A Tandem Ester Cleavage-Michael Addition Reaction for the Synthesis of Oxygen Heterocycles

Simple Thiazocine-2-acetic Acid Derivatives via Ring-Closing Metathesis

A new protocol for synthesis of 2-heterocylylacetic acid derivatives involving conjugate addition of allyl mercaptan to an acrylate containing a tethered olefinic site followed by RCM (ring-closing metathesis) is described. In this series, sulfanyl derivatives were unreactive, while sulfoxide and sulfone analogues provided the corresponding thiazocines in fair to excellent yields. Use of the sulfoxide oxidation state as a protecting group for sulfides inert to RCM is demonstrated also. Thus, oxidation of sulfide 9 [N-allyl-N-[2-(allylthio)-4-(1H-indol-1-yl)-4-oxobutyl]-4-methylbenzenesulfonamide] followed by cyclization yielded the corresponding thiazocine sulfoxide 12. Deprotection (deoxygenation) of 12 was accomplished using Lawesson's reagent, producing 1-[[4-[4-(methylphenyl)sulfonyl]-3,4,5,8-tetrahydro-2H-1,4-thiazocin-2-yl]acetyl]-1H-indole (21) in 67% unoptimized yield.

2,2,3-Trisubstituted Tetrahydrofurans and 2H-Tetrahydropyrans by Tandem Demethoxycarbonylation-Michael Addition Reactions

A tandem demethoxycarbonylation-Michael addition reaction has been developed as a synthetic route to highly functionalized five- and six-membered oxygen heterocycles. Methyl esters, activated toward decarboxylation by a C-2 ethoxycarbonyl group and tethered by a three- or four-atom chain to an acrylate Michael acceptor, have been prepared and used as the cyclization substrates. Treatment of these compounds with lithium chloride in DMEU (1,3-dimethyl-2-imidazolidinone) at 120 degrees C for 4-8 h results in chemoselective S(N)2 dealkylation of the methyl esters, decarboxylation, and cyclization of the intermediate enolates by a Michael addition to the pendent acrylate moiety. This affords tetrahydrofuran and 2H-tetrahydropyran derivatives in 60-90% yields with diastereoselectivities up to 7.5:1 in favor of the product having the C-2 ethoxycarbonyl group trans to the C-3 acetic ester side chain. The reaction works best for the preparation of five- and six-membered rings, and cyclizations proceed most cleanly from substrates which cyclize through a tertiary enolate. Synthetic and mechanistic details are presented.

Sequenced Reactions with Samarium(II) Iodide. Sequential Nucleophilic Acyl Substitution/Ketyl Olefin Coupling Reactions for the Preparation of Oxygen Heterocycles

Samarium(II) iodide has been employed to promote a sequential intramolecular nucleophilic acyl substitution/intramolecular ketyl olefin coupling cyclization sequence to provide bicyclic, tricyclic, and spiro-fused oxygen heterocycles in excellent yield and with high diastereoselectivity.