Highly diastereoselective tandem photoenolization-hetero-Diels-Alder cycloaddition reactions of o-tolualdehydes in the solid state

The (E)-photoenols generated in situ by photolysis of o-tolualdehydes 1-5 in the solid state react with the precursor aldehydes as dienophiles in a hetero-Diels-Alder cycloaddition fashion to afford trans-3-arylisochromanols in excellent yields and in a high diastereoselectivity. An examination of the reactivity of three different classes of rationally designed aldehydes shows that the tandem enolization-Diels-Alder cycloaddition occurs in the solid state efficiently for aldehydes whose (E)-photoenols (i) are more stable than their corresponding benzocyclobutenols and (ii) are not sterically congested. However, rapid cyclization to benzocyclobutenols is found to be the sole pathway for sterically encumbered (E)-enols derived from aldehydes 6-8. Given that the execution of heteromolecular reactions in the solid state is a challenge, the results obtained with simple crystalline o-tolualdehydes are remarkable and the strategy involving in situ generation of a dienol in the crystal lattice of a dienophile to achieve hetero-Diels-Alder reaction in a highly diastereoselective manner is heretofore unprecedented. In the context of enormous interest in tandem/domino reactions in contemporary synthetic organic chemistry, the results observed with o-tolualdehydes exemplify successful execution of tandem reactions in the solid state.     

Conformational control and photoenolization of pyridine-3-carboxaldehydes in the solid state: stabilization of photoenols via hydrogen bonding and electronic control

We have investigated the solid-state photobehavior of a broad set of pyridine-3-carboxaldehydes 1-5. The introduction of a heteroatom into mesitaldehydes as in aldehydes 1 raises the question of conformational preference in the solid state. The preferred conformations have been unequivocally established from X-ray crystal structure analyses of two of the aldehydes, 1c and 2c; it is shown that intramolecular hydrogen bonding could be utilized to achieve conformational control. In contrast to mesitaldehydes, which undergo efficient photocyclization to benzocyclobutenols in the solid state, the heteroatom analogues 1b and 1c exhibit a perceptible color change (from colorless to pale yellow for 1b and yellow-orange for 1c) upon UV irradiation; the color attributed to (E)-enols is persistent for several hours. Continued irradiation leads to an intractable polymeric material. The AM1 calculations, which have been reliably applied to the thermal cyclization of xylylenols to benzocyclobutenols, reveal that the (E)-enols of 1 are more stable than those of the mesitaldehydes relative to their corresponding benzocyclobutenols. The stabilization is interpreted as arising from the possibility of engaging the heteroatom in resonance delocalization. That the contribution from such a role of the nitrogen atom is so pronounced is elegantly demonstrated by forming the fluoroborate salts; 1a-HBF(4) and 1b-HBF(4) readily exhibit highly red-shifted absorption upon exposure to UV radiation as a result of stabilization of the photoenols. Notably, such a remarkable stabilization via electronic control of the photoenols is unprecedented. All of the 2-methoxy- and 2-chloro-substituted aldehydes 2-5 exhibit photochromism. Ab initio calculations show that the methoxy group in aldehydes 2 and 3 stabilizes the (E)-enols via O[bond]H...O hydrogen bonding as compared to those of 1 by 5-6 kcal/mol relative to their corresponding benzocyclobutenols. Thus, the presence of methoxy and halo groups at position 2 serves not only to direct the formyl oxygen toward the methyl group for H-abstraction but also to stabilize the (E)-enols.     

Solid-state diphotocyclization of iso- and terephthalaldehydes via dihalogen substitution

The supramolecular nonbonded C-H...X interactions between formyl hydrogens and ortho-halogen atoms (Br/Cl) have been exploited to achieve conformational control in the solid state of dimethyl-substituted iso- and terephthaladehydes (1-3) for unprecedented diphotocyclization. It is shown that the dihalogen substitution also contributes to the stability of the benzocyclobutenols relative to their precursor photoenols, so that the solid-state photolysis of dialdehydes 2b, 2c, and 3b leads to diphotocyclization to afford respectable yields of bis-benzocyclobutenols.     

Unique solvent effect on photochemistry of ortho-alkylphenacyl benzoates

Photolysis of 2,4,6-trialkylphenacyl benzoates gives not only the corresponding indanones and benzoic acid, but also the corresponding benzocyclobutenols (CBs), which are also detected in the photolysis of mono-alkylphenacyl benzoates for the first time. The product selectivity was heavily dependent upon solvents and o-alkyl group. H-bonding acceptor solvents strongly favor the formation of the CB. As the size of the o-alkyl group increases, the relative amount of the CB increases.     

Solid phase synthesis of highly substituted tetrahydropyrans by tandem ene-reaction/intramolecular Sakurai cyclization

A solid-phase tandem ene-reaction/intramolecular Sakurai cyclization sequence has been developed to synthesize highly substituted tetrahydropyran derivatives in two steps from aldehydes and with complete control of the relative stereochemistry of the three newly formed stereocenters. The compounds are obtained with high purity after release from the solid support and can be easily isolated in multimilligram amounts. Moreover, we have shown that asymmetric induction is possible on solid phase and that enantiomerically pure tetrahydropyrans containing four stereocenters can be effectively synthesized with this method.     

Synthesis of Multiply Substituted Polycyclic Aromatic Hydrocarbons by Iridium‐Catalyzed Annulation of Ring‐Fused Benzocyclobutenol with Alkyne through C−C Bond Cleavage

The first iridium‐catalyzed intermolecular cyclization between alkynes and ring‐fused benzocyclobutenols (RBCB) through C?C bond cleavage is described. A variety of elusive polycyclic aromatic hydrocarbons (PAHs) with multiple substituents are obtained in good yields under mild conditions. This procedure provides a unique and expeditious tool for the synthesis of PAHs.     

Intramolecular Cyclization of Carbonate and Thiocarbonate Derivatives of myo‐Inositol in the Solid State: Implications for Acyl Group Transfer Reactions in Molecular Crystals

Racemic 4‐O‐phenoxycarbonyl and 4‐O‐phenoxythiocarbonyl derivatives of myo‐inositol orthoformate undergo thermal intramolecular cyclization in the solid state to yield the corresponding 4,6‐bridged carbonates and thiocarbonates, respectively. The thermal cyclization also occurs in the solution and molten states, but less efficiently, suggesting that these cyclization reactions are aided by molecular pre‐organization, although not strictly topochemically controlled. Crystal structures of two carbonates and a thiocarbonate clearly revealed that the relative orientation of the electrophile and the nucleophile in the crystal lattice facilitates the intramolecular cyclization reaction and forbids the intermolecular reaction. The correlation observed between the chemical reactivity and the non‐covalent interactions in the crystal of the reactants provides a way to estimate the chemical stability of analogous molecules in the solid state.     

Reactions de cyclisation photochimique D'α-alcoxy cyclohexenones

α-Ketooxetanes and α-alkylidene oxetanols are the main products of the photolysis of 2-alkoxy 2-cyclohexenones; γ-hydrogen abstraction and cyclization of the intermediate biradical are general processes for these enones. Regioselectivity of the reaction and several aspects of the photolysis of a ketooxetanes are discussed. The corresponding 2-alkoxy cross-conjugated dienones rearrange in cyclopropyl ketones when photolyzed in the same conditions.     

Nickel-Catalyzed Tunable Enantioconvergence and Kinetic Resolution in the Coupling of Tertiary Cyclobutenols with Arylboroxines

We report that a nickel catalyst system with a modified 1,1′-spirobiindane-7,7′-diol-phosphoramidite (SPINOL) as the chiral ligand can enable the coupling of tertiary cyclobutenols and arylboroxines in an enantioconvergent manner, providing cyclobutenes with an all-carbon quaternary stereocenter in good yields (up to 84 % yield) with excellent enantioselectivities (up to >99 % ee). Moreover, the catalytic system can be applied in the kinetic resolution of cyclobutenols under slightly modified conditions, giving enantioenriched tertiary cyclobutenols with an s factor of up to >200. The reaction uses free hydroxyl groups as the leaving group without additional activation while the strained ring remains untouched. Preliminary mechanistic studies reveal that the inherent discrepant reactivity of the two enantiomers is the key to the controllable enantioconvergent and kinetic resolution process.     

Control of the regioselectivity of sulfonamidyl radical cyclization by vinylic halogen substitution

The radical cyclization reactions of unsaturated sulfonamides were investigated. The photolysis of N-(4-halo-4-pentenyl)sulfonamides (X=I, Br, or Cl) with (diacetoxyiodo)benzene (DIB) and iodine at room temperature afforded exclusively the corresponding piperidines in 73-98% yield via 6-endo radical cyclization. On the other hand, the reactions of N-(5-halo-4-pentenyl)sulfonamides with DIB/I2 led to the only formation of the pyrrolidine products in 84-99% yield via 5-exo radical cyclization. The vinylic halogen substitution not only successfully inhibits the competing ionic iodocyclization process to allow the radical cyclization to proceed smoothly but also shows a remarkable effect in controlling the regioselectivity of cyclization.     

Efficient synthesis of aminomethylated azaindoles and corresponding pyrrole-fused derivatives by copper-catalyzed domino multicomponent coupling and cyclization

Efficient methods for the synthesis of aminomethylated azaindole derivatives via domino copper-catalyzed multicomponent coupling and cyclization have been developed. Using various secondary amines and aldehydes, N-substituted 3-ethynyl-4-aminopyridine was converted to substituted azaindoles in moderate to excellent yields. By use of a 3,4-diaminopyridine derivative bearing two alkynyl groups, the corresponding pyrrole-fused azaindoles were synthesized by controlled stepwise cyclization.     

Characterization of the photodegradation products of metolachlor: structural elucidation,potential toxicity and persistence

Aqueous solutions of metolachlor and metolachlor‐d₆ were photolyzed with UV‐visible radiations. The structures of 15 by‐products of metolachlor were determined through gas chromatography‐mass spectrometry analyses using electron and chemical ionization combined with multistage mass spectrometry. The photolysis by‐products of metolachlor resulted mainly from dehalogenation and hydroxylation, in some cases accompanied by cyclization. In silico tests for toxicity prediction showed that the toxicity of some photolysis products is expected to be greater than that of metolachlor. Persistence studies showed that the by‐product relative abundances vary in large amounts with the irradiation time. The post‐photolysis evolution of the solution was also studied, in order to determine the persistence of the main by‐products. It allowed to establish that most of the by‐products can be found more than 12 h after the end of the photolysis, which is of a great concern as treated water is generally available for consumption only a few hours after treatment in most of industrial processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Double conjugate addition of dithiols to propargylic carbonyl systems to generate protected 1,3-dicarbonyl compounds

The work describes the efficient double conjugate addition of ethane and propane dithiols in the presence of sodium methoxide to a wide variety of propargylic carbonyl containing compounds. The products of these reactions are differentiated, 1,3-dicarbonyl systems useful for various synthesis programs. By judicious use of hydroxylated substrates tandem cyclization occurs to afford tetrahydropyran lactols or, in the case of hydroxy-substituted propargylic esters, lactones. The corresponding amino-substituted propargylic aldehydes gives piperidine derivatives upon double conjugate addition tandem cyclization.     

Comparative semiempirical and DFT study of styrylnaphthalenes and styrylquinolines and their photocyclization products

Semiempirical molecular orbital (PM3, PM6, and RM1) and density functional theory (DFT) (B3LYP/6‐31G*) studies are carried out for 1‐ and 2‐styrylnaphthalenes and their aza‐derivatives—2‐ and 4‐styrylquinolines. Relative stabilities of three isomeric forms: E‐ and Z‐isomers and the closed‐ring dihydrocyclophotoproduct (derivative of dihydrophenanthrene) are calculated. Compared to PM3, PM6 and especially RM1 understate heats of formation; in some cases, PM6 and RM1 even place Z‐isomer in energy below E‐isomer. PM3 rather close to DFT predicts heats of isomerization reaction, whereas PM6 and especially RM1 underestimate these values. Semiempirical methods in comparison with DFT markedly underestimate heats of cyclization reaction; however, reproduce trends in relative stabilities of different isomers in dependence on the structure of styrylnaphthalenes and styrylquinolines. Qualitative correlation is found between calculated relative stabilities of the closed‐ring forms (heats of cyclization reaction) and experimental data: cyclized products with low heats of cyclization are observed in steady‐state photolysis and those with high heats of cyclization are not. In the latter case, the closed‐ring compounds, if formed in the excited state, due to thermal instability decompose rapidly with ring opening in the ground state that prevents their observation. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

5-Exocyclic products, 2,3,5-trisubstituted tetrahydrofurans via Prins-type cyclization

[reaction: see text] 5-Exocyclic products, 2,3,5-trisubstituted tetrahydrofurans, were synthesized from homopropargylic alcohols with terminally substituted alkynes and various aldehydes via Prins-type cyclization. It is of interest that the exocyclic vinyl cation generated as a result of Prins-type cyclization could be trapped as a vinyl triflate when CH2Cl2 was used as a solvent, whereas in ethereal solution the vinyl cation underwent hydrolysis to give the corresponding ketone product.     

Radical reactions initiated by the photochemical cleavage of carbon-indium bonds of organoindium compounds

Intra- and intermolecular reactions of carbon-centered radicals generated by photolysis of organoindium compounds were examined. The photolysis of vinylindium compounds and indium acetylides provided vinyl and alkynyl radicals, respectively, which were trapped with ethyl iodoacetate giving the corresponding β,γ-unsaturated esters. Allylic indium compounds, prepared from 8-bromo- or 8-iodooct-1,6-dienes and powdered indium metal, underwent an intramolecular radical cyclization to afford the 5-exo-trig product.     

5- And 6-exocyclic products, cis-2,3,5-trisubstituted tetrahydrofurans, and cis-2,3,6-trisubstituted tetrahydropyrans via Prins-type cyclization

Exocyclic products having cis-2,5 and cis-2,6 substitution were synthesized from terminally substituted alkynyl alcohols with various aldehydes via Prins-type cyclization in good yields. It is of interest that synthesized 5- and 6-exocyclic vinyl cations generated as a result of Prins-type cyclization could be trapped as a vinyl triflate in CH2Cl2 to give 3-furanylidenes and 3-pyranylidenes. Those 3-furanylidenes and 3-pyranylidenes underwent hydrolysis to give the corresponding 3-acyl-substituted products having all-cis-configured isomers, such as 2,3,5-trisubstituted tetrahydrofurans and 2,3,6-trisubstituted tetrahydropyrans.     

Revision of the scheme for the formation of heterocyclic quinones upon the photolysis of 3-amino-2-cycloalkylamino-1,4-naphthoquinones

Conclusions The formation of naphthimidazole derivatives in the photolysis of 3-amino- and 3-acetyl-amino-2-cycloalkylamino-1,4-naphthoquinones is the result of the dark intramolecular cyclization of the corresponding 3-aminoenaminoquinones.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 694–699, March, 1989.     

Diastereoselective synthesis of tetrahydrofurans via mead reductive cyclization of keto-beta-lactones derived from the tandem Mukaiyama aldol lactonization (TMAL) process

The development of a diastereoselective, three-step strategy for the construction of substituted tetrahydrofurans from alkenyl aldehydes based on the tandem Mukaiyama aldol-lactonization process and Mead reductive cyclization of keto beta-lactones is reported. Stereochemical outcomes of the TMAL process are consistent with models established for Lewis acid-mediated additions to alpha-benzyloxy and beta-silyloxy aldehydes while reductions of the five-membered oxocarbenium ions are consistent with Woerpel's models. Further rationalization for observed high diastereoselectivity in reductions of alpha-silyloxy 5-membered oxocarbenium ions based on stereoelectronic effects are posited. A diagnostic trend for coupling constants of gamma-benzyloxy beta-lactones was observed that should enable assignment of the relative configuration of these systems.     

Synthesis of difluorinated carbocyclic analogues of 5-deoxypentofuranoses and 1-amino-5-deoxypentofuranoses: en route to fluorinated carbanucleosides

The synthesis of difluorinated carbocyclic analogues of 5-deoxypentofuranoses and 1-amino-5-deoxypentofuranoses is described. The sequence involves an addition of PhSeCF₂TMS to carbohydrate-derived aldehydes or their corresponding tert-butanesulfinylimines followed by a radical cyclization. Optimized conditions for the PhSeCF₂TMS addition to α-chiral aldehydes have been disclosed and its unusual diastereoselectivity is discussed. Application of the sequence using Ellman's auxiliary allows a more direct access to 1-aminopentose analogues with a complete control of the pseudo-anomeric center configuration.