Enol thioethers cycloadditions

An interesting rearrangement, which appears to be anion-accelerated, takes place in the enol thioether, anion-terminated vinylcyclopropanes of type 14. ° The rearrangement proceeds at — 78 C and is reasonably stereoselective with regard to the final cyclopentene products (syn selectivity 16 1). Regioisomers are encountered in the formation of the dihydrothiopyran cycloaddition adducts 13 in several instances. The mechanism of this rearrangement appears to involve the enol thioether anion in accord with the well-documented donor acceptor principles " and may be related to similar rearrangements observed with trimethylsilyl enol ether terminated vinylcyclopropanes under fluoride ion or Lewis acid catalysis." " ... 

More recently, Maignan and Hayes have reported the asymmetric hetero-Diels-Alder reaction of chiral (5)-3-/ -tolylsulfinylbut-3-en-2-one (278) with simple enol thioethers 279. Moderate yields of cycloaddition products 280 were obtained, but with low enantioselectivities (Scheme 4.90). 

Kobayashi et al. found that lanthanide triflates were excellent catalysts for activation of C-N double bonds —activation by other Lewis acids required more than stoichiometric amounts of the acids. Examples were aza Diels-Alder reactions, the Man-nich-type reaction of A-(a-aminoalkyl)benzotriazoles with silyl enol ethers, the 1,3-dipolar cycloaddition of nitrones to alkenes, the 1,2-cycloaddition of diazoesters to imines, and the nucleophilic addition reactions to imines [24], These reactions are efficiently catalyzed by Yb(OTf)3. The arylimines reacted with Danishefsky s diene to give the dihydropyridones (Eq. 14) [25,26], The arylimines acted as the azadienes when reacted with cyclopentadiene, vinyl ethers or vinyl thioethers, providing the tet-rahydroquinolines (Eq. 15). Silyl enol ethers derived from esters, ketones, and thio-esters reacted with N-(a-aminoalkyl)benzotriazoles to give the /5-amino carbonyl compounds (Eq. 16) [27]. The diastereoselectivity was independent of the geometry of the silyl enol ethers, and favored the anti products. Nitrones, prepared in situ from aldehydes and N-substituted hydroxylamines, added to alkenes to afford isoxazoli-dines (Eq. 17) [28]. Addition of diazoesters to imines afforded CK-aziridines as the major products (Eq. 18) [29]. In all the reactions the imines could be generated in situ and the three-component coupling reactions proceeded smoothly in one pot. 

Racemic 4-acetoxy-3-unsubstituted azetidinone 10 and homochiral (3R,4R)-3-(IR)-hydroxyethyl derivative 11, commercially available intermediates industrially prepared by the so-called CSI route , testify to the importance of this ring formation strategy. By using vinyl acetate as the olefin partner in the chlorosulfonylisocyanate-alkene cycloaddition, in 1974, Clauss et al. [10] laid down the basis of this approach in the field of p-lactam antibiotics. The most recent efforts have been directed to the key intermediate 11 and its synthetic equivalents 12, 13. Starting from methyl 3R-hydroxybutyrate [11], three independent methodologies have been realized, the main difference being the type (ether, thioether, ester) of enolate used. 

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