ライフサイエンスコーパス: dienophile

1 itu formation of acyl nitroso intermediates (dienophile).

2 ls-Alder reaction with an alkenyl or alkynyl dienophile.

3 additional reagents apart from a diene and a dienophile.

4 ing acetylene or a "masked acetylene" as the dienophile.

5 ubstrates and electronic distribution in the dienophile.

6 in as a masked diene and iso-maleimycin as a dienophile.

7 iately reacts with a trans-cyclooctene (TCO) dienophile.

8 upon bioorthogonal reaction with a strained dienophile.

9 ediated redox modifications of the activated dienophile.

10 -demand Diels-Alder reaction with a suitable dienophile.

11 3-fold) upon Tz ligation with BCN-a strained dienophile.

12 zing furan as the diene and maleimide as the dienophile.

13 opanol moiety with the carbonyl group of the dienophile.

14 t unoccupied molecular orbital (LUMO) of the dienophile.

15 anilines and aldehydes possessing a pendent dienophile.

16 of the Diels-Alder reactions with an allene dienophile.

17 s a stepwise pathway even for less activated dienophiles.

18 diene and moderately to highly electrophilic dienophiles.

19 is indeed favored for the most electrophilic dienophiles.

20 tility, expanding the range of participating dienophiles.

21 enes with 1,4-quinone monoketal and acrolein dienophiles.

22 with polymer-supported acyl- and arylnitroso dienophiles.

23 ch, electron-deficient, and electron-neutral dienophiles.

24 e in Diels-Alder cycloaddition with reactive dienophiles.

25 rom less nucleophilic dienes and unactivated dienophiles.

26 go thermal cycloadditions with electron-rich dienophiles.

27 d undergo Diels-Alder reactions with various dienophiles.

28 lerant toward a variety of alkyl-substituted dienophiles.

29 the difference in LUMO energies for the two dienophiles.

30 tially reacts with electron-rich or strained dienophiles.

31 substituents originating from the acetylene dienophiles.

32 divergence between cyclic and acyclic enone dienophiles.

33 trazine in the presence of trans-cyclooctene dienophiles.

34 have been established as highly reactive aza-dienophiles.

35 yltetrazine with high orthogonality to other dienophiles.

36 varov reaction, employing phenolic dienes as dienophiles.

37 rnene, Nb) and fast (trans-cyclooctene, TCO) dienophiles.

38 antial differences were observed between the dienophiles.

39 dition in the presence of electron-deficient dienophiles.

40 gand for the enantioselective hDA of acyclic dienophiles.

41 undergo Diels-Alder reactions with suitable dienophiles.

42 developed that expanded the scope to cyclic dienophiles.

43 ompounds may be due to the distortion of the dienophiles.

44 rning on reactivity toward trans-cyclooctene dienophiles.

45 onylchromones, isocyanides, and nonsymmetric dienophiles.

46 than that of IEDDA cycloadditions with other dienophiles.

47 ng in stronger interactions with the HOMO of dienophiles.

48 zation with the preformed glycosyl diene and dienophiles.

49 In the case of alkynyl dienophiles, [4 + 2] cycloaddition initially generates a

50 -Alder reaction of the diene 4 and the novel dienophile 50.

51 iels-Alder reaction between diene (R)-61 and dienophile 55, followed by an intramolecular nucleophili

52 ction features the first use of an azepinone dienophile (8), with high diastereofacial selectivity ac

53 tom at the gamma position, an amine-aldehyde-dienophile (AAD) [4 + 2] cycloaddition takes place by fo

54 Not only are the reactions of electron-rich dienophiles accelerated but those of strained and even u

55 Diels-Alder cyclization by 8.3 kcal/mol via dienophile activation.

56 udy the ability of the isoimidium group as a dienophile activator in DA reactions, as well as its beh

57 lobutadiene can function as both a diene and dienophile, affording a mixture of vinylcyclobutane (2 +

58 e methide interception by a nucleophile or a dienophile allows for one-pot conversion of benzylic C-H

59 azines for amidines but slower with strained dienophiles), an exclusive C4/N1 mode of cycloaddition,

60 to a three-membered diene and a two-membered dienophile, analogous to a retro-Diels-Alder reaction, m

61 driven Diels-Alder reaction of a fluorinated dienophile and a borole that shows remarkable diastereos

62 -Alder reaction between a pyranobenzoquinone dienophile and a silyl ketene acetal diene.

63 ents of the aryne component iteratively as a dienophile and as an electrophile.

64 reported for a range of N-acyloxazolidinone dienophile and diene substrate combinations.

65 ycloaddition (relative approach of the diene-dienophile and endo/exo approach of the nitro group) not

66 sting substituents judiciously placed on the dienophile and intrinsic to the widely used tetrazine 2,

67 anduratin A employing a highly electron-rich dienophile and Lewis acid sensitive diene.

68 , in which the tether linking the initiating dienophile and oxadiazole bears a chiral substituent tha

69 chiral substituent on the tether linking the dienophile and oxadiazole was used to control the facial

70 cursors the substitution pattern of both the dienophile and the 2-azadiene were examined.

71 rable electrostatic interactions between the dienophile and the charged catalyst stabilize the highly

72 the seven-membered enone system bearing the dienophile and the diene in the side chain.

73 and 7-deazapurin-2,6-diamine nucleosides as dienophiles and 3,6-dipyridyl-1,2,4,5-tetrazine as diene

74 applicability by 20 examples with different dienophiles and a variety of dienes.

75 review we discuss the range of commonly used dienophiles and aza-dienes for this process whilst highl

76 r (DTDA) reaction by employing two different dienophiles and eventually generating a small repository

77 hemo- and regioselective cycloadditions with dienophiles and heterodienes.

78 ro-Diels-Alder reaction features unactivated dienophiles and o-QM precursors tethered by a simple phe

79 With the recent discovery of novel dienophiles and optimal tetrazine coupling partners, att

80 es with both electron-poor and electron-rich dienophiles and reveal reaction pathways involving eithe

81 Starting with aryl acrylic esters as dienophiles and siloxybutadienes as diene counterparts,

82 ddition reaction involving in situ arynes as dienophiles and substituted 1,2,4,5-tetrazines as dienes

83 el these reactions by parametrizing both the dienophiles and the dienes partners.

84 azolidinone appended with a number of common dienophiles and various dienes demonstrate the utility o

85 derived from a rearranged lanostane moiety (dienophile) and an abietane unit (diene).

86 e range of reactions as a Diels-Alder diene, dienophile, and [2 + 2] addend.

87 re possible for both the heterodiene and the dienophile, and the target products are obtained in good

88 nvolve highly substituted dienes and juglone dienophiles, and there is an opposite regiochemical outc

89 Furthermore, suitable dienophiles are commercially available in the form of ma

90 ed synthetic literature: while electron poor dienophiles are expected to be the most reactive partner

91 The iminium dienophiles are predicted to have a 17 kcal/mol lower ba

92 Alkyne-equivalent dienophiles are strategically important coupling partner

93 on reactions between tetrazines and strained dienophiles are widely used for protein, lipid and glyca

94 midines and 1,2,3,5-tetrazines with strained dienophiles) are sufficiently low to indicate they may b

95 ilities were established with respect to the dienophile as well as azadiene; H-bonding solvent effect

96 iene intermediates using maleic anhydride as dienophile as well as kinetic studies confirm the calcul

97 to be significantly more reactive than imino dienophiles, as a consequence of less distorted transiti

98 upon mixing the dendralene and an excess of dienophile at ambient temperature in a common solvent, s

99 We study dienophiles attached to isoimidium auxiliaries derived f

100 nificant polarization of the n-system of the dienophile away from the incoming diene.

101 onalization of the selectivity obtained with dienophiles based on (R)-bis((R)-1-phenylethyl)amine.

102 reactive conformation of the vinyl glycoside dienophile being controlled by an exo-anomeric effect.

103 ive Diels-Alder reactions with monoactivated dienophiles, but require fairly forcing conditions, thus

104 oaddition reaction with norbornadiene and as dienophiles (C=S) in the reaction with cyclopentadiene.

105 stereogenic center between the diene and the dienophile can control the relative and absolute configu

106 ted by (2R,5R)-2,5-diphenylpyrrolidine based dienophiles can easily be rationalized by a C2 symmetric

107 xidation level of the alpha,beta-unsaturated dienophile (carbonyl and carboxyl), whereas proton catal

108 and resulting in a wider range of potential dienophile choices or the use of milder reaction conditi

109 ine and (+/-)-neostenine from the same diene/dienophile combination.

110 n of spiro[4.4]nona-2,7-diene-1,6-dione as a dienophile component followed by immediate aromatization

111 Through an appropriate choice of dienophile configuration and backbone substitution eithe

112 require either electronic activation of the dienophile, conformational restriction of the tether, or

113 allylic C-H alkylation and approximated to a dienophile contained within the tertiary nucleophile to

114 egioselectivity for reactions performed with dienophile containing electron-releasing groups.

115 the course of cycloadditions of Diels-Alder dienophiles containing linked enyne sites, each substitu

116 at furan/maleimide is the most studied diene/dienophile couple, this perspective article reports stra

117 t advances in the synthesis of tetrazine and dienophile coupling partners.

118 of unactivated cyano groups as enophile and dienophile cycloaddition partners.

119 The first selective dienophile cycloadditions to substituted [4]dendralenes

120 In reactions with weak dienophiles, cyclooctatetraene (COT) often yields 2:1 ad

121 Thus, while the structures of the dienophiles derived from these two amines seem similar,

122 in the Diels-Alder reaction and reacts with dienophiles dimethyl acetylenedicarboxylate (DPF.DMAD, 6

123 ers, they can be trapped in situ by suitable dienophiles during or immediately after the gold(I)-cata

124 eteroatom substitution and utilizing alkynyl dienophiles enhances the reaction rates up to 10(5)-fold

125 iels-Alder reactions with exceptionally rare dienophile/enophile combinations are reported.

126 ectron-withdrawing groups on the aryl alkyne dienophile facilitated the reaction, and these substrate

127 tem toward a number of indene derivatives as dienophiles, first in intermolecular and thence intramol

128 neat dimethyl acetylene dicarboxylate as the dienophile, followed by catalytic hydrogenation.

129 sine-L36 acts as a Lewis acid activating the dienophile for nucleophilic attack, and asparagine-L91 a

130 atom, comprise a new and valuable family of dienophiles for servicing Diels-Alder reactions.

131 omprise a new and versatile family of chiral dienophiles for the venerable Diels-Alder (DA) cycloaddi

132 ituted orthoquinone monoketals with olefinic dienophiles furnished functionalized ortho-endo bicyclo[

133 These furo[3,4-c]furans react with various dienophiles, furnishing anisole derivatives derived by l

134 e at 80 degrees C in the presence of several dienophiles gave [4 + 2]-cycloadducts derived from the D

135 the Diels-Alder reaction with simple ketone dienophiles has been accomplished.

136 o-Diels-Alder reactions of iminium and imine dienophiles has been explored with density functional th

137 ion of 1,3,5-triazines with amino-containing dienophiles has been obtained.

138 ene S,S-dioxide, a highly reactive diene and dienophile, has been synthesized.

139 nes, and three tetrazines) with the ethylene dienophile have been explored with density functional th

140 dition reactions involving vinylazaarenes as dienophiles have been quantitatively explored in detail

141 reactions between s-tetrazines and strained dienophiles have numerous applications in fluorescent la

142 ization reaction or exclusively as dienes or dienophiles if they are trapped with imines or cyclopent

143 of N-glycosides of enone sugars as reactive dienophile in asymmetric synthesis of bicyclic adduct th

144 part of the tether system works to place the dienophile in closer proximity to the furan ring, thereb

145 Cyclobutenone was employed as a dienophile in Diels-Alder cycloadditions, provide divers

146 reactivity trend: cyclobutadiene serves as a dienophile in intramolecular reactions when it is connec

147 model study required the modification of the dienophile in the [4+2] cycloaddition to accommodate the

148 of star anise oil from a natural source as a dienophile in the multicomponent double Povarov reaction

149 n between a nitrobenzene, an aldehyde, and a dienophile in the presence of iron powder as a reductant

150 (E)-Cyclohept-2-enone is a reactive dienophile in thermal [4 + 2] cycloaddition reactions wi

151 nstead of gaseous ethylene, as the source of dienophile in this one-pot synthesis, makes the aromatic

152 amples of ynamides behaving as electron-rich dienophiles in [4 + 2] cycloaddition reactions.

153 DMTB and related reagents engage diverse dienophiles in an initial Diels-Alder or metal-catalyzed

154 lpha,beta-unsaturated N-acyl iminium ions as dienophiles in Diels-Alder reactions and electrophilic a

155 nones were prepared and subsequently used as dienophiles in Diels-Alder reactions to form building bl

156 nones are demonstrated to function as potent dienophiles in inter- and intramolecular Diels-Alder cyc

157 trophilic nature, benzynes serve as reactive dienophiles in numerous cycloaddition reactions.

158 -Alder cycloaddition with electron-deficient dienophiles in the absence of any activator or catalyst,

159 The use of chiral rigid 1-azadienes as dienophiles in the cycloaddition reaction with cyclopent

160 t-2-enone were employed in their (E)-form as dienophiles in the Diels-Alder reaction with 1,3-cyclope

161 hat the regiopreferences of these dienes and dienophiles in these cases are not a fixed property of e

162 servation that furans and an indole serve as dienophiles in this cascade reaction permitted the synth

163 Exploiting furans as unusual dienophiles in this inverse electron demand Diels-Alder

164 of complex Diels-Alder adducts with various dienophiles, including alkynes.

165 thesis of para-benzoquinones from acetylenic dienophiles, including benzynes, is also demonstrated, a

166 ion, which is dependent on the nature of the dienophile, initially led to the development of an unant

167 Analysis of the diene-dienophile interactions reveal that the reactivity trend

168 served when the termini of the diene and the dienophile involved in the shorter of the forming bonds

169 mportant reactions of 1,2,3-triazines with a dienophile is inverse electron demand Diels-Alder (IEDDA

170 Historically, the use of quinones as dienophiles is highly significant, being the very first

171 tion of several alkyne and alkyne-equivalent dienophiles is presented, showing that the relative reac

172 ly facilitated by phenyl substitution in the dienophile, is highlighted.

173 2] cycloaddition of a conjugated diene to a dienophile, is one of the most powerful reactions in syn

174 resting reactivity was observed with certain dienophiles leading to ring-opening of the initially for

175 e (CHT) and cyclooctatetraene (COT) with the dienophiles maleic anhydride and 4-phenyl-1,2,4-triazoli

176 s are reacted with the isotopically enriched dienophiles maleic anhydride and a C18-alkyl chain-modif

177 ) reactions that occur between the diene and dienophile moieties located on a single macrocyclic trie

178 -Alder reactions with tethered electron-poor dienophiles more rapidly and in higher yield than non-ni

179 proceed with a predictable regioselectivity (dienophile most electron-rich atom attaches to C4), and

180 lenes (as multidienes) with an electron poor dienophile, N-methylmaleimide (NMM), has been studied.

181 f various 3,4-diarylfurans with a variety of dienophiles, neat and under mild conditions in the absen

182 1,2,3-trisubstituted diene to a symmetrical dienophile obtained from Meldrum's acid.

183 Herein, we investigate the effect of the dienophile on the fluorescence enhancement obtained upon

184 s of tether length, heteroatoms, and alkynyl dienophiles on reactivity were analyzed.

185 e radical cation can be formed at either the dienophile or the diene.

186 Compared to the traditional diene-dienophile or two-component approach that requires preci

187 ic diene component either with water-soluble dienophiles or with dienophiles requiring mixtures of wa

188 nomaly is related to the fact that the diene/dienophile orientation in the charge-transfer [MeOSty, o

189 redicting regioselectivity for untried diene-dienophile pairs are made.

190 with deeper features on other types of diene/dienophile pairs being either petro-sourced (cyclopentad

191 Further testing on diene-dienophile pairs that underwent successful cycloaddition

192 oxidation state) of the 4pi (diene) and 2pi (dienophile) pairs of reactants dictates the oxidation st

193 ns involving cyclodimerizations, dienes, and dienophiles, parameters affecting the reaction, while th

194 involving unsymmetrical alkenyl and alkynyl dienophiles proceed with good to excellent regioselectiv

195 rimethylsiloxy)-dienes and a,B-alkynyl ester dienophiles provided facile access to resorcinol precurs

196 ith both achiral and chiral acrylate-derived dienophiles provides rapid access to the bicyclo[2.2.2]d

197 Limited reactions were observed with other dienophiles, providing the expected cycloadducts in most

198 cycloaddition are defined; new viable diene/dienophile reaction partners in the cycloaddition reacti

199 Diels-Alder cyclization of diverse diene-dienophile reactive pairs proceeds rapidly, in high yiel

200 of C-C and C-heteroatom bonds, their use as dienophile-reactive groups has been rare.

201 ither with water-soluble dienophiles or with dienophiles requiring mixtures of water and organic solv

202 cycloaddition reactions with a wide range of dienophiles, resulting in the formation of bridged bicyc

203 ns with amidine, electron-rich, and strained dienophiles reveal unique fundamental reactivity pattern

204 ents with structural perturbations; modified dienophile segments; and different acid catalysts.

205 and rate (kinetics) dependent on the alkene dienophile selected.

206 Nitroalkene 26 containing a diene as the dienophile served well in the tandem cycloaddition to af

207 gid 1-azadienes can behave as both diene and dienophile species through a spontaneous cyclodimerizati

208 ses was found not to negatively affect their dienophile specificity.

209 and the influence of various factors such as dienophile structure and the approach mode of the nitro

210 carbonyl-containing series, a wide range of dienophile substituents were screened including esters,

211 trostatic interactions between the diene and dienophile substituents.

212 action was investigated, including diene and dienophile substitution patterns and dienophile tether l

213 Dienophile substitution results in complex trends becaus

214 t both the amino acid-Cu(2+) complex and the dienophile substrate are included inside the macrocyclic

215 ty and stereoselection is observed with both dienophile substrates.

216 e turn-on (up to 400-fold) when reacted with dienophiles such as cyclopropenes and trans-cyclooctenes

217 dioneketene, which reacts with electron-rich dienophiles such as enol ethers to afford [4 + 2] cycloa

218 ol of rapid bioorthogonal cycloaddition with dienophiles such as trans-cyclooctenes.

219 ere the "unmasked" Cp can rapidly react with dienophiles, such as maleimides, through a conventional

220 ene and dienophile substitution patterns and dienophile tether length.

221 presence of an amido carbonyl as part of the dienophile tether, as opposed to the annealed ring (66)

222 raordinary cycloaddition rates with strained dienophiles (tetrazine ligation), the new and isomeric 1

223 Cyclopropenyl ketones are highly reactive dienophiles that can engage a range of cyclic dienes and

224 ween surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand.

225 s of cyclohexadiene and cycloheptadiene with dienophiles that give relatively synchronous transition

226 A diene and either the HSty, ClSty, or MeSty dienophile, the donor/acceptor arrangements in the charg

227 the lack of reaction of the non-fluorinated dienophile, the stereochemical outcomes, and the rate ac

228 elative basicities of the PEDA precursor and dienophile, the stoichiometry and concentration of the L

229 For both dienophiles, the highest stereoselectivities (>or=89% en

230 h the energies of the vacant orbitals of the dienophiles, the lower reactivity of the allenes relativ

231 With dihydropyridone 5 and cis-disubstituted dienophiles, the resulting cycloadducts were obtained as

232 complexes containing both the diene and the dienophile; the cycloaddition reaction occurs in the cav

233 beta,gamma) double bond of one allene as the dienophile; the double bonds at the alpha-aryl-(beta',ga

234 several symmetrical as well as unsymmetrical dienophiles, these dendralenes afforded diversity-orient

235 t rates of reaction of these products with a dienophile through the Diels-Alder reaction confirmed th

236 e heterodiene component and the other as the dienophile to afford different types of dimeric products

237 the interactions of MaDA with the diene and dienophile to catalyse the [4+2] cycloaddition.

238 ective Diels-Alder reaction with a different dienophile to form 2-fold cycloadducts.

239 on between a cyclopentadiene and an olefinic dienophile to form norbornene, which is among the best-s

240 s synthesized from L-lactide and used as the dienophile to prepare spiro[6-methyl-1,4-dioxane-2,5-dio

241 l Diels-Alder reaction relied on an "ynoate" dienophile to rapidly assemble the required resorcylinic

242 along with an alkyne, have been utilized as dienophiles to afford a wide range of different THP deri

243 undergo Diels-Alder cyclization with various dienophiles to afford novel octahydroquinolines containi

244 -Alder cycloaddition with electron-deficient dienophiles to afford, without any significant selectivi

245 We have found 2-brominated cycloalkenone dienophiles to be both highly endo selective and signifi

246 dienes and undergo Diels-Alder reaction with dienophiles to form stable non-aromatic Diels-Alder addu

247 went Diels--Alder cycloaddition with various dienophiles to furnish vinyl sulfone resins 10-14.

248 itions with electron-rich and neutral alkene dienophiles to generate a tetrahydropyridinium ion as th

249 [4+2] cycloaddition reactions with different dienophiles to generate bridged azabicycles in high endo

250 eta-enals can act as well as highly reactive dienophiles to render adducts endowed with nitrogenated

251 ronsted acid catalyst, it also activates the dienophile toward reaction with the diene.

252 The strained dienophiles, trans-cyclooctene and cyclooctyne, are much

253 Cycloaddition with olefinic dienophiles, under exceptionally mild conditions, enable

254 Whereas electron-deficient dienophiles undergo irreversible Diels-Alder reactions,

255 furan as the diene unit and maleimide as the dienophile unit at its termini.

256 The diene and dienophile units of adjacent monomer molecules are align

257 s, providing gels that can be patterned with dienophiles upon 365 nm uncaging of cyclopentadiene.

258 n Diels-Alder cycloaddition of electron-rich dienophiles upon irradiation with visible light.

259 ty of pyrones toward a collection of popular dienophiles used in bioorthogonal reactions.

260 d Diels-Alder cycloaddition with an external dienophile usually exists, except in the case of 5-subst

261 ng that the relative reaction rates of these dienophiles vary over multiple orders of magnitude.

262 ying highly electron rich 2'-hydroxychalcone dienophiles via electron transfer-initiated Diels-Alder

263 nce from benzene oxide and a nitroso- or azo-dienophile was investigated as a tool for access to high

264 site K127 and the aldehyde group of the enal dienophile was revealed by structural analysis and captu

265 n cycloaddition, and a methylenecyclopropane dienophile was used for a stereoselective intramolecular

266 The scope of dienophiles was comprehensively explored; relative react

267 of isatins compared to other carbonyl based dienophiles was demonstrated and rationalized with the a

268 togenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, elec

269 e" [4 + 2] cycloaddition with representative dienophiles was followed by the "west-side" construction

270 eic acids incorporating 7-oxanorbornene as a dienophile were reacted with tetrazines linked to either

271 The reactions with arylnitroso dienophiles were also carried out in solution.

272 Combinations of dienes and dienophiles were examined in order to elicit possible co

273 ral and chiral dienes with several different dienophiles were examined.

274 The reactivities of cycloalkenes as dienophiles were found by a distortion/interaction analy

275 eketals (7 or 8) or with methacrolein as the dienophiles were investigated.

276 adiene, and cycloheptadiene with a series of dienophiles were studied with quantum mechanical calcula

277 lving 1,3-disubstituted nonequivalent allene dienophiles were studied, and some surprising results we

278 nyl-1,2,4-triazoline-3,5-dione served as the dienophile which provided the adjacent N-N unit in hexah

279 The relative configuration adjacent to the dienophile, which led to C4 of the decalin system, as we

280 eric (Pauli) repulsion between the diene and dienophile, which originates from both a more asynchrono

281 ubstrate promiscuity towards both dienes and dienophiles, which enables the expedient syntheses of st

282 to predict even more reactive vinylazaarene dienophiles, which may be useful in organic synthesis.

283 in the conserved hexacene unit works as the dienophile while the tetraazahexacene participates as th

284 f the diene to the more hindered face of the dienophile, while electrostatic interactions control the

285 ate and cyclic ketenimine equivalents as aza-dienophiles, while extension to a one-pot aza-Diels-Alde

286 was also explored on the carbonyl-containing dienophiles with a series of cyclic dienes.

287 nd ArNO), and nitrosoformaldehyde (CHONO) as dienophiles with butadiene and a series of 1- and 2-subs

288 siderations favor endo cycloadducts for both dienophiles with DHP, while total energy considerations,

289 ion via a Diels-Alder reaction of acetylenic dienophiles with subsequent Michael additions of aromati

290 A new method for the generation of N-acylazo dienophiles with tetra-n-butylammonium periodate is repo

291 nproductive consumption of the electron-rich dienophiles without productive activation of the electro