Tin(II) Enolates in Organic Synthesis

Abstract

Utilizing the characteristic properties of tin(II) species, various types of new synthetic reactions are developed and the results are summarized in this article.
It was found that tin(II) enolates are readily generated on treatment of carbonyl campounds with tin(II) triflate and N-ethylpiperidine, which in turn react smoothly with aldehydes to afford aldol products in highly stereoselective manner. In comparison with lithium and boron enolates, tin(II) enolates display enhanced reactivity towards ketones, and a facile method for the directed aldol reaction between two different ketones in realized.
he generation of tin(II) enolates from 3-acyl-2-thiazolidinethiones als o proceeds successfully on treatment with tin(II) triflate and N-ethylpiperidine, and the resulting aldol products are readily converted to various β-hydroxy carboxylic acid derivatives and β-hydroxy aldehydes. This reaction is further aβlied to the stereoselective construction of carbapenem skeleton, and the intermediate of thienamycin synthesis is synthesized with high stereoselectivity.
Utilizing the specific affinity of fin(II) with sulfur, a novel catalytic metal enolate mediated aldol reaction is realized. Thus, just by the action of a catalytic amount of fin(II)triflate salt of ethanethiol, the mixture of α, β-unsaturated ketone, aldehyde, and trimethylsilylether of ethanethiol affords the aldol type product in good yield.
The Michael addition reaction of metal enolate to α, β-unsatura ted ketone still remains as one of difficult problems due to the difficulty in controlling 1, 2- and 1, 4-addition. Really, tin(II)enolate alone did not add to α, β-unsaturated ketone efficiently. However, when trimethylsilyl chloride is added as an activator of the ketone, the Michael reaction proceeds smoothly to give the corresponding product in good yield.
When p-benzoquinone is employed as an acceptor, a novel aldol-reduction reaction proceeds when silyl chloride is added as an activator. According to this procedure, α-aryl carbonyl compound is easily prepared from the corresponding carbonyl compound and quinone.
Finally, a mild and efficient catalyst system is developed by the combined u se of trimethylsilyl chloride and fin(II) chloride. Various reactions of silyl enol ether with aldehydes, acetals, and α, β-unsaturated ketones are effectively catalyzed by the combined use of mild Lewis acid (tin(II) chloride) and neutral molecule(trimethylsilyl chloride).