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Hetero-quinone methides

The last reaction commonly evoked to support the involvement of radical species 10 in tocopherol chemistry is the disproportionation of two molecules into the phenol a-tocopherol and the ort/zo-quinone methide 3 (Fig. 6.8), the latter immediately dimerizing into spiro dimer 9. This dimerization is actually a hetero-Diels-Alder process with inverse electron demand. It is largely favored, which is also reflected by the fact that spiro dimer 9 is an almost ubiquitous product and byproduct in vitamin E chemistry.28,29 The disproportionation mechanism was proposed to account for the fact that in reactions of tocopheroxyl radical 2 generated without chemical coreactants, that is, by irradiation, the spiro dimer 9 was the only major product found. [Pg.172]

In a study of annelation reactions of 4-alkynylcyclobutenones <91JOC6104>, thermolysis of a 4-alkynyl-4-(propargyloxy)cyclobutenone was observed to give a high yield of a quinone methide as evidenced by the formation of a hetero-Diels-Alder product in the presence of butoxyethene as a... [Pg.893]

Another interesting example of ort/zo-addition to phenol complexes is its reaction with aldehydes. In the presence of a weak base, the aldehyde undergoes an aldol condensation at C2 to give a rare example of a stable o-quinone methide complex. In Fig. 9 citronellal (34) is combined with phenol complex 20 to generate the quinone methide 35. Upon oxidative decomplexation a hetero-Diels-Al-der reaction occurs to form the benzochromene core 37 in 52% yield [48]. [Pg.105]

The synthesis of chromenes through the reaction of phenols with oc,P-unsaturated aldehydes has been modiOed in a variety of ways to improve the value of the route. In the presence of phenylboronic acid, a 1,3,2-benzodioxaborin is formed in high yield which on heating yields the chromene via an o-quinone methide. The robustadial framework has been assembled in this manner (94CJC1866). Hetero-fused derivatives of chromenes result from the reaction of Ti(lV) phenoxides with 3-phenylcinnamaldehyde (94JCS(P1)2591) and the 1,2-addition... [Pg.271]

SCHEME 4 o-Quinone methide-mediated hetero Diels-Alder reaction. [Pg.339]

Napthoquinone-3-methides are conveniently generated via the efficient photodehydration (O = 0.2) of 3-(hydroxymethyl)-2-naphthols. These intermediates undergo facile hetero-Diels-Alder addition kxA x 10 M s ) to electron-rich olefins in an aqueous solution. In this way, photostable ben-zo[g]chromans are formed in excellent yield. The fraction of the quinone methide that is not trapped is rapidly hydrated (A 125 s ) and regenerates... [Pg.18]

In 2013, Lei s group described an elegant methodology that utilizes the in situ formation of o-quinolinone quinone methide from suitable precursor under physiological conditions. The in situ-generated methide reacts with vinyl thioethers as electron rich dienophile in hetero-Diels-Alder reaction. This chemistry was successfully used for live cell imaging [78]. [Pg.23]

Hetero atom-containing dienophiles also take part in the intramolecular Diels-Alder reaction [121-122]. With o-quinone methides [123], cyclization proceeds at the temperature required to generate the diene. The reaction of a nitrile as a dienophile with a simple acyclic diene has also been reported [124]. [Pg.29]

Atroposelective cycloaddition reactions of A-2-(r-butylphenyl)- and A-2,5-(di-r-butylphenyl)-maleimide show good to excellent stereoselectivities and the high rotation barriers prevent cycloadducts from interconverting. The stereospeciflc hetero-Diels-Alder reaction of o-quinone methides (80) with o-quinones (79) in MeOH at room temperature produce the 4a,8-di(hydroxymethyl)chromane derivatives (81) and (82) in high yields (Scheme 29). The intramolecular inverse-electron-demand Diels-Alder reaction of o-quinone methides (84) derived from 2-(l-hydroxy-5-alkenyl)phenol derivatives (83) produces l,2,3,3a,4,9b-hexahydrocyclopenta[c][l]benzopyrans (85) under mild acidic conditions (Scheme 30). The Diels-Alder reactions between dimethyl-cyclohexadiene derivatives and di-(-)-menthyl acetylenedicarboxylate exhibit modest diastereoselectivity. ... [Pg.517]

Recently, the synthetic utility of dimethyl formamide (DMF) as the nucleophilic trigger in aryne MCRs was developed independently by the Miyabe and Yoshida groups. The three-component coupling reaction between aryne, DMF, and an active methylene compound such as acetyl acetone, dibenzoyl methane, cyclohexane 1,3-dione, etc. resulted in the formation of either 2H-chromenes 105 or coumarin derivatives 106 [75,76]. The insertion of fhe carbonyl group of DMF to the aryne generates the o-quinone methide intermediate 104. Subsequent reaction of 104 with cyclic or acyclic 1,3-diketones as the third component resulted in the formation of 2H-chromenes 105. Intriguingly, furnished coumarin derivatives 106 were formed when 3-ketoester or a-(hetero)aryl esters were used as the third component (Scheme 53). [Pg.63]

In 2012, Liao et al. disclosed total synthesis of ( )-schefflone 97 and tocopherol trimers 100 and 101 by the hetero-Diels-Alder trimerization of o-quinone methides 96 and 99, generated from the silver oxide-mediated oxidation of espintanol 95 and tocopherol 98, respectively (Scheme 28.33) [46]. [Pg.792]

The hetero-Diels-Alder reaction (HDA) has been used extensively for the construction of spiroacetals [4], notably in the synthesis of the reveromycins [150-153]. In particular, the recent development of mild methods for the generation of ort/to-quinone methides (o-QM) in the presence of sensitive five-membered exocyclic enol ethers has revealed an alternative route to benzannulated... [Pg.239]

Stolz and co-workers and Funk and co-workers successfully performed model studies toward the related indole natural products communesin and perophoramidine featuring biogenetically inspired Diels-Alder reactions with ortho quinone methide imines. Based on these important preliminary discoveries. Quin and co-workers recently achieved the enantioselective total synthesis of (-l-)-pero-phoramidine via an asymmetric intermolecular hetero Diels-Alder reaction. ... [Pg.1267]


See other pages where Hetero-quinone methides is mentioned: [Pg.190]    [Pg.272]    [Pg.136]    [Pg.95]    [Pg.404]    [Pg.52]    [Pg.177]    [Pg.272]    [Pg.146]    [Pg.13]    [Pg.372]    [Pg.160]    [Pg.98]    [Pg.97]    [Pg.26]    [Pg.97]   
See also in sourсe #XX -- [ Pg.125 ]




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