Alkynes intermolecular enyne cross-metathesis

The intermolecular enyne cross metathesis, and consecutive RCM, between a terminal alkyne and 1,5-hexadiene produces cyclohexadienes, by cascade CM-RCM reaction, and trienes, formed during the sole CM step. Studies of various parameters of the reaction conditions did not show any improvement of the ratio of desired cyclohexadiene product [25] (Scheme 12). The reaction with cyclopentene instead of hexadiene as the alkene leads to 2-substituted-l,3-cycloheptadienes [26]. After the first cyclopentene ROM, the enyne metathesis is favored rather than ROMP by an appropriate balance between cycloalkene ring strain and reactivity of the alkyne. 

Under enyne cross-metathesis conditions, the intermolecular reaction of the a,(D-dienes 153, derived from the MBH reaction, with different terminal alkynes 154 afforded triene intermediates that cyclized spontaneously under the reaction conditions to give substituted cis-hexahydro-l/f-indenes 155 (Scheme 4.45), which can be further transformed into steroid analogues via TBS deprotection and oxidation. However, metathesis reactions starting with 156 only furnished trienes 157 [as EfZ) mixtures] and no spontaneous intramolecular cycloaddition occurred. Even at elevated reaction temperatures, trienes 157 cyclized only slowly to give octahydronaphthalene diastereomers. With deprotection of the TBS and subsequent Dess-Martin oxidation, trienes 157 could be converted exclusively into cw-fused 7-substituted 6,7-dehy-drodealone-l-one-lO-carboxylic esters 158 in 50-60% yields. Moreover, c ross-metathesis of TBS-unprotected MBH adduct 159 with alkynes 154 along with treatment with Dess-Martin periodinane (DMP) in one pot could conveniently produce the corresponding bicyclic ketones 160 in moderate yields. ... 

Cross-enyne metathesis has also been applied to solid-phase synthesis by immobilizing either the alkyne or the alkene (498,499). By combination with a Diels-Alder reaction, this process has been used to create a combinatorial library of various six-membered ring structures (500). Within this theme, intermolecular enyne metathesis is the integral synthetic method used to produce novel amino acid derivatives (501, 502). To this end, the aromatic ring of highly substituted phenylalanines was synthesized through enyne crossmetathesis and cycloaddition. 

Cross-Metathesis (CM) of Alkyne with Olefine Ethylene-mediated intermolecular enyne metathesis of various alkynes, pioneered by Mori et al., is a convenient procedure for the formation of 2-substituted or 2,3-disubstituted 1,3-dienes 248 (Scheme 24.60). The low... 

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