Esters 1,4-enyne, cycloaddition

Alkoxyoxazoles have also proven to be valuable heterodienes in the synthesis of highly substituted furans and biologically active natural products via oxazole-aUtyne Diels-Alder reactions. Thus the synthesis of a synthon for the DEF ring system of fredericamycin began with the cycloaddition of 5-ethoxy-4-methyl-oxazole 8 with enyne ester 149 (Fig. 3.45). This reaction proceeded in 24 h in refluxing toluene to afford a 65% yield of the 2-ethoxyfuran 150. Furan 150 was then converted to the isoquinoline 151 in six steps. ... 

The Au-catalysed intermolecular 4-1-2-cycloaddition reactions of 2-arylynamides with alkenes produced dihydronaphthalenes, whereas the Au-catalysed 2-1-2-1-2-cycloadditions of terminal ynamides with enol ethers produced tetrahydrobenzenes. " The Rh-catalysed 4-1-1- and 5 4-1-cycloaddition reactions of 1,4-enyne esters (7) with CO yielded cyclopentenones (8) and functionalized resorcinols (9), respectively. A key step in these reactions is an acyloxy migration (Scheme 3). ... 

The enantioselective variants of the corresponding [2+2+2] cycloaddition were already achieved by chiral Rh catalyst however, the coupling partners of enynes were limited to monoalkynes with functionality, such as ester or alcohol [18]. 

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>. 

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. ... 

Enynes with an amide or ester group in the tether between the multiple bonds undergo (formal) intramolecular [2+2]-cycloadditions in the presence of PhsPAuCl and AgSbFe (Scheme 4-43). The analogous substrates lacking the... 

Since the 1990s, the olefin metathesis reaction has become a major synthetic tool in organic chemistry. Organoboranes were first employed in the construction of carbo-cyclic and heterocyclic alkenylboronates by ring closure of the corresponding acyclic precursors [104], Ruthenium-catalyzed enyne metathesis of acetylenic boronates was later demonstrated as a concise route for the construction of cyclic 1,3-dienylboronic esters, which can be further engaged in [4-i-2] cycloadditions (Scheme 9.51) [86]. 

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