The <Emphasis Type="Italic">Mitsunobu</Emphasis> Inversion Reaction of Sterically Hindered 17-Hydroxy Steroids

Summary. The Mitsunobu inversion reaction of 3-methoxyestra-1,3,5(10)-trien-17-ol is dramatically influenced by the acidic component. There appears to be a relationship between the dissociation constant of the electron-withdrawing substituent on the aryl acid and the overall effectiveness of the reaction, with more acidic species generally providing a higher yield of inverted product.     

The Use of Sterically Hindered Benzylamines in the Sommelet Reaction

A number of 2- and 2,6-disubstituted benzylamines have been successfully converted to the corresponding benzaldehydes via the Sommelet reaction in yields of 17–68%.     

The Wittig Reaction Using Potassium-tert-butoxide High Yield Methylenations of Sterically Hindered Ketones

Potassium-tert-butoxide is recommended for general use in Wittig reactions with nonstabilized ylides. High yield methylenations of cyclohexanone and a series of five sterically hindered ketones demonstrate its utility.     

Microwave Activation of Reaction between 1,3,5-Trisubstituted Pyrazole-4-carbaldehydes and Sterically Hindered Aminoalcohols

Structure of products of reaction between 1,3,5-trisubstituted pyrazole-4-carbaldehydes and 2-aminoalkan-1-oles is determined by the structure of the alkyl chain of aminoalcohol. The reaction with 2-aminobutan-1-ol proceeds for 2 h in conditions of microwave activation at 150°C with the formation of 2-hydroxyalkylimines of pyrazole-4-carbaldehyde. At reaction of 1,3,5-trisubstituted pyrazole-4-carbaldehydes with 2-amino-2-methylpropan-1-ol mixtures form of the corresponding 2-hydroxyalkylimines and pyrazol-4-yl-1,3-oxazolidines in ratios 66 : 33–80 : 20. Heating of 2-hydroxyalkylimines of pyrazole-4-carbaldehyde in the presence of dehydrating agents, in particular, trimethylchlorosilane, does not result in 1,3-oxazolidines.     

Synthesis of Spirocyclohexadienone Lactones from the Reaction of Ortho-Lithiated Oxazolines with Sterically Hindered 1,4-Benzoquinones

γ-Spirolactones have been synthesised from 1,2-addition reactions of ortho-lithiated oxazolines to 1,4-benzoquinones.     

Preparation and Synthetic Applications of Sterically Hindered Secondary Amines

The preparation of sterically hindered secondary amino esters from the reaction of N-protected α-amino aldehydes and 3-, or 2-oxo esters with α-amino esters by reductive amination is described. The resulting amino esters were converted to the β-lactam or acylated to form N-acyl secondary amides.     

Pseudohalogeno Complexes of Sterically Hindered Tripyrrinatopalladium (TrpyPd) Fragments

Eight new pseudohalogeno complexes of sterically hindered TrpyPd fragments 4 – 11 have been prepared by salt metathesis in one step from known trifluoroacetates TrpyPdOAc^F 1 – 3 , using sodium salts of N₃, NCO, NCS, NCSe, dicyanamide (dca) and tricyanomethanide (tcm) anions. Despite the propensity of the co‐ligands to bind to two or more metal centres in a bridging coordination mode all new complexes have been shown by NMR spectroscopy to be essentially monomeric in solution. X‐ray diffraction studies performed on TrpyPdNCS( 6 ), TrpyPddca( 8 ), TrpyPddca( 9 ), and TrpyPdtcm( 10 ) revealed the presence of two different conformers in the solid state, one with a helically distorted Trpy backbone for 6 and 10 , and one with an almost planar tripyrrole but a tetrahedral distortion of the Pd^II coordination, a so‐called pseudoplanar form, for 8 and 9 . Interestingly the helical forms are also detected by unusual wavenumbers for the ν_CN stretching bands in their IR spectra. The reason for the occurrence of two different conformers in the solid state appears to be the crystal packing which depends mainly on the shape of the anionic co‐ligand and on the presense of alkyl groups on the TrpyPd fragment.     

Isotope Effects in the Solvolysis of Sterically Hindered Arenesulfonyl Chlorides

Solvent isotope effects in the ethanolysis of sterically hindered arenesulfonyl chlorides ruled out a proton transfer in the rate‐determining step and agreed with a S_N2 mechanism involving at least a second solvent molecule in the transition state (TS). The lack of a secondary kinetic isotope effect in the o‐alkyl groups allows us to disregard the possible contribution of σ–π hyperconjugation. The measured activation parameters are consistent with a S_N2 mechanism involving the participation of solvent molecules in the TS, possibly forming a cyclic TS through a chain of solvent molecules.     

Photocatalyzed Aminomethylation of Alkyl Halides Enabled by Sterically Hindered N-Substituents

The catalytic C(sp³)−C(sp³) coupling of alkyl halides and tertiary amines offers a promising tool for the rapid decoration of amine skeletons. However, this approach has not been well established, partially due to the challenges in precisely distinguishing and controlling the reactivity of amine-coupling partners and their product homologues. Herein, we developed a metal-free photocatalytic system for the aminomethylation of alkyl halides through radical-involved C(sp³)−C(sp³) bond formation, allowing for the synthesis of sterically congested tertiary amines that are of interest in organic synthesis but not easily prepared by other methods. Mechanistic studies disclosed that sterically hindered N-substituents are key to activate the amine coupling partners by tuning their redox potentials to drive the reaction forward.     

Synthesis of Sterically Hindered Amine Containing Azide via Chemoselective Hydride Reduction

The chemoselective reduction of an ester in the presence of an azido group by LiCl/NaBH₄ is described. A sterically hindered secondary amine containing an azido group is synthesized by this method.     

A Simple Method for the Synthesis of the Sterically Hindered Chloramines

A simple route to the hindered chloramines, starting from the corresponding hindered amines is reported. Sodium dichlorisocyanurate is the chlorinating agent.     

Effective High‐Pressure Cleavage of Sterically Hindered Steroid Esters

A simple and effective method to deprotect of sterically hindered steroid esters is described. Deprotection was carried out in MeOH in the presence of a catalytic amount of Et₃N under high‐pressure conditions. Enzymatic, anionite, and high‐pressure methods are compared.     

Synthesis and Antioxidative Properties of New Sulfur-Containing Derivatives of Sterically Hindered Phenols

3-(3,5-Di-tert-butyl-4-hydroxyphenyl)-1-chloropropane was converted into derivatives containing S(II) in various functional groups. The inhibiting power of the compounds with respect to thermal autooxidation of animal fat was evaluated.     

Structure of Metal Bischelates with Enaminoketone Derivatives of Sterically Hindered 2-Imidazoline

Crystal structures of the first Cu(II), Ni(II), and Pd(II) bischelates with enaminoketone derivatives based on the 2-imidazoline heterocycle have been determined. In all the compounds, the coordinated heterocycles have an envelope conformation leading to a puckered structure of the complex molecules.     

Comparison of Silylation Reaction Rates of Different Reagents : Catalytic Effect of Methoxyamine on the Silylation of Sterically Hindered Hydroxyl Groups

Abstract

A comparison of different silylating agents was conducted by gas-chromatography, using methandienon e(17α-methyl-androsta-1, 4-dien-17β-ol-3-one) as a model substance for steroids with hindered hydroxyl groups. A very significant catalytic effect of methoxy-amine on the reactions was found with all examined silylating agents. A 100% yield was found using N-trimethylsilyl-imidazole in combination with the catalyst in an “on-column” reaction.     

Synthesis of Sterically Hindered Secondary and Tertiary Alkyl(Aryl)Phosphines

Cross coupling of silylphosphines with 2-halobenzenecarboxylates or 2-halophenyl ethers, catalyzed with zero-valent palladium complexes can not be used as a procedure for preparing sterically hindered tertiary phosphines. The latter compounds can be successfully prepared by means of phosphorylation of corresponding o-lithiated derivatives.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 2, 2005, pp. 239–243.Original Russian Text Copyright © 2005 by Veits, Neganova, Vinogradova.