4,4-dimethyl-2-R-2-oxazolines from carboxylic acids and 2-amino-2- methyl-1-propanol (1) using 2-chloro-4,6-dimethoxy-1,3,5-triazine (2) via direct coupling / cyclisation is questioned. The results reported herein indicate that the products are actually ?-hydroxyamides resulting from dehydrative R-COOH / 1 (peptide) coupling only, which has been known to be mediated by 2 for some time.

Synthesis of new dihydrospiro[quinoline-2,4'-piperidines] by a two-step synthetic route based on 4-piperidone imine reactivity is reported. An acetyl migration in 1-acetyl-1'-benzyl-4-methyl-3,4- dihydrospiro[quinoline-2,4'-piperidines] under debenzylation conditions (HCOONH4 / Pd / C / MeOH) is found.

Chiral secondary amines of the 3,4-dimethoxyphenylethyl and the tryptamine system were reacted with chiral acetylenic sulfoxides and achiral acetylenic sulfone. In the case of 3,4-dimethoxyphenylethyl amine, much improved diastereoselectivity (96 : 4) was observed as compared to the reaction of achiral secondary amine with chiral acetylenic sulfoxides.

Pyrazolium-3-carboxylates, which belong to the class of pseudo-cross-conjugated heterocyclic mesomeric betaines, decarboxylate to give nucleophilic carbenes, which can be trapped as pyrazolium salts. An aldol addition, a Knoevenagel reaction, and a Cannizzaro-type reaction are presented, which are initiated by the strong basicity of the in situ generated pyrazol-3-ylidene.

Reaction systems peripheral to the 1:2 condensation reaction between o-phthalaldehyde and primary amine were studied. Newly identified were 1,3-dihydroxyisoindoline, o-iminomethylbenzaldehyde and oamidiniobenzoate, the last of which exhibited moderate anticancer activity.

1,3,5 Trisubstituted hydantoins having two different substituents at the nitrogen atoms could be smoothly prepared by a domino condensation / aza-Michael / O→N acyl migration of asymmetric carbodiimides with activated α,β-unsaturated carboxylic acids. The regiochemical outcome of the process, as well as scope and limits, are discussed.

1,3-Oxazolidine-2-thiones fused to ketohexose backbone were prepared from 1-O-, 1-N- or 1-Sderivatives in reasonable yields. Spontaneous ring closure leads to spiro-tricyclic unexpected aminal and amide structures.

Cerium is the less rare “rare earth”; its salts are commercially available and inexpensive. However, screening of lanthanide triflates as Lewis acid catalysts in the Friedel-Crafts acylation of aromatics skipped cerium triflate. Now, catalytic Friedel-Crafts acylations of activated benzenes are performed by using cerium (III) triflate as catalyst. For less activated arenes, addition of lithium perchlorate as co-catalyst is required. In Read More

In a 25% methanolic aqueous solution the metal ion specificity of ratiometric fluorescence response of a bis-anthrylpolyamine host is increased versus a purely aqueous medium. It is proposed that the geometry of the host, metal ion size, and hydrophobicity work in concert to give the geometry leading to excimer fluorescence.

A highly selective and efficient method for the chemoselective deprotection of t-butyldimethylsilyl (TBS) ethers using 20 mol% of zirconium(IV) chloride as Lewis acid in isopropanol is reported. Remarkable selectivity was observed towards the TBS group deprotection while TPS remains unaffected. The reaction is fast and the conditions are tolerable to a variety of protecting groups, carbohydrates and terpenes. The prod Read More

A novel tri-metal system (Zn / CdCl2 / InCl3) has been developed as an effective mediator in the benzylations of various aldehydes in tap water to afford the corresponding alcohols. It can selectively mediate the benzylation of aldehyde in the presence of ketone. CdCl2 and InCl3 are easily recycled and used in this reaction without further purification.

The copper-catalysed asymmetric conjugate addition of diphenylzinc to 2-cyclohexenone has been carried out with good results. An enantiometric excess of up to 91% was obtained using 5 mol% CuI and 5 mol% phosphine ligand derived from L-proline.

The regioselective [2+2] cycloaddition of a substituted benzyne possessing a fused four-membered ring to a ketene acetal has been theoretically studied. This cycloaddition presents a two-step mechanism that is initiated by the nucleophilic attack to the benzyne to give a zwitterionic intermediate. The analysis performed on the basis of the global and local electrophilicity of reagents correctly explain the observed reac Read More

Fumaric and maleic acid esters are functionalised with propargylamine. A subsequent Pd / Cucatalysed oxidative homo-coupling of two alkyne moieties yields the corresponding dimers. Hence, a model of an unsaturated polyester chain is cross-linked in a well-defined, metal-mediated reaction in a very high yield.

Oximes, hydrazones and semicarbazones are converted to the corresponding carbonyl compounds using imidazolium dichromate adsorbed on alumina (IDC / alumina).