The role of Lawesson's reagent in the total synthesis of macrocyclic natural products

A chromatography-free and aqueous waste-free process for thioamide preparation with Lawesson's reagent

After completing the thio-substitution with Lawesson’s reagent, ethanol was found to be effective in the decomposition of the inherent stoichiometric six-membered-ring byproduct from the Lawesson’s reagent to a highly polarized diethyl thiophosphonate. The treatment significantly simplified the following chromatography purification of the desired thioamide in a small scale preparation. As scaling up the preparation of two pincer-type thioamides, we have successfully developed a convenient process with ethylene glycol to replace ethanol during the workup, including a traditional phase separation, extraction, and recrystallization. The newly developed chromatography-free procedure did not generate P-containing aqueous waste, and only organic effluents were discharged. It is believed that the optimized procedure offers the great opportunity of applying the Lawesson’s reagent for various thio-substitution reactions on a large scale.

Synthesis and Chiral Separation of Some 4-thioflavones

A thionation reaction was performed on some chiral flavanones using Lawesson's reagent (LR) and leads to the formation of new chiral thiocarbonyl flavanes. LR in this thionation reaction with Hesperetin and Naringenin gives new flavan-4-thiones with yields ranged between 41 and 52%. Based on the Wittig reaction principle, LR is currently the most widely used reagent for this type of reaction. Enantiomeric separation by high-performance liquid chromatography methods was then set-up using three different polysaccharide-based chiral stationary phases (CSPs). Chiral separations were successfully accomplished with high resolution (1.22 ≤ Rs ≤ 5.23). The chiral discrimination mechanism(s) between the analytes under study, mobile phase, and the CSPs were discussed.

Applications of Lawesson's reagent in the synthesis of naturally occurring steroids and terpenoids

Steroids and terpenoids are among the most biologically significant classes of natural products possessing a variety of biological activities. The replacement of one or more oxygen atoms in a steroid or terpenoid molecule by a heteroatom affects the chemical properties of that particular steroid or terpenoid, and that replacement often results in alterations of its biological properties, which is sometimes valuable. One possible modification is the thionation that could have some influence on such activity. Among the various thionating reagents, Lawesson's reagent was found to be most suitable and showed versatile properties, including chemoselectivity and functional group tolerance. In this review, we present the role of Lawesson's reagent in the synthesis of thioanalogues of natural steroids and terpenoids.