Benzosiloles with Crystallization-Induced Emission Enhancement of Electrochemiluminescence: Synthesis, Electrochemistry, and Crystallography.

Crystallization-induced emission enhancement (CIEE) was demonstrated for the first time for electrochemilunimescence (ECL) with two new benzosiloles. Compared with their solution, the films of the two benzosiloles gave CIEE of 24 and 16 times. The mechanism of the CIEE-ECL was examined by spooling ECL spectroscopy, X-ray crystal structure analysis, photoluminescence, and DFT calculations. This CIEE-ECL system is a complement to the well-established aggregation-induced emission enhancement (AIEE) systems. Unique intermolecular interactions are noted in the crystalline chromophore. The first heterogeneous ECL system is established for organic compounds with highly hydrophobic properties.

Synthesis of Functionalized Tetrahydropyridines by SnCl4 -Mediated [4+2] Cycloaddition between Donor-Acceptor Cyclobutanes and Nitriles.

Cycloadditions of strained carbocycles promoted by Lewis acids are powerful methods to construct heterocyclic frameworks. In fact, the formal [3+2] cycloadditions of donor-acceptor (DA) cyclopropanes with nitriles has seen particular success in synthesis. In this work, we report on the first [4+2] cycloaddition of nitriles with DA cyclobutanes by Lewis acid activation. Tetrahydropyridine derivatives were obtained in up to 91 % yield from various aryl-activated cyclobutane diesters and aliphatic or aromatic nitriles.

Synthesis of Hexahydropyridazines by [4 + 2] Cycloaddition of Donor-Acceptor Cyclobutanes and cis-Diazenes.

The GaCl3-catalyzed [4 + 2] cycloaddition between alkoxy- and aryl-activated donor-acceptor cyclobutane diesters and cis-diazene 1a (4-phenyl-1,2,4-triazoline-3,5-dione, PTAD) is disclosed. The reaction provides hexahydropyridazine derivatives as single diastereomers in good to excellent yields in most cases. The structural assignment of the cycloadduct 3b was unambiguously established by single-crystal X-ray diffraction.

Total synthesis of (+/-)-goniomitine via a formal nitrile/donor-acceptor cyclopropane [3 + 2] cyclization.

The total synthesis of (+/-)-goniomitine has been accomplished in 17 linear steps with 5.2% overall yield starting from commercially available delta-valerolactam. A synthetic highlight includes the first application of a formal [3 + 2] cycloaddition between a highly functionalized nitrile and a donor-acceptor cyclopropane to prepare an indole nucleus. The use of a microwave reactor is shown to greatly improve the reaction times for two steps.

First total synthesis and structural reassignment of (-)-aplysiallene.

The first total synthesis of (-)-aplysiallene has been completed in 16 steps and features a key sequential Mukaiyama aerobic oxidative cyclization to prepare the fused bis-THF core. The original stereochemical assignment has been revised as shown.

Advances in Lewis acid controlled carbon-carbon bond-forming reactions enable a concise and convergent total synthesis of bullatacin.

Lewis acids control regioselectivity in the alkylation of epi-chlorohydrin and the stereochemistry of an alkyne addition to set the C24/C23 anti relationship. These advances facilitate an efficient total synthesis of bullatacin in 13.3% overall yield from commercial starting materials.

Cascade Reaction of Donor-Acceptor Cyclopropanes: Mechanistic Studies on Cycloadditions with Nitrosoarenes and cis-Diazenes.

Tandem ring opening, elimination, and cycloaddition of donor-acceptor cyclopropanes were observed in Yb(OTf)3-catalyzed cycloaddition with nitrosoarenes. The reaction results in formation of tetrahydro-1,2-oxazine instead of the normal cycloadduct isoxazolidine via in situ nitrone formation. A similar cascade sequence was observed with cis-diazines. Mechanistic studies on this unique transformation offer an entirely new approach for reaction design with donor-acceptor cyclopropanes.

Allylation of donor--acceptor cyclopropanes.

[reaction: see text] Direct allylation of glycal-derived donor-acceptor cyclopropanes has been achieved with TiCl(4) activation followed by addition of allyltrimethylsilane. The alpha diastereomer is the major product, with selectivities ranging from 3:1 to 10:1 and yields around 80%.

A powerful new strategy for diversity-oriented synthesis of pyrroles from donor-acceptor cyclopropanes and nitriles.

Lewis acid activated donor-acceptor cyclopropanes react with aliphatic, aromatic, and alpha,beta-unsaturated nitriles in a novel cascade [3 + 2] dipolar cycloaddition, dehydration, and tautomerization sequence to afford pyrroles in moderate to excellent overall yield. This cost-effective and regiospecific method is ideally suited for the preparation of combinatorial libraries. [reaction: see text]

A new class of substituted aryl bis(oxazoline) ligands for highly enantioselective copper-catalyzed asymmetric aldol addition of dienolsilane to pyruvate and glyoxylate esters.

[reaction: see text] A new class of bis(oxazoline) ligands are introduced that feature o-alkoxyaryl substituents and provide the highest enantioselectivities yet reported for the copper-catalyzed asymmetric dienosilane aldol addition to pyruvate and glyoxylate esters. Enantioselectivities up to 98% ee (before recrystallization) and isolated yields up to 91% were observed. Additionally, chloride counterions were found to be superior to triflate for this reaction.

Synthesis of the C(1)-C(12) segment of peloruside A by an alpha-benzyloxymethyl ketone aldol strategy.

The C(1)-C(12) segment of 16-membered antitumor macrolide peloruside A has been prepared by a BF(3).OEt(2)-catalyzed Mukaiyama aldol reaction between a glucose-derived C(1)-C(7) aldehyde and a C(8)-C(12) alpha-benzyloxymethyl ketone. Exclusive 2,3-anti and moderate 3,5-anti/syn facial selectivity (3.5:1) was observed in the aldol reaction. The key C(1)-C(7) aldehyde contains the required stereochemistry at carbons two, three, and five, and has been efficiently prepared on multigram scales from commercial triacetyl D-glucal. [reaction: see text]

Selective oxidation of zirconocyclopentenes via organoboranes.

[reaction: see text] A new, convenient, one-pot protocol is described for oxidation of zirconocyclopent-2-enes selectively at the sp(3) carbon by efficient transfer to electrophilic ((c)Hex)(2)BCl followed by oxidation with H(2)O(2)/NaOH to afford 1-alkylidene-2-hydroxymethylcyclopentanes. Results with several substrates show that overall reaction efficiencies for the zirconocene-mediated enyne cyclization, boron transmetalation, and oxidation sequence are generally comparable to yields obtained from protonation of intermediate zirconocycles. The formation of E/Z olefin isomers from the cyclization-oxidation sequence and an acid-catalyzed pinacol-type rearrangement of a vinylsilane are described.

Synthesis of 2,2'-bipyrroles and 2,2'-thienylpyrroles from donor-acceptor cyclopropanes and 2-cyanoheteroles.

[reaction: see text] Two new series of 2,2'-bipyrroles and 2,2'-thienylpyrroles have been prepared by trimethylsilyl trifluoromethanesulfonate (TMSOTf)-mediated reaction of donor-acceptor cyclopropanes with 2-cyanopyrroles and 2-cyanothiophene, respectively. This method opens the door toward a wide variety of unsymmetrical bipyrroles and thienylpyrroles.

Stereospecific and efficient alkynylation at the more hindered carbon of trisubstituted epoxides.

The stereospecific and regioselective C(3) alkynylation of trisubstituted epoxides has been achieved with lithium alkynyl trimethylaluminium ate complexes in the presence of BF3 x OEt2.

Mechanistic Insights into Cu-Catalyzed Asymmetric Aldol Reactions: Chemical and Spectroscopic Evidence for a Metalloenolate Intermediate.

In situ IR spectroscopy and transmetalation experiments confirm a postulated catalytic cycle. The metalloenolate 1 describes the active intermediate in the aldol reaction catalyzed by [CuF2 {(S)-tol-binap}] (see reaction scheme). (S)-tol-binap=(S)-(-)-2,2'-bis(di-p-tolylphosphanyl)-1,1'-binaphthyl.

The [4+2] cycloaddition of donor-acceptor cyclobutanes and nitrosoarenes.

The Yb(OTf)3 catalyzed [4+2] cycloaddition between donor-acceptor cyclobutanes and nitrosoarenes is disclosed. This method facilitates the synthesis of tetrahydro-1,2-oxazines in good to excellent yields as single diastereomers. Except for a few electron-deficient nitrosoarenes, excellent regioselectivity was observed throughout these studies.

Gram scale synthesis of the C(18)-C(34) fragment of amphidinolide C.

The synthesis of the C(18)-C(34) fragment of amphidinolide C has been achieved via two routes, culminating in both the shortest (11 steps) and highest yielding (26% overall yield) approaches to this segment. The highly convergent approach will facilitate the synthesis of analogues, including the C(18)-C(29) fragment of amphidinolide F. Synthetic highlights include the selective methylation of a diyne, and the highly efficient use of a second generation cobalt catalyst in the Mukaiyama oxidative cyclization to form the trans-THF ring.

The formal [4+3] cycloaddition between donor-acceptor cyclobutanes and nitrones.

The formal [4+3] cycloaddition of 2-alkoxy-1,1-dicarboxylate activated donor-acceptor cyclobutanes with nitrones is disclosed. The reaction forms structurally unique oxazepines in moderate to high yield with a wide scope of nitrones. In most cases either a diastereomeric mixture or a single diastereomer may be formed, depending on the reaction conditions.

Diels-Alder chemistry of siloles and their transformation into cyclohex-2-ene-1,4-cis-diols.

The synthesis of siloles with substitution patterns that are continuative toward natural product synthesis are described. Their reactivity in Diels-Alder chemistry was explored through thermal, Lewis acid, and high-pressure reactions. Furthermore, bicyclic adducts were oxidatively cleaved to reveal a highly functionalized cyclohexene core.