Chiral dithioesters

A review of the asymmetric thia-Diels-Alder reactions of chiral dithioesters with dienes has been published. The asymmetric organocatalytic thio-Diels-Alder reac- 0 tion of thiocarbonyls with a-methylene carbonyl dienes produced dihydrothiopyrans in high yields and high to excellent selectivities. The inverse-electron-demand 0 hetero-Diels-Alder cycloadditions of norbomene (106) with 5-benzylidene-2-arylimino-3-aryl-thiazolidine-4-thiones (107) at 25 °C produced cycloadducts (108)... 

Keywords. Phosphorus and sulfur derivatives, Dithioesters, Hetero Diels-Alder, Sigmatropic rearrangements. Chiral ligands... 

In 2004, ruthenium-catalysed asymmetric cyclopropanations of styrene derivatives with diazoesters were also performed by Masson et al., using chiral 2,6-bis(thiazolines)pyridines. These ligands were prepared from dithioesters and commercially available enantiopure 2-aminoalcohols. When the cyclopropanation of styrene with diazoethylacetate was performed with these ligands in the presence of ruthenium, enantioselectivities of up to 85% ee were obtained (Scheme 6.6). The scope of this methodology was extended to various styrene derivatives and to isopropyl diazomethylphosphonate with good yields and enantioselectivities. The comparative evaluation of enantiocontrol for cyclopropanation of styrene with chiral ruthenium-bis(oxazolines), Ru-Pybox, and chiral ruthenium-bis(thiazolines), Ru-thia-Pybox, have shown many similarities with, in some cases, good enantiomeric excesses. The modification... 

Zwanenburg and his group [105] have addressed the question of the tautomeric interconversion of aliphatic sulfines into vinyl sulfenic acids. They have synthesised dithioesters bearing an a asymmetric carbon centre. Oxidation with mCPBA did produce chiral sulfines, which did not racemise except when a phenyl group was attached to the a carbon. 

The aldol reaction of dithioester enethiolates has been used for the synthesis of dithiolactones [123], chiral substrates for the Claisen rearrangement [124, 125], and oxathianes [126]. Thioamide enethiolates may be employed in this reaction as well as shown with /Tthiolactams [26], or with a precursor of the antitumour agent vinblastine [127]. 

It is remarkable that elimination of sulfenic acid was not observed during the formation of unsaturated sulfinyl dithioesters. These results are now being extended to enantiopure starting materials, with diacetone glucose as a chiral source of the starting sulfoxides [203]. 

Tin (II) enolates have been widely used by Mukaiyama and coworkers in aldol and Michael reactions. These enolates are generated by action of Sn(OTf)2 on ketones, esters, N-acy -1,3-oxazolidin-2-ones or -l,3-thiazolidin-2-thiones, thi-olesters or dithioesters in the presence of chiral diamines 2.13 (R = CH2NHAr or CH2N(CH2)5) bearing the (S)-proline skeleton [253, 559, 682]. Diastereoiso-meric and enantiomeric excesses observed in these reactions are often excellent. 

Dithioesters bearing a chiral auxiliary have been used in asymmetric thia-Diels-Alder reactions with acyclic 1,3-dienes to produce cycloadducts with a diastereomeric excess of 90%. A Cu(II)bis(oxazoline)/dithioesters complex has been proposed to rationalize the chiral induction ... 

---