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Ring conformational constraint

C12-C13 would facilitate the ring closure, monocyclic diene 170 was chosen as the metathesis substrate. Indeed, exposure of 170 to catalyst A provided cycliza-tion product 171 in 94% yield within 6 h, without the aid of a cyclic conformational constraint. [Pg.301]

The bias of a given diene for RCM depends on the ring size formed, on conformational constraints of the substrate, on the presence of functional groups, and on interactions with the specific catalyst used. [Pg.52]

An illustrative example for the potency of catalyst C is Paquette s highly efficient total synthesis of the natural products teubrevin G 180 and teubrevin H 181, which feature a cyclooctane core fused and spiroannulated to smaller oxygen containing rings. In the retrosynthetic analysis, the viability of an RCM step for annulation of a cyclooct-enone ring to the furan played a central role. Despite the presence of a conformational constraint by the furan ring in... [Pg.227]

The above discussion provides a background for a discussion of the conformations of various azocanes, oxocanes, thiocanes and related compounds which lack strong ring torsional constraints. [Pg.699]

Contrary to most allocolchicinoids which lack a C-ll substituent, compounds 78 and 80 show stable axial chirality. The above study constitutes the first example in the allocolchicinoid series where both configurations at the biaryl axis can be obtained from a given stereochemistry at C-7. In the natural alio series, free rotation around the biaryl axis is often possible at room temperature, and the configuration of the biaryl axis is controlled by the stereochemistry at C-7 due to conformational constraints in the C-ring [14]. While several total enantioselective syntheses of colchicine address the control of the stereochemistry at C-7 [106], no direct enantioselective synthesis of natural allocolchicines has been reported to date. ... [Pg.380]

The conformational behavior of cyclic molecules is conveniently discussed separately from that of acyclic molecules. Ring closure indeed imposes conformational constraints and limitations not existing in linear systems. [Pg.36]


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See also in sourсe #XX -- [ Pg.174 ]




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Conformational constraints

Rings conformations

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