The conformational rigidity of butane-1,2-diacetals as a powerful synthetic tool

Butane-1,2-diacetals are selective protecting groups for trans-diequatorial-1,2-diols and have been widely used in carbohydrate chemistry. The scope of diacetal protection has been extended more recently to include other important hydroxylated chiral templates containing trans-1,2-diols, such as quinic and shikimic acids, the protection of which as diacetals leads to a strong conformational rigidity that induces excellent diastereoselectivity control. In addition, the chiral information stored in the diacetal backbone has also been exploited in the synthesis of important building blocks, such as glycerate, glycolate and tartrate diacetal derivatives. In this critical review, the synthetic power of the conformational rigidity and the chirality stored in the diacetal backbone is described. This phenomenon will be illustrated with recent examples of applications in the synthesis of natural products or biologically interesting compounds (80 references).     

Diastereoselective diallylation of various butane-2,3-diacetals by allylsilane

Five butane-2,3-diacetals obtained from the reaction of 1,2-diols and methanol with biacetyl have been treated with allylsilane in the presence of titanium tetrachloride to afford bis-allyldioxanes as single diastereoisomers.     

Macrocycles with 1,2-dicyano-1,2-dithioethene units

Abstract

The new unsaturated macrocyclic tetrathioethers (Z,Z)-4 (n = 0), (Z,Z)-5 (n = 1), (Z,Z)-6 (n = 2) and (Z,Z)-7 (n = 3) were synthesized by the cyclization of (Z)-disodium-1,2-dicyanoethene-1,2-dithiolate (Z)-3 with ω,ω'-dibromoalkanes BrCH₂CH₂(CH₂)nCH₂Br (n = 0;1;2;3) on refluxing in dioxane in yields up to 15%. By reaction of the dithiolate (Z)-3 with 1,3-dibromopropane the unsaturated hexathioether (Z,Z,Z)-6 was also obtained. By the cyclization of dithiolate (Z)-3 with 1,5-dibromopentane and 1,6-dibromohexane the (Z,E)- and (E,E)-isomers, respectively, were formed in addition to the (Z,Z)-isomers. The (E,E)- and (Z,E)-isomers are photochemically convertable to the corresponding themodynamically more stable (Z,Z)-isomers by irradiation with UV-light. The (E,E)-isomers can be synthesized in a straightforward manner using the (E)-disodium-1,2-dicyanoethene-1,2-dithiolate (E)-3. Crystal structures of (Z,Z)-5, (Z,Z)-6, (E,E)-6, (Z,E)-7 and (E,E)-7 are reported.     

Beauty and fascination of morphologies of the micro- and macrocosmos

Summary Twenty years of experience of the authors with scanning electron microscopy (SEM) led to the idea to present a collection of striking examples of micrographs of intrinsic structural beauty as a birthday present to Professor Fresenius. Harmonic structures, marvellous or scurrilous SEM micrographs of inorganic materials are followed by some examples from the living microcosmos. Sometimes, striking parallelisms to our macroscopic world can be detected, which ultimately lead to some outstanding macroscopic morphologies of the Tyrolean Alps. This is an experiment which tries to encompass fascinating morphologies of the microcosmos and morphologies of the environment of the authors, the Tyrolean Alps. It should empasize the totalitarian aspect of our existence as scientists in a specific environment although one of the authors has recently moved to a less dramatic landscape.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday     

Reactions of azoles with tetrachloro-1,2-difluoroethane and 1,2-dichlorodifluoroethylene

Tetrachloro-1,2-difluoroethane reacted with pyrazole and imidazole sodium salts to give mixtures of the corresponding N-(1,2,2-trichloro-1,2-difluoroethyl) derivatives and (E)-1,2-difluoro-1,2-dihetarylethenes. (E)-1,2-Difluoro-1,2-di(3,5-dimethyl-1H-pyrazol-1yl)ethene was also obtained as a result of replacement of chlorine atoms in 1,2-dichloro-1,2-difluoroethene. Analogous reaction with more nucleophilic imidazole involved replacement of not only chlorine but also fluorine atoms in 1,2-dichloro-1,2-difluoroethene, yielding tetraimidazolyl-substituted ethylene.     

Spectrophotometric estimation of molybdenum with disodiom cis-1,2-dicyanoetfaylene-1,2-dithiolate

A rapid spectrophotometnc method is described for estimation of molybdenum by use of the molybdenum-dicyanoethytene dithiolate complex. The bright green complex has its absorption maximum at 665 nm and in 50% aqueous acetonitrilc is stable for several days at 3M hydrochloric add concentration, The estimation is more sélective if longer wavelengths and high acidity are used. Beer's law is obeyed over the range 1-16 ppm molybdenum, optimum range 4-16 ppm. The overall formation constant Of the complex with metal to ligand ratio of 1:4 is 5 +/- 2 x 10(18). The molar absorptivity is 5.05 x 10(3) l.mole(-1).cm(-1).     

Preparation of enantiopure butane-2,3-diacetals of glycolic acid and alkylation reactions leading to alpha-hydroxyacid and amide derivatives

The preparation of butane-2,3-diacetal protected glycolic acid and related systems is described together with highly selective alkylation reactions of (R,R) and (S,S) butanediacetal protected glycolic acid. These compounds are readily deprotected to give enantiopure alpha-hydroxyacids, alpha-hydroxyesters or alpha-hydroxyamides by suitable choice of conditions.     

A Short Syntheses of 1,2-Dimethylazulenes

Dimethyl azulene-1,2-dicarboxylates are reduced with a 4–5 molar excess of DIBAH in Et₂O/hexane at 0° to yield the corresponding 2-(hydroxymethyl)-1-methylazulenes which can be further reduced to 1,2-dimethylazulenes on treatment with Et₃SiH in TFA at 60° (cf. the Table).     

A novel synthesis of 1,2-diphosphorylbenzenes

Synthesis of phosphorylchloroacetylenes, diphosphorylacetylenes and the Diels-Alder reactions of the latter to produce $\text{o}$-diphosphorylbenzenes are described.     

Lewis Acid Catalyzed Reactions of Thioketones with 1,2-Epoxycyclohexane and 1,2-Epoxycyclopentane

Non-enolizable thioketones and 1,2-epoxycycloalkanes undergo a Lewis acid catalyzed addition reaction to give 1,3-oxathiolanes. Appropriate reaction conditions are CH₂Cl₂ as the solvent, BF₃⋅Et₂O as the Lewis acid, and a temperature between −78° and r.t. Under the reaction conditions, the 1,3-oxathiolanes are only moderately stable. They decompose to yield the corresponding epithiocycloalkane and ketone. In general, 1,3-dithiolanes are isolated as minor products or, after prolonged reaction, as the main product. These secondary products are formed via the Lewis acid catalyzed reaction of the intermediate epithiocycloalkane and a second molecule of the thioketone. In the reaction of thiobenzophenone and 1,2-epoxycyclohexane, trans-8,8-diphenyl-7,9-dioxabicyclo[4.3.0]nonane is formed in small amounts as an additional side product (Scheme 12). In all cases, the newly formed heterocycle and the carbocycle are trans-fused. This result is consistent with a nucleophilic ring-opening of the complexed oxirane by the thioketone via inversion of the configuration and subsequent formation of the O(1)−C(2) bond of the 1,3-oxathiolane (Scheme 13). The surprising formation of the fused 1,4-oxathiepan derivative 23 (Scheme 9) is in accordance with an ionic reaction mechanism (cf. Scheme 15).     

Reaction of Z isomers of alkylaromatic 1,2-hydroxylamino oximes with 1,2-diketones

The reactions of Z isomers of alkylaromatic 1,2-hydroxylamino oximes containing the hydroxylamino group at the primary or secondary carbon atom with diacetyl afford 6-acetyl-5,6-dihydro-4H-1,2,5-oxadiazines. The reactions of these compounds with alkylaromatic 1,2-diketones produce N-substituted α-aroylnitrones or 6-aroyl-5,6-dihydro-4H-1,2,5-oxadiazines or, alternatively, their tautomeric mixtures. Dedicated to the memory of Academician V. A. Koptyug on the occasion of the 75th anniversary of his birth. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1008–1013, June, 2006.     

Reaction of chlorine fluorosulfate with 1-trifluoromethoxy-1,2-dichlorotrifluoroethane and 1,2-dichlorohexafluoropropane

Conclusions The reactivity of chlorine fluorosulfate increases with increasing acidity of the medium. In HSO₃F/SbF₅, chlorine fluorosulfate under mild conditions replaces the terminal chlorine atom in 1,2-dichlorohexafluoropropane and both chlorine atoms in 1-trifluoromethoxy-1,2-dichlorotrifluoroethane by fluorosulfate groups.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1669–1672, July, 1985.