Gold(I) complexes are the most active catalysts for alkoxy- or hydroxycyclization and for skeletal rearrangement reactions of 1,6-enynes. Intramolecular alkoxycyclizations also proceed efficiently in the presence of gold(I) catalysts. The first examples of the skeletal rearrangement of enynes by the endocyclic cyclization pathway are also documented. Iron(III) is also able to catalyze exo and endo skeletal rearrangements of 1,6-enynes, although the scope of this transformation is more limited. The gold(I)-catalyzed endocyclic cyclization proceeds by a mechanism different from those followed in the presence of PdII, HgII, or RhI catalysts.

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Three pathways actually compete in metal-catalyzed cyclizations of enynes in which the metal selectively activates the alkyne: an endocyclic process and two exo-cyclizations, one proceeding by anti attack of the alkene and a second one resulting in a syn addition. Although cyclobutenes may be formed in transition-metal-catalyzed cyclization of some enynes, particularly, 1,7-enynes, these compounds are not necessarily the intermediates in the skeletal rearrangement. Cyclobutenes are formed by ring expansion of syn-cyclopropyl metal-carbenes formed in the syn pathway.

In this feature article we cover most recent efforts in gold-catalysed transformations, highlighting the wide molecular diversity that can be achieved, in particular with regard to the formation of C-C bonds. Mechanistic interpretations of some cyclisations are based on our own work on the skeletal rearrangement of 1,6-enynes.

Gold(I) complexes are the most active catalysts for the biscyclopropanation of dienynes to form tetracyclic compounds. PtII and ZnII are also able to promote the biscyclopropanation, although less efficiently. The configurations obtained in all cases with the use of gold(I) catalysts can be explained by the pathway proceeding through anti cyclopropyl gold carbenes. Similar intermediates are most probably involved in reactions catalyzed by RuII and PtII. Two different cyclopropanation pathways have been found; they depend on the structures of the cyclopropyl gold carbenes (anti or syn) and the relative arrangements of the metal carbenes and the alkenes.

Skeletal rearrangements of simple 1,6-enynes have been studied in order to determine the factors that control the formation of five-or six-membered rings. Simple 1,6-enynes substituted only at C-4 preferentially give six-membered rings on skeletal rearrangement in the presence of gold(I) catalysts, whereas increasing electron-withdrawing character of substituents at C-4 leads to five-membered rings. Reactions of these simple enynes in the presence of PtCl 4 as catalyst give exclusively exo-double skeletal rearrangements. Enynes substituted at the alkyne also react with Au I catalysts to give exclusively products of exo-double rearrangement.

The hydroxy-or alkoxycyclization of enynes I catalyzed by electrophilic transition-metal complexes usually takes place through cyclopropyl metal carbenes II, which react with nucleophiles R'OH to give intermediates III (Scheme 1). The reaction is then terminated by proto-demetalation of the alkenyl metal intermediate III to give IV. [1, 2] We have now found that in the Au I -catalyzed cyclization of enynes [3, 4] the alkenyl metal intermediate can be trapped with appropriate substituents, as shown in V and VI in a Prins cyclization. These new cyclizations allow the one-step synthesis of tricyclic skeletons, such as those of b-kessyl ketone [5a] and orientalol E (Scheme 2), [5b] from enynes with carbonyl groups and octahydrocyclobuta[a]pentalenes, such as fascicularone B, [6] kelsoene, [7] and sulcatine G, [8] starting from cyclopropyl enynes.Enynes 1 a-c, bearing a carbonyl group at the alkenyl side chain, are cyclized to give oxatricyclic derivatives 2 a-c and rearranged ketones 3 a-c by using Au I catalysts ( Table 1). The reactions were completed at room temperature in 5-30 min.Aldehyde 1 a gave a mixture of tricycle 2 a (35 %) and ketone 3 a (50 %) with [AuCl(PPh 3 )]/AgSbF 6

A stereoselective gold-catalyzed [2 + 2 + 2] cycloaddition of ketoenynes substituted at the propargylic position with OR groups has been applied for the synthesis of (+)-orientalol F and pubinernoid B.