1,3-Cyclopentadiene, cycloaddition with ethyne

The bisiodonium ethyne 24 is even more reactive than 18 and readily undergoes Diels-Alder cycloaddition with cyclopentadiene and furans as shown in equation 41. The remarkable fact of the cycloaddition of 24 is that, unlike the usual Diels-Alder reactions that require extensive heating and/or pressure, these reactions occur in a matter of minutes at -35 °C. 

Low-temperature trapping experiments of the photolysis products of disilacycloheptene (75), which was prepared by insertion of ethyne into the Si—Si bond of l,l,2,2-tetramethyl-l,2-disilacyclopentane <75JA931>, provided evidence for the trans isomer of (75) <90JA6601>. GC-MS analyses indicated that, as the decay of trans-(75) progressed, a mixture of six dimers of mjz 368 was formed. Only one of the dimeric products was successfully isolated in pure form by preparative GC this dimer was assigned the unsymmetrical c/s,fra 5-fused cyclobutene structure (76) on the basis of NMR data. Trapping experiments of metastable trans-(75) by Diels-Alder cycloadditions with cyclopentadiene and 9,10-dihydro-l l,12-dimethylene-9,10-ethanoanthracene afforded (77) and (78), respectively. The crystal and molecular structures of (78) were determined. 

Neither ethene nor ethyne is a very good dienophile but L4 + 2] cycloadditions of either with 1,3-cyclopentadiene go well at temperatures of 160-180° because 1,3-cyclopentadiene is a very reactive diene. Achieving the overall result of addition of ethene or ethyne to a less reactive diene could necessitate a synthetic sequence wherein one of the reactive dienophiles listed in Table 13-1 is used to introduce the desired two carbons, and the activating groups are subsequently removed. An example follows ... 

The bisiodonium ethyne 30 is even more reactive than 11 and undergoes Diels-Alder cycloaddition [24] with cyclopentadiene and furans in a matter of minutes at low temperatures (Scheme 3-10). The structure of adduct 122 was unambiguously established by X-ray analysis [24]. Cycloadducts 122 and 123 can be reacted with RC = CLi to give enediynes, 125, or with nucleophiles [90] to give 126 (Scheme 3-10). 

Scheme 3-10 Diels-Alder cycloaddition of bis-iodonium ethyn 30, with cyclopentadiene and furans and subsequent reaction of the adducts.

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