Showing papers on "Intramolecular reaction published in 1998"

Stereocontrolled total synthesis of hemibrevetoxin b

Abstract: The stereocontrolled total synthesis of hemibrevetoxin B (1) has been achieved in 56 steps and 0.75% overall yield from d-mannose. The intramolecular reaction of γ-alkoxyallylstannane with aldehyde is a key step for the present total synthesis. Thus, the BF3·OEt2-mediated reaction of 24 gave 6 as a sole product. We encountered difficulty in the synthesis of γ-alkoxyallylstannane 30 from the corresponding allyl ether 29 in which the γ-alkoxy substituent became sterically quite bulky. This problem was solved by developing the acetal cleavage method for the synthesis of γ-alkoxyallylstannanes. The cyclization of 38 proceeded smoothly to give the key intermediate 5 in a highly stereoselective manner. Construction of the α-vinyl aldehyde and (Z)-diene moieties were performed using Nicolaou's protocol.

Intramolecular Hydrogen Transfers in Vinyl Chloride Polymerization: Routes to Doubly Branched Structures and Internal Double Bonds

Abstract: During the preparation of poly(vinyl chloride) (PVC) under low monomer pressures, intramolecular hydrogen transfers (“backbites”) lead to the formation of small amounts of 1,3-bis(2-chloroethyl) br...

Synthetic Studies on Ciguatoxin: A Convergent Strategy for Construction of the F-M Ring Framework.

Abstract: Exceptional neurotoxicity is associated with ciguatoxin. The ciguatoxin mimic 1, which contains the F-M framework of the natural product, has been prepared through a convergent synthesis. The two key steps were a Lewis acid mediated intramolecular reaction of a γ-alkoxyallylsilane with an acetal group and an SmI2 -mediated intramolecular Reformatsky reaction that permitted construction of the annelated hexahydrooxonin ring system.

A synthesis of mono- and dimethoxy-1,2,3,4-tetrahydroisoquinolines via Pummerer reaction: Effects of methoxyl groups on intramolecular cyclization

Abstract: A synthesis of 1, 2, 3, 4-tetrahydroisoquinolines (TIQs)(23) with one and two methoxyl groups at various positions of the benzene ring was achieved via the intramolecular cyclization of N-(aryl)methyl-2-(phenylsulfinyl)ethylamines (9) using the Pummerer reaction as a key step. The reaction was carried out by using trifluoroacetic anhydride (TFAA)(method A) of TFAA-BF3·Et2O (method B). The cyclization to 4-SPhTIQs (11) proceeded effectively when the reaction center at the benzene ring was electronically activated by a methoxyl group. In the reaction of the sulfoxide (9e) having two OMe groups at ortho- and para-positions a different cyclization reaction leading to a benzothiazepine (12) was observed, indicating that the high nucleophilicity of the benzene ring caused the unexpected reaction prior to the cyclization to 4-SPhTIQ (11e). The route starting from methoxylated benzaldehydes (5) was proved to provide an efficient and convenient method of TIQ synthesis which should be complementary to the well known Pictet-Spengler method.

General Method for Synthesis of Erythrinan and Homoerythrinan Alkaloids (1) : Synthesis of a Cycloerythrinan, as a Key Intermediate to Erythrina Alkaloids, by Pummerer-Type Reaction

Abstract: A new synthetic route to Erythrina alkaloids by formation of ring C utilizing a Pummerer-type intramolecular cyclization was developed. The N-(2-phenylthioethyl)-dioxopyrroline (3) was converted to 13 in five steps with satisfactory overall yield (69%) by means of [2+2] photocycloaddition followed by 1, 3-anionic rearrangement as crucial reactions. Treatment of the sulfoxide (13) with trifluoroacetic anhydride gave the cyclization product, 11-phenylthioerythrinan (15), in 87% yield. This was converted to the cycloerythrinan (22) by reductive elimination of the PhS group, thus constituting a formal total synthesis of (±)-erysotrine.

Approaches to the Cephalotaxine Skeleton Using an Intramolecular Heck Reaction

Abstract: 1-[2-(o-Iodophenyl)acetyl]-2-(prop-2-enyl)-2-vinylpyrrolidine (17), upon treatment with palladium(II) acetate [Pd(OAc)2] in the presence of triphenylphosphine and triethylamine in refluxing acetonitrile, gave the monocyclization product 20 as the major product along with a small amount of the tandem cyclization product 21. On the other hand, treatment of the 1-[2-(o-iodophenyl)acetyl]-1-azaspiro[4.4]non-8-en-7-one (25) with Pd(OAc)2, 1, 3-bis-(diphenylphosphino)propane, tributylphosphine, and silver carbonate in boiling N, N-dimethylformamide afforded the cyclized product 27 in 51% yield.

A new intramolecular reaction for the regioselective debenzylation or protection of alcohols

Abstract: Strained benzylidene acetals such as 1 can be prepared with a new intramolecular reaction that allows the selective removal of benzyl protecting groups neighboring free OH groups, without attacking other benzyl groups. The mechanism of this reaction, which is useful for carbohydrate synthesis, probably proceeds via ionic intermediates with neighboring group participation.

Electron ionization mass spectrometry of curcumin analogues: an olefin metathesis reaction in the fragmentation of radical cations

Abstract: The natural compound curcumin, used in cosmetics, traditional medicines and as a spice in food, is known as a multi-factorial anti-inflammatory agent. To study the anti-inflammatory activity of curcumin derivatives, 24 analogues were synthesized and their structures were confirmed by 1H NMR and electron ionization (EI) mass spectrometry. Most signals in the EI mass spectra can be attributed to commonly known fragmentations, but the formation of ring-substituted 1,2-diphenylethene (stilbene)-type radical cations, observed in the spectra of all compounds investigated and resulting in the base peak for some compounds, requires a peculiar rearrangement. Metastable ion spectra and 13C labelling studies show that the stilbene-type ions are formed directly from the molecular ions and contain the two original aryl groups and the 1 and 7 carbon atoms of the olefinic system. It is proposed that the formation of stilbene-type ions results from an intramolecular olefin metathesis reaction; this suggestion is supported by semi-empirical (MNDO/PM3) calculations. © 1998 John Wiley & Sons, Ltd.

Molecular orbital studies of the intramolecular reaction of protonated cis- and trans-3,4-epoxypentan-1-ol

Abstract: The transition structures for the intramolecular reaction of protonated cis- and trans-3,4-epoxypentan-1-ol (13) and (16), which result in the formation of protonated cis- and trans-2-methylfuran-3-ols (14) and (18) with inversion and retention, have been determined at the ab initio MP2/6-31G* and hybrid density functional B3LYP/6-31G* levels of theory. Intrinsic reaction coordinate calculations for the lower energy inversion pathways for formation of the 2-methylfuran-3-ols show that intramolecular attack occurs in concert with ring opening. The reaction pathways are far from optimum for best orbital overlap, reflecting the strained bicyclic nature of the transition structures.

Photoreaction of Arenecarbothioamides with 2-Vinylfuran Analogues. The Formation of Tetracyclic Indoles and 2, 3-Diaryl-2-pyrrolin-5-ones

Abstract: Photoreaction of arenecarbothioamides (1) with 2-vinylfuran analogues (2, 6) in benzene aolution gave tetracyclic indole systems (3, 8) via photogenerated diarylethylene intermediates. In the case of reaction of furan- or thiophenecarbothioamide (1c, 1d) with 2-furanacrylic acid (2-i), 2, 3-diaryl-2-pyrrolin-5-ones (5c-i, 5d-i) were obtained.

Synthesis of Novel 2-Substituted-1, 3, 4-thiadiazoles

Abstract: New 2-monosubstituted and 2,2-disubstituted 2,3-dihydro-3,5-diphenyl-1,3,4-thiadiazoles (2) were synthesized by the reaction of N'-phenylthiobenzoylhydrazide with aliphatic, aromatic and heterocyclic aldehydes and ketones in the presence of trimethylsilyl chloride in benzene. The reaction proceeded smoothly to afford the corresponding 1,3,4-thiadiazoles (2a-2j) in excellent yields (84-96%) except for a few cases, such as the 5-methyl-2-furfural and 2-acetylthiophene systems. 2,3-Dihydro,2,2-dimethyl,3,5-diphenyl-1,3,4-thiazole (2f) showed curative activity against powdery mildew of wheat.

Efficient Synthesis of (R)-6-Benzyloxycarbonylamino-1-methyl-4-(3-methylbenzyl)hexahydro-1, 4-diazepine. I

Abstract: An efficient and practical method for large scale synthesis of (R)-6-benzyloxycarbonylamino-1-methyl-4-(3-methylbenzyl)hexahydro-1, 4-diazepine (R-3), which is a key intermediate in the synthesis of DAT-582, a potent and selective serotonin-3 receptor antagonist, is described. The precursor of R-3, the (S)-2, 3-diaminopropylaminoacetate S-5, was obtained from the chiral triaminopropane derivative R-19. Nucleophilic reaction of the chiral mesylate R-11 with 3-methylbenzylamine gave the racemic 2, 3-diaminopropylaminoacetate (±)-5 via the achiral azetidinium cation 12, while the reaction of the N-protected mesylate R-14 produced the desired triamine S-15 but in poor yield.However, reaction of the N-protected mesylate S-18 with a large excess of methylamine proceeded smoothly to afford R-19 in good yield. S-5 was converted into R-3 with >99% enantiomeric excess using an intramolecular reductive cyclization method.

Photochemical synthesis of medium-ring azalactams from N-(aminoalkyl)-2-stilbenecarboxamides

Abstract: The photochemical reactions of several N-(aminoalkyl)-2-stilbenecarboxamides have been investigated. The amide groups quench the Stilbenecarboxamide fluorescence intensity, presumably via intramolecular electron transfer. The relatively low quenching efficiencies and the absence of intramolecular exciplex formation are attributed to the molecular structure which prevents effective overlap between the stilbene and amine chromophores. The secondary amines are observed to undergo regioselective intramolecular addition to afford 9- or 10-ring azalactams in 25–43% isolated yield. The dependence of both fluorescence quenching and adduct formation upon molecular structure are discussed.

Possible Reaction Mechanism for the Formation of C60 From Naphthalene: A Semi-Empirical and Ab Initio Molecular Orbital Study

Abstract: Mechanism of C60 formation from naphthalenes was studied by quantum chemical calculations. a patch up process of T-shape dimerization of naphthalenes followed by the intramolecular ring fusion, all through radical reactions, was proposed for a possible mechanism for the C60 formation from naphthalenes. the reaction barrier was found to be less than 60-70 kcal/mol for each reaction step in this mechanism.

Radical polymerization of ortho-(1,3-dioxolan-2-yl)phenyl ethyl fumarate involving intramolecular hydrogen abstraction and ring opening of cyclic acetal

Abstract: The polymerization of o-(1,3-dioxolan-2-yl)phenyl ethyl fumarate (DOPEF) initiated with dimethyl 2,2 '-azobisiso-butyrate (MAIB) was studied kinetically in benzene. The polymerization rate (R p ) at 60°C was given by R p = k [MAIB] 0.76 [DOPEF] 0.71 . The overall activation energy of polymerization was calculated to be 98.3 kJ/mol. The number-average molecular weight of resulting poly(DOPEF) was in the range of 1000-3100. 'Hand 13 C-NMR spectra of resulting polymers revealed that the radical polymerization of DOPEF proceeds in a complicated manner involving vinyl addition, intramolecular hydrogen abstraction, and further ring opening of the cyclic acetal at higher temperatures. From the copolymerization of DOPEF (M 1 ) and styrene (St) (M 2 ) at 60°C, the monomer reactivity ratios were obtained to be r 1 = 0.02 and r 2 = 0.20, the values of which are similar to those of the copolymerization of ethyl o-formylphenyl fumarate and St.

Intramolecular photoaddition reactions of (aminoalkyl)styrylamides

Abstract: The photochemical reactions of a family of styrene—spacer—amine molecules in which the spacer consists of a rigid amide group in the middle of a flexible alkane chain have been investigated Our study of the photophysics of these molecules has shown that the amide linker can facilitate intramolecular styrene—amine exciplex formation (J Phys Chem, in press) We report here the photochemical behavior of these amide-linked (aminoalkyl) styrylamides Several tertiary amines are found to undergo regioselective α -C-H addition to the styrene double bond resulting in the formation of medium-ring lactams A secondary amine undergoes N-H addition yielding an azalactam The dependence of these reactions upon spacer structure, amine and amide substitution, and solvent polarity is discussed

Synthesis of Macrocyclic Lactams in Organized Media

Abstract: Lactamization was studied in a micellar environment using the novel carboxyl-activating agent, N-hexadecyl-2-chloropyridinium iodide (C16PyCl, I¯). ω-amino acids were macrocyclized successfully in various surfactantless microemulsions containing formamide or traces of water and using the classical Mukayama reagent(C1PyCl, I¯). In these media, selectivity was attributed to the poor solubility of the precursors in the continuous phase. The w-amino acids were thought to be localized at the interface of the molecular aggregates, favoring intramolecular reactions.

Complete Diastereocontrol in [3 + 2] Cycloaddition Reactions: Synthesis of Enantiomerically Pure Fluoromethyl Substituted β-Amino Acids.

Abstract: The intramolecular reaction of nitrones of crotonic and cinnamic esters of (S)-3-fluoro-2-hydroxypropanal afforded corresponding cycloadducts in good chemical yields and complete stereoselection.

Total Synthesis of (+)‐Desoxoprosopinine via the Intramolecular Reaction of γ‐Aminoallylstannane.

Abstract: A stereoselective total synthesis of (+)-desoxoprosopinine starting from L-glutamic acid as a chiral source was accomplished, in which the intramolecular reaction of γ-aminoallylstannane with aldehyde was used as a key step.

Novel Intramolecular Introduction of Nucleophiles to 2,2′‐Anhydrouridine.

Abstract: 2′-N-alkyluridine nucleosides are synthesized via intramolecular reaction of alkyl isocyanates with 5′-O-protected-2,2′-anhydrouridine.

Selectivity in inter- and intra-molecular photocycloaddition of ethenes to benzenoid compounds

Abstract: The formation of isomers from the inter- and intra-molecularmeta photocycloaddition of ethenes to the benzene ring has been specifically directed using the addend substituents in order to cause an asymmetric distortion of theC 6 ring during the reaction, and to induce non-synchroneity into the addition process. The latter influence in the intramolecular reaction is achieved by the presence of a heteroatom in the linking tether which also provides a convenient access to linear and angular heterotriquinanes. Themeta photoaddition can be induced by sunlight exposure of appropriately substituted arene-ethene bichromophores.