Ruthenium porphyrin catalyzed tandem sulfonium/ammonium ylide formation and [2,3]-sigmatropic rearrangement. A concise synthesis of (±)-platynecine

Abstract

meso-Tetrakis(p-tolyl)porphyrinatoruthenium(II) carbonyl, [Ru II(TTP)(CO)], can effect intermolecular sulfonium and ammonium ylide formation by catalytic decomposition of diazo compounds such as ethyl diazoacetate (EDA) in the presence of allyl sulfides and amines. Exclusive formation of [2,3]-sigmatropic rearrangement products (70-80% yields) was observed without [1,2]-rearrangement products being detected. The Ru-catalyzed reaction of EDA with disubstituted allyl sulfides such as crotyl sulfide produced an equimolar mixture of anti- and syn-2-(ethylthio)-3-methyl-4- pentenoic acid ethyl ester. The analogous "EDA + N,N- dimethylcrotylamine" reaction afforded a mixture of anti- and syn-2-(N,N-dimethylamino)-3-methyl-4-pentenoic acid ethyl esters with a diastereoselectivity of 3:1. The observed catalytic activity of [Ru II(TTP)(CO)] for the ylide [2,3]-sigmatropic rearrangement is comparable to the reported examples involving [Rh2(CH 3CO2)4] and [Cu(acac)2] as catalyst. Similarly, cyclic sulfonium and ammonium ylides can be produced by intramolecular reaction of a diazo group tethered to allyl sulfides and amines under the [RuII-(TTP)(CO)]-catalyzed reaction conditions. The subsequent [2,3]-sigmatropic rearrangement of the cyclic ylides furnished 2-allyl-substituted sulfur and nitrogen heterocycles in good yields (>90%). By employing [RuII(TTP)(CO)] as catalyst, the cyclic ammonium ylide [2,3]-sigmatropic rearrangement reaction was successfully applied for the total synthesis of (±)-platynecine starting from cis-2-butenediol.