Novel regiochemistry in the aqueous singlet oxygen ene reactions of carboxylic acid salts: a comparison of substrate structure

The singlet oxygen (¹Δ_g) photooxidations of angelic acid salt (1), tiglic acid salt (2), 2,3-dimethyl-2-butenoic acid salt (3), 3-ethoxycarbonyl-5,6-dihydro-2-methyl-4H-pyrane acid salt (4), cis-3-hexenoic acid salt (5), and trans-3-hexenoic acid salt (6) were conducted in deuterated water. The major and minor ene allylic hydroperoxide products were quantified and indicate that the allylic hydrogen geminal to the carboxylate group is preferentially abstracted in 1-4, whereas the allylic hydrogen α to the carboxylate is slightly favored for 5 and 6. We have attributed the observed regiochemistry in 1-4 to stabilizing hydrogen bonding interactions between the solvent and the perepoxide, which leads to the major ene product.     

Dye-sensitized intrazeolite photooxygenation of 4-substituted cyclohexenes. Remote substituent effects in regioselectivity and diastereoselectivity

The product distribution in the dye-sensitized photooxygenation of α-terpinyl acetate and terpinen-4-ol is quite similar in solution, however, by zeolite Y confinement, is substantially influenced by the position of the remote polar substituents relative to the reacting double bond. The intrazeolite results were rationalized in terms of Na⁺-substrate and Na⁺-singlet oxygen interactions.     

Stereoelectronic and solvent effects on the allylic oxyfunctionalization of alkenes with singlet oxygen

The factors that control the stereochemistry of sensitized photooxygenation of alkenes via singlet oxygen (ene reaction) are selectively reported. We also introduce our most recent stereoelectronic effects on the singlet oxygen-ene reaction. The origin of site selectivity and solvent-dependent stereoselectivity in this classical ene reaction with simple as well as functionalized alkenes is highlighted. These studies and other similar studies have enhanced substantially the utility of singlet oxygen in the synthesis of natural and non-natural products.     

Origin of the stereo- and regioselectivities in the Diels-Alder reactions of azaphospholes: a DFT investigation

Computations of the Diels-Alder (DA) reactions of azaphosphole representative namely, thiazolo[3,2-d][1,4,2]diazaphosphole with 1,3-butadiene and isoprene at the density functional theory level reveal concerted mechanisms via asynchronous transition states. The activation energies (B3LYP/6-311++G**// B3LYP/6-311G**), 16-19 kcal mol⁻¹, are much smaller than the value (32.57 kcal mol⁻¹) calculated for the DA reaction of the non-phosphorus analogue, imidazo[2,1-b]thiazole with 1,3-butadiene. An electron-withdrawing group at the 3-position of the dienophile enhances both stereo- and regioselectivities, which agree nicely with the experimental values. Inclusion of solvent effect (PCM model) reveals that the stereo- and regioselectivities are not affected appreciably. The relative stabilities of the transition structures corresponding to the endo/exo stereoisomers and meta (P/Me, 1:3)/para (P/Me, 1:4) regioisomers have been rationalized on the basis of the secondary molecular orbital interactions.     

Intrazeolite photooxygenation of chiral alkenes. Control of facial selectivity by confinement and cation-π interactions

Depending on the nature of the substituents on the stereogenic carbon atom, the ene reaction of singlet oxygen with several chiral alkenes by confinement within thionin-supported zeolite NaY, may exhibit significant changes on facial selectivity by comparison to their photooxygenation reaction in solution. It is proposed that, apart from the conformational consequences as a result of the alkene confinement within the zeolite cavities, a synergism between Na⁺-π interactions and singlet oxygen-Na⁺ interactions plays a significant role in the transition states of ene hydroperoxidation. Within NaY, the diastereoselectivity may significantly depend on the site selectivity, as probed through specific deuterium labelling of trisubstituted alkenes bearing a gem-dimethyl group. In certain cases, a remote stereogenic centre relative to the reacting double bond may induce enhanced diastereoselection and regioselectivity.     

A new regio- and stereoselective intermolecular Friedel-Crafts alkylation of phenolic substrates with aryl epoxides

A conceptually new regioselective and highly syn-stereoselective intermolecular Friedel-Crafts-type O-alkylation of phenols with aryl epoxides by the use of appropriately substituted aryl borates is reported. The carbon-carbon bond formation occurs in neutral and mild conditions without the need for external Lewis acids or transition metal catalysts.     

Regioselective and stereoselective photodimerization of securinine-type and norsecurinine-type alkaloids

The photodimerization of both securinine-type and norsecurinine-type alkaloids has been investigated. The dimers of these alkaloids were obtained in a unique form of anti-head-head with high regioselectivity and stereoselectivity. Different reaction conditions were investigated and thermodynamical calculation was also carried out. It is believed that the selective dimer formation is related to the thermodynamical stability of dimeric products and the steric hindrance caused by the complex bridge rings and particular spatial structure. In addition, the norsecurinine-type alkaloids are more prone to dimerize under photochemical conditions than securinine-type alkaloids, which might result from the transannular conjugation confirmed in the securinine-type molecules.     

The Homoaldol Reaction,or How to Overcome Problems of Regio- and Stereo-selectivity

Even today, there are conspicuous gaps in the register of organic synthetic methods. Thus, numerous types of homoenolate reagents are listed; yet no generally applicable method for the homologue of the aldol addition, viz. the homoaldol addition, is to be found. The methods documented up to 1980 offer only singular solutions, and, moreover, are not stereoselective. The present article gives an overview of the existing possibilities and then describes how the gap can be closed by using a new class of homoenolate reagents: 2-alkenyl esters of N,N-diisopropylcarbamic acid are lithiated and the resulting allyllithium compounds tailored to high regio- and diastereoselectivity in addition reactions to the carbonyl group via metal exchange. Altogether, the homoaldol reactions now offer a general and reliable, highly stereoselective entry to γ-hydroxycarbonyl compounds, whereby the C(β)? C(γ) bond is formed. Furthermore, it is demonstrated how problems associated with the application of allyl anions as synthetic reagents can be solved today.     

Regio- and stereoselective functionalization of alkenes with emphasis on mechanistic insight and sustainability concerns

Alkene is an attractive substrate for chemists due to its easy availability and reactivity towards large number of reactants affording diverse range of organic compounds. It reacts under ionic and free radical mechanisms including single electron transfer (SET). In this review, strategies used for C–C and C-heteroatom functionalization of alkene has been discussed with emphasis on the regio, stereoselectivity, mechanistic detail and sustainability aspects. These strategies mainly follow the free radical mechanism, and the highly reactive carbon radicals show uncontrollable regio- and stereoselectivities. Thus these strategies still need to be focused; especially in the asymmetric versions. The regio- and stereoselectivities of functionalization of alkenes have been highlighted and debated. In addition, the hazardous reagents such as Cl₂, Br₂ I₂, CO, peroxides, and benzene have also been discussed with the emphasis on their impact on the environment. Their plausible green alternatives have also been suggested, such as MX as halogen replacement; CO surrogates (formaldehyde etc.); sustainable aromatic solvents as benzene replacement. The non-green strategies relying on pre-formed silyl hydride and their green alternative strategies such as transfer hydrogenations have also been indicated. The applications of the functionalization of alkenes for the total synthesis of bioactive compounds have also been discussed in detail. In addition, future perspectives are also highlighted for further developments in the functionalization of alkenes.     

Regio- and stereocontrolled synthesis and conformational analysis of benzimidazole nucleosides

Regio- and stereocontrolled synthesis and conformational analysis of a series of benzimidazole nucleosides were achieved. A simple method by ¹H NMR 1D NOE experiment was developed for estimation of syn or anti conformation of benzimidazole nucleosides. Substituents at C2 of benzimidazole demonstrated to play a key role both in the unexpected regioselectivity of the glycosidic reaction and in the conformation distributions of the final products.     

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.     

Singlet oxygen reactions of 3-methoxy-2-pyrrole carboxylic acid tert-butyl esters. A route to 5-substituted pyrrole precursors of prodigiosin and analogs

Reaction of the tert-butyl ester of 3-methoxy-2-pyrrole carboxylic acid with singlet oxygen yields a peroxidic intermediate which undergoes coupling with a range of nucleophiles to yield 5-substituted pyrroles. Among these products are α,α′-bipyrroles which serve as precursors of prodigiosin, including A-ring substituted analogues.     

Singlet oxygen addition to homoallylic substrates in solution and microemulsion: novel secondary reactions

The photooxygenation of three homoallylic substrates, the γ,δ-unsaturated ketone 1a, nitrile 2a, and the γ,δ-unsaturated ester 3a was investigated in homogeneous solution and in microemulsion (for 1a). Two secondary reaction pathways were detected for the allylic hydroperoxides of type b and c, respectively. The cyclization reactions of 1b and 2b to the 1,2-dioxanes 1d and 2d followed well-known reaction patterns, whereas the base-catalyzed epoxide (1e-3e) formation from the tertiary allylic hydroperoxides 1c-3c is a unprecedented reaction type.     

Synthesis of 2-arylacrylic esters from aryl methyl ketones via Wittig reaction/singlet oxygen ene reaction

An efficient synthetic method has been developed for the synthesis of 2-arylacrylic esters from the corresponding aryl methyl ketones via Wittig reaction and singlet oxygen ene reaction. Wittig reaction to aryl methyl ketones with (methoxymethyl)triphenylphosphonium chloride in basic condition afforded the methyl enol ethers, and then 2-arylacrylic esters were obtained by singlet oxygen ene reaction, followed by tosylation and elimination in one-pot to the methyl enol ethers in good yields.     

Regio-controlled pentadienylation of sulfinylimines by pentadienyltin reagent. Efficient Lewis acid activation of 4-methoxyphenylsulfinyl imines

Nucleophilic addition of pentadienyltin reagent toward N-(4-methoxyphenylsulfinyl)-imine was effectively accelerated by the use of InCl₃ or TMSOTf as a Lewis acid. The former afforded the γ-adduct with high stereoselectivity, while the latter gave the ε-adduct with moderate stereoselectivity, depending on the reaction mechanisms.     

Ring opening of cyclic sulfamidates with bromophenyl metal reagents: complementarity of sulfamidates and aziridines

Bromophenyl magnesium reagents generated via a Knochel type magnesium-halogen exchange of aryl iodides undergo regioselective ring opening of cyclic primary and secondary N-Boc sulfamidates in good to excellent yields. With secondary sulfamidates the reaction proceeds with clean inversion of the stereochemistry. This protocol complements the ring opening of aziridines with bromophenyl metal reagents and extends its scope to secondary substrates.     

Reprint of: Recent advances in the functionalization of allenes via radical process

This review is focused on the recent advances in the functionalization of allenes via radical process. Different radical partners including carbon radicals and heteroatom radicals are discussed in the reactions of allenes. Generally, the radical formed in situ would attack the allene at the central carbon leading to allyl radical intermediate. However, the formation of alkenyl radical intermediate from allene could be observed as well in some cases with high regioselectivity and stereoselectivity.     

Recent advances in the functionalization of allenes via radical process

This review is focused on the recent advances in the functionalization of allenes via radical process. Different radical partners including carbon radicals and heteroatom radicals are discussed in the reactions of allenes. Generally, the radical formed in situ would attack the allene at the central carbon leading to allyl radical intermediate. However, the formation of alkenyl radical intermediate from allene could be observed as well in some cases with high regioselectivity and stereoselectivity.     

Regio-/stereoselective formation of monosulfoxides from thiacalix[4]arenes in all possible conformations

The partial S-oxidation of all four basic conformations of thiacalix[4]arene was carried out using NaBO₃·4H₂O as the oxidizing agent. It was found that despite the possible formation of many regio- and stereoisomers, the reactions leading to mono- or disulfoxides were highly regio- and stereoselective depending on the starting conformation. Our results clearly show that the sulfur atoms possessing syn-orientation of the appended phenolic units are remarkably more reactive than the sulfur atoms with anti-oriented aromatic subunits. Moreover, the most easily formed syn-oriented equatorial configuration is highly preferred over the corresponding axial arrangement which was never observed. As demonstrated by the resolution of racemic products, partial S-oxidation represents a very interesting tool to access novel inherently chiral building blocks based on thiacalix[4]arenes.     

1,3-dipolar cycloadditions of arylnitrile oxides and 2-diazopropane with 5-hydroxy-3-methyl-1,5-dihydropyrrol-2-one derivatives

The reactions of arylnitrile oxides 2 and 2-diazopropane 5 with 5-hydroxy-3-methyl-1,5-dihydropyrrol-2-one derivatives 1 have been studied. 1,3-dipolar cycloaddition of arylnitrile oxides and 2-diazopropane with 5-hydroxy-3-methyl-1,5-dihydropyrrol-2-one derivatives is taking place regiospecifically. The asymmetric induction expected by the chiral centre of the 5-hydroxy-3-methyl-1,5-dihydropyrrol-2-one derivatives was very effective, diastereoisomers 3 and 4 were formed in an approximate 90:10 ratio. The stereoselectivity of the 1,3-dipolar cycloaddition of the 2-diazopropane with the 5-hydroxy-3-methyl-1,5-dihydropyrrol-2-one derivatives are investigated. The attack of the 1,3-dipole occurred from the less hindered face of the dipolarophile, giving the isomer 6.