Stereoselectivity in oxyallyl–furan (4 + 3) cycloadditions: control of intermediate conformations and dispersive stabilisation in cycloadditions involving oxazolidinone auxiliaries

Krenske, Elizabeth H.; Houk, K. N.; Lohse, Andrew G.; Antoline, J.; Hsung, Richard P.

doi:10.1039/c0sc00280a

Cited by 62 publications

(43 citation statements)

References 47 publications

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“…Consequently, Krenske, Houk, and Hsung 143 conducted a detailed mechanistic study of this nitrogenstabilized oxyallyl cation [4 + 3] cycloadditions to provide a rationale. DFT calculations revealed that the E -oxyallyl cations were actually the preferred species regardless of the presence of ZnCl 2 , and there was no bidentate coordination (Scheme 149).…”

Section: Reactions Of Allenamidesmentioning

confidence: 99%

Allenamides: A Powerful and Versatile Building Block in Organic Synthesis

et al. 2013

Chem. Rev.

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321

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Section: Reactions Of Allenamidesmentioning

confidence: 99%

Allenamides: A Powerful and Versatile Building Block in Organic Synthesis

et al. 2013

Chem. Rev.

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321

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“…[13] The phenyl-substituted furfuryl alcohol 7 (see Table 1, entry 5) was chosen for this purpose, as this substrate gives mostly exo-selective cycloaddition. Several competing pathways have been investigated in an effort to deduce a plausible mechanism (Scheme 3).…”

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confidence: 99%

Scope and Mechanism of the (4+3) Cycloaddition Reaction of Furfuryl Cations

et al. 2011

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The (4+3) cycloaddition reaction between a conjugated diene and an allylic cation constitutes a convenient and conceptually straightforward method to prepare seven-membered rings. [1,2] In recent years, investigations into this type of cycloaddition, including a number of elegant applications in total synthesis, [3] have demonstrated its potential for development as a synthetic method that could approach the (4+2) cycloaddition (Diels-Alder) reactions in terms of selectivity and efficiency. Of particular interest are allylic cations bearing a cation-stabilizing substituent (Z group) at the 2-position [Eq. (1)] [4] and stabilized allylic cations such as vinyloxocarbenium ions [Eq. (2)] [5] or, typically, cationic reaction partners incorporating both of these features [Eq. (2), R = Z]. [6] Whereas reactions of vinyloxocarbenium species are believed to proceed by a concerted (4+2) cycloaddition followed by a Wagner-Meerwein-type ring expansion, [5] allylic cations with a Z group at the 2-position have been suggested to undergo "true" (4+3) cycloaddition reactions, either through a concerted or a stepwise process.[7] The synthetic utility of these cycloadditions is limited however by issues concerning the generation of the allylic cation reaction partner. The precursors required for allylic cations can be synthetically challenging, and the carbocationic species themselves are generally quite unstable, thus giving rise to various by-products.In the course of synthetic studies towards the natural product rameswaralide, [8] Pattenden and Winne uncovered an unexpected smooth intramolecular dehydrative reaction between a diene and a furfuryl alcohol that gave a novel fused polycycle containing a seven-membered ring [Eq. (3), TFA = trifluoroacetic acid].We surmised that the mechanism of this intriguing cycloaddition reaction involves the initial formation of a furfuryl cation intermediate, [9] which then undergoes an intramolecular (4+3)-type cycloaddition with the appended diene (cf. Scheme 1). The initially formed bicyclic oxocarbenium ion can then be converted into the observed product through straightforward loss of a proton and regeneration of the furan ring. The cationic reaction partner in this sequence can thus be regarded as an allylic cation with a stabilizing oxygen substituent at the 2-position [cf. Eq. (1)], and moreover, if the oxonium ion resonance structure is considered (Scheme 1, bottom), it can be viewed as a vinylogous vinyloxocarbenium ion [cf. Eq. (2)]. If it is concerted, the observed cycloaddition can then be regarded as a rare example of a "higher order" pericyclic reaction, that is, a [6p + 4p] cycloaddition with an unprecedented 2-oxafulvene-type 6p reactant. [10] As the prospect of an efficient (4+3) cycloaddition method using simple furfuryl alcohol starting materials is highly attractive from a synthetic point of view, we decided to investigate the scope and mechanism of this unprecedented Scheme 1. Proposed mechanism of cycloaddition via a furfuryl cation intermediate.

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“…[11] We also computed the NICS values at the (3, + 1) ring critical point of the electron density to prove the aromaticity of TS a. Given the unsymmetrical character of our cyclic systems, we needed to define the inner points unambiguously.…”

Section: Resultsmentioning

confidence: 99%

Concerted and Stepwise Mechanisms in Metal‐Free and Metal‐Assisted [4+3] Cycloadditions Involving Allyl Cations

Fernández

Cossío

Cózar

et al. 2010

Chemistry A European J

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The thermal [4+3] cycloaddition reaction between allenes and tethered dienes (1,3-butadiene and furan) assisted by transition metals (Au(I), Au(III), Pd(II), and Pt(II)) was studied computationally within the density functional theory framework and compared to the analogous non-organometallic process in terms of activation barriers, synchronicity and aromaticity of the corresponding transition states. It was found that the metal-mediated cycloaddition reaction is concerted and takes place via transition structures that can be even more synchronous and more aromatic than their non-organometallic analogues. However, the processes exhibit slightly to moderately higher activation barriers than the parent cycloaddition involving the hydroxyallylic cation. The bond polarization induced by the metal moiety is clearly related to the interaction of the transition metal with the allylic π* molecular orbital, which constitutes the LUMO of the initial reactant. Finally, replacement of the 1,3-butadiene by furan caused the transformation to occur stepwise in both the non-organometallic and metal-assisted processes.

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Stereoselectivity in oxyallyl–furan (4 + 3) cycloadditions: control of intermediate conformations and dispersive stabilisation in cycloadditions involving oxazolidinone auxiliaries

Cited by 62 publications

References 47 publications

Allenamides: A Powerful and Versatile Building Block in Organic Synthesis

Allenamides: A Powerful and Versatile Building Block in Organic Synthesis

Scope and Mechanism of the (4+3) Cycloaddition Reaction of Furfuryl Cations

Concerted and Stepwise Mechanisms in Metal‐Free and Metal‐Assisted [4+3] Cycloadditions Involving Allyl Cations

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