Intriguing Influence of the Solvent on the Regioselectivity of Sulfoxide Thermolysis in β-Amino-α-sulfinyl Esters

Abstract

The sulfoxide thermolysis of the diastereoisomeric methyl (3R,4aS,10aR)-6-methoxy-1-methyl-3-(phenylsulfinyl)-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinoline-3-carboxylates 3a and 3'b in toluene yields, by loss of benzenesulfenic acid, an almost 1 : 1 mixture of the vinylogous urethane 2b and the isomeric a-aminomethyl enoate 2a. When this elimination is performed in acetic acid, the enoate 2a is formed rather selectively. The same solvent effects on the regioselectivity of the elimination of benzenesulfenic acid are observed with a simple sulfoxide of ethyl piperidine-3-carboxylate (7).Introduction. ± In the course of new product developments, we were interested in the conversion of methyl (3R,4aR,10aR)-6-methoxy-1-methyl-1,2,3,4,4a,5,10,10a-octahydrobenzo[g]quinoline-3-carboxylate (1) [1] to the corresponding unsaturated methyl (4aS,10aR)-6-methoxy-1-methyl-1,2,4a,5,10,10a-hexahydrobenzo[g]quinoline-3-carboxylate (2a) [2] (Scheme 1).

Radical Mechanism in the Elimination of 2-Arylsulfinyl Esters

“…Elimination reactions of sulfoxides and sulfones are well-known reactions in synthetic organic chemistry . In particular, the transformation of 2-arylsulfinyl esters to afford unsaturated carbonyl compounds by elimination is a common synthetic transformation. − The elimination reaction of sulfoxides follows an Ei (elimination internal) mechanism, as demonstrated by the seminal work by Cram and Kingsbury, who also suggested the existence of a competing homolytic mechanism for some sulfoxides having a group to stabilize a radical intermediate. Although extensive work has been reported for the elimination of sulfoxides through an Ei mechanism, , no convincing evidence has appeared for a radical mechanism involving any sulfinyl substrate.…”