Terminal moieties alkenic units

This unique reactivity was subsequently utilized in the functionalization of unsaturated hydrocarbon substrates with CO (Scheme 39) [285, 286]. In the course of these studies the stereospecific synthesis of a diolato unit was observed via a samarium-mediated CO and CH (alkene) activation. A bridging trans-5,10-dihydroindeno[2,l-a]-5,10-diolate ligand forms terminal Sm-O bonds in [Cp Sm]2(O2C16H10) and its THF adduct. Two molecules of CO are formally inserted into the carbon carbon double bond of l,2-di-2-pyridylethene to form a terminal bisenolate moiety [286] (Table 18). 

To deduce the location of the double bond within the lipid backbone, the mixture (500 ng) was subjected to consecutive bisthiomethylation of the alkene85 and O-methyloxime formation (Equation 3). GC—MS study of the fragmentation of these derivatives (e.g., see 31, derived from 24) allowed simultaneous determination of the cleavage site (between C24 and C25) and of which portion contained the original ketone (i.e., the odd versus even mass fragments of 17 3 and 426 for 31). All of the monounsaturated lipid ketones had the alkene in the same downstream location in other words, they varied in the number of methylene units between the ketone and alkene functional groups but were constant in their -octyl terminal alkyl moiety. The four most major components (24, 25, 27, and 28) were prepared by chemical synthesis and used to confirm their identity in the natural pheromone and their pheromonal activity both alone and in admixtures. 

Borane, monoalkylboranes and dialkylboranes (such as thexylborane and 9-BBN) were shown to be powerful reducing agents for carbonyl derivatives (sec. 4.6.A). When the B—H unit is removed, as in trialkylboranes, addition to the alkene moiety of a,p-unsaturated ketones or aldehydes occurs. An alkyl group of the borane is transferred to the terminal position of the alkene moiety (1,4-addition) and boron is transferred to the oxygen, giving a boron enolate (sec. 9.4.D). Initial hydroboration of acrolein with tricyclopentylborane gave boron enolate 196.1 0... 

D.i.b. Insertion of Another Alkenyl Unit In certain cases, however, the 3-exo-trig process may be retarded and an additional alkene moiety participates in the cascade carbopalladation. A pioneering example of this kind has been demonstrated by Overman and co-workers in their total synthesis of scopadulcic acid A, starting from an iodoaUcenyl-substituted methylenecycloheptene derivative (Scheme 24). The first intramolecular carbopalladation occurs across the disubstituted double bond of the exomethylene group, and the trisubstituted endocyclic double bond acts as the terminator to give a tricyclic system, which was further elaborated to the natural product (Scheme 24). It is remarkable that all three quaternary carbon centers can be created by intramolecular Heck reactions. 

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