1.5- Enynes Cope rearrangement

In the reaction of disubstituted alkynes, 1,3-migration of the acetate takes place to give allenyl esters that can be versatile substrates, especially for [3,3]-Cope rearrangements.333 1,5-Enynes have proved to be versatile substrates for the preparation of perfumery agents such as sabinol334 and sabina ketone.335 Transannular systems undergo similar reactions.336... 

Participation of alkyne moieties in Cope rearrangements leads to the formation of al-lenes1115 1116. With simple 1,5-enynes. thermal reactions are often without practical value as the primarily formed allenes cyclize at high temperatures, or else an equilibrium is formed1117-1118. 

In this latter reaction mode, which is observed much more rarely than /3-dehydropal-ladation, a wide variety of ligands can be coupled to each other with the formation of new C—C, C—H, C— N, C—O, and C—Hal bonds. This section does not cover the numerous cascade couplings in which a number of successive intramolecular additions of 2 onto double bonds is eventually completed by /3-dehydropalladationt as well as the numerous [2 -I- 2 -I- 2] and [4 + 2] cyclotri- and cyclodimerizations of alkynes, enynes, and related compounds. " The Pd(0)-catalyzed Cope rearrangement also will not be considered here, as it proceeds via bis(i7 -allyl)palladium(ll) intermediates. The carbopalladation reactions of aUenes, which have been reviewed recently, are covered in Sect. IV.7. (For new examples see also refs. [10]-[12]). On the other hand, the numerous Pd-catalyzed formal [3 + 2] cycloadditions of trimethylenemethane (TMM) complexes may be classified as carbopalladations of alkenes without subsequent dehydropalladation. As the subject of this section has partially been covered in several newly published reviews, " the attention here will be on the most recent and interesting communications. 

Intermediate 1-6 can evolve by metal loss to give tricyclic compound 1-2, or by fragmentation to form methyl ketone 1-3 (Scheme 2.2). The competitive 2-oxonia-Cope rearrangement of intermediate 1-6 via 1-7 forms 1-8, which results in the minor epimer 1-2 of the tricychc compotmd. As other gold(I)-catalyzed reactions of enynes, this reaction is stereospecific [Refs. 4, 8, 154 in Chap. 1] [1]. 

Terminal Z-enynes (140) are produced by selective hydrogenation of the unsily-lated triple bond of the corresponding diynes (141) prepared by oxidative coupling of an acetylene with trimethylsilylacetylene (Scheme 82).Enynes are also obtained from a-cyclopropylacetylenic carbinols (128), and by Cope rearrangement of dienynols (137). 

Cope and Sila-Cope Rearrangements Computational studies by Yates et ed. on 1,5-enynes were used to rationalize the different regioselectivities observed for the Au(I)-catalyzed addition of alcohols to the all carbon (X = C) and silyl 1,5-enynes (X = Si) as shown in Eq. (5.9) [35]. 

A useful application of the oxy-Cope rearrangement has been used to produce conjugated enynes [e.g. (114) - (115)]. The enyne (115) is an intermediate in a synthesis of the sex pheromone of the Pine Sawfly Neodiprion lecontei. Potassium t-butoxide in liquid ammonia has been found to be the preferred base to produce high yields of the conjugated 1-alkylthio- or l-arylthiobut-3-en-l-ynes (117). Elimination of thiols from the acetylenic disulphides (116) under these conditions completely suppresses the competing formation of cumulene or diene-sulphides. 

The initial isomerization product is now the (trimethylallyl)allene 30 which subsequently participates in a Cope-type rearrangement leading to the enyne 31. 

---