2007
DOI: 10.1016/j.tet.2007.06.117
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Effects of α-alkoxy substitution and conformational constraints on 6-exo radical cyclizations of hydrazones via reversible thiyl and stannyl additions

Abstract: Access to multifunctional hydrazones of relevance to dysiherbaine synthesis studies is described. Subsequent radical cyclizations of multifunctional hydrazones via a Si-and C-linked tethering strategy are shown to function effectively in 6-exo fashion. Conformational constraints are proposed to play a key role in suppressing unproductive premature reduction pathways. The stereochemical outcomes suggest that minimizing the dipole repulsion between neighboring C=N and C-O bonds favors a C α -C(=N) dihedral angle… Show more

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Cited by 19 publications
(7 citation statements)
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“…It was assumed that minimizing the dipole repulsion between neighboring CN and C−O bonds favored a C α C(N) dihedral angle, placing the CN bond axial within a chairlike transition state. The hypothesis was substantiated by experimentally observed lowering of diastereoselectivity for diastereomeric substrate 368a (from dr 98:2 for product 368 to dr 70:30 for product 369a ) . Two key structural features in the substrates were proposed to play a crucial role in successful, silicone-tethered 6- exo -cyclizations: (i) the presence of α-alkoxy substituents and (ii) conformational constraints such as the benzylidene acetal ring.…”
Section: Free Radical Reactionsmentioning
confidence: 93%
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“…It was assumed that minimizing the dipole repulsion between neighboring CN and C−O bonds favored a C α C(N) dihedral angle, placing the CN bond axial within a chairlike transition state. The hypothesis was substantiated by experimentally observed lowering of diastereoselectivity for diastereomeric substrate 368a (from dr 98:2 for product 368 to dr 70:30 for product 369a ) . Two key structural features in the substrates were proposed to play a crucial role in successful, silicone-tethered 6- exo -cyclizations: (i) the presence of α-alkoxy substituents and (ii) conformational constraints such as the benzylidene acetal ring.…”
Section: Free Radical Reactionsmentioning
confidence: 93%
“…It is noteworthy that a protodestannylation of crude 372 (filtration through silica, n-BuLi followed by propionic acid) provided the functionalized product 373 with complete diastereoselectivity and in 48% yield from Dgalactose. 269 …”
Section: Radical Addition Of Silicon-tethered Vinyl and Acetylene Groupsmentioning
confidence: 99%
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“…However, in conformationally constrained systems, successful 6- exo -cyclizations have been reported. Depending on the relative configurations of the precursors, excellent levels of stereocontrol can be attained (Scheme , eq b) . The observed diastereoselectivity was explained by a chairlike transition state in which the hydrazone moiety adopts a contra-intuitive pseudoaxial disposition to minimize vicinal dipole repulsions with the C–O bond.…”
Section: Addition Of Thiols To Cc Bondsmentioning
confidence: 99%
“…The structure of 15 was determined by extensive spectroscopic studies to be an unprecedented diamino dicarboxylic acid, which is characterized by a structurally novel cis -fused hexahydrofuro[3,2- b ]pyran ring system containing a glutamate substructure. The potent biological activity and structural novelty has led to many approaches toward the synthesis of dysiherbaine . On the other hand, malayamicin A 17 , a bicyclic C -nucleoside that exhibits fungicide activity, was isolated from the soil organism Streptomyces malaysiensis , and its structure was proposed by detailed NMR studies and by degradation.…”
Section: Resultsmentioning
confidence: 99%