Enynes 1,1,2-trisubstituted

II, while 9 failed. For substrates with H, both catalysts were found to be effective providing preferentially the rrawi-configured products 15a. Enynes carrying trisubstituted olefin moieties failed to react. 

Subsequently, high chemoselectivity and enantioselectivity have been observed in the asymmetric epoxidation of a variety of conjugated enynes using fructose-derived chiral ketone as the catalyst and Oxone as the oxidant. Reported enantioselectivities range from 89% to 97%, and epoxidation occurs chemoselectively at the olefins. In contrast to certain isolated trisubstituted olefins, high enantioselectivity for trisubstituted enynes is noticeable. This may indicate that the alkyne group is beneficial for these substrates due to both electronic and steric effects. 

Cross-coupling of allenes with 1 -alkynes.1 Pd(OAc)2 in combination with a triarylphosphine (cat. A) can effect cross-coupling of 1-alkynes with 1,1-di- and 1,1,3-trisubstituted allenes to provide as the major product conjugated enynes (equations I, II). The regioselectivity in the case of coupling with methyl 2,3-alkadienoates... 

Enynes were prepared in good yield from alkynyl iodonium salts and alkenylcopper reagents, stereospecifically. This approach was suitable for the synthesis of conjugated enynes, using a trisubstituted alkene with complete retention of its geometry [47], 1,3-Diynes were similarly obtained by coupling alkynyl iodonium... 

Mechanistically, this unusual multicomponent trimerization can be rationalized as a sequence of a Pd-catalyzed alkyne dimerization [154,155] giving rise to the regioselective formation of the enyne 218, which undergoes subsequent Pd-catalyzed [4 + 2]-benzannulation [158,159] with a butadiyne as an enynophile to furnish the benzene 216 via an allenyl-Pd species 219 (Scheme 90). Upon submitting an electron-deficient alkyne together with a terminal alkyne in equimolar amounts to the sequence, the unsymmetri-cal alkyne dimerization gives a trisubstituted enyne to set the stage for the formation of pentasubstituted benzene derivatives 217. 

Another cascade reaction using conjugated enynals as starting materials with a palladium catalyst was reported. Good to high yields of 2,3,4-trisubstituted furans were afforded in the reaction. An example is shown in the following scheme 070L1191>. 

Reaction of vinylcopper reagents, 62, with alkynyliodonium tosylates results in conjugated enyne 63, [53] [Eq. (24)]. The reaction is stereospecific with retention of olefin geometry. By appropriate order of addition, either of the two possible isomeric trisubstituted olefin isomers, 63, can be obtained in good isolated yields and excellent (>99%) stereoselectivity. Likewise, conjugated diynes, 65, are obtained [54] in the reaction of dialkynylcuprates, 64, with alkynyliodonium tosylates [Eq. (25)]. This method may be used for the preparation of unsym-metrical diynes. The mechanism of these coupling reactions is not understood at present. 

The aniline derivative 208 was obtained by cross-benzannulation of the amino-enyne 207 with dodeca-5,7-diyne (50) [64]. Cross-benzannulation of the 1,4-disubstituted enyne 209 with fte diyne 50 afforded the 1,2,3,4-tetrasubstituted benzene 210 [63]. Pentasubstituted benzene 212 can be synthesized by the coupling of the trisubstituted l-buten-3-yne 211 with the diyne 50 [63]. 

Another type of benzannulation, which might be related to the benzannulation of conjugated enynes, is the Pd-catalyzed cyclotrimerization of 1,3-diynes (Scheme 1,3-Diynes cyclotrimerize in the presence of a Pd catalyst to give 1,3.5-trisubstituted benzene derivatives. In this reaction, too, the regioselectivity is perfectly controlled as shown in Scheme 4, and other products were never isolated. The reaction of various diynes is summarized in Table 7. Some functional groups such as olefins and ether linkages may be present in the side chain. An example for the cross-cyclotrimerization of a diyne and an aUcyne has also been reported (Scheme... 

Conjugated enynes cyclodimerize in the presence of Pd(0) catalysts to give di- or trisubstituted benzene derivatives with high regioselectivity. This reaction could be carried out in an intramolecular fashion to give [njparacyclophanes. 

Allylic acetates couple with dimethyl propargylmalonate anion in the presence of [(Ph2P)i Pd] to give 1,6-enynes. In the presence of [(o-tolyl )P]2Pd(0Ac)2 these 1,6-enynes isomerize to 1,4-dienes unless the 1,4-diene product would contain a trisubstituted olefin in which case 1,3-dienes are obtained (Scheme 13). 1,3-Dienes... 

The cycloisomerization of 1,4-enynes in the presence of platinum(II) catalyst gives 1,2,3-trisubstituted IH-indenes via a [l,2]-alkenyl rearrangement (Scheme 157). ... 

Using late transition metals, trisubstituted pyrrole products have been assembled using a Rh-catalyzed regioselective head-to-tail alkyne dimerization of protected propargylic amine substrates (Scheme 15.98) [323]. Then, upon isolation of the resultant substituted enyne product, Au(III) intramolecular hydroamination with these protected amine substrates could be used to effectively prepare amine-functionalized pyrroles in up to 88% yield. The nature of the N-protecting group dramatically impacts the yields obtained in these reactions [323]. 

As illustrated in Scheme 3.16, Sakai et al. have recently reported the synthesis of 1,1,2-trisubstituted enynes 19 based on the palladium-catalyzed arylalkynylation of aryl... 

SCHEME 3.16 Palladium-catalyzed arylalkynylation of aryl iodides, internal alkynes, and alkynylsilanes. Synthesis of 1,1,2-trisubstituted enynes. 

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