Organocatalyzed Cyclopropanation of α-Substituted α,β-Unsaturated Aldehydes: Enantioselective Synthesis of Cyclopropanes Bearing a Chiral Quaternary Center

Sulfoxonium Ylides in Aminocatalysis: An Enantioselective Entry to Cyclopropane-Fused Chromanol Structures

The 1,1a,2,7b-tetrahydrocyclopropa[c]chromene, arising from fusion of chromane and cyclopropane rings is the core of medicinally relevant compounds. Engaging sulfoxonium ylides in enantioselective aminocatalytic reactions for the first time, a convenient entry to this scaffold is presented. Several ring-fused derivatives were obtained in moderate-to-good yields and enantioselectivities and with perfect diastereoselectivity at the cyclopropane, using an α,α-diphenylprolinol aminocatalyst. The versatility of the hemiacetal moiety in the products was leveraged to effect various synthetic manipulations.

Functionalized Cyclopropanes as Versatile Intermediates for the Diversity-Oriented Synthesis of γ-Lactones, γ-Lactams and δ-Lactams

A two-step procedure for the preparation of cyclopropanecarboxaldehyde-1,1-diester from a γ,δ-epoxyester and its synthetic versatility are described herein. The epoxide ring-opening/cyclopropanation process occurs in the presence of Mg(ClO4)2 under heating, resulting in cyclopropanemethanol-1,1-diester in 65% yield. A mild TEMPO-mediated oxidation of this substrate readily generated the corresponding aldehyde in 75% yield, which was applied in the one-pot synthesis of four cyclopropylidene-γ-lactams and three δ-lactams. In addition, vinylcyclopropanes were obtained through the Wittig reaction of the aldehyde with phosphonium salts and used as precursors for tetrahydrofurans.

Copper-Catalyzed Ring Expansion of Vinyl Aziridines under Mild Conditions

Vinyl aziridines are versatile starting materials toward ring-expansion transformations. Such processes are widely used to give various medium or larger N-heterocycles of synthetic interest. This letter describes a copper-catalyzed ring expansion of vinyl aziridines to 3-pyrrolines by using a Cu(I) salt [(CuOTf)2·toluene or Cu(MeCN)4·PF6]. In particular, this transformation occurs under mild conditions (THF, rt).

An Organocatalytic Cope Rearrangement

The first example of an organocatalytic Cope rearrangement is reported. Acyclic and cyclic acyl hydrazides catalyze the rearrangement of 1,5-hexadiene-2-carboxaldehydes via iminium ion formation. A correlation between ring size and catalyst activity was observed for the cyclic hydrazides, with seven- and eight-membered-ring catalysts being the most active. Diazepane carboxylate 5 c (10 mol %) catalyzed the rearrangement of a range of dienes at room temperature in acetonitrile using triflic acid as a co-catalyst. Preliminary proof of principle for asymmetric catalysis was provided by rearrangement of 3,3-dimethyl-7-phenyl-1,5-heptadiene-2-carboxaldehyde in the presence of a novel 7-substituted diazepane carboxylate.

Asymmetric synthesis of spiro[chroman-3,3′-pyrazol] scaffolds with an all-carbon quaternary stereocenter via a oxa-Michael–Michael cascade strategy with bifunctional amine-thiourea organocatalysts

The chiral spiro[chroman-3,3′-pyrazol] derivatives have been synthesized through a catalytic oxa-Michael–Michael cascade reaction of 2-hydroxynitrostyrenes with 4-alkenyl pyrazolin-3-ones in good yields with moderate to high stereoselectivities.

Chiral primary-amine-catalyzed conjugate addition to α-substituted vinyl ketones/aldehydes: divergent stereocontrol modes on enamine protonation

Enantioselective protonation with a catalytic enamine intermediate represents a challenging, yet fundamentally important process for the synthesis of α-chiral carbonyls. We describe herein chiral primary-amine-catalyzed conjugate additions of indoles to both α-substituted acroleins and vinyl ketones. These reactions feature enamine protonation as the stereogenic step. A simple primary-tertiary vicinal diamine 1 with trifluoromethanesulfonic acid (TfOH) was found to enable both of the reactions of acroleins and vinyl ketones with good activity and high enantioselectivity. Detailed mechanistic studies reveal that these reactions are rate-limiting in iminium formation and they all involve a uniform H2 O/acid-bridged proton transfer in the stereogenic steps but divergent stereocontrol modes for the protonation stereoselectivity. For the reactions of α-branched acroleins, facial selections on H2 O-bridged protonation determine the enantioselectivity, which is enhanced by an OH⋅⋅⋅π interaction with indole as uncovered by DFT calculations. On the other hand, the stereoselectivity of the reactions with vinyl ketones is controlled according to the Curtin-Hammett principle in the CC bond-formation step, which precedes a highly stereospecific enamine protonation.

An efficient organocatalytic enantioselective synthesis of spironitrocyclopropanes

An organocatalytic asymmetric synthesis of spironitrocyclopropanes has been demonstrated starting from 2-arylidene-1,3-indandiones and bromonitroalkanes catalyzed by a cinchona-derived bifunctional organocatalyst. The products were obtained with excellent enantioselectivities, diastereoselectivities and with good yields.