Synthesis of α-(aminoethyl)-α,ß-enones via alkyne aza-Prins cyclization and their synthetic application to pyrrolidines.

We developed a synthetic method for α-(aminoethyl)-α,ß-enones from aryl-substituted homopropargyl sulfonamides and aldehydes, representing the first synthesis of conjugated enones via alkyne aza-Prins cyclization. These products could be converted into pyrrolidines by a formal 5-endo-trig cyclization.

Stereodivergent Synthesis of Bispyrrolidinoindoline Alkaloidal Scaffolds and Generation of a Lead Candidate with Stereospecific Antiproliferative Activity.

The fungal secondary metabolites (+)-WIN 64821 and (-)-ditryptophenaline are biosynthesized through condensation of l-tryptophan and l-phenylalanine, followed by reductive dimerization with generation of stereochemical variations. Inspired by the stereodivergent biogenetic process, we designed and synthesized a collection of bispyrrolidinoindoline diketopiperazine alkaloids and their analogues with systematic diversification of the stereochemistry of the privileged structural motif of the fungal alkaloids. Not only the stereochemical modifications of (+)-WIN 64821 at the 3-/3'-, 11-/11'-, and 15-/15'-positions, but also ring cleavage of the diketopiperazine moieties, allowed the generation of a lead compound exhibiting potent growth inhibitory activity (IC50 =3.03 µm) toward human colon cancer cells. Structure-activity relationship studies revealed that all six stereogenic centers were essential for the pharmacophore. High cell densities dramatically intensified the cytotoxic activities of the lead compound.

Chiroptical Inversion of Europium(III) Complexes by Changing a Remote Stereogenic Center of a C2-Symmetric Bispyrrolidinoindoline Manifold.

As an effort to integrate natural products chemistry and coordination chemistry, a diastereomeric pair of chiral alkaloidal manifolds composed of a bispyrrolidinoindoline (BPI) framework was designed and synthesized to generate luminescent EuIII complexes with switchable chiroptical properties. The C2-symmetric alkaloidal manifolds were linked with bis(benzimidazolyl)pyridine (BBIPy) as an achiral metal-binding component through substituents installed at the stereogenic 2/2' positions of the BPI manifolds. The resulting diastereomeric pair of ligands, syn-L1 and anti-L2, allow pseudomirror symmetrical presentation of the metal-binding BBIPy units due to the stereogenic centers on the alkaloidal manifold. The ligand syn-L1 induces intramolecular coordination to form the 1:1 complex EuIII( syn-L1) composed of a single stranded metal helicate which exhibits a negative split Cotton effect. In contrast, the ligand anti-L2 led to a supramolecular assembly comprising the 2:2 complex EuIII2( anti-L2)2 consisting of a bimetallic double-stranded helicate which shows a positive split Cotton effect. Thus, the sp3 stereogenic centers in the BPI manifolds play pivotal roles in controlling both metal-ligand equilibria and chirality-switching of luminescent EuIII complexes. This approach, which exploits diastereomeric natural product-based manifolds, provides a relatively unexplored means for diversifying metal coordination modes and for controlling the chiroptical properties of the resultant luminescent lanthanoid complexes.

Peptide coupling using recyclable bicyclic benziodazolone.

We report a greener peptide coupling using bicyclic benziodazolone and triarylphosphine as coupling reagents. Bicyclic benziodazolone also works as a base and can be recovered as the corresponding iodine(I) compound after use, which can be converted to the original iodine(III) reagent by electrolytic oxidation.

Highly diastereoselective construction of acyclic systems with two adjacent quaternary stereocenters by allylation of ketones.

Unsymmetrical ketones and allyltitanocenes generated by the desulfurizative titanation of γ,γ-disubstituted allyl phenyl sulfides react under highly diastereoselective construction of adjacent quaternary stereocenters (see scheme; R(L) = large group, R(S) = small group). The title reaction is stereospecific: the anti- and syn-homoallylic alcohols are obtained by the reaction of E- and Z-allylic sulfides, respectively.

In Situ Generation of N-Triflylimino-λ3-iodanes: Application to Imidation of Phosphines and Catalytic α-Amidation of 1,3-Dicarbonyl Compounds.

We describe the imidation of phosphines and α-amidation of 1,3-dicarbonyl compounds using N-triflylimino-λ3-iodane, which is generated in situ from iodosylarene and triflylamide without any other additives. Furthermore, the imino-λ3-iodane catalytically generated from an iodoarene precatalyst with oxone and triflylamide promotes α-amidation of 1,3-dicarbonyl compounds, representing the first method catalyzed by imino-λ3-iodane.

Highly diastereoselective addition of allyltitanocenes to ketones.

A route with less congestion: A practical method for the highly diastereoselective preparation of anti tertiary homoallylic alcohols has been developed. The reaction of allyltitanocenes, generated by the reductive titanation of various allylic substrates with a titanocene(II) species, with a variety of ketones produced the anti tertiary homoallylic alcohols in good diastereoselectivity, even when using sterically less congested ketones (see scheme; Cp: cyclopentadienyl; Piv: pivaloyl).The reaction of allyltitanocenes, generated by the reductive titanation of allylic sulfides or allylic alcohol derivatives with a titanocene(II) species, with phenyl and sterically hindered alkyl methyl ketones produced anti tertiary homoallylic alcohols with complete diastereoselectivity. Even when sterically less congested methyl ethyl ketone and methyl vinyl ketone were employed, the anti homoallylic alcohols were obtained with unprecedented high diastereoselectivity. The observed anti selectivity suggests that the reaction proceeds by the addition of primary (E)-sigma-allyltitanocenes to ketones through chairlike cyclic transition states.

Regio- and stereoselective formation of conjugated dienes by titanocene(ii)-promoted alkylation of propargyl carbonates.

Conjugated dienes were produced with complete regio- and stereoselectivity by the titanocene(ii)-promoted alkylation of propargyl carbonates via the formation of 2,3,4-trisubstituted titanacyclobutenes.

Zn(OTf)2-mediated annulations of N-propargylated tetrahydrocarbolines: divergent synthesis of four distinct alkaloidal scaffolds.

Intramolecular hydroarylations of N-propargylated tetrahydrocarbolines were efficiently mediated using a unique combination of Zn(OTf)2 with t-BuOH under neutral conditions. Use of the artificial force induced reaction method in the global reaction route mapping strategy provided insights into the Zn(OTf)2-mediated hydroarylations and the associated intriguing solvent effects of t-BuOH facilitating a protodezincation process without a Brønsted acid activator. We systematically implemented three distinct hydroarylations as well as an unanticipated α-alkenylation of a carbonyl group to obtain the four alkaloidal scaffolds 2-4, and 18. Zn(OTf)2-mediated annulation of 1c proceeded through kinetic formation of the spiroindole 3c followed by an alkenyl shift and concomitant retro-Mannich-type fragmentation to furnish azepino[4,5-b]indole 2 framework. Substituents on substrate 1 in the vicinity of the reaction sites substantially affected the mode of the divergent annulations. Judicious choices of the substituents, solvent and reaction conditions enabled programmable divergent synthesis of the four distinct skeletons.

Formation of titanacyclobutenes with a spiro-bonded cyclopropane.

The reaction of titanium cyclopropylidene complexes, prepared by the reductive titanation of 7,7-dichlorobicyclo[4.1.0]heptanes, with alkynes produced air and moisture stable titanacyclobutenes with a spiro-bonded cyclopropane.

Titanocene(II)-promoted, one-pot, three-component coupling of thioacetals, alkynyl sulfones, and carbonyl compounds: highly stereoselective formation of tert-homopropargyl alcohols.

Titanocene alkylidene complexes, generated by desulfurizative titanation of thioacetals with Cp2Ti[P(OEt)3]2, reacted with alkynyl methyl sulfones to produce organotitanium species, which gave tert-homopropargyl alcohols with high diastereoselectivity on treatment with aromatic and alpha,beta-unsaturated ketones.

Nickel(0) triethyl phosphite complex-catalyzed allylic substitution with retention of regio- and stereochemistry.

Nickel(0) triethyl phosphite complex-promoted reaction of allylic acetates with thiols produced allylic sulfides with retention of configuration without allylic rearrangement. A similar reaction of allylic acetates with alcohols and phenols also proceeded with retention of regio- and stereochemistry.

Diversity-oriented stereoselective synthesis of ß,γ-disubstituted tert-homoallylic alcohols.

The successive treatment of ß-(trimethylsilyl)allyl phenyl sulfides with titanocene(II)-1-butene complex and ketones produced tertiary γ-(trimethylsilyl)homoallylic alcohols with good anti-selectivity, which reacted with a variety of organic halides in the presence of copper(I) tert-butoxide to afford the cross-coupling products, γ-substituted homoallylic alcohols.

Trialkylsilyl group-directed regioselective transformations of 2-(alk-1-yn-1-yl)-2-(trialkylsilyl)-1,3-dithianes to alkynylcyclopropanes and enynes.

[reaction: see text] The Cp(2)Ti[P(OEt)(3)](2)-promoted reaction of 2-(alk-1-yn-1-yl)-2-(trialkylsilyl)-1,3-dithianes with 1-alkenes regioselectively produced [(trialkylsilyl)ethynyl]cyclopropanes with a formal allylic rearrangement. The reaction of the thioacetals with ketones proceeded with the same regioselectivity to produce 1-(trialkylsilyl)alk-3-en-1-ynes predominantly. It is suggested that these reactions proceed via the formation of titanium alpha-(silylethynyl)carbene complexes Cp(2)Ti=C(R)CCSi in preference to their regioisomers, alpha-silylalkynylcarbene complexes Cp(2)Ti=C(Si)CCR.

First anionic silyl migration from sp2 carbon to carbonyl oxygen. Stereospecific allylation of (Z)-beta-trimethylsilyl-alpha,beta-unsaturated ketones.

1,4-Silyl migration from sp2 carbon to carbonyl oxygen proceeded by the treatment of (Z)-beta-trimethylsilyl-alpha,beta-unsaturated ketones with copper(I) tert-alkoxide to afford vinylmetal species, which reacted with allylic halides to produce enol trimethylsilyl ethers of beta-alk-2-enyl-alpha,beta-unsaturated ketones with complete retention of configuration.

Carbonyl olefination utilizing an alkyl halide-titanocene(II) system.

Ketones, aldehydes, esters and lactones are transformed into olefins by treatment with the organotitanium species formed from alkyl halides and titanocene(II).

Copper(I) 2-hydroxyethoxide-promoted cross-coupling of aryl- and alkenyldimethylsilanes with organic halides.

Fluoride-free cross-coupling of aryl- and alkenyldimethylsilanes with organic halides proceeded in the presence of monocopper(I) alkoxide of ethylene glycol.