Six different N-alkynyl carbamates containing Boc groups were prepared. Their gold(I)-catalyzed cyclization to oxazolinones proceeded radily at room temperature or even lower temperatures.Among gold-catalyzed reactions 2 the intramolecular oxyauration of C-C multiple bonds is an excellent tool for the formation of oxygen-containing heterocycles. The first example was the cycloisomerization of allenyl ketones or propargyl ketones, 3 similar results were obtained with 2-alkynyl-2-alken-1-ones. 4 Subsequently, the isomerization of N-propargylcarboxamides to oxazoles or even the rare alkylidene oxazolines was investigated. 5 Propargylic and homopropargylic trichloroacetimidates react analogously. 6 Carboxylates and tert-butyl carboxylates also react. 7 A study of the cyclization of tert-butyl allenoates showed that the tert-butyl group is eliminated and a butenolide is formed. 8 This principle was then extended to tert-butyl carbonates, 9 where for example a propargylic carbonate was shown to cyclize faster than a homopropargylic one. 9a A similar behavior was reported for propargylic and homopropargylic tert-butyl carbamates. 9c,dWe now wanted to investigate N-alkynylcarbamates as substrates, where instead of a 5-exo-dig or 6-exo-dig, a 5-endo-dig cyclization would lead to oxazolinones. To the best of our knowledge, so far only one reference has described the use of derivatives of alkynylamines in goldcatalyzed reactions. 10The synthesis of the substrates started from the Boc-protected amines 1, using the phenyliodonium salts 2, 11 the tert-butyl N-alkynylcarbamates 3 and 4 were obtained (Table 1). For the ynamine synthesis two acceptors are needed on the amine, the Boc group is either accompanied by a second Boc group, a Ts group, or a pivaloyl group. When the terminal position of the alkyne was occupied by the TMS group, partial desilylation was observed. Changing to the more robust TBDMS group avoided this problem. Overall, the yields of these substrates were not high, but still allowed the investigation of the cyclization of 3 and 4.Gold catalysis with Gagosz's catalyst 12 [Ph 3 PAu]NTf 2 readily delivered the corresponding oxazolinones 5 in 65-93% yield. As shown in Table 2, the terminal (entries 1 and 3), the TMS (entries 2, 4, and 6), and the TBDMS (entry 5) carbamates all react well. With the combination Ts/ Boc it was very clear that the Boc carbonyl oxygen atom is the more nucleophilic partner (entries 1 and 2), but in the combination Piv/Boc also a high selectivity towards Table 1 Alkynyl Carbamates Entry Carbamate Iodonium salt Product Yield (%) 1 1a; R 1 = Ts 2a; R 2 = TMS 3a/4a 24/46 2 1b; R 1 = Boc 2a 4b 23 a 3 1b 2a 3b 51 4 1b 2b; R 2 = TBDMS 3c 27 5 1c; R 1 = Piv 2a 3d 27 a Cs 2 CO 3 (1.3 equiv) instead of KHMDS; DMF instead of toluene.Table 2 Gold-Catalyzed Oxazolinone Synthesis Entry Alkynyl carbamate Product Yield (%) 1 4a; R 1 = Ts, a R 2 = H 5a 71 2 3a; R 1 = Ts, a R 2 = TMS 5b 67 3 4b; R 1 = Boc, R 2 = H 5c 65 4 3b; R 1 = Boc, R 2 = TMS 5d 90 5 3c; R 1 = Boc, R 2 = TBDMS 5e 93 6 3d; R 1 = Piv,...

A series of furyl alcohols and homofuryl alcohols was synthesized by reduction of furfurals or reaction of furyllithium compounds with epoxides and subsequent propargylation. The gold-catalyzed cycloisomerization of these products furnished dihydroisobenzofurans and isochromanes. Crystal structure analyses proved the sequence of the substituents for both classes of products. Unsaturated dicarbonyl compounds as side-products show the mechanistic relationship to the analogous platinum-catalyzed reactions. Neither ester groups, even on the 4-position of the furan ring, nor aryl bromides hinder the catalysis by gold. In the case of a substrate with an allyl ether in the side chain, a side-product, which provides evidence for a reaction of the alkyne with an inverse regioselectivity, was observed.

A series of 2-alkynylaryl epoxides were prepared by a sequence of Sonogashira coupling, Wittig olefination and epoxidation or a Darzens glycid ester synthesis. The conversion of these substrates with gold(I) catalysts furnished 3-acylindenes and, in occasional cases as side-products, the products of an isomerization of the oxirane ring to a ketone. Isotope labelling of the epoxide oxygen indicates an intramolecular oxygen transfer.

Different dihydroisoindol-4-ols and 8-hydroxytetrahydroisoquinolines were prepared by the gold-catalyzed phenol synthesis. The reaction was investigated with several sterically demanding groups in the 5-position of the furan starting material. The influence of the reaction time and the catalyst on the yield was investigated.

Gold‐catalyzed reactions of pyrrole derivatives with methyl vinyl ketone preferentially give rise to doubly substituted products, with an excess of methyl vinyl ketone even triple substitutions can be observed. This is independent of the electronic nature of substituents on the pyrrole ring, even substrates with electron‐withdrawing acyl substituents in 1‐, 2‐ or 3‐position show this behavior. The NH group of the pyrrole ring may be unprotected, no competing hydroamination is observed. In an intramolecular competition experiment with a furan ring only the pyrrole ring reacted. The constitutions of the highly‐substituted pyrroles were proven by NMR spectroscopy and one X‐ray structure analysis. Control experiments show that Yb(OTf)3 does not catalyze these reactions, but Ag+ and H+ does. Contrary to pyrroles, under similar reaction conditions indoles show a clean monosubstitution reaction. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)