An α‐Cyclopropanation of Carbonyl Derivatives by Oxidative Umpolung

The reactivity of iodine(III) reagents towards nucleophiles is often associated with umpolung and cationic mechanisms. Herein, we report a general process converting a range of ketone derivatives into α‐cyclopropanated ketones by oxidative umpolung. Mechanistic investigation and careful characterization of side products revealed that the reaction follows an unexpected pathway and suggests the intermediacy of non‐classical carbocations.     

Borocyclopropanation of Styrenes Mediated by UV‐light Under Continuous Flow Conditions

Herein, we report a user‐friendly and metal‐free UV‐A light mediated borocyclopropanation of styrenes using continuous flow technology. A broad range of styrene derivatives can be cyclopropanated in good yields within 1 h residence time to produce highly valuable cyclopropylboronate esters with modest to good diastereoselectivities. The reaction is also applicable to α‐substituted styrenes. Mechanistic studies support a photoredox process during which xanthone, a well‐known organic photosensitizer, can easily reach a photoexcited state that is available for both an oxidative and a reductive quenching.     

gem‐Dimethylcyclopropanation of dibenzylideneacetone using triisopropyl sulfoxonium tetrafluoroborate

The reaction between dibenzylideneacetone (dba) and triisopropyl sulfoxonium tetrafluoroborate has been reinvestigated. The stereochemistry of the major diasteromeric bis(gem‐dimethylcyclopropane) adduct has now been assigned as [(1RS,3RS)‐2,2‐dimethyl‐3‐phenylcyclopropyl][(1SR,3SR)‐2,2‐dimethyl‐3‐phenylcyclopropyl]methanone, C₂₃H₂₆O, by X‐ray crystallographic studies on a twinned crystal. The asymmetric unit contains two molecules of the adduct, the conformations of which differ in the orientation of the phenyl ring relative to the adjacent cyclopropanated double bond. The carbonyl groups of each adduct are aligned approximately along the a axis and in opposite directions to each other. The molecules pack to give a sinusoidal pattern along the b axis. This is the first acyclic bis(dimethylcyclopropyl) ketone for which an X‐ray crystal structure determination has been reported, and is also the first bis‐cyclopropanated dba analogue. The knowledge that the major diastereomer has the meso structure (and therefore the confirmation that the minor isomer is the racemate) will prove invaluable in future studies to utilize bis(dimethylcyclopropyl) ketones as reagents, in rearrangement processes, and as potential ligands and ligand precursors in organometallic chemistry.     

A Geomimetic Approach to the Formation and Identification of Fossil Sterane Biomarkers in Crude Oil: 18‐nor‐D‐homo‐Androstane and 5α,14β‐Androstane

A diazonium ion derived from 18‐aminoandrostane rearranged upon decomposition by a carbonium and a carbenium ion to furnish a mixture of a cyclopropanated compound and two D ‐homo‐androstenes. Hydrogenation of this mixture gave the saturated hydrocarbons, 18‐nor‐D ‐homo‐androstane and 5α,14β‐androstane, which are both fossil sterane biomarkers in Neoproterozoic crude oil. The so far unknown constitution and configuration as well as the geochemical genesis were established by this experiment. The starting material for this investigation, 18‐aminoandrostane, was prepared in twelve steps from androstan‐17‐one (12.5 % overall yield) with a Barton reaction as the key step.     

Stereoselective Synthesis of Tropanes via a 6π‐Electrocyclic Ring‐Opening/ Huisgen [3+2]‐Cycloaddition Cascade of Monocyclopropanated Heterocycles

The synthesis of tropanes via a microwave‐assisted, stereoselective 6π‐electrocyclic ring‐opening/ Huisgen [3+2]‐cycloaddition cascade of cyclopropanated pyrrole and furan derivatives with electron‐deficient dipolarophiles is demonstrated. Starting from furans or pyrroles, 8‐aza‐ and 8‐oxabicyclo[3.2.1]octanes are accessible in two steps in dia‐ and enantioselective pure form, being versatile building blocks for the synthesis of pharmaceutically relevant targets, especially for new cocaine analogues bearing various substituents at the C‐6/C‐7 positions of the tropane ring system. Moreover, the 2‐azabicyclo[2.2.2]octane core (isoquinuclidines), being prominently represented in many natural and pharmaceutical products, is accessible via this approach.     

A New Procedure for Highly Regio‐ and Stereoselective Iodoacetoxylation of Protected Glycals and α‐1,2‐Cyclopropanated Sugars

Protected glycals and α‐1,2‐cyclopropanated sugars were converted in high yields and selectivities in less than 2 h at low temperatures to 2‐deoxy‐2‐iodoglycosyl acetates or novel 2‐deoxy‐2‐iodomethylglycosyl acetates using the simple, inexpensive reagent mixture of ammonium iodide, hydrogen peroxide, and acetic anhydride/acetic acid in acetonitrile. The protected glycals gave rise to 2‐deoxy‐2‐bromoglycosyl acetates when ammonium bromide was used instead of the iodide, although longer reaction times were required and selectivities were inferior. Other simple olefins such as styrene and indene were also converted to their corresponding 1,2‐trans‐iodoacetates.     

Catalytic cyclopropanation of polybutadienes

Catalytic cyclopropanation of commercial 1,2‐ or 1,4‐cis‐polybutadiene, respectively, with ethyl diazoacetate catalyzed by [Tp^Br3Cu(NCMe)] (Tp^Br3 = hydrotris(3,4,5‐tribromo‐1‐pyrazolyl)borate) at room temperature afforded high molecular weight (M_n > 10⁵ mol^?1) side‐chain or main‐chain, respectively, carboxyethyl cyclopropyl‐substituted polymers with variable and controlled degrees of functionalization. Complete functionalization of 1,4‐cis‐polybutadiene afforded poly[ethylene‐alt‐(3‐ethoxycarbonyl‐cyclopropene)]. Catalytic hydrogenation of residual double bonds of partially cyclopropanated polybutadienes provided access to the corresponding saturated polyolefins. Thermal properties are reported. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

Stipitatic acid: synthesis via cyclopropanated quinones

A new synthesis of stipitatic acid incorporating a cyclopropanated quinone intermediate is reported.     

Functionalization of single-walled carbon nanotubes via the Bingel reaction

Single-walled carbon nanotubes have been cyclopropanated under Bingel reaction conditions, and the functionalized nanotubes have been characterized by atomic force microscopy using "chemical tagging" techniques.     

Diastereoselective michael initiated ring closure on vinyl sulfone-modified carbohydrates: a stereospecific and general route to alpha-substituted cyclopropanes

Suitably designed vinyl sulfone-modified furanosides act as substrates for Michael initiated ring closure reactions yielding cyclopropanated carbohydrates. The strategy is general in nature and gives access to cyclopropanes with predefined chiralities on three consecutive carbons with varying substitutions at the alpha-position.     

Highly diastereoselective synthesis of spiro[cyclopropane-1,3′-indolin]-2′-ones via catalyst-free cyclopropanation using ethyl diazoacetate

Substituted 3-methyleneindolin-2-ones were efficiently cyclopropanated with ethyl diazoacetate to yield spiro[cyclopropane-1,3′-indolin]-2′-ones in a catalyst-free and highly diastereoselective reaction in a mixture of ethanol–acetonitrile (1:1 v/v) as solvent.     

Functionalized oxepines via fragmentation of highly strained epoxides

[Structure: see text] Epoxidation of highly strained cyclobutenes followed by thermal rearrangement provides a new entry into oxepine-containing bicyclo[5.3.0] ring systems. In contrast to the rearrangement of the corresponding cyclopropanated systems, the strained epoxides in this study are believed to fragment through two competing pathways leading to a mixture of diastereomeric 5-7 ring systems.     

Site and Stereoselectivity of the Cyclopropanation of Unsymmetrically Substituted 1,3-Dienes by the Simmonssmith Reaction. +

In the Simmons-Smith reaction, 1,3-dienes are preferentially cyclopropanated at the more electron-rich double bond to afford the trans-vinylcyclopropane; an allylic hydroxyl group increases the reactivity and directs the cyclopropanation to the adjacent double bond.

     

Synthesis and bioactivities of paclitaxel analogs with a cyclopropanated side-chain

Four paclitaxel analogs with a cyclopropanated side-chain were synthesized by coupling of the spirocyclopropanated oxazoline-5-carboxylic acid 4 with 7-TES-baccatin III, followed by hydrolytic ring opening and rearrangement. The absolute configuration of the 2′-position was determined by NMR analysis of the corresponding Mosher esters. The two new paclitaxel analogs with the R configuration at C-2′ were both active in the A2780 mammalian cell line cytotoxicity assay, but much less than paclitaxel itself.     

Trimetallic nitride endohedral metallofullerenes: reactivity dictated by the encapsulated metal cluster

The first derivatives of Y(3)N@C(80) have been synthesized and fully characterized. 1,3-Dipolar cycloaddition of N-ethylazomethine ylide yielded mainly the pyrrolidine monoadduct of the icosahedral (I(h)()) symmetry cage exclusively at a [6,6] double bond. The same regioselectivity on a [6,6] double bond was observed when the endohedral compound was cyclopropanated with diethyl bromomalonate. These results are in pronounced contrast to those observed for icosahedral symmetry Sc(3)N@C(80), for which all reported derivatives add completely regioselectively to [5,6] double bonds. (1)H NMR, (13)C NMR, and HMQC spectroscopy revealed that the addition pattern on Y(3)N@C(80) resulted in a pyrrolidinofullerene derivative with unsymmetric pyrrolidine carbons and symmetric geminal protons. The cyclopropanated monoadduct exhibited symmetric ethyl groups on the malonate, consistent with regioselective addition at a [6,6] double bond. Attempts to perform the same cyclopropanation reaction on (I(h)()) Sc(3)N@C(80) failed to yield any identifiable products. These observations clearly indicate that the reactivity of trimetallic nitride endohedral metallofullerenes toward exohedral chemical functionalization is profoundly affected and effectively controlled by the nature of the endohedral metal cluster.     

Unusual rearrangement in the reaction of cyclopropanated cyclopentadienes with Et3Al/CH2I2 in CH2Cl2

The reaction of Et₃Al/CH₂I₂ reagent with cyclopropanated spiro[2.4]hepta-4,6-diene and 6-substituted fulvenes in CH₂Cl₂ was found to lead to the selective formation of rearrangement products through sequential cyclopropyl-allyl rearrangement and cyclopropanation.     

Cyclopropyl-allyl isomerization of cyclopropanated norbornadiene dimers

Conclusions We have studied the thermal and boron trifluoride etherate catalyzed cyclopropyl-allyl isomerization of cyclopropanated norbornadine dimers leading to novel types of C₁₅ and C₁₆ carbocycles.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1083–1087, May, 1988.     

Mechanistic studies of rearrangements during the ring expansions of cyclopropanated carbohydrates

Deuterium-labeling studies have been performed on the ring expansion of cyclopropanated carbohydrates. From these studies, mechanisms have been proposed for two unusual rearrangements. In addition, a selective deprotection of the 1,3-di-tert-butylsilyl ether protecting group at the secondary position versus the primary position has been observed. This is a high yielding transformation with the potential for general synthetic utility.     

Reactions of diazoalkanes with unsaturated compounds 8. Catalytic cyclopropanation of allyl alcohols and allylamines with diazomethane

Allyl alcohols and allylamines have been cyclopropanated directly with diazomethane in the presence of palladium compounds to give 60–88% of cyclopropylmethanols and cyclopropylmethylamines, respectively, almost free from the products of formal insertion of methylene into the heteroatom-hydrogen bond.For previous communication, see [1].For preliminary report, see [2].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2752–2755, December, 19S9.     

Unusual oxidative free-radical additions of 1,3-dicarbonyl compounds to benzonorbornadiene and oxabenzonorbornadiene

Benzonorbornadiene and oxabenzonorbornadiene were reacted with dimedone and acetylacetone in the presence of Mn(OAc)₃ and Cu(OAc)₂. The reaction of benzonorbornadiene with dimedone gave the dihydrofuran addition product, whereas the reaction with acetylacetone produced, in addition to the dihydrofuran derivative, a rearranged product. On the other hand, oxanorbornadiene gave unusual products such as the cyclopropanated compound and the product arising from the addition of two moles of dimedone. The mechanism of formation of the products is discussed.