Domino metathesis reactions dienynes

Domino metathesis reactions of the dienynes 97 and 99 catalyzed by the phosphine-free ruthenium catalyst 4 in the presence of ethylene allowed a rapid access to the naturally occurring marine trisnorsesquiterpenes (—)-clavukerin A (98) and (—)-isoclavukerin A (100), respectively, by preferential initiation at the disubstituted alkene (Scheme 2.36) [18i]. As the dienyne substrates 97 and 99 were easily available from (S)- and (J )-citronellal, respectively, in only three high yielding operations, the 1,3-dienes 98 and 100 now offer themselves as enantiopure building blocks for the preparation of structurally more complex hydroazulene targets. 

Scheme 6.71 Dienyne (reaction a) and triyne (reaction b) domino ring-closing metathesis reactions.

Domino metathesis of siloxy-tethered dienynes offers attractive opportunities for the synthesis of stereochemically defined 1,3-dienes after desilylation [21]. These reactions also allow the efficient preparation of highly substituted cycloolefins, which is illustrated in Scheme 2.43 [21b,c]. Selective double cydization of silaketal 118 via initiation at the monosubstituted alkene led to a fused 6/7-bicychc product which was subsequently desilylated to afford the triol 119 in high overall yield. 

As illustrated by the schemes above, metathesis reactions in domino processes have swiftly become an established tool in modern organic synthesis. Especially, RRM and domino dienyne metathesis have found extensive uses and will surely continue to catch the attention of synthetic chemists as elegant and atom-economic transformations that allow a quick enhancement of molecular complexity. Next to combinations of metathesis events only, the incorporation of non-metathesis steps into true domino processes has advanced to a remarkable level and will certainly provide even more opportunities in the years to come. 

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