Ramani Gurubrahamam

Highly Enantioselective Synthesis of Chiral Allenes by Sequential Creation of Stereogenic Center and Chirality Transfer in a Single Pot Operation

Chiral allenes are readily accessed in a single pot operation in the reaction of terminal alkynes, aldehydes, chiral secondary amines, and zinc halides in good yields (up to 77% yield) and excellent enantioselectivities (up to 99% ee) in toluene at 120 °C. The reaction proceeds through initial formation of chiral propargylamine intermediates with creation of a new stereogenic center and subsequent chirality transfer via an intramolecular hydride shift to produce chiral allenes with high enantiomeric purities.

Copper(I) Halide Promoted Diastereoselective Synthesis of Chiral Propargylamines and Chiral Allenes using 2-Dialkylaminomethylpyrrolidine, Aldehydes, and 1-Alkynes

Copper bromide promoted reactions of aldehydes, 1-alkynes, and chiral 2-dialkylaminomethylpyrrolidine at 25 °C give the corresponding chiral propargylamine derivatives in up to 96% yield and 99:1 dr that are readily converted to the corresponding disubstitued chiral allenes in up to 81% yield and 99% ee upon reaction with CuI in dioxane at 100 °C.

Organocatalytic kinetic resolution of racemic secondary nitroallylic alcohols combined with simultaneous desymmetrization of prochiral cyclic anhydrides.

This study describes an organocatalytic kinetic resolution of racemic secondary nitroallylic alcohols (2) combined with simultaneous desymmetrization of prochiral cyclic anhydrides (1). The experimental results revealed that enantioselective alcoholysis of 3-substituted glutaric anhydrides afforded hemiesters (3) with high levels of enantioselectivities (up to 99% ee) in the presence of cinchonidine-derived thiourea catalyst (IV). The highly optical enrichment (up to 95% ee) of (S)-nitroallylic alcohols (2) was recovered.

Enantioselective Synthesis of Functionalized Polycarbocycles via a Three-Component Organocascade Quadruple Reaction

An efficient organocascade quadruple reaction was conducted to synthesize a functionalized spiropolycyclic scaffold in high chemical yields (43-80%) and excellent levels of stereoselectivity (up to >19:1 dr and 99% ee). The quadruple reaction proceeded smoothly between 1,3-indanedione and aromatic aldehydes with concomitant desymmetrization of prochiral 4-substituted cyclohexanones through the Knoevenagel/Michael/aldol/aldol reaction sequence catalyzed by a bifunctional thiourea catalyst. Two of the formed products were transformed into spirocyclic epoxides containing four contiguous quaternary centers.

Organocascade Synthesis of Annulated (Z)-2-Methylenepyrans: Nucleophilic Conjugate Addition of Hydroxycoumarins and Pyranone to Branched Nitro Enynes via Allene Formation/Oxa-Michael Cyclization/Alkene Isomerization Sequence

An efficient organocatalytic reaction using 1,3-nitro enynes with 4-hydroxycoumarin and 4-hydroxy-6-methyl-2-pyrone to afford pyrano-annulated scaffolds in high yield (up to 88% yield) and excellent stereoselectivities (up to >20:1 dr and >99% ee) is described. The reaction proceeded through sequential conjugate addition, allene formation, intramolecular oxa-Michael 6-endo-dig cyclization and DABCO-catalyzed olefin isomerization. A kinetic profile for isomerization was established. The mechanism for the organocascade reaction was proposed according to requisite computational and mechanistic experimental studies.

Control of five contiguous stereogenic centers in an organocatalytic kinetic resolution via Michael/acetalization sequence: synthesis of fully substituted tetrahydropyranols.

An organocatalytic kinetic resolution of racemic secondary nitroallylic alcohols via Michael/acetalization sequence to give fully substituted tetrahydropyranols is described. The process affords the products with high to excellent stereoselectivities (up to 19.9:1.5:1 dr and 98% ee). The highly enantioenriched, less reactive (S)-nitroallylic alcohols 3 were isolated with good to high chemical yields (30-44%). The synthetic application of the resolved substrate is shown toward the synthesis of enantioenriched (+)-(2S,3R)-3-amino-2-hydroxy-4-phenylbutyric acid.

Recent Advances in Organocatalytic Kinetic Resolution for the Synthesis of Functionalized Products

This Minireview describes organocatalytic kinetic resolution (OCKR) that has evolved on the basis of using small organic catalysts through enamine/iminium ion, N‐heterocyclic carbene, and hydrogen‐bonding catalysis. The review includes the use of recent methodologies for which the evolution of OCKR mainly involves creating new stereogenic center(s) in functionalized products through the formation of a C−C, C−N, C−O, C−X, or C−S bond. The combination of OCKR with sequential reactions and its association with desymmetrization and dynamic kinetic resolution are also described.

Dihydrooxazine N-Oxide Intermediates as Resting States in Organocatalytic Kinetic Resolution of Functionalized Nitroallylic Amines with Aldehydes

Kinetic resolution of nitroallylic amines was established using chiral α,α-l-diphenylprolinol silyl ether auxiliary through isolation of the dihydrooxazine N-oxide intermediates. Further hydrolyzing the resting states provided tetrahydropyridines in high chemical yields and high to excellent stereoselectivities (up to >20:1 dr and 98% ee). A detailed mechanistic explanation for stereoselective protonation in the dihydrooxazine was probed computationally. In addition, the probable intermediates in α-halogenation of aldehydes (masked with enamines) were isolated to provide crystallographic evidence.