Ylide compounds directed syntheses

In a very recent example, Chiu and co-workers (84-86) used the tandem ylide-cycloaddition methodology to prepare advanced intermediates directed toward the synthesis of the pseudolaric acids. Pseudolaric acids are a family of diterpenes isolated from the root bark of Pseudolarix kaempferi. These novel compounds have shown antimicrobial activity comparable to that of amphotericin B and have demonstrated cyctotoxicity against several cancer cell lines (Fig. 4.5). 

The revived interest in this area led to new efforts for improved methods of synthesis for pure alkylidene trialkyls (ylides), for the detection and possible characterization of a pentaalkylphosphorane species, and for a better understanding of the state of structure and bonding in ylides and pentaalkyls. This recent work is summarized here with the main focus directed to some of the very simple compounds and their silicon derivatives, which have turned out to be of special significance. 

Lothar Weber was born in 1944 in Langenols in Schlesien. He studied at the Universitat Marburg and received his doctorate there under the direction of Professor Gunter Schmid in 1973. Thereafter he carried out postdoctoral studies with Professor Barry M. Trost at the University of Wisconsin in Madison, USA. On his return to Marburg, he began the experimental work leading to his habilitation, which was completed in 1982 at the Universitat Essen. His work focused on the coordination chemistry of sulfur ylides. In 1985, he became a C2 Professor and then joined the Fakultat fiir Chemie der Universitat Bielefeld. His research interests include the chemistry of compounds with low-coordinate elements of the fifth main group, the synthesis of homo- and heterocycles with heavy elements, as well as new aspects in boron chemistry. 

The synthesis of the arsonium ylides most commonly involves deprotonation of arsonium salts utilizing a variety of bases. Simple arsonium salts are available via direct alkylation of triphenylarsine, but more highly substituted compounds require specialized methods. These include the use of highly electrophilic triflate salts, alkylation of triphenylarsonium methylide, and double alkylation of lithiodiphenyl-arsine. For some purposes, formation of the tetrafluoroborate salts is desirable and can be effected in good yield by cation exchange. ... 

The Wittig reaction involves the interaction of an aldehyde or ketone with a phosphorus-containing carbanionic species, in which the phosphorus is bonded directly to the carbanionic site. At the time of discovery of the reaction, that specification described the triphenylphosphonium alkylides 343 later developments employed the anions from tert-phosphine oxides, the use of which has been described elsewhere in this series and also from a wide variety of phosphonic and phosphinic esters. This latter application will be considered more fully in Chapter 6. An early application of the reaction to the synthesis of alkenephosphonic acids, and which involved an ylide, employed the stable compound 344, generated from 345 by the action of a strong base. Reactions between 344 and aliphatic or aromatic aldehydes at 100 °C in toluene or dmso gave the diphenyl esters of (alk-1-enyl)phosphonic acids or (2-arylethenyl)phosphonic acids. ... 

Maulide et al. developed a unified method for the direct transfer of ylides and the metal-free arylation of carbonyl compounds. This was applied in the synthesis of pyrrole 76 from 75 upon treatment with Martin s sulfurane in toluene at room temperature. The dearomatization of both indole and pyrrole could be effected with a variety of electron-donating or -withdrawing groups in good to excellent yields (13JA7312). 

Reactions.—The reaction of sulphonium ylides with carbonyl compounds to form oxirans is now a standard reaction. Among the more unusual examples reported are an intramolecular version directed towards the preparation of arene epoxide (12) (not isolable), the synthesis of the epoxide of 8-phenylthio-methylenecycloheptane, and the direct conversion of a diketone by dimethyl-sulphonium methylide into a doubly homologated hydroxymethyl aldehyde (13) by the rearrangement shown. ... 

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