Cross-metathesis chains

Blechert and co-workers have also reported the application of cross-metathesis to the synthesis of jasmonic acid derivatives containing modified alkene side chains [28]. Molybdenum or ruthenium catalysed cross-metathesis of acetal 20 with various alkenes gave the desired cross-metathesis products in high yields (Eq. 21) (Table 2). 

Two repeated exposures of resin 38 to the catalyst (9% mol) for 18 h in dichloromethane at room temperature afforded the expected allyl lactoside in an encouraging isolated yield of 81% from resin 35 (90% per step). Traces of dimerized compounds resulting from cross-metathesis were detected as the only side products. Extension of the oligosaccharide chain was subsequently performed first by deacetylation (excess NaOMe in 4/1 CH2Cl2/MeOH at r.t.) and glycosylation with known lactosyl donor 40 in conditions similar to those mentioned above. Cleavage was performed twice as described above, but with a reduced reaction time of 6 h in this case tetrasaccharide 42 was isolated in 51% yield from 35 (84% per step). No dimerized products were detected. 

After successful completion of all rearrangement reactions, the incorporation of the different side chains of the tetraponerines was attempted by employing a cross metathesis reaction. However, the cross metathesis of 19 and 22 with allyltrimethylsilane in the presence of 10% [Ru-1] was unsuccessful due to the formation of a carbene with low reactivity. The use of Schrock s molybdenum catalyst26 [Mo] (Figure 7) also failed to show any conversion. The terminal double bonds of 19 and 22 were assumed to be too hindered for cross metathesis. An alternative route to incorporate the different alkyl chains of the tetraponerines was necessary (Scheme 8). 

A synthesis of the Annonaceous acetogenin asimicin and a side-chain analogue has been achieved by a highly convergent route in which Grubbs cross-metathesis played a key role.98... 

A palladium-catalyzed intramolecular benzannulation of bis-enynes 1135 proceeds chemoselectively to afford dihydroisocoumarins 1136 (Equation 441) <2002JOC2653>. A reaction sequence involving ruthenium-catalyzed yne-ene cross-metathesis of a polystyrene supported undecynoic acid ester followed by a Diels-Alder cycloaddition reaction with DMAD provides the basis for a combinatorial approach to dihydroisocoumarins featuring a variety of side chains at C-6 and C-8 <1999SL1879>. 

With the advent of efficient cross metathesis catalysis, this reaction is being seen in a variety of total syntheses and analogue synthesis strategies. Two areas where cross metathesis has been particularly useful are the addition of side chains or heterodimerization and the formation of homodimers. 

A new procedure for GSL synthesis via olefin cross metathesis (164) is highly versatile in terms of the hydrophobic agly-cone. A protected 5 carbon amino alkene diol is the central building block to which the protected carbohydrate donor, long chain fatty acid, or, by olefin cross metathesis, the long alkenyl chain of the base can be coupled, in a variety of sequences. This atypical synthetic flexibility should allow a stmctural approach to dissecting the role of the lipid moiety in GSL receptor function and intracellular trafficking. 

Tandem reactions. The multiple activities of the Ru catalysts enable development of valuable tandem reactions. Further extension of the carbon chain of a conjugated enal obtained from a cross-metathesis reaction is readily achieved on treatment with diazoacetic esters to give alkadienoic esters. ... 

The stability of 4A in the presence of BF3 OEt2 is apparently critical to the synthesis of azacycles containing a 2-oxoalkyl chain at C-2 by cross-metathesis of l-alken-3-ones with w-alkenamines. The Lewis acid is responsible for the intramolecular Michael reaction. ... 

Via stereoselective ethenolysis of 1,5-cyclooctadiene (COD), Bykov et al. [25] prepared l,d5-5,9-decatriene, a precursor for the synthesis of many ds-isomeric insect sex pheromone compounds. In the presence of the MoCl5/Si02/Me4Sn catalyst system, at 20 °C and an ethene pressure of 25 bar, a 80 % conversion of COD was obtained with a selectivity of 68.4% for l,ds-5,9-decatriene. From this triene, many long-chain (CiQ-Cig) unsaturated acetates, alcohols and aldehydes can be obtained with the required biologically active cis conformation. Cross-metathesis of cyclooctene with a-olefins in the presence of the same catalyst gave... 

Cross-metathesis between cyclic and acyclic olefins may involve some or all of the propagation and chain-transfer steps shown in Scheme 15.2 for the particular case... 

Scheme IS.2 Mechanism of cross-metathesis between cyclopentene and pent-l-ene P = polymer chain.

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