Addition to Enynes

Not only electrophilic 1,4-addition, as shown above, but also radical 1,4-addition to conjugated enynes such as selenosulfonation is known to yield acceptor-substituted allenes [118]. Finally, monotitanation of conjugated diynes followed by treatment with benzaldehyde and aqueous workup leads to an ester of penta-2,3,4-tri-enoic acid, which is formally also a product of 1,4-addition [147]. 

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Other conjugated systems, including trienes, enynes, diynes, and so on, have been studied much less but behave similarly. 1,4 Addition to enynes is an important way of making allenes ... 

Although the resulting vinylallenes 48 were usually obtained as mixtures of the E and Z isomers, complete stereoselection with regard to the vinylic double bond was achieved in some cases. In addition to enyne acetates, the corresponding oxiranes (e.g. 49) also participate in the 1,5-substitution (Scheme 2.18) and are transformed into synthetically interesting hydroxy-substituted vinylallenes (e.g. 50) [42], Moreover, these transformations can also be conducted under copper catalysis by simultaneous addition of the organolithium compound and the substrate to catalytic amounts of the cuprate (see Section 3.2.3). 

Additions to conjugated systems are further facilitated by some of the features discussed previously e.g., additions to dienes" " and enynes" in the presence of transition-metal catalysts and additions to enynes containing a suitably placed intramolecular function such as hydroxyl. Although uncatalyzed intermolecular additions to alkynylsilanes are not yet demonstrated, additions are seen in the presence of a transition-metal catalysts" and intramolecular additions to alkynylsilane functions by suitably placed internal organomagnesium-halide functions are facile. ... 

The catalytic P-H bond addition to enynes showed selective reactions, which involved the triple bond only. The presence of two triple bonds in diynes resulted in bis-hydrophosphorylation product under these conditions (Scheme 8.3). 

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]. 

The light-induced rearrangement of 2-phenyl- to 3-phenyl-thiophene may occur by a similar mechanism an equilibrium between the bicyclic intermediate (26) and the cyclopro-penylthioaldehyde (27) has been suggested (Scheme 2). The formation of IV-substituted pyrroles on irradiation of either furans or thiophenes in the presence of a primary amine supports this suggestion (Scheme 3). Irradiation of 2-phenylselenophene yields, in addition to 3-phenylselenophene, the enyne PhC=C—CH=CH2 and selenium. Photolysis of 2-phenyltellurophene furnishes solely the enyne and tellurium (76JOM(108)183). 

The first examples of macrocyclization by enyne RCM were used in Shair s impressive biomimetic total synthesis of the cytotoxic marine natural product longithorone A (429) [180]. This unique compound features an unusual hep-tacyclic structure which, in addition to the stereogenic centers in rings A-E, is also chiral by atropisomerism arising from hindered rotation of quinone ring G through macrocycle F (Scheme 85). It was assumed that biosynthesis of 429 could occur via an intermolecular Diels-Alder reaction between [12]paracy-... 

In recent years, in addition to the ring-opening metathesis (ROM) and ring-closing metathesis (RCM), the enyne metathesis and the cross-metathesis (CM) have... 

The active Rh(i) catalyst was generated in situ by the addition of AgSbF6 to [Rh(ligand)Gl]2 in the presence of substrate. Zhang postulates that the coordinatively unsaturated metal complex generated by this process facilitates coordination to enyne substrates and smooth conversion into the Alder-ene products. l,4-Bis(diphenylphosphino)butane (dppb) and... 

Mechanistically, a (silyl)(stannyl)palladium initially formed undergoes regioselective silylpalladation to the alkyne moiety of the enyne (Scheme 66). Then, two possible pathways are conceivable for addition to the alkene moiety, that is, stannylpalladation and carbopalladation. It has not been established that which pathway operates. 

If the acceptor molecule contains both an alkene and an alkyne group, hydrostannation occurs at the alkyne moiety. 1,6-Enynes undergo ring closure initiated by addition to the alkyne, as shown by the fate of the deuterium in Equation (26).108... 

Introduction of a double bond between the triple bond and the leaving group leads to enyne electrophiles 45, which would give access to vinylallenes 46 if the attack of the nucleophile takes place at the triple bond in an SN2" (1,5) substitution reaction (Scheme 2.16). In addition to the regioselectivity, two types of stereoselectivity also have to be considered in this transformation, i.e. the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. 

Scheme 2.23 1,6-Cuprate addition to different acceptor-substituted enynes 66.

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