Metal-catalyzed cycloisomerization reactions of 1,n-enynes have appeared as conceptually and chemically highly attractive processes as they contribute to the highly demanded search for atom economy and allow the discovery of new reactions. Since the pioneering studies with palladium by the research group of Barry Trost in the mid-1980s, several other metals have been identified as excellent catalysts for the rearrangement of enyne skeletons. Moreover, the behavior of 1,n-enynes may be influenced by other functional groups such as alcohols, aldehydes, ethers, alkenes, or alkynes, thus enhancing the molecular complexity of the synthesized products. Apart from the intrinsic rearrangements of 1,n-enynes, several tandem reactions incorporating intramolecular trapping agents or intermolecular partners have been discovered. This Review aims to highlight the main contributions in this field of catalysis and to propose and comment on the mechanistic insights of the recent discoveries.

The enantioselective formation of a quaternary stereogenic center coinciding with a hydroxylation process is a very rare reaction from a homogeneous catalysis point of view. Indeed, to our knowledge, no asymmetric transition-metal-catalyzed direct hydroxylation has been reported before. We describe here our initial study concerning the enantioselective ␣-hydroxylation of various ␤-ketoesters catalyzed by Lewis-acidic complexes. Specifically, it was found that the Ti complex [TiCl2((R,R)-1-Np-TADDOLato)(MeCN)2] affords the hydroxylated products in high yield and enantioselectivities up to 94% enantiomeric excess when using 2-(phenylsulphonyl)-3-(4-nitrophenyl)oxaziridine as the oxidizing agent. Chiral enantiopure compounds of the latter type have been used previously in stoichiometric asymmetric hydroxylation reactions. We also show that, in a complementary approach with H2O2 as the oxidant, the Ru(II) complex [RuCl(OE 2)((S,S)-PNNP)]PF6 catalyzes the same type of transformation in a case of substrates showing a very substantial extent of enolization under reaction conditions; being, however, unreactive toward only weakly enolized ␤-ketoesters.R eactions involving both the formation of new carbonheteroatom bonds and the concomitant generation of a new stereogenic center are prototypical transformations in asymmetric catalysis. Thus, for example, hydroboration, hydrosilylation, epoxidation, and aziridination of olefins, as well as allylic substitutions with heteroatom nucleophiles, have been extensively studied (1). However, catalytic halogenations, in particular fluorinations, and direct hydroxylations have received much less attention in the asymmetric catalysis community.Recently, we developed an enantioselective catalytic process in which ␤-ketoesters are fluorinated at the 2-position with an electrophilic fluorine donor such as Selectfluor [also called F-TEDA (TEDA, triethylenediamine), 1-chloromethyl-4-fluoro-1,4-diazoniabicyclo[2.2.2]octane bis{tetrafluoroborate}] in the presence of a Lewis-acidic chiral titanium catalyst (1, Scheme 1, Fig. 1 (2-5). Our work inspired further developments involving chiral Pd catalysts (6) and chiral phase transfer catalysts for the same type of fluorination (7).The ␣-hydroxycarbonyl functional unit is ubiquitous in natural products and bioactive compounds, such as, e.g., carbohydrates, antibiotics, and antitumor agents; therefore, it is also present in synthetic intermediates and in chiral auxiliaries in view of the stereodirecting ability of the hydroxy group (8). The ␣-functionalization of carbonyl compounds most often relies on the reaction of enolate anions, or enol derivatives with electrophiles. The direct activation of the enol form of keto derivatives has recently been introduced as a new synthetic methodology to achieve ␣-amination (9) or, as already mentioned, ␣-halogenation (2-5). So far, the only straightforward enantioselective ␣-hydroxylation is the reaction of enolate salts with enantiopure N-sulfonyloxaziridines, acting as electrophilic oxygen sources, as dev...

A highly efficient gold-catalyzed cyclization reaction of various functionalized acetylenic acids is described. The cyclizations are conducted in the presence of Au(I) catalyst in acetonitrile at room temperature in a short reaction time. The reaction conditions are compatible with several functional groups, such as ester, alkene, alkyne, chloro, and free or protected alcohol, and lead to original gamma-lactones in good to excellent yields.

A novel water-soluble Au(III)-NHC complex has been synthesized and successfully applied in the intramolecular cyclization of γ-alkynoic acids into enol-lactones under biphasic toluene/water conditions, thus representing a rare example of an active and selective catalyst for this transformation in aqueous media. Remarkably, competing alkyne hydration processes were not observed, even during the desymmetrization reaction of challenging 1,6-diyne substrates. In addition, after phase separation, the water-soluble Au(III) catalyst could be recycled 10 times without loss of activity or selectivity.