Letters in Organic Chemistry - Volume 5, Issue 6, 2008

The AM1 “in silico tool box” is capable of correctly predicting the preferred stereochemistry and the stereoenhancement effects of Lewis acids during free radical hydrogen transfer reactions for a series of chiral non-racemic organostannanes to several prochiral radicals. While other methods overestimate the difference in energy involved, the accuracy of the predicted magnitude, using the above-mentioned process, for observed enantioselectivities of certain cases is noteworthy. The methodology has shown a remarkable adaptability to various structurally diverse chiral non-racemic hydrides and given reproducibility of the trends in enantioselectivity observed by several independent experimental studies.

RCM mediated stereoselective synthetic approaches for the branched C-C disaccharide and its aza analog via Ring Closing Metathesis protocol are described.

A rapid, high yielding and regioselective process has been developed for the synthesis of aryl sulfones via sulfonation reactions using a mixture of graphite/MeSO3H.

Glycosidation reactions of 5-thioxylopyranosides were examined. The reaction of a 1,5-dithioxyloside derivative with N-iodosuccinimide and trifluoromethanesulfonate gave ring-opened products, but no glycosylated products. On the other hand, a 5-thioxylopyranosyl trichloroacetimidate derivative was found to react with aliphatic alcohols and a sugar alcohol to give a 5-thio analog of biologically important β(1,4)xylopyranoside structures.

Treatment of various aliphatic and functionalized aromatic thiols with a mixture of sodium bromate/sodium hydrogen sulfite gave the corresponding disulfides in good yields.

A variety of TBDMS ethers were directly transprotected into their acetates/benzoates with acetyl chloride/ benzoyl chloride in a one-pot procedure using CuBr2 (30 mol%). Chemo-selective transprotection of aliphatic TBDMS ethers in the presence of aromatic TBDMS ethers was achieved using 10 mol% of the catalyst. Various sensitive functional groups such as ketals, ketones, alkenes, allyl and benzyl ethers were found to be compatible under the reaction conditions.

Synthesis of photoluminescent lanthanide(III) chelates that allow selective solution phase labeling of bioactive molecules using phosphate elimination, native chemical ligation, and ‘click-chemisty’ is described.

A convenient procedure has been developed for the synthesis of heteroaryl, aryl and alkyl 14-[H]-dibenzo [a,j] xanthenes by one-pot condensation of β-naphthol with heteroaryl, aryl and alkyl aldehydes in the presence of CBr4, under solvent free conditions in very good yields. The catalyst is readily available and is inexpensive to promote the synthesis of xanthenes.

Epoxides undergo smooth ring opening with N-heterocyclic compounds such as indoles and pyrroles in the presence of a catalytic amount of bismuth triflate under mild conditions to produce C-alkylated indole and pyrrole derivatives in high yields with a high regioselectivity. Imidazole and benzotriazole affords N-alkylated products under identical conditions.

A novel simple two-step synthesis of phenolic benzosultams is first reported. o-Lithiation of N-Boc-otoluenesulfonamide followed by reaction with methoxybenzaldehydes gave carbinol sulfonamides, which were converted to the cyclic phenolic compounds via TMSCl/NaI/MeCN reagent mediated sequence involving N-deprotective cyclization and O-demethylation.

Yb(NTf2)3 catalyzed Michael reactions of β-ketoesters to α,β-unsaturated carbonyl compounds more efficiently than did Yb(OTf)3 without any additives in pure water, providing Michael adducts in moderate to good yields.

A simple and convenient synthesis of (±)-bakuchiol has been developed. The key intermediate was obtained in only one step from the easily available starting material, citral, by an 1,4-addition procedure. (±)-Bakuchiol was finally produced in 4 steps to give 50% overall yield.

A new and short total synthesis affording (±)-Xanthorrhizol in 90% overall yield is described. The method comprises three steps: 1) C-alkylation of 3-methoxy-4-methyl-benzylcyanide with 5-iodine-2-methyl-pent-2-ene; 2) Cmethylation of the resulting cyanide; and 3) one-pot decyanation and demethylation carried out by the action of lithium.

The catalytic activity of Molybdenum(V) chloride as a Lewis acid in carbon-carbon bond formation reactions has been described. The reactions of various substrates viz. aldehydes, benzyl alcohols, glucals and aminals with carbon nucleophiles were studied in the presence of MoCl5 as a catalyst. Progress of all the reactions proceeded smoothly to furnish the corresponding products in good yields.

A facile synthetic approach to 3'-substituted-5-cyclopropanespirohydantoins has been developed and used to synthesize some new spirohydantoins. The key aspect is the preparation of α-carboethoxy isocyanate basing on Curtius rearrangement, which can be ring-closed to the expected spirohydantoins after the reaction with various amines and hydrazines. Interestingly the cyclopropane ring doesn't open during the whole process.

A facile route to synthesize amide type N-9-purinyl-acetic acid derivatives both in guanine and hypoxanthine series is described. The regioselective alkylation of the 6-chloro-purine derivatives followed by their conversion into guanine or hypoxanthine nucleus in mild acidic conditions, are the key steps that allow the efficient preparation of compounds with an acetic function coupled with various amino acids.

Oxalic acid as a solid organic acid catalyzed the synthesis of bis(indolyl)methanes by the reaction of indole derivatives and different carbonyl compounds. All the reactions were performed at ambient temperature and completely heterogeneous conditions and the products were obtained in high yields.

Silica-supported dichlorophosphate had been proved to be an efficient, recoverable and reusable catalyst for Beckmann rearrangement of a variety of ketoximes and dehydration of various aldoximes in THF under microwave irradiation. This protocol has advantages of high conversion, high selectivity, short reaction time, no environmental pollution and simple work-up procedure.

Novel spiro[thioxanthene-naphthopyrans] were prepared and their photochromic properties in solution investigated under UV-Vis continuous irradiation. When compared to known similar naphthopyrans these new compounds showed a significant bathochromic shift of the maximum wavelength of absorption and faster ring closure kinetics.

A new, convenient and high yield route to luzindole, the most commonly used melatonin receptor antagonist, is described. The new method involves the Sonogashira coupling reaction between 2-iodoaniline and 3-phenyl-1-propyne followed by cyclisation of the adduct formed, C-3 indole nitroolefination with 1-(dimethylamino)-2-nitroethylene/TFA, reduction to the respective tryptamine and finally acetylation.