Acid-catalyzed Ring Expansion Research Articles

Efficient Synthesis of Cyclohepta[b]indoles and Cyclohepta[b]indole-Indoline Conjugates via RCM, Hydrogenation, and Acid-Catalyzed Ring Expansion: A Biomimetic Approach.

Cyclohepta[b]indoles, prevalent in natural products and pharmaceuticals, are conventionally accessed via metal or Lewis acid-mediated cycloadditions with prefunctionalized substrates. Our study introduces an innovative sequential catalytic assembly for synthesizing cyclohepta[b]indoles from readily available isatin derivatives. The process involves three catalytic sequences: ring-closing metathesis, catalytic hydrogenation, and acid-catalyzed ring expansion. The RCM of 2,2-dialkene-3-oxindoles, formed by butenyl Grignard addition to 3-allyl-3-hydroxy-2-oxindoles, yields versatile spirocyclohexene-3-oxindole derivatives. These derivatives undergo further transformations, including dibromination, dihydroxylation, epoxidation, Wacker oxidation at the double bond. Hydrogenation of spirocyclohexene-3-oxindole yields spirocyclohexane-3-oxindoles. Their subsequent acid-catalyzed ring expansion/aromatization, dependent on the acid catalyst, results in either cyclohepta[b]indoles or cyclohepta[b]indole-indoline conjugates, adding a unique synthetic dimension. The utility of this methodology is exemplified through the synthesis of an A-FABP inhibitor, showcasing its potential in pharmaceutical applications.

Strong acid-catalyzed electrophilic ring expansion of oxetanes and sulfoxonium ylides

• Protic acid-catalyzed synthesis of functionalized trans -disubstituted tetrahydrofurans. • Electrophilic ring expansion reaction of sulfoxonium ylides to oxetanes was developed. • A transition metal-free ring expansion reaction of oxetanes to tetrahydrofurans. • Ring expansion of oxetanes to tetrahydrofurans with safe and stable ylides. • Stereospecific synthesis of functionalized trans -2,3-disubstituted tetrahydrofurans. A strong protic acid-catalyzed electrophilic ring expansion of oxetanes into trans -2,3-disubstituted tetrahydrofuran derivatives using sulfoxonium ylides has been developed. This reaction produces functionalized trans -2,3-disubstituted tetrahydrofuran derivatives stereospecifically by using safe and stable sulfoxonium ylides without metal catalysts and the protection of inert gas.

Tandem Achmatowicz Rearrangement and Acetalization of 1-[5-(Hydroxyalkyl)-furan-2-yl]-cyclobutanols Leading to Dispiroacetals and Subsequent Ring-Expansion to Form 6,7-Dihydrobenzofuran-4(5 H)-ones.

Herein, we report a one-pot protocol for the synthesis of dispiroacetals 4 bearing a cyclobutane motif via tandem Achmatowicz rearrangement and acetalization of 1-[5-(hydroxyalkyl)-furan-2-yl]-cyclobutanols 3 with m-CPBA as the oxidant and AgSbF6 as an additive to promote the cyclization step in an aqueous medium. Dispiroacetals 4 could subsequently undergo Lewis acid-catalyzed ring expansion and skeletal rearrangement to afford 6,7-dihydro-5 H-benzofuran-4-ones 5.

Synthesis and Stereochemical Determination of Complestatin A and B (Neuroprotectin A and B)

Recently, we reported the first total synthesis of chloropeptin II (1, complestatin), the more strained and challenging of the two naturally occurring chloropeptins. Central to the design of the approach and by virtue of a single-step, acid-catalyzed ring expansion rearrangement of chloropeptin II to chloropeptin I, the route also provided a total synthesis of chloropeptin I. Herein, we report a complementary and divergent oxidation of chloropeptin II (1, complestatin) to either complestatin A (2, neuroprotectin A) or complestatin B (3, neuroprotectin B), providing the first synthesis of the natural products and establishing their remaining stereochemical assignments. Key to the approach to complestatin A (2, neuroprotectin A) was the development of two different single-step indole oxidations (HCl-DMSO and NBS, THF-H(2)O) that avoid the rearrangement of chloropeptin II (1) to chloropeptin I (4), providing the 2-oxindole 2 in superb yields (93% and 82%). With a mechanistic understanding of features that impact the latter oxidation and an appreciation of the intrinsic reactivity of the chloropeptin II indole, its modification (NCS, THF-H(2)O; Cs(2)CO(3), DMF-H(2)O) provided a two-step, single-pot oxidation of chloropeptin II (1) to afford directly the 3-hydroxy-2-oxindole complestatin B (3, neuroprotectin B). Extensive studies conducted on the fully functionalized synthetic DEF ring system of chloropeptin II were key to the unambiguous assignment of the stereochemistry as well as the exploration and subsequent development of the mild oxidation conditions used in the synthesis of complestatin A and B.

A Highly Efficient and Selective Route to Isomeric Cyclic Diazadienes.

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A Highly Efficient and Selective Route to Isomeric Cyclic Diazadienes

A highly selective Lewis acid-catalyzed ring expansion of methylenecyclopropyl hydrazones to isomeric cyclic diazadienes is reported. In general, good to excellent yields and selectivities were obtained for a broad range of hydrazone derivatives.

L-Proline-Catalyzed Direct Intermolecular Asymmetric Aldol Reactions of 1-Phenylthiocycloalkyl Carboxaldehydes with Ketones. Easy Access to Spiro- and Fused-Cyclobutyl Tetrahydrofurans and Cyclopentanones

[reaction: see text] Acid-catalyzed ring expansion of chiral cyclopropyl and cyclobutyl derivatives for the synthesis of carbo- and heterocyclic compounds is reported. The starting materials have been successfully prepared by l-proline-catalyzed direct asymmetric aldol reactions of 1-phenylthiocycloalkyl carboxaldehydes with ketones.

Synthesis and Lewis acid induced isomerization of mono-, di-, and tri-spiro α-keto tetrahydro-furans and -pyrans

The stereoselectivity of the acid-catalyzed ring expansion of dihydrofuranyl and dihydropyranyl 1,2-adducts to spirocyclic ketones 8 and 9 has been examined. These substrates have become readily available by the utilization of 1,3-dichloroacetone as a synthetic equivalent of cyclopropanone. The kinetically controlled isomerizations result in ring expansion to dispirocyclopentanones. Third-stage conversion to trispirocyclohexanones was shown to be possible in select examples. On a different front, the syn and anti pairs proved capable of interconversion by heating in the presence of boron trifluoride etherate. A push–pull fragmentation of the ketone ring is proposed to account for these epimerizations. Calculations have been performed to simulate transition state energies in the oxonium ion intermediates and to gain some idea of the preferred ground-state geometries of the final products.Key words: oxonium ion rearrangements, ring expansions, acid-catalyzed epimerizations, cyclopropanone equivalent, spirocyclic ethers.

2,2‐Dimethyl Cyclopentanones by Acid Catalyzed Ring Expansion of Isopropenylcyclobutanols. A Short Synthesis of (.+‐.)‐α‐Cuparenone and (.+‐.)‐Herbertene.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Lewis Acid‐Catalyzed Stereospecific Ring Expansion of Aziridine‐2‐carboxylates to Imidazolidin‐2‐ones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Lewis acid-catalyzed stereospecific ring expansion of aziridine-2-carboxylates to imidazolidin-2-ones

Lewis acid-catalyzed ring expansion reaction of chiral aziridine-2-carboxylate proceeds regio- and stereospecifically to yield enantiomerically pure 4-functionalized imidazolidin-2-ones in high yields.

2,2-Dimethyl cyclopentanones by acid catalyzed ring expansion of isopropenylcyclobutanols. A short synthesis of (±)-α-cuparenone and (±)-herbertene

2,2-dimethyl cyclopentanones are readily prepared by acid catalyzed ring expansion of isopropenylcyclobutanols; the method allows ready access to the family of sesquiterpenes cuparanes and herbertanes, as demonstrated by the synthesis of (+/-)-alpha-cuparenone and the direct precursor of (+/-)-herbertene.

Novel Acid‐Catalyzed Rearrangement of Methanofullerenes Bearing an α‐Ylidic Ester to Cyclopentanofullerenes: A Vinyl Cyclopropane‐Type Ring Expansion.

A novel acid-catalyzed ring expansion of methanofullerenes bearing an α-ylidic ester has been investigated. Treatment of dialkyl acetylenedicarboxylate and tricycloalkylphosphine with C60 led to the isolation of a methanofullerene ylide after passing the reaction mixtures through a basic alumina column. If the reaction mixture was passed through a silica gel column, a cyclopentanofullerene was isolated instead. These new cyclopentanofullerenes consisted of a fused cyclopentanone ring bearing an α-hydroxy ester and a phosphonium ylide and were confirmed by their NMR, mass, and X-ray diffraction data. The cyclopentanofullerenes were formed by the ring expansion of the corresponding methanofullerenes in the presence of silica gel. The ring expansion also proceeded by treating methanofullerene with acetic acid in chloroform. On the other hand, the methanofullerenes from RO2CCCCO2R, PAr3, and C60 were stable in silica gel. However, upon heating with acetic acid at 50 °C, they underwent ring expansion and depho...

Novel acid-catalyzed rearrangement of methanofullerenes bearing an alpha-ylidic ester to cyclopentanofullerenes: a vinyl cyclopropane-type ring expansion.

A novel acid-catalyzed ring expansion of methanofullerenes bearing an alpha-ylidic ester has been investigated. Treatment of dialkyl acetylenedicarboxylate and tricycloalkylphosphine with C(60) led to the isolation of a methanofullerene ylide after passing the reaction mixtures through a basic alumina column. If the reaction mixture was passed through a silica gel column, a cyclopentanofullerene was isolated instead. These new cyclopentanofullerenes consisted of a fused cyclopentanone ring bearing an alpha-hydroxy ester and a phosphonium ylide and were confirmed by their NMR, mass, and X-ray diffraction data. The cyclopentanofullerenes were formed by the ring expansion of the corresponding methanofullerenes in the presence of silica gel. The ring expansion also proceeded by treating methanofullerene with acetic acid in chloroform. On the other hand, the methanofullerenes from RO(2)CCCCO(2)R, PAr(3), and C(60) were stable in silica gel. However, upon heating with acetic acid at 50 degrees C, they underwent ring expansion and dephosphination to give cyclopentenofullerenes. An implicit vinyl cyclopropane ring expansion mechanism was proposed to account for this novel acid-catalyzed rearrangement.

Practical Synthesis of Spirocyclic Bis-C,C-glycosides. Mechanistic Models in Explanation of Rearrangement Stereoselectivity and the Bifurcation of Reaction Pathways

The susceptibility of glycal-derived carbinols to acid-catalyzed ring expansion is described. In the systems prepared from cyclopentanone, Ferrier ionization precedes the pinacol-like Wagner−Meerwein shift, thermodynamic control operates, and high stereoselectivity is seen if a C(6) substituent is present. In contrast, the adducts to cyclobutanone exhibit release of ring strain under kinetically controlled conditions and intercept the oxonium species reversibly formed via direct proton transfer. The results show that the substituents positioned on the glycal ring have a pronounced influence on whether a chair-like or twist-boat transition state geometry is adopted primarily. The composite reaction profiles reveal for the first time the fundamental importance of exothermicity and of substitution in these spiro glycosidation reactions. Since optical activity is preserved in all instances, the utility of this chemistry for the synthesis of bis-C,C-glycosides and more complex oxacyclics appears promising.

Concise Synthesis of cis- and trans-Theaspirones via Oxonium Ion-Initiated Pinacol Ring Expansion

The odoriferous principle of black tea has been produced from 2,2-dimethylcyclopentanone. The reaction sequence begins with 1,2-addition of 5-lithio-2-methyl-2,3-dihydrofuran to this ketone and immediate acid-catalyzed ring expansion of the resulting carbinols to a separable pair of spiro ethers. Individual conversion of these diastereomers to α,β-unsaturated ketones is followed by tandem condensation with the methyllithium−lithium bromide complex and oxidation with pyridinium chlorochromate.

Total synthesis of some marasmane and lactarane sesquiterpenes.

A general and efficient synthetic route to the marasmane sequiterpenes (±)-isovelleral (2) and (±)-stearoylvelutinal (1b) is described. Total syntheses of two other naturally occurring sesquiterpenes, deconjugated anhydrolactarorufin A (5) and lactarorufin A (6), were achieved using an acid-catalyzed ring expansion. All four syntheses are highly stereoselective and do not require the use of any protecting groups. Finally, the protic acid-catalyzed degradation of velutinal (1a) was investigated in an effort to chemically induce the biologically important conversion of velutinal (1a) to isovelleral (2). The experimental results thus obtained indicate that an enzymatic mechanism for the key transformation of velutinal(1a) into isovelleral(2) is more plausible than one that is acid-catalyzed

Regio- and stereochemical course of the ring expansion of bridged bicyclic ketones to spirocyclic .alpha.-keto tetrahydrofurans.

The regio- and stereochemical aspects of oxonium-promoted pinacol-like rearrangements have been investigated starting from the bridged bicyclic ketones (±)-norcamphor, (1R)-(-)-fenchone, and (1R)-(-)-3,3-dimethyl-1-vinyl-2-norbomanone. 1,2-Addition of 5-lithio-2,3-dihydrofuran to these substrates provided alcohols that smoothly underwent acid-catalyzed ring expansion. Whereas bridgehead carbon migration was observed in the first and third examples, the alternative available 1,2 Wagner-Meerwein shift operated in the fenchone series

Acid-catalyzed ring expansion of 1-(1-methoxy-1,2-propadienyl)-2-cyclobuten-1-ols. Synthesis of 5-hydroxy-5-vinyl-2-cyclopenten-1-ones and their stereoselective transformation to 5-(2-acetoxyethylidene)-2-cyclopenten-1-ones

Acid-catalyzed ring expansion of 1-(1-methoxy-1,2-propadienyl)-2-cyclobuten-1-ols. Synthesis of 5-hydroxy-5-vinyl-2-cyclopenten-1-ones and their stereoselective transformation to 5-(2-acetoxyethylidene)-2-cyclopenten-1-ones

ChemInform Abstract: CHEMO‐ AND REGIOSELECTIVITIES IN ACID‐CATALYZED RING EXPANSION OF 1‐(1‐METHYLSULFINYL‐1‐(METHYLTHIO)ALKYL)CYCLOBUTANOL DERIVATIVES

Chemischer InformationsdienstVolume 13, Issue 26 Isocyclic Compounds ChemInform Abstract: CHEMO- AND REGIOSELECTIVITIES IN ACID-CATALYZED RING EXPANSION OF 1-(1-METHYLSULFINYL-1-(METHYLTHIO)ALKYL)CYCLOBUTANOL DERIVATIVES K. OGURA, K. OGURASearch for more papers by this authorM. YAMASHITA, M. YAMASHITASearch for more papers by this authorM. SUZUKI, M. SUZUKISearch for more papers by this authorG. TSUCHIHASHI, G. TSUCHIHASHISearch for more papers by this author K. OGURA, K. OGURASearch for more papers by this authorM. YAMASHITA, M. YAMASHITASearch for more papers by this authorM. SUZUKI, M. SUZUKISearch for more papers by this authorG. TSUCHIHASHI, G. TSUCHIHASHISearch for more papers by this author First published: June 29, 1982 https://doi.org/10.1002/chin.198226155Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume13, Issue26June 29, 1982 RelatedInformation