Silicon-initiated carbonylative carbotricyclization and [2+2+2+1] cycloaddition of enediynes catalyzed by rhodium complexes

The reaction of dodec-11-ene-1,6-diynes or their heteroatom congeners with a hydrosilane catalyzed by Rh(acac)(CO)2 at ambient temperature and pressure of CO gives the corresponding fused 5-7-5 tricyclic products, 5-oxo-1,3a,4,5,7,9-hexahydro-3H-cyclopenta[e]azulenes or their heteroatom congeners, in excellent yields through a unique silicon-initiated cascade carbonylative carbotricyclization (CO-SiCaT) process. It has also been found that the 5-7-5 fused tricyclic products can be obtained from the same type of enediynes and CO through a novel intramolecular [2+2+2+1] cycloaddition process. The characteristics of these two tricyclization processes and the fundamental differences in their reaction mechanisms are discussed. This novel higher-order cycloaddition reaction has also been successfully applied to the tricyclization of undeca-5,10-diyn-1-als, affording the corresponding 5-7-5 fused-ring products bearing a seven-membered lactone moiety. Related [2+2+2] tricyclizations of enediyne and diynal substrates are also discussed. These newly discovered reactions can construct multiple bonds all at once, converting linear starting materials to polycyclic compounds in a single step. Thus, these new processes provide innovative routes to functionalized polycyclic compounds that are useful for the syntheses of natural and unnatural products.     

Rhodium N-heterocyclic carbene-catalyzed [4 + 2] and [5 + 2] cycloaddition reactions

[reactions: see text] A rhodium complex of N-heterocyclic carbene (NHC) has been developed for intra- and intermolecular [4 + 2] and intramolecular [5 + 2] cycloaddition reactions. This is the first use of a transition-metal NHC complex in a Diels-Alder-type reaction. For the intramolecular [4 + 2] cycloaddition reactions, all the dienynes studied were converted to their corresponding cycloadducts in 91-99% yields within 10 min. Moreover, up to 1900 turnovers have been obtained for the intramolecular [4 + 2] cycloaddition at 15-20 degrees C. For the intermolecular [4 + 2] cycloadditions, high yields (71-99%) of the corresponding cycloaddition products were obtained. The reaction time and yield were highly dependent upon the diene and the dienophile. For the intramolecular [5 + 2] cycloaddition reactions, all the alkyne vinylcyclopropanes studied were converted to their corresponding cycloadducts in 91-98% yields within 10 min. However, the catalytic system was not effective for an intermolecular [5 + 2] cycloaddition reaction.     

Total Syntheses of (+)‐Polygalolide A and (+)‐Polygalolide B: Elucidation of the Absolute Stereochemistry and Biogenetic Implications

The total syntheses of (+)‐polygalolide A and (+)‐polygalolide B have been completed by using a carbonyl ylide cycloaddition strategy. Three of the four stereocenters, including two consecutive tetrasubstituted carbon atoms at C2 and C8, were incorporated through internal asymmetric induction from the stereocenter at C7 by a [Rh₂(OAc)₄]‐catalyzed carbonyl ylide formation/intramolecular 1,3‐dipolar cycloaddition sequence. The arylmethylidene moiety of these natural products was successfully installed by a Mukaiyama aldol‐type reaction of a silyl enol ether with a dimethyl acetal, followed by elimination under basic conditions. We have also developed an alternative approach to the carbonyl ylide precursor based on a hetero‐Michael reaction. This approach requires 18 steps, and the natural products were obtained in 9.8 and 9.3 % overall yields. Comparison of specific rotations of the synthetic materials and natural products suggests that polygalolides are biosynthesized in nearly racemic forms through a [5+2] cycloaddition between a fructose‐derived oxypyrylium zwitterion with an isoprene derivative.     

Organocatalytic, enantioselective intramolecular [6+2] cycloaddition reaction for the formation of tricyclopentanoids and insight on its mechanism from a computational study

Diphenylprolinol silyl ether was found to be an effective organocatalyst for promoting the asymmetric, catalytic, intramolecular [6 + 2] cycloaddition reactions of fulvenes substituted at the exocyclic 6-position with a δ-formylalkyl group to afford synthetically useful linear triquinane derivatives in good yields and excellent enantioselectivities. The cis-fused triquinane derivatives were obtained exclusively; the trans-fused isomers were not detected among the reaction products. The intramolecular [6 + 2] cycloaddition occurs between the fulvene functionality (6π) and the enamine double bond (2π) generated from the formyl group in the substrates and the diphenylprolinol silyl ether. The absolute configuration of the reaction products was determined by vibrational circular dichroism. The reaction mechanism was investigated using molecular orbital calculations, B3LYP and MP2 geometry optimizations, and subsequent single-point energy evaluations on model reaction sequences. These calculations revealed the following: (i) The intermolecular [6 + 2] cycloaddition of a fulvene and an enamine double bond proceeds in a stepwise mechanism via a zwitterionic intermediate. (ii) On the other hand, the intramolecular [6 + 2] cycloaddition leading to the cis-fused triquinane skeleton proceeds in a concerted mechanism via a highly asynchronous transition state. (iii) The fulvene functionality and the enamine double bond adopt the gauche-syn conformation during the C-C bond formation processes in the [6 + 2] cycloaddition. (iv) The energy profiles calculated for the intramolecular reaction explain the observed exclusive formation of the cis-fused triquinane derivatives in the [6 + 2] cycloaddition reactions. The reasons for the enantioselectivity seen in these [6 + 2] cycloaddition reactions are also discussed.     

Recent advances in [2+2+2] cycloaddition reactions

The [2+2+2] cycloaddition is an elegant, atom-efficient and group tolerant process for the synthesis of carbo- and heterocycles, mostly aromatic, involving the formation of several C-C bonds in a single step. Cyclotrimerisation is catalyzed by a variety of organometallic complexes, including more than 15 different metals. The aim of this tutorial review is to point out the most recent advances in this field and to encourage the use of this reaction enroute to complex molecules. After summarizing the most common catalysts and reaction conditions generally used, we survey the mechanistic features currently accepted for this reaction. Section 4 covers the scope of the different [2+2+2] cycloaddition versions starting with the cyclotrimerisation of three triple bonds, including nitriles, with especial emphasis on asymmetric reactions that create central, axial or planar chirality. Then, reactions that use double bonds are addressed. Finally, the most outstanding examples of natural products synthesis using [2+2+2] cycloadditions as a key step reported recently are shown.     

Density functional theory study of the intramolecular [2 + 3] cycloaddition of azide to nitriles

Density functional theory calculations using the hybrid functional B3LYP have been performed to study tetrazole formation by intramolecular [2 + 3] dipolar cycloaddition of organic azides and nitriles. Experimental reactivity trends are explained and rationalized in terms of a number of parameters, such as strain, tether length, and solvation and entropy effects. Interestingly, no correlation was found between the overall free energies and the free energies of activation of the reactions, due to the significant difference in strain and geometry between the transition states and products.     

An atom-economic synthesis of bicyclo[3.1.0]hexanes by rhodium N-heterocyclic carbene-catalyzed diastereoselective tandem hetero-[5+2] cycloaddition/Claisen rearrangement reaction of vinylic oxiranes with alkynes

The first synthetic application of a vinylic oxirane as a heteroatom-containing five-atom component in transition-metal-catalyzed cycloaddition reactions is reported. A new, efficient, diastereoselective tandem intramolecular hetero-[5+2] cycloaddition/Claisen rearrangement of vinylic oxirane-alkyne substrates that uses a rhodium NHC complex and provides strategically novel, atom-economic, regiospecific, and diastereoselective access to [3.1.0] bicyclic products has been developed.     

Mode selectivity in the intramolecular cyclization of ketenimines bearing N-acylimino units: a computational and experimental study

[reaction: see text] The mode selectivity in the intramolecular cyclization of a particular class of ketenimines bearing N-acylimino units has been studied by ab initio and DFT calculations. In the model compounds the carbonyl carbon atom and the keteniminic nitrogen atom are linked either by a vinylic or an o-phenylene tether. Two cyclization modes have been analyzed: the [2+2] cycloaddition furnishing compounds with an azeto[2,1-b]pyrimidinone moiety and a 6pi-electrocyclic ring closure leading to compounds enclosing a 1,3-oxazine ring. The [2+2] cycloaddition reaction takes place via a two-step process with formation of a zwitterionic intermediate, which has been characterized as a cross-conjugated mesomeric betaine. The 6pi-electrocyclic ring closure occurs via a transition state whose pseudopericyclic character has been established on the basis of its magnetic properties, geometry, and NBO analysis. The 6pi-electrocyclic ring closure is energetically favored over the [2+2] cycloaddition, although the [2+2] cycloadducts are the thermodynamically controlled products. A quantitative kinetic analysis predicts that 1,3-oxazines would be the kinetically controlled products, but they should transform rapidly and totally into the [2+2] cycloadducts at room temperature. In the experimental study, a number of N-acylimino-ketenimines, in which both reactive functions are supported on an o-phenylene scaffold, have been successfully synthesized in three steps starting from 2-azidobenzoyl chloride. These compounds rapidly convert into azeto[2,1-b]quinazolin-8-ones in moderate to good yields as a result of a formal [2+2] cycloaddition.     

Novel Kumada coupling reaction to access cyclic (2-azaallyl)stannanes. Cycloadditions of cyclic nonstabilized 2-azaallyllithium species derived from cyclic (2-azaallyl)stannanes

A Kumada cross-coupling reaction involving organomagnesium reagents and (3-methylthio-2-azaallyl)stannanes with a Ni(0) catalyst provided cyclic nonstabilized (2-azaallyl)stannanes in moderate to good yields. Primary alkyl, aryl, and allylic organomagnesium reagents can be used as the cross-coupling partner. In general, NiCl(2)dppp in toluene at room temperature provided the shortest reaction times and most consistent yields. The azomethine ylides and 2-azaallyllithium species derived from these stannanes were shown to undergo efficient [3 + 2] cycloaddition reactions to provide azabicyclo[n.2.1]alkanes as the endo cycloadducts. These cycloadducts were found to be useful as starting materials for further elaboration into aza-bridged bicyclic natural and unnatural products of biological interest. Although cyclic 2-azaallyllithium species have been generated previously, this work reports the first generation and cycloaddition of entirely nonstabilized 2-azaallyllithium species. In addition a novel extension of the Kumada coupling was developed to allow for the preparation of the cyclic (2-azaallyl)stannanes, which are precursors to the nonstabilized 2-azaallyllithium species.     

A biomimetic approach to the rocaglamides employing photogeneration of oxidopyryliums derived from 3-hydroxyflavones

A unified biomimetic approach to the aglain-forbaglin-rocaglamide classes of natural products is reported. The approach involves photogeneration of oxidopyryliums via excited-state intramolecular proton transfer (ESIPT) of 3-hydroxyflavones followed by [3+2] dipolar cycloaddition to the aglain core. An alpha-ketol (acyloin) rearrangement was employed to transform the aglain core to the rocaglamide framework. This approach was successfully used for the synthesis of the natural product (+/-)-methyl rocaglate.     

Retro [2 + 2 + 2] ring opening in tricyclobutabenzene derivatives. Thermochemistry and reaction barriers. A theoretical hybrid density functional study

The starting materials, transition states, and products of the concerted retro [2 + 2 + 2] ring-opening reactions of several tricyclobutabenzene derivatives substituted by double-bonded substituents were calculated at the B3LYP/6-31G theoretical level. It was found that the geometries of the products (cyclododecatriyne derivatives) are governed by the electronegativity of the substituents and hyperconjugation effects. The transition-state geometries of all the derivatives are similar. It was found that the retro [2 + 2 + 2] ring-opening reactions are endothermic (ca. 30-85 kcal mol(-1), depending on the substituents) with high barriers, ranging between ca. 95 and 115 kcal mol(-1). It is predicted that the tricyclobutabenzene derivatives under study will be stable when prepared and that they can be made by [2 + 2 + 2] intramolecular ring closure of the respective cyclododecatriyne derivative, especially with the use of transition metals as mediators or catalysts.     

Rhodium-catalyzed intramolecular [4 + 2] cycloadditions of alkynyl halides

[reaction: see text] Cationic rhodium(I)-catalyzed intramolecular [4 + 2] cycloadditions of diene-tethered alkynyl halides were found to occur in good yields (70-87%). The halide moiety is compatible with the cycloaddition reactions, and no oxidative insertion to the alkynyl halide was observed. The halogen-containing cycloadducts could be transformed into a variety of products that are difficult or impossible to obtain via direct cycloaddition.     

Unprecedented intramolecular [3 + 2] cycloadditions of azido-ketenimines and azido-carbodiimides. Synthesis of indolo[1,2-a]quinazolines and tetrazolo[5,1-b]quinazolines

N-(2-azidomethyl)phenyl ketenimines and N-(2-azidomethyl)phenyl-N'-alkyl(aryl) carbodiimides undergo, under mild thermal conditions, intramolecular [3 + 2] cycloaddition reactions between the azido group and either the C=C or the distal C=N double bonds of the ketenimine and carbodiimide functions respectively. The reaction products are indolo[1,2-a]quinazolines and/or indolo[2,1-b]quinazolines in the case of azido-ketenimines, and tetrazolo[5,1-b]quinazolines in the case of azido-carbodiimides. The formation of the two classes of indoloquinazolines implies the ulterior dinitrogen extrusion from the non-isolated, putative [3 + 2] cycloadducts between the azide and ketenimine functions, whereas in the case of azido-carbodiimides the initial cycloadducts, tetrazoloquinazolines, were cleanly isolated and further converted into 2-aminoquinazolines by thermally induced dinitrogen extrusion.     

Asymmetric Brønsted Base Catalyzed and Directed [3+2] Cycloaddition of 2‐Acyl Cycloheptatrienes with Azomethine Ylides

Conjugated cyclic trienes have the potential for different types of cycloaddition reactions. In the present work, we will, in a novel asymmetric cycloaddition reaction, demonstrate that the organocatalytic reaction of 2‐acyl cycloheptatrienes with azomethine ylides proceeds as a [3+2] cycloaddition, which is in contrast to the Lewis acid‐catalyzed reaction, in which a [3+6] cycloaddition takes place. In the presence of a chiral organosuperbase, 2‐acyl cycloheptatrienes react in a highly enantioselective manner in the [3+2] cycloaddition with azomethine ylides, providing the 1,3‐dipolar cycloaddition product in high yields and up to 99 % ee. It is also shown that the diene formed by the reaction can undergo stereoselective dihydroxylation, bromination, and cycloaddition reactions. Finally, based on experimental observations, some mechanistic considerations are discussed.     

The Flash Vacuum Thermolysis of N-Acyl Imidates

In the flash vacuum thermolysis of methyl N-acyl imidates, the loss of methanol is consistent with the intermediacy of an N-acylketenimine. Recombination and further reactions may lead to the array of products observed in these thermal reactions. An attempt to trap the intermediate N-acylketenimine by an intramolecular [2 + 2] cycloaddition was unsuccessful.     

Gold-catalyzed, intramolecular, oxygen-transfer reactions of 2-alkynyl-1,5-diketones or 2-alkynyl-5-ketoesters: scope, expansion, and mechanistic investigations on a new [4+2] cycloaddition

The gold-catalyzed intramolecular oxygen-transfer reactions of 2-alkynyl-1,5-diketones or 2-alkynyl-5-ketoesters-obtained from tetra-n-butylammonium fluoride mediated Michael addition of activated allenes to electron-deficient olefins-furnished cyclopentenyl ketones under very mild conditions. These reactions proceeded much easier and faster than similar reactions reported in literature, and the corresponding products were obtained in very good yields. Mechanistic investigations on the cycloisomerization were carried out by means of both (18) O isotopic experiments and quantum chemical calculations. The results from both, the designed isotopic experiments and theoretical calculations, satisfactorily supported the novel proposed intramolecular [4+2] cycloaddition of a gold-containing furanium intermediate to a carbonyl group, instead of the previous well-accepted [2+2] pathway.     

Synthesis of 2H-indazoles by the [3 + 2] dipolar cycloaddition of sydnones with arynes

A rapid and efficient synthesis of 2H-indazoles has been developed using a [3 + 2] dipolar cycloaddition of sydnones and arynes. A series of 2H-indazoles have been prepared in good to excellent yields using this protocol, and subsequent Pd-catalyzed coupling reactions can be applied to the halogenated products to generate a structurally diverse library of indazoles.     

Total syntheses of (+)-polygalolide a and (+)-polygalolide b: elucidation of the absolute stereochemistry and biogenetic implications

The total syntheses of (+)-polygalolide?A and (+)-polygalolide?B have been completed by using a carbonyl ylide cycloaddition strategy. Three of the four stereocenters, including two consecutive tetrasubstituted carbon atoms at C2 and C8, were incorporated through internal asymmetric induction from the stereocenter at C7 by a [Rh(2) (OAc)(4)]-catalyzed carbonyl ylide formation/intramolecular 1,3-dipolar cycloaddition sequence. The arylmethylidene moiety of these natural products was successfully installed by a Mukaiyama aldol-type reaction of a silyl enol ether with a dimethyl acetal, followed by elimination under basic conditions. We have also developed an alternative approach to the carbonyl ylide precursor based on a hetero-Michael reaction. This approach requires 18 steps, and the natural products were obtained in 9.8 and 9.3?% overall yields. Comparison of specific rotations of the synthetic materials and natural products suggests that polygalolides are biosynthesized in nearly racemic forms through a [5+2] cycloaddition between a fructose-derived oxypyrylium zwitterion with an isoprene derivative.     

Development of catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions using chiral calcium complexes

Catalytic asymmetric 1,4-addition and [3 + 2] cycloaddition reactions using chiral calcium species prepared from calcium isopropoxide and chiral bisoxazoline ligands have been developed. Glycine Schiff bases reacted with acrylic esters to afford 1,4-addition products, glutamic acid derivatives, in high yields with high enantioselectivities. During the investigation of the 1,4-addition reactions, we unexpectedly found that a [3 + 2] cycloaddition occurred in the reactions with crotonate derivatives, affording substituted pyrrolidine derivatives in high yields with high enantioselectivities. On the basis of this finding, we investigated asymmetric [3 + 2] cycloadditions, and it was revealed that several kinds of optically active substituted pyrrolidine derivatives containing contiguous stereogenic tertiary and quaternary carbon centers were obtained with high diastereo- and enantioselectivities. In addition, optically active pyrrolidine cores of hepatitis C virus RNA-dependent polymerase inhibitors and potential effective antiviral agents have been synthesized using this [3 + 2] cycloaddition reaction. NMR spectroscopic analysis and observation of nonamplification of enantioselectivity in nonlinear effect experiments suggested that a monomeric calcium species with an anionic ligand was formed as an active catalyst. A stepwise mechanism of the [3 + 2] cycloaddition, consisting of 1,4-addition and successive intramolecular Mannich-type reaction was suggested. Furthermore, modification of the Schiff base structure resulted in a modification of the reaction course from a [3 + 2] cycloaddition to a 1,4-addition, affording 3-substituted glutamic acid derivatives with high diasterero- and enantioselectivities.     

Intramolecular and intermolecular diels-alder reactions of acylhydrazones derived from methacrolein and ethylacrolein

The intramolecular Diels-Alder reactions of hydrazones derived from methacrolein or ethylacrolein and terminally unsaturated N-acyl-N-methylhydrazines have been investigated. The hydrazones 7b and 7c derived from N-methyl-N-pent-4-enoylhydrazine 3b were found to undergo intramolecular [4 + 2] cycloaddition above 140 °C and the pyridopyridazines 12 were isolated. The corresponding hydrazones 8b and 8c from N-methyl N-pent-4-ynoylhydrazone 4a reacted similarly and gave as the final products the pyridines 13. The scope of the reaction is limited, as was shown by the failure of several other terminally unsaturated hydrazones of β-unsaturated aldehydes to undergo intramolecular cycloaddition. These hydrazones did, however, undergo intermolecular [4 + 2] cyctoaddition to N-phenylmaleimide. Other hydraiones 15 of methacrolein. including the benzoylhydrazone and the phenylhydrazone, also reacted with N-phenylmaleimide to give the pyridine 14b by way of an isolable dihydropyridine 16.