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

Stereodivergent Palladium- and Rhodium-Catalyzed Intramolecular Addition of Tosylureas to Allenes: Diastereoselective Synthesis of Tetrahydropyrimidinones

The intramolecular addition of tosylureas to allenes is highly syn-/anti-diastereoselective when employing a palladium or rhodium-based catalytic system and affords 1,3-cyclic ureas. Under palladium catalysis a range of thermodynamic anti-tetrahydropyrimidinones are accessible, while rhodium catalysis allows synthesis of the kinetic syn-tetrahydropyrimidinones. For a representative scope of substrates both cyclic ureas were obtained in excellent yields and diastereoselectivities. The obtained tetrahydropyrimidinones were shown to be easily deprotected and modified to demonstrate the synthetic value.

O2 -Assisted Four-Component Reaction of Vinyl Magnesium Bromide with Chiral N-tert-Butanesulfinyl Imines To Form syn-1,3-Amino Alcohols

An O₂ -assisted, four-component reaction has been developed to synthesize a wide range of syn-1,3-amino alcohols in one step. The reaction proceeds by oxygenation of vinyl magnesium bromide (component-I) with O₂ (component-II) to give a magnesium enolate of acetaldehyde, which undergoes addition to a chiral N-tert-butanesulfinyl imine (component-III) followed by a sequential addition with excess vinyl magnesium bromide (component-IV). The approach allows diastereoselective synthesis of anti/syn- and syn/syn-3-amino-1,5-diols in good yields with high diastereoselectivity. The method was illustrated in an efficient, four-step synthesis of piperidine alkaloid (-)-2'-epi-ethylnorlobelol.

Addition of Chloroprene Grignards to Aromatic Aldehydes: Synthesis of Homoallenyl Alcohols

A general procedure for the one-pot synthesis of racemic homoallenyl alcohols from the corresponding aldehyde and chloroprene-derived Grignards is described. Employing bis[2-dimethylaminoethyl]ether (BDMAEE) as an additive at low temperatures shifts the selectivity of the chloroprene Grignard addition to aldehydes such that it is almost exclusive toward allene formation. In a set of follow-up experiments, simple and more elaborate methods for further derivatization have been demonstrated, allowing quick access to more complex structures.

Rhodium-Catalyzed Cyclization of Terminal and Internal Allenols: An Atom Economic and Highly Stereoselective Access Towards Tetrahydropyrans

A comprehensive study of a diastereoselective Rh-catalyzed cyclization of terminal and internal allenols is reported. The methodology allows the atom economic and highly syn-selective access to synthetically important 2,4-disubstituted and 2,4,6-trisubstituted tetrahydropyrans (THP). Furthermore, its utility and versatility are demonstrated by a great functional-group compatibility and the enantioselective total synthesis of (-)-centrolobine.

Rhodium-Catalyzed Diastereo- and Enantioselective Tandem Spirocyclization/Reduction of 3-Allenylindoles: Access to Functionalized Vinylic Spiroindolines

A highly selective rhodium-catalyzed tandem spirocyclization/reduction of 3-allenylindoles is reported. By employing a Hantzsch ester as reductant, vinylic spiroindolines are obtained in excellent yields as well as diastereo- and enantioselectivity. In addition, the reaction's synthetic utility is highlighted by broad functional group compatibility and exemplified by a gram scale reaction with subsequent assorted transformations.

Rhodium-Catalyzed Enantioselective Cyclization of 3-Allenyl-indoles: Access to Functionalized Tetrahydrocarbazoles

A highly selective rhodium-catalyzed cyclization of tethered 3-allenylindoles is reported. In a smooth reaction, 1-vinyltetrahydrocarbazoles are obtained in excellent yields and enantioselectivities. Aside from a great functional group tolerance, this method requires neither the Schlenk technique nor the use of anhydrous solvents. Preliminary mechanistic investigations proved that the reaction proceeds via an intermediary formed spiroindolenine which rapidly undergoes an acid-catalyzed stereospecific migration.

Chemo-, Regio-, and Enantioselective Rhodium-Catalyzed Allylation of Pyridazinones with Terminal Allenes

The rhodium-catalyzed addition of pyridazinones to terminal allenes furnished the corresponding branched N²-allylated products in good yields with high regio- and enantioselectivities. A broad functional group compatibility was observed, and assorted synthetic transformations of the N-allylpyridazinones led to the preparation of a small library of N²-functionalized pyridazinones. Labeling experiments with deuterated substrates provided insights into the underlying reaction mechanism.