Stabilization, Isomerization and Rearrangement of Enyne [4 + 4]-Cycloadducts

Abstract

Reactive 1,2,5-cyclooctatrienes, formed by photocycloaddition of 2-pyridones with enynes, are stabilized by steric shielding, slowing or preventing an otherwise facile [2 + 2]-dimerization reaction. Diisopropylsilyl ether-tethered reactants paired with an alkene substituent (R) produce allenes that are stable (R = TMS) or that isomerize to 1,3-dienes by hydrogen migration (R = alkyl). Under acidic conditions, hydrolysis of the photoproduct's silyl ether can lead to a [3,3]-sigmatropic rearrangement.

Enyne [4+4] Cycloaddition/Oxidation: Ring Contraction via Cyclopropanones and Their Anionic Ring‐Opening Reactions

“…[2] Thethermal cycloadditions yield allenes embedded in six-membered rings (2)t hat isomerize to arenes (e.g. [5,6] Stability through steric shielding can be designed into strained allenes,asdescribed by Johnson and Price. Recently we reported that 1,3-enynes can be participants in [4+ +4] cycloadditions and found them to work well with pyridones (4, 7), pyrones and anthracene.…”

“…Recently we reported that 1,3-enynes can be participants in [4+ +4] cycloadditions and found them to work well with pyridones (4, 7), pyrones and anthracene. [6] When the enyne carries two bulky silyl groups,compound 7,they cap the termini of the allene in the resulting cyclooctatriene 8.This molecule is formed as asingle stereoisomer.A llene 8 undergoes as low decomposition in solution, with an ambient temperature half-life of five days. [5,6] Stability through steric shielding can be designed into strained allenes,asdescribed by Johnson and Price.…”

“…[5,6] Stability through steric shielding can be designed into strained allenes,asdescribed by Johnson and Price. Initially we had found that irradiation of 9 for one hour led to allene 10,w hich was unstable to chromatography and dimerized with ah alf-life of 1.3 h. [6] If,h owever, the irradiation was continued for an additional 3h,anew product formed that was spectroscopically similar to 10 but could be isolated by silica gel chromatography in 68 %yield. [6] When the enyne carries two bulky silyl groups,compound 7,they cap the termini of the allene in the resulting cyclooctatriene 8.This molecule is formed as asingle stereoisomer.A llene 8 undergoes as low decomposition in solution, with an ambient temperature half-life of five days.…”

“…Thes tereochemical arrangements shown in Scheme 3were determined by nOe studies of 8 [6] and 14. Thep roduct of this oxidation was cyclopropanone 14,d isplaying typical cyclopropanone absorbances in the IR at 1796 cm À1 and 13 CNMR spectrum at 211.5 ppm.…”

Enyne-2-pyrone [4 + 4]-Photocycloaddition: Sesquiterpene Synthesis and a Low-Temperature Cope Rearrangement

“…12 Treatment of 20 with NBS converted the silane to silyl bromide 21. Addition of the alcohol, base, and DMAP gave the silyl ethers 13 and 22 in good yield.…”