Thermally induced skeletal inversion and ring opening of hexamethylbicyclo[2.2.0]hexanes

Hexamethylbicyclo[2.2.0]hexanes underwent endo,exo isomerization (skeletal inversion) and stereoselective supra, antara cleavage at 140–250°. The kinetics were studied and the products isolated and identified. The reactions are discussed in terms of a common 1,4-π-bonded intermediate which evolves to the invertomer or opens conrotatorially to methyl-substituted hexadienes.     

A simple and efficient synthesis of enantiomeric (3aRS,4RS,6aSR)-4-hydroxy-3,3a,4,6a-tetrahydro-1H-cyclopenta[c]furan-1-ones

Diastereomeric amides produced via the cleavage of easily available (±)-7,7-dichloro-4-exo-trimethylsilylbicyclo[3.2.0]hept-2-en-6-one by treatment with (+)-α-methylbenzylamine were transformed into bicyclic lactam-aminals, which can easily be separated by column chromatography on SiO₂. The latter products lead to enantiomeric (3a,6a)-4-hydroxy-3,3a,4,6a-tetrahydro-1H-cyclopenta[c]furan-1-ones after the removal of the chiral auxiliary and epoxidation.     

Azabrendanes IV. Synthesis and characterization of N-(alkyl- and benzylsulfonyl)-exo-2-hydroxy-4-azatricyclo[4.2.1.0]nonanes

exo- and endo-5-Aminomethylbicyclo[2.2.1]hept-2-enes have been obtained from stereoisomeric exo- and endo-5-cyanobicyclo[2.2.1]hept-2-enes and the corresponding sulfonamides were obtained through reaction of amines with methyl-, n-propyl-, n-butyl-, benzyl-, and cyclohexylsulfonyl chlorides. From the stereoisomeric sulfonamides with peroxy acids, various products were obtained: exo-sulfonamides were transformed into epoxy derivatives, and, in contrast, most of the endo-stereoisomers underwent heterocyclization resulting in substituted exo-2-hydroxy-4-azatricyclo[4.2.1.0^3,7]nonanes. The type of the products obtained did not depend on the type of peroxy acid used (peroxyacetic, peroxyphthalic, and m-chloroperoxybenzoic one). In contrast to other endo-sulfonamides, N-(cyclohexylsulfonyl)-endo-5-aminomethylbicyclo[2.2.1]hept-2-ene in reaction with peroxyacetic acid did not undergo heterocyclization, probably, due to steric factors. The structure and stereochemical homogeneity of the sulfonamides and the structure of the products of their oxidation with peroxy acids were confirmed by spectroscopic methods. The molecular structure of N-(cyclohexylsulfonyl)-endo-5-aminomethyl-exo-2,3-epoxybicyclo[2.2.1]heptane was determined by X-ray diffraction analysis. The mechanism of the intramolecular heterocyclization reaction of N-substituted endo-5-aminomethyl-exo-2,3-epoxybicyclo[2.2.1]heptanes was studied at the BHandHLYP/6-31G(d) level of theory.     

The Beckmann fragmentation of quadricyclanone oxime

The attempted O-tosylation of tetracyclo[3.2.0.0^2,7.0^4,6]heptan-3-one (quadricyclanone) oxime with p-toluenesulfonyl chloride in dichloromethane in the presence of triethylamine/DMAP or pyridine resulted in the Beckmann fragmentation to give a mixture of 4-exo-/4-endo-tosyloxy- and 4-exo-/4-endo-chlorobicyclo[3.1.0]hex-2-ene-6-endo-carbonitriles in 90% overall yield. Solvolysis of all four products in 2,2,2-trifluoroethanol afforded the corresponding 4-exo-trifluoroethoxy derivative as the sole product. Quadricyclanone itself undergoes the fragmentation reaction with hydroxylamine-O-sulfonic acid, selectively affording the 4-exo-hydroxy-6-endo-nitrile in 90% isolated yield.     

Synthesis of the C8-C20 and C21-C30 segments of pectenotoxin 2

In this study, we synthesized the C8-C20 and C21-C30 segments of the diarrhetic shellfish toxin pectenotoxin 2. The C8-C20 segment was assembled from a phosphonate corresponding to the C8-C15 segment (prepared from l-malic acid in 19 steps) and an aldehyde corresponding to the C16-C20 segment (synthesized from 3-methyl-3-butenol in nine steps) by a twelve-step process including the Horner-Wadsworth-Emmons reaction, regio- and stereoselective reduction of the resulting enone, diastereoselective epoxidation, and 5-exo epoxide cleavage forming the C-ring. The C21-C30 segment was constructed in 13 steps from (S)-glycidol via a route involving E-ring formation by 5-exo epoxide cleavage and stereoselective methylation at C27 by the Evans method.     

Iron carbonyl complexes of 2,3,5,6-tetrakis(methylene)-7-oxabicyclo[2.2.1]heptane. Crystal and molecular structure of (C10H10O)Fe(CO)3 and (C9H10CO)Fe2(CO)6

The reaction of 2,3,5,6-tetrakis(methylene)-7-oxabicyclo[2.2.1]heptane (I) with iron carbonyls in various solvents yields the (η⁴-1,3-diene)Fe(CO)₃ isomers (II: exo; III: endo) and the bimetallic isomers bis[(η⁴-1,3-diene)Fe(CO)₃] (IV: bis(exo); V: endo,exo). In weakly coordinating solvents, a parallel rearrangement of I occurs through CO bond cleavage of the allylic ether by Fe₂(CO)₉ yielding an unsaturated ketone (VI) bonded to two Fe(CO)₃ groups through a trimethylenemethane and a 1,3-diene system, respectively. The geometries of III and VI have been ascertained by X-ray crystal structure determinations.     

Synthesis and isomer distribution of 2-alkyltropinones and 2-alkylgranatanones

Tropinone (8-methyl-8-azabicyclo[3.2.1]octan-3-one) metal (Li, Na, K, Mg) enolates were used to achieve α-alkylation. The reactions, regardless of the metal or conditions used, were low-yielding. N,N-Dimethylhydrazones of tropinone and granatanone (pseudopelletierine, 9-methyl-9-azabicyclo[3.3.1]nonan-3-one) were prepared and α-alkylated using n-butyllithium as the lithiating reagent. Lithium amides, including a polymer-supported lithium amide, were less effective. The reactions were modelled using DFT calculations at the B3LYP 6-31G(d) level and the CPCM solvent model, revealing that the face-selective alkylations of tropinone and granatanone hydrazones favoured the exo-isomers. Granatanone and α-isopropyl tropinone derivatives resisted typical mild hydrolytic hydrazone cleavage (aqueous trifluoroacetic acid) and required more forceful conditions (p-TsOH, boiling dioxane). Using the hydrazone alkylation strategy, 16 α-alkyl derivatives (benzyl, methyl, propyl, isopropyl, allyl, pentyl, heptyl, p-methoxybenzyl) were prepared in 52–90% yields overall. For the α-alkylated tropinones and granatanones (10 examples), the DFT calculations and experimental thermodynamic distributions in base catalysed equilibrations showed that except for the α-isopropyl derivatives, the endo-isomers were more stable than the exo-isomers and were the major products. For 2-isopropyltropinone, the bulky substituent favoured the exo (axial) position in the bicyclic skeleton. The thermodynamic distribution for the α,α′-dibenzyl tropinone isomers was also evaluated.     

Diastereoselective addition of monoterpenic alcoholates and thiolates to 2,3-dicarbomethoxynorbornadiene

Addition of excess lithium salts of (−)-menthol or (−)-borneol to 2,3-dicarbomethoxynorbornadiene 1 affords the transesterification–addition products 4 and 5. The thiols derived from the same monoterpenes give only the addition products to the activated double bond, 6 and 7. In all cases, the addition reaction is totally exo-selective with respect to the norbornadiene moiety and moderately selective in discriminating the si- and re-face of the double bond. Both diastereomeric products are formed as a mixture of endo–exo epimers at C3. The major addition product of (−)-menthol to 1 is crystalline, of known absolute configuration, and has potential as a precursor of chiral ligands.     

Epoxidation and Heterocyclization of Arenesulfonamides of the Norbornene Series

Reactions of previously known and newly synthesized N-(bicyclo[2.2.1]hept-5-en-endo-2-ylmethyl)arenesulfonamides with monoperoxyphthalic acid generated in situ from phthalic anhydride and 30% hydrogen peroxide lead mostly to the corresponding N-arylsulfonyl-exo-2-hydroxy-4-azatricyclo[4.2.1.0^3,7]nonanes (azabrendanes). In some cases, N-(exo-5,6-epoxybicyclo[2.2.1]hept-5-en-exo-2-ylmethyl)arenesulfonamides were isolated as the only products or mixtures of alternative oxidation products were obtained. The presence of electron-acceptor nitro groups in the benzene ring and bulky substituents, primarily in the ortho position, is considered to be a structural factor preventing the primary oxidation products (epoxy derivatives) from undergoing heterocyclization.     

1,3-Dipolar cycloaddition reaction of a d-galactose derived nitrone with allyl alcohol: synthesis of polyhydroxylated perhydroazaazulene alkaloids

Diastereofacial intermolecular 1,3-dipolar cycloaddition of d-galactose derived nitrone with allyl alcohol followed by tosylation afforded, in a 1:1 ratio endo- and exo-isooxazolidines 4a and 4b with complete diastereoselectivity at the nitrone carbon. The N–O bond reductive cleavage and S_N2 displacement afforded the pyrrolidine ring with a galactose appendage that on acetonide cleavage and reductive amino-cyclization afforded pentahydroxylated perhydroazaazulenes 1a and 1b.     

Hydrazide-catalyzed 1,3-dipolar nitrone cycloadditions

The triflate salts of cyclic hydrazides function as asymmetric catalysts for the [3+2]cycloadditions of nitrones with α,β-unsaturated aldehydes. The camphor-derived hydrazides show a preference for the exo isomers during these reactions, providing a compliment to other organically catalyzed dipolar cycloadditions. Enantiomeric excesses as high as 93% were realized for the exo isomers, while some endo products were obtained in 94% ee.     

Quantum-chemical study on the mechanism of catalytic dimerization of norbornadiene in the presence of hydride nickel(I) complex

[2+2]-Cyclodimerization of norbornadiene to pentacyclic products with exo-trans(cis)-exo structure in the presence of model catalytically active η⁴-norbornadiene nickel hydride complex was simulated at the DFT/PBE level of theory. According to the calculations, the rate-determining step in the cyclodimerization process is reductive elimination of the metallacycle whose decomposition yields norbornadiene dimer. The formation of cis-dimer is unfavorable for both kinetic and thermodynamic reasons.     

Highly regioselective Wittig reactions of cyclic ketones with a stabilized phosphorus ylide under controlled microwave heating

Significant base and temperature effects on the Wittig reactions of cyclohexanones with (carbethoxymethylene)triphenylphosphorane under microwave irradiation were observed. For the Wittig reactions carried out in a domestic microwave oven under solvent-free conditions, the initially formed exo-olefin products isomerized into the thermodynamically more stable endo-olefins due to uncontrolled high reaction temperature. In sharp contrast, under controlled microwave heating, both exo- and endo-olefins have been selectively synthesized by accurately regulating the reaction temperature with or without a base.     

One-pot reductive cleavage of exo-olefin to methylene with a mild ozonolysis-Clemmensen reduction sequence

A one-pot exo-olefin reductive cleavage was for the first time developed. The reaction could proceed under a mild condition avoiding the use of hazardous and expensive reagents. Meanwhile, a TMSCl-mediated Clemmensen reduction in alcoholic solvent was also examined.     

Acid catalyzed rearrangements in bicyclo[3.3.1]nonanes : 2-Substituted-6-(1,3-dioxolan-2-yl)-cyclooctanones from 1-substituted-2-hydroxy-9,9-(ethylenedioxy)-bicyclo[3.3.1]-nonanes

Treatment of endo-2 or exo-2-hydroxy-1-substituted ketals 1a–d with p-toluensulfonic acid in dry benzene results in a reversible C₉ bridge cleavage and affords equilibrium mixtures where 2-substituted-6-(1,3-dioxolan-2-yl)cyclooctanones 6a–d are present as main products. Yields in 6a–d are present as the steric hindrance of the substituents at C₁ in the substrate increases as well. Deuterium exchange experiments are in favour of an intramolecular 1,3-hydride shift from C₂ to C₉.     

Thermal decomposition products of methyl 1,5-diphenyl- and 5-methyl-1-phenyl-2,3-diazabicyclo[3.1.0]hex-2-ene-6-exo-carboxylates

Methyl 1,5-diphenyl- and 5-methyl-1-phenyl-2,3-diazabicyclo[3.1.0]hex-2-ene-6-exo-carboxylates at 138°C undergo decomposition via elimination of nitrogen molecule with formation in each case of five products. Two products are methyl 1,3-diphenyl(or 1-methyl-3-phenyl)bicyclo[1.1.0]butane-2-endo- and -exo-carboxylates, and the three others are derivatives of buta-1,3-diene, methyl (Z)-2-benzylidene-3-phenyl(or 3-methyl)but-3-enoate and methyl (E)- and (Z)-3,4-diphenyl(or 4-methyl-3-phenyl)penta-2,4-dienoates. The formation of these products may be rationalized assuming intermediacy of substituted allylcarbene which undergoes both intramolecular cycloaddition and rearrangements involving 1,2-hydride and 1,2-vinyl shifts.     

Cycloadditions of chiral carbonyl ylides with imine dipolarophiles as a route to enantiomerically pure α-amino-β-hydroxy acids

The preparation of enantiomerically pure threo-β-amino-α-hydroxy acids via 1,3-dipolar cycloadditions of imine dipolarophiles with the chiral isomünchnone derived from (5R)-5-phenylmorpholin-3-one 1 is described. The cycloadducts were obtained with excellent diastereofacial- and exo-selectivity. Subsequent hydrolysis and chemoselective exocyclic amide cleavage afforded the threo-β-amino-α-hydroxy acids with recovery of the initial chiral auxiliary.     

1,3-Dipolar cycloaddition of 1H-pyrazinium-3-olate and N1- and C-methyl substituted pyrazinium-3-olates with methyl acrylate: a density functional theory study

A DFT study of the 1,3-dipolar cycloaddition of methyl acrylate to 1H-pyrazinium-3-olate and N1- and C-methyl substituted pyrazinium-3-olates, in the gas phase and in THF, has been carried out at the B3LYP/6-31G(d) level. Two stereoisomeric pathways, endo and exo, and two regioisomeric channels, 2-oxo-3,8-diazabicyclo[3.2.1]octane-6-ester and 7-ester products, have been considered. Thermodynamic and kinetic parameters calculated at room temperature have been analyzed. The regioselectivity has been interpreted using reactivity indices. It is generally found that the exo pathway is preferred and the formation of the 6-esters is dominant. The theoretical data obtained for the cycloaddition reaction of 1,5-dimethylpyrazinium-3-olate with methyl acrylate are consistent with the literature where the 6-exo regioisomer is formed as the major cycloadduct.     

Tandem molecular rearrangement in the alkylation of phenol with camphene

The alkylation of phenol with camphene in the presence of boron trifluoride in glacial acetic acid was accompanied by tandem molecular rearrangement with formation of a mixture of ortho- and para-substituted phenols having 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yl and 5,5,6-trimethylbicyclo[2.2.1]hept-exo-2-yl substituents. The same products were obtained by rearrangement of 1,7,7-trimethylbicyclo[2.2.1]hept-exo-2-yloxybenzene under analogous conditions. Similar reactions performed in the presence of aluminum phenoxide as catalyst resulted in predominant formation of the corresponding ortho-substituted phenols.     

An intramolecular oxo Diels-Alder approach to 1-oxo-1,2,3,3a,4,7a-hexahydro-pyrano[3,4-c]pyrrole-4-carboxylic acid ethyl esters

The diastereoselective synthesis of a series of 1-oxo-1,2,3,3a,4,7a-hexahydro-pyrano[3,4-c]pyrrole-4-carboxylic acid ethyl esters via an oxo Diels-Alder reaction is described. Ab initio calculations predicted the products of the exo cycloaddition to be the thermodynamic products while the products resulting from the endo cycloaddition were predicted to be the kinetic products. The calculations were born out by experimental data.