Sampak Samanta

Asymmetric Diels–Alder Reactions of α,β‐Unsaturated Aldehydes Catalyzed by a Diarylprolinol Silyl Ether Salt in the Presence of Water

The Diels–Alder reaction is a powerful synthetic method for the construction of regioand stereochemically defined cyclohexane frameworks. There are several catalytic enantioselective methods, and MacMillan and co-workers developed the first Diels–Alder reaction involving an organocatalyst, which proceeds by a LUMO-lowering activation mechanism. Since then several asymmetric Diels–Alder reactions involving organocatalysts have been reported. Our group and that of Jørgensen developed a diarylprolinol silyl ether as an effective organocatalyst in 2005, and this type of catalyst has since been employed widely in several asymmetric reactions. Recently, we found that diarylprolinol silyl ether 1 combined with CF3CO2H is an effective Diels–Alder catalyst in toluene. In contrast, water has attracted a lot of interest as a reaction medium in current organic chemistry because of its unique properties. In the Diels–Alder reaction, for instance, the reaction is accelerated “in water” (homogeneous dilute conditions) and “on water” (biphasic conditions). We reported the positive effect of water on diastereoand enantioselectivities for the asymmetric aldol reaction in the presence of water. Palomo et al. and Ma and co-workers reported the enantioselective Michael reaction catalyzed by dialkyland diphenylprolinol silyl ethers, respectively in the presence of water. Some organocatalyzed reactions are known to be affected by dissolved water, and on the basis of our interest in reactions in the presence of water, we have examined the enantioselective Diels–Alder reaction by using diarylprolinol silyl ether as an organocatalyst. Although Northrup and MacMillan and Ogilvie and co-workers reported the asymmetric Diels–Alder reaction in the presence of water, we developed a green and practical procedure that does not require an organic solvent, even for the purification step. We also observed an interesting phenomenon, namely the positive effect of water on the rate and enantioselectivity of the reaction, which is different from that of the “on water” reaction and will be described herein. First, we chose the model reaction between cinnamaldehyde and cyclopentadiene, which we had found to be promoted by a combination of 1 (Figure 1) and CF3CO2H in

Asymmetric, catalytic, and direct self-aldol reaction of acetaldehyde catalyzed by diarylprolinol.

An asymmetric, catalytic, and direct self-aldol reaction of acetaldehyde was catalyzed by diarylprolinol in NMP, affording the trimer acetal, which was generated by the reaction of the self-aldol product with another acetaldehyde molecule in a moderate yield with good enantioselectivity. Acetal is the synthetic equivalent of the self-aldol product, which can be converted into other synthetically useful compounds in one pot without compromising the enantioselectivity.

Catalysis by ionic liquids: solvent-free efficient transthioacetalisation of acetals by molten tetrabutylammonium bromideElectronic supplementary information (ESI) avialable: spectral data of S,S-acetals. See http://www.rsc.org/suppdata/p1/b2/b204363g/

Tetrabutylammonium bromide in molten state has been demonstrated to be a very efficient and recyclable catalyst for transthioacetalisation of O,O-acetals to S,S-acetals under solvent-free conditions.

Organocatalytic Enantioselective Synthesis of Both Diastereomers of α-Hydroxyphosphinates

Racemic alpha-acylphosphinates and formylphosphinate hydrate were used directly as the substrates in a proline derivative-catalyzed cross aldol reaction with ketones. Because of the preexisting phosphorus stereogenic center, a mixture of two diastereomers of the corresponding alpha-hydroxyphosphinates was obtained in this reaction. Good to high enantioselectivities (up to 99% ee) were obtained simultaneously for the two diastereomers in good yields. Good diastereoselectivities were also obtained when the reaction generates an additional carbon stereogenic center.

Use of indium hydride (Cl2InH) for chemoselective reduction of the carboncarbon double bond in conjugated alkenes

Abstract Indium hydride (Cl 2 InH) generated in situ from a combination of a catalytic amount of indium(III) chloride and sodium borohydride selectively reduces the carboncarbon double bond in conjugated alkenes such as α,α-dicyano olefins, α,β-unsaturated nitriles, cyano esters, cyanophosphonate, diesters and ketones.

Reduction of trihalomethyl carbinols and their acetates, mesylates and tosylates by indium metal

A study of reduction of trihalomethyl carbinols and their derivatives by indium metal shows that while trichloromethyl carbinols produce a mixture of the corresponding dichloromethyl carbinol and vinylidene dichloride, the acetates, mesylates and tosylates of the trichloro- and tribromomethyl carbinols undergo clean reduction to provide the respective vinylidene dichloride only.

Stereoselective synthesis of highly functionalized tetrahydrocarbazoles through a domino Michael–Henry reaction: an easy access to four contiguous chiral centers

For the first time, a very simple, efficient, mild, catalytic and one-step procedure for the synthesis of a series of densely functionalized 1-methoxycarbonyl-2-aryl-3-nitro-4-hydroxy-1,2,3,4-tetrahydro-9H-carbazole derivatives has been achieved via a domino Michael–Henry reaction of methyl 3-formyl-1H-indole-2-acetates with β-nitrostyrenes using DABCO as an organocatalyst. Furthermore, the high enantio-(≤92% ee) and diastereoselective (≤12 : 1 dr) synthesis of the title compounds has been achieved with excellent yields using 9-O-benzylcupreidine (10 mol%) as a catalyst.

Self-assembled monolayer coated gold-nanoparticle catalyzed aerobic oxidation of α-hydroxy ketones in water: an efficient one-pot synthesis of quinoxaline derivatives

For the first time, 4-aminothiophenol self-assembled monolayer-coated gold-nanoparticles (Au-NPs) which catalyze the aerobic oxidation of aryl substituted α-hydroxy ketones to aryl 1,2-diketones are reported. In addition, a one-pot synthesis of quinoxalines has been successfully achieved via in situ oxidation of α-hydroxy ketones and subsequent condensation with aryl 1,2-diamines in water. This method offers the potential for simple self-assembled monolayer-coated Au-NPs to exhibit catalytic activity for the aerobic oxidation reaction in a green and efficient manner.

Access to 4,6-Diarylpicolinates via a Domino Reaction of Cyclic Sulfamidate Imines with Morita-Baylis-Hillman Acetates of Nitroolefins/Nitrodienes.

An interesting domino reaction of 5-membered cyclic sulfamidate imines with a variety of Morita-Baylis-Hillman acetates of nitroolefins/nitrodienes in the presence of DABCO as an organic base at 55 °C is reported for the first time. This new synthetic strategy provides a series of pharmacologically interesting 4,6-diarylpicolinates in high to excellent yields and allows several compatible functionalities on aryl rings. Moreover, the biologically interesting imidazo[1,2-a]pyridine (alpidem derivative) has been prepared in high chemical yield through a unique procedure.

A remarkable solvent effect on the reaction of 4-hydroxycoumarin with (E)-3-aryl-2-nitroprop-2-enol: Facile synthesis of highly substituted furo/pyrano[3,2-c]chromenes

A remarkable solvent effect on the reaction of 4-hydroxycoumarin derivatives with (E)-3-aryl/hetero-aryl-2-nitroprop-2-enols has been observed in water and DMSO media. This result was employed for the straightforward syntheses of new functionalized furo/pyrano[3,2-c]chromenes in 63–93% yields and diasteromeric ratio up to ≤99 : 1. Moreover, a simple, mild, efficient and catalyst-free one-pot method may offer an alternative synthetic strategy for annulating the furan/pyran rings on the coumarin nucleus. Furthermore, water has shown a significant positive effect on the rate and selectivity (product) of this reaction.