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Intramolecular coupling reductions

The reductive coupling of aldehydes or ketones with 01, -unsaturated carboxylic esters by > 2 mol samarium(II) iodide (J.A. Soderquist, 1991) provides a convenient route to y-lactones (K. Otsubo, 1986). Intramolecular coupling of this type may produce trans-2-hy-droxycycloalkaneacetic esters with high stereoselectivity, if the educt is an ( )-isomer (E.J. Enholm, 1989 A, B). [Pg.69]

The allylstannane 474 is prepared by the reaction of allylic acetates or phosphates with tributyltin chloride and Sml2[286,308] or electroreduction[309]. Bu-iSnAlEt2 prepared in situ is used for the preparation of the allylstannane 475. These reactions correspond to inversion of an allyl cation to an allyl anion[3l0. 311], The reaction has been applied to the reductive cyclization of the alkenyl bromide in 476 with the allylic acetate to yield 477[312]. Intramolecular coupling of the allylic acetate in 478 with aryl bromide proceeds using BuiSnAlEti (479) by in situ formation of the allylstannane 480 and its reaction with the aryl bromide via transmetallation. (Another mechanistic possibility is the formation of an arylstannane and its coupling with allylic... [Pg.353]

Reduction of the nitro group in 70 with zinc in hot sodium hydroxide results in formation of pyrrolo[2,l-(f][l,2,5]benzotriazepine 71 by intramolecular coupling of the amino group with the newly formed nitroso group (Scheme 16). If the reduction is carried out under the less rigorous conditions of zinc in aqueous ammonium chloride the intermediate hydroxy compound is formed <96T10751>. [Pg.330]

The inter- and intramolecular catalytic reductive couplings of alkynes and aldehydes recently have experienced rapid growth and are the topic of several recent reviews.5 h-8k 107 With respect to early transition metal catalysts, there exists a single example of the catalytic reductive cyclization of an acetylenic aldehyde, which involves the titanocene-catalyzed conversion of 77a to ethylidene cyclopentane 77b mediated by (EtO)3SiH.80 This process is restricted to terminally substituted alkyne partners (Scheme 53). [Pg.524]

ET-induced cycloadditions of polycyclic olefins and cycloreversions of cyclobutane species have been studied by ESR spectroscopy [266]. Upon chemical and electrochemical reduction, 2,2 -distyrylbiphenyl rearranges by intramolecular coupling into a bis-benzylic dihydrophenanthrene dianion (Scheme 1), which can be either protonated to a 9,10 -dibenzyl-9,10-dihydrophenanthrene or oxidatively coupled to a cyclobutane species. It is interesting to note that the intramolecular bond... [Pg.117]

Radical anions resulting from cathodic reductions of molecules react with electrophilic centers. As an example (Scheme 8), the reduction of compounds in which a double bond is not conjugated with a carbonyl group, involves an intramolecular coupling reaction of radical anion with alkene [12]. [Pg.344]

Reductive cross-dimerization has been established with ketones and 0-meth-oximes upon reduction in isopropanol with a Sn cathode as a convenient route to yS-amino alcohols, diastereoselectivities of up to 85 15 were obtained. A chiral ligand was obtained this way from the coupling of (-) - menthone with O-methyl acetaldoxime. Similarly, ketones could be coupled to hydrazones and nitrones. Also, intramolecular couplings were achieved with good yields and diastereoselectivity (Fig. 56) [308]. [Pg.433]

Intramolecular coupling Some aromatic diketones have been stereoselectively cy-clized under various electrolysis conditions, which, together with the substrate structure, strongly influence the stereochemistry of the formed cyclic diol. Reductive cyclization of 1,8-diaroylnaphthalenes led to trans-diols, 2,2 -diaroylbiphenyls and a, )-diaroylalkanes yielded cis-diols with different stereoselectivities depending on substrate structure and electrolysis conditions (pH, cosolvent) (Fig. 57) [310-312]. [Pg.433]

An interesting reaction takes place when diketones with the keto groups in positions 1,4 or more remote are refluxed in dimethoxyethane with titanium dichloride prepared by reduction of titanium trichloride with a zinc-copper couple. By deoxygenation and intramolecular coupling, cycloalkenes with up to 22 members in the ring are obtained in yields of 50-95%. For example, 1-methyl-2-phenylcyclopentene was prepared in 70% yield from 1-phenyl-1,5-hexanedione, and 1,2-dimethylcyclohexadecene in 90% yield from 2,17-octa-decanedione [206, 210]. [Pg.128]

Reduction of dicarbonyl compounds can be expected to lead to a cyclic 1,2-glycol formed by the intramolecular coupling of radical intermediates. [Pg.350]

A further reductive intramolecular coupling methodology via organocuprate intermediates has recently been described see B. H. Lipshutz, Z.-P. Liu, F. Kayser, Tetrahedron Lett. 35,5567 (1994). [Pg.581]

The intramolecular coupling of an aldehyde and alkene at —78 °C, promoted by Sml2, in the presence of a ketone, has led to a tandem coupling-addition process. This reaction has proved to be synthetically very useful for the preparation of poly-oxygenated compounds (equation 139)523. This reductive coupling process may also be promoted in dry methanol,... [Pg.744]


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See also in sourсe #XX -- [ Pg.8 , Pg.528 ]




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Intramolecular reduction

Reduction Reductive coupling

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