Enynes, conjugated

Pyrolysis of diethynylketone tosylhydrazones gives a mixture of dimeric products (142- 144). The alkylation of several enamines of cycloalkanones with 5-halogeno-3-ene-l-ynes gives a mixture of isomeric ketones (145) and (146). The composition of this mixture is independent of the enamine but is related to the nature of the halide. Primary halides lead essentially to (145) and secondary halides give (146). 

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Treatment of conjugated enyne systems, H C-C=C-CeC-CH-OR, with a suitable base in principle may give rise to 1,6-elimination of HOR with primary formation of the system C=C-C=C=C=C, which subsequently may undergo isomerization to a dienyne. ... 

The alkynyl iodide 150 undergoes the oxidative addition to form an alky-nylpalladium iodide, and subsequent insertion of an alkene gives the conjugated enyne 151 under phase-transfer conditions[120]. 

In the coupling of the allenyl ester 7 with a terminal alkyne, an electron-deficient phosphine (Ph3P) gave the enyne-conjugated ester 8 as the major product, while an electron-rich phosphine (TDMPP or TTMPP) yielded the non-conjugated enyne esters ( )- and (Z)-9[4],... 

The insertion of alkynes into a chromium-carbon double bond is not restricted to Fischer alkenylcarbene complexes. Numerous transformations of this kind have been performed with simple alkylcarbene complexes, from which unstable a,/J-unsaturated carbene complexes were formed in situ, and in turn underwent further reactions in several different ways. For example, reaction of the 1-me-thoxyethylidene complex 6a with the conjugated enyne-ketimines and -ketones 131 afforded pyrrole [92] and furan 134 derivatives [93], respectively. The alkyne-inserted intermediate 132 apparently undergoes 671-electrocyclization and reductive elimination to afford enol ether 133, which yields the cycloaddition product 134 via a subsequent hydrolysis (Scheme 28). This transformation also demonstrates that Fischer carbene complexes are highly selective in their reactivity toward alkynes in the presence of other multiple bonds (Table 6). 

Although the path (a) has been verified by a stoichiometric reaction [23], the details of exact reaction mechanism remain unsettled. Triggered by this publication [and the Pd-catalyzed doublethiolation of alkynes described in Eq. (7.7) in Section 7-3], a number of transition metal-catalyzed additions of S-X or Se-X bonds to C-C unsaturated organic compounds started to be published. In 1994, BackvaU et al. applied the Pd(OAc)2-catalyzed hydrothiolation to conjugated enynes and obtained 17,... 

It was described that the use of the combination of Pd(OAc)2/dppp /THE resulted in the highest yield. It is not clear whether such a combination of catalyst, ligand and solvent is restricted to the hydrothiocarboxylation of conjugated enynes or also may be applied to other alkynes. Furthermore, it remains to be explored whether, in the Pt-catalyzed reaction performed in CHjCN [see Eq. (7.18)], conjugated enynes can be used. 

A hydroboration-protonolysis procedure for the conversion of conjugated enynes to dienes is far superior to partial hydrogenation over Lindlar s catalyst, in terms of stereoselectivity and yields280. Ratovelomanana and Linstrumelle reported the synthesis of methyl a-eleostearate (equation 163) and methyl punicate by employing this strategy280. 

Reductive Coupling of Conjugated Enynes and Diynes with Activated Aldehydes and Imines... 

For use of conjugated enynes as nucleophilic partners in catalytic intermolecular reductive couplings to aldehydes, see K.M. Miller, T. Luanphaisarnnont,... 

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. 

The 1,4-addition of an appropriate reagent, such as a hydrosilane or a hydroborane, to a conjugated enyne produces an allenic derivative. Occasionally, 1,2-addition to the alkynyl moiety of the substrate competes as an undesirable side reaction to give a conjugated diene (Scheme 3.66). Suppression of the latter reaction path is required for the selective preparation of the allenic products. 

For the selective 1,4-hydrosilylation to 133 giving the allenic product 134, the presence and orientation of the two -SiMe3 groups at the 1- and 4-positions in 133 play important roles. Reaction of the hydrosilanes with the closely related conjugated enynes 136 and 137 in the presence of the Pt or Rh catalyst proceeded in a 1,2-addi-tion manner to produce conjugated dienes instead of allenes [108],... 

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