ZnI2‐promoted homologation of ferrocenylacetylene and aldehydes: an efficient synthesis of ferrocene‐containing allenes

A concise and efficient protocol for the first synthesis of ferrocene‐containing allenes based on the homologation reaction of ferrocenylacetylene and aldehydes promoted by ZnI₂ was developed. The present method was applicable to many kinds of substituted aldehydes, providing good to excellent yields of ferrocene‐containing allenes. Copyright © 2012 John Wiley & Sons, Ltd.     

Rhodium‐Catalyzed [(3+2)+2] Carbocyclization of Alkynylidenecyclopropanes with Substituted Allenes: Stereoselective Construction of Tri‐ and Tetrasubstituted Exocyclic Olefins

The development of the stereoselective rhodium‐catalyzed [(3+2)+2] carbocyclization of alkynylidenecyclopropanes (ACPs) with substituted allenes is described. This work demonstrates that activated and unactivated allenes preferentially undergo carbometalation at the distal terminus to generate tri‐ and tetrasubstituted exocyclic olefins with a neutral rhodium catalyst. In addition, this method provides a strategy for the total synthesis of the guaiane family of sesquiterpenes, which are not directly accessible using alkynes as exogenous π‐components. Finally, the preparation of the bicyclo[5.4.0]undecane ring system using a homologated ACP tether serves to further illustrate the versatility of this approach.     

Chemo‐ and Regioselective Rhodium(I)‐Catalyzed [2+2+2] Cycloaddition of Allenynes with Alkynes

A highly chemo‐ and regioselective partially intramolecular rhodium(I)‐catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.     

Rhodium‐Catalyzed Enantioselective Intermolecular Hydroalkoxylation of Allenes and Alkynes with Alcohols: Synthesis of Branched Allylic Ethers

Regio‐ and enantioselective additions of alcohols to either terminal allenes or internal alkynes provides access to allylic ethers by using a Rh^I/diphenyl phosphate catalytic system. This method provides an atom‐economic way to obtain chiral aliphatic and aryl allylic ethers in moderate to good yield with good to excellent enantioselectivities.     

RhI‐Catalyzed Bimolecular Cyclization between Two Different 1,5‐Bisallenes: A Combinatorial One‐Step Approach to Heterosteroids and Mechanistic Implications

In this paper, a bimolecular‐cyclization reaction between two different bis(allene)s with at least one heteroatom as the tether under the catalysis of trans‐[RhCl(CO)(PPh₃)₂] is described. This protocol provides an efficient entry to different heterocyclic 18,19‐norsteroid‐like scaffolds. The tricyclic product was formed highly selectively from the cyclization reaction of bis(2,3‐butadienyl)sulfide with dimethyl 2‐bis(2′,3′‐butadienyl)malonate, which sheds light on the mechanism involving the metalla‐[4.3.0]‐bicyclic intermediate formed by the cyclometallation of the terminal and the internal C=C bonds of each of the two allene moieties in 2‐bis(2′,3′‐butadienyl)malonate.     

Metal Alkoxide Promoted Regio‐ and Stereoselective CO and CC Metathesis of Allenoates with Aldehydes

The reaction of 2,3‐allenoates and aldehydes in the presence of an alkoxide affords alkyl 4,5‐diaryl‐3‐oxo‐2‐propylpent‐4(E)‐enoates and cis‐3,4‐diaryloxetanes through a formal C?O and C?C metathesis. A mechanism for this reaction has been proposed.     

Rhodium‐Catalyzed Diastereoselective Cyclization of Allenyl‐Sulfonylcarbamates: A Stereodivergent Approach to 1,3‐Aminoalcohol Derivatives

A diastereoselective and stereodivergent rhodium‐catalyzed intramolecular coupling of sulfonylcarbamates with terminal allenes is described and it provides selective access to 1,3‐aminoalcohol derivatives, scaffolds found in bioactive compounds. The reaction is compatible with a large range of different functional groups, thus furnishing products with high diastereoselectivities and yields. Moreover, multigram scale reactions, as well as the application of suitable product transformations were demonstrated.     

Stereocontrolled Syntheses of Seven‐Membered Carbocycles by Tandem Allene Aziridination/[4+3] Reaction

A tandem allene aziridination/[4+3]/reduction sequence converts simple homoallenic sulfamates into densely functionalized aminated cycloheptenes, where the relative stereochemistry at five contiguous asymmetric centers can be controlled through the choice of the solvent and the reductant. The products resulting from this chemistry can be readily transformed into complex molecular scaffolds which contain up to seven contiguous stereocenters.     

Catalyst‐Free Dehydrative α‐Alkylation of Ketones with Alcohols: Green and Selective Autocatalyzed Synthesis of Alcohols and Ketones

Direct dehydrative α‐alkylation reactions of ketones with alcohols are now realized under simple, practical, and green conditions without using external catalysts. These catalyst‐free autocatalyzed alkylation methods can efficiently afford useful alkylated ketone or alcohol products in a one‐pot manner and on a large scale by C?C bond formation of the in situ generated intermediates with subsequent controllable and selective Meerwein–Pondorf–Verley–Oppenauer‐type redox processes.     

Iodoindazoles with Selective Magnesiation at Position 3: A Route to Highly Functionalized Indazoles

A unique route to highly functionalized indazoles is described. A regioselective magnesiation at position 3 of 4‐, 5‐, 6‐ and 7‐iodo‐2‐THP‐indazoles (THP=tetrahydropyranyl) has been developed using TMPMgCl ? LiCl (TMP=2,2,6,6‐tetramethylpiperidyl). The obtained magnesiate can be trapped by different electrophiles to introduce a wide range of functional groups including halogens, thioalkyls, alcohols, aldehydes, ketones, amides, or esters at position 3. Once this position is functionalized, the iodine atoms can be further reacted through metal–halogen exchange or cross‐coupling strategies. Finally, N‐substitution reactions allow the synthesis of a variety of highly functionalized indazoles giving access to these valuable scaffolds through a simple and unique route.