The Chiral Pool in the Pictet-Spengler Reaction for the Synthesis of ß-Carbolines.

The Pictet-Spengler reaction (PSR) is the reaction of a ß-arylethylamine with an aldehyde or ketone, followed by ring closure to give an aza-heterocycle. When the ß-arylethylamine is tryptamine, the product is a ß-carboline, a widespread skeleton in natural alkaloids. In the natural occurrence, these compounds are generally enantiopure, thus the asymmetric synthesis of these compounds have been attracting the interest of organic chemists. This review aims to give an overview of the asymmetric PSR, in which the chirality arises from optically pure amines or carbonyl compounds both from natural sources and from asymmetric syntheses to assemble the reaction partners.

Strategies for the asymmetric functionalization of indoles: an update.

During the past four years, the research of new synthetic methodologies for the rapid construction of enantiomerically pure substituted indole has had a fruitful and important growth. This research line continues to produce stunning arrays of enantioselective technologies either metal or organocatalyzed. Thus, an update of our previous review (Chem. Soc. Rev., 2010, 39, 4449-4465) has become necessary and this critical review documents the literature on this topic, until the end of 2013.

Applications of Bartoli indole synthesis.

In 1989, the reaction of vinyl magnesium halides with ortho-substituted nitroarenes leading to indoles was discovered. This reaction is now frequently reported as the "Bartoli reaction" or the "Bartoli indole synthesis" (BIS). It has rapidly become the shortest and most flexible route to 7-substituted indoles, because the classical indole syntheses generally fail in their preparation. The flexibility of the Bartoli reaction is great as it can be extended to heteroaromatic nitro derivatives and can be run on solid support. This review will focus on the use of the Bartoli indole synthesis as the key step in preparations of complex indoles, which appeared in the literature in the last few years.

Recent advances in organocatalytic methods for the synthesis of disubstituted 2- and 3-indolinones.

In recent years, organocatalysis has enhanced its importance as a tool for the synthesis of enantiomerically enriched compounds. Among the candidates for organocatalysis, the construction of asymmetric quaternary carbons is regarded as a challenging problem in organic synthesis. In particular, 3,3'-disubstituted oxindoles have one or more asymmetric quaternary carbon atoms and they represent a large family of bioactive compounds and synthetic derivatives that mimicry natural products. Therefore they are good targets for drug candidates and in the last two years many papers have appeared on organocatalytic methods for the synthesis of 3,3'-disubstituted oxindoles. Moreover, in the last few years 2-substituted and 2,2'-disubtituted 3-indolinones have also attracted the interest of chemists. This review aims to cover the literature on these topics from its origin to the end of 2011.

Organocatalytic strategies for the asymmetric functionalization of indoles.

The desire for new synthetic methodologies for the rapid construction of enantiomerically pure substituted indole has been a fruitful driving force for chemical research in the last few years. This research line has produced a stunning array of enantioselective technologies either metal or organocatalyzed. This critical review documents the development of organocatalytic indole alkylation strategies, until the end of 2009 (127 references).

The defensive secretion of Carabus lefebvrei Dejean 1826 pupa (Coleoptera, Carabidae): gland ultrastructure and chemical identification.

This study documents the defensive function of flavored humor secreted by the abdominal glands of Carabus lefebvrei pupae. The morphology and the ultrastructure of these glands were described and the volatile compounds of glands secretion were identified by gas chromatography/mass spectrometry. The ultrastructure analysis shows an acinose complex formed by about 50 clusters. Each cluster has 20 glandular units and the unit-composed of one secretory and one canal cell lying along a duct-belongs to the class 3 cell type of Quennedey (1998). In the cytoplasm, the secretory cell contains abundant rough endoplasmatic reticula, glycogen granules, numerous mitochondria, and many well-developed Golgi complexes producing electron-dense secretory granules. Mitochondria are large, elongated, and often adjoining electronlucent vesicles. The kind and the origin of secretory granules varying in size and density were discussed. The chemical analysis of the gland secretion revealed the presence of a mixture of low molecular weight terpenes, ketones, aldehydes, alcohols, esters, and carboxylic acids. Monoterpenes, especially linalool, were the major products. We supposed that ketones, aldehydes, alcohols, esters, and carboxylic acids have a deterrent function against the predators and monoterpenes provide a prophylaxis function against pathogens.

A facile Er(OTf)3-catalyzed synthesis of 2,3-unsaturated O- and S-glycosides.

Er(OTf)(3) is a useful catalyst for the Ferrier rearrangement furnishing high yields of O- and S-glycosides. The transformation has wide applicability, cleaner reaction profiles, mild reaction conditions, and high stereoselectivity and the catalyst, which is also commercially available, can be recovered and reused.

Alcohols and di-tert-butyl dicarbonate: how the nature of the Lewis acid catalyst may address the reaction to the synthesis of tert-butyl ethers.

The reaction between alcohols and Boc2O leads to the formation of tert-butyl ethers and/or Boc-alcohols, depending on the nature of the Lewis acid catalyst. Product distribution is mainly tuned by the anionic part of the salt. Perchlorates and triflates, anions with highly delocalized negative charge, give prevalent or exclusive ether formation. On the other hand, Boc alcohols are the main or exclusive products with un-delocalized isopropoxide or low-delocalized acetate ions. The metal ion influences only the reaction rate, roughly following standard parameters for calculating Lewis acidity. A reaction mechanism is supposed, and a series of experimental evidences is reported to support it. These studies allowed us to conclude that, to synthesize tert-butyl ethers, in reactions involving aliphatic alcohols, Mg(ClO4)2 or Al(ClO4)3 represents the best compromise between costs and efficiency of the reaction, while, in reactions involving phenols, Sc(OTf)3 is the best choice, since aromatic tert-butyl ethers are not stable in the presence of perchlorates.

Mild and efficient method for the cleavage of benzylidene acetals by using erbium (III) triflate.

Er(OTf)3 is proposed as new efficient Lewis acid catalyst in a mild deprotection protocol of benzylidene derivatives. In a modified procedure, where acetic anhydride is used as the reaction solvent, the simultaneous cleavage of the benzylidene acetal and the peracetylation of the substrates is obtained in quantitative yields and very short reaction times.

Simple and efficient chemoselective mild deprotection of acetals and ketals using cerium(III) triflate.

A new and chemoselective method for the cleavage of alkyl and cyclic acetals and ketals at room temperature in wet nitromethane by using catalytic cerium(III) trifluoromethane sulfonate is presented. The high yields, the observed selectivity, the very gentle reaction conditions, and the almost neutral pH make this procedure particularly attractive for multistep synthesis.

An efficient procedure for the preparation of (E)-alpha-alkylidenecycloalkanones mediated by a CeCl(3) x 7H(2)O-NaI system. Novel methodology for the synthesis of (S)-(-)-pulegone.

2-Alkylidenecycloalkanones are powerful synthons used as the key intermediates in many important syntheses. Because of their potential, a general method of preparation from readily available starting materials, under very mild conditions, was considered to be worthwhile. Cerium(III) chloride heptahydrate in combination with sodium iodide in refluxing acetonitrile promotes a regio- and stereoselective beta-elimination reaction to (E)-2-alkylidenecycloalkanones in 2-(1-hydroxyalkyl)cycloalkanones. The synthetic value of the present procedure is demonstrated by the synthesis of monoterpene (S)-(-)-pulegone (8) in its optically active form.

Reversed Stereochemical Control in the Presence of CeCl(3) and TiCl(4) in the Lewis Acid Mediated Reduction of alpha-Alkyl-beta-keto Esters by Metal Hydrides. A General Methodology for the Diastereoselective Synthesis of syn- and anti-alpha-Alkyl-beta-hydroxy Esters.

The Lewis acid-mediated reduction of alpha-alkyl-beta-keto esters has been shown to proceed by different stereochemical control depending on the nature of the metal atom. Strongly chelating TiCl(4) led to the syn isomer in high diastereomeric excess in noncoordinating solvents (CH(2)Cl(2)) at -78 degrees C with BH(3).py as reducing agent, while nonchelating CeCl(3) gave a high excess of the anti isomer in coordinating solvents (THF) at the same temperature with lithium triethylborohydride (LiEt(3)BH) as reducing agent. The methodology has been successfully utilized for obtaining important syn- and anti-alpha-alkyl-beta-hydroxy esters with high diastereoselectivity.