On the basis of Woodward-Hoffmann frontier molecular orbital interactions and steric interactions between dienes and dienophiles during the formation of [2+4] pericyclic transition states, endo/exo selectivity in the Diels-Alder reaction strongly depends on the substrates.[1] Therefore, it is quite difficult to control both enantioselectivity [2] and anomalous endo/exo selectivity by conventional chiral catalysts, which can discriminate only the enantiofaces of the dienophiles. For example, in the reaction between cyclopentadiene (1) and acrolein (2 a), an endo preference is observed with regard to second-order orbital interactions without significant steric interactions [Eq. (1)]. In sharp contrast, in the reaction between 1 and an a-substituted acrolein (R ¼ 6 H), such as methacrolein (2 b), an exo preference is observed with regard to steric interactions between the methylene moiety of 1 and the substituent R at the a position of the dienophile [Eq. (2)]. Therefore, enantiomerically enriched endo-3 a and exo-3 b have been synthesized by using many conventional chiral catalysts.[2] Moreover, thermodynamically more stable and enantiomerically enriched exo-3 a can be generated by the epimerization of endo-3 a [Eq. (1)]. Alternatively, catalystinduced anomalous exo-selective Diels-Alder reactions that contravene the endo rule have been performed by Yamamoto and co-workers [3] in a non-asymmetric manner, and later by Maruoka and co-workers, [4] Sibi et al., [5] and Hayashi et al. [6] in an asymmetric manner. In contrast, enantiomerically enriched endo-3 b with a quaternary carbon center can not be generated by the epimerization of exo-3 b or by other known synthetic methods [Eq. (2)]. To the best of our knowledge, no examples of catalyst-induced anomalous endo-selective enantioselective Diels-Alder reactions with a-substituted acroleins have been reported to date. To address this major yet unexplored subject, catalysts must discriminate chiral transition-state structures by precisely recognizing the re or si face of dienophiles, and the endo or exo approach of dienes, thus, the rational design of conformationally flexible chiral supramolecular catalysts, such as enzymes, is necessary. [7] As such, conformationally rigid metal-organic frameworks (MOFs) are not suitable as artificial enzymes because they have few induced-fit properties to adapt dynamics in transition states. [8] A chiral supramolecular catalyst (4 a) was readily prepared in situ from three components, which included 10 mol % of chiral (R)-3,3'-bis(5,5-dimethyl-2-oxido-1,3,2-dioxaphosphorinan-2-yl)-BINOL (5 a; BINOL = 1,1'-bi(2-naphthol)), [9] 10 mol % of 3,5-bis(trifluoromethyl)phenylboronic acid (6 a), and 20 mol % of tris(pentafluorophenyl)borane (7), by taking advantage of the typical preparation of boron BINOLates [10] (Scheme 1). Intermolecular acid-base coordinate bonds in the two P=O···B(C 6 F 5 ) 3 moieties [11] are critical for the design of conformationally flexible complex 4 a; compound 7 acts as a bulky functional group to form a ch...

BBr3-chiral phosphoric acid complexes are highly effective and practical Lewis acid-assisted Brønsted acid (LBA) catalysts for promoting the enantioselective Diels-Alder (DA) reaction of α-substituted acroleins and α-CF3 acrylate. In particular, the DA reaction of α-substituted acroleins with 1,2-dihydropyridines gave the corresponding optically active isoquinuclidines with high enantioselectivities. Moreover, transformations to the key intermediates of indole alkaloids, catharanthine and allocatharanthine, are demonstrated.

A catalytic enantioselective three-component Mannich-type reaction of alkenyl trichloroacetates, ethyl glyoxalate, and aniline derivatives was achieved using an (S)-BINOL-derived chiral tin dibromide possessing a 4-trifluoromethylphenyl group at the 3- and 3'-positions as the chiral precatalyst in the presence of sodium ethoxide, sodium iodide, and ethanol. Optically active beta-amino ketones with up to 98% ee were syn-selectively obtained in high yields even from imines possessing a polar amino group under the influence of the in situ generated chiral tin bromide ethoxide.

A chiral Brønsted acid containing two different acidic sites, chiral carboxylic acid-monophosphoric acid 1a, was designed to be a new and effective concept in catalytic asymmetric hetero-Diels-Alder reactions of azopyridinecarboxylate with amidodienes. The multipoint hydrogen-bonding interactions among the carboxylic acid, monophosphoric acid, azopyridinecarboxylate, and amidodiene achieved high catalytic and chiral efficiency, producing substituted 1,2,3,6-tetrahydropyridazines with excellent stereocontrol in a single step. This constitutes the first example of regio-, diastereo-, and enantioselective azo-hetero-Diels-Alder reactions by chiral Brønsted acid catalysis.

A s y m m e t r i c C y c l o a d d i t i o n o f A l k e n y l T r i c h l o r o a c e t a t e s w i t h N i t r o n e sAbstract: A catalytic enantioselective cycloaddition reaction of alkenyl trichloroacetates with nitrones was achieved using an (S)-BINOL-derived chiral tin dibromide possessing a 4-trifluoromethylphenyl group at 3-and 3¢-positions as the chiral precatalyst in the presence of sodium ethoxide, sodium iodide, and ethanol. Optically active isoxazolidines with up to 95% ee were diastereoselectively obtained in high yields even from aliphatic aldehyde derived nitrones under the influence of the in situ generated chiral tin ethoxide iodide.Asymmetric 1,3-dipolar cycloaddition is a convenient method for preparing nonracemic heterocyclic compounds. 1 A representative example is the reaction between nitrones and alkenes to provide chiral isoxazolidines, which can be further transformed by reductive N-O bond cleavage into optically active g-amino alcohols that serve as useful synthetic intermediates for natural products or pharmaceuticals. Since the first report of the catalytic enantioselective 1,3-dipolar cycloaddition of nitrones by Gothelf and Jørgensen in 1994, 2 various chiral catalysts, including chiral Lewis acid catalysts 3-5 and chiral organocatalysts, 6 have been developed for the asymmetric transformation; however, most of those protocols use electrondeficient olefins as dipolarophiles and as far as we know, there have been only very few examples on the catalytic reaction of electron-rich alkenes to give nonracemic isoxazolidines. 7 We report here a novel example of the enantioselective cycloaddition reaction between alkenyl trichloroacetates and nitrones catalyzed by a chiral tin ethoxide iodide (Scheme 1).We have previously reported an asymmetric Mannichtype reaction 8 of imines with alkenyl trichloroacetates, which are converted into chiral tin enolates in the presence of a catalytic amount of a chiral tin alkoxide. The chiral tin catalyst is efficiently recycled with the assistance of an alcohol. This addition reaction gives the corresponding optically active b-amino carbonyl compounds with syn-selectivity. We envisioned that if a nitrone was used instead of an imine, the electrophile would also undergo the aforementioned chiral tin catalyzed syn-selective asymmetric Mannich-type reaction to provide enantiomerically enriched b-hydroxyamino carbonyl compounds. Thus, we attempted to react cyclohexanonederived alkenyl trichloroacetate 1a with nitrone 2a employing chiral tin dibromide 4d and sodium ethoxide as catalysts and as a result, unanticipated cycloadduct 3aa was obtained with significant asymmetric induction. For example, when a mixture of 1a (2 equiv) and 2a (1 equiv) was treated with chiral tin dibromide 4d (5 mol%) and NaOEt (5 mol%) in the presence of EtOH (10 equiv) in THF at 60 °C for 30 minutes, a 71:29 diastereomeric mixture of 3aa was obtained in 32% yield (Table 1, entry 1). The major diastereomer of 3aa had 77% ee. Then, we tested some additives and as a result, found that ...

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Single-crystalline organic semiconductors exhibiting band transport have opened new possibilities for the utilization of efficient charge carrier conduction in organic electronic devices. The epitaxial growth of molecular materials is a promising route for the realization of wellcrystallized organic semiconductor p−n junctions for optoelectronic applications enhanced by the improved charge carrier mobility. In this study, the formation of a high-quality crystalline interface upon "quasi-homoepitaxial" growth of bis(trifluoromethyl)dimethylrubrene (fmRub) on the singlecrystal surface of rubrene was revealed by using out-of-plane and grazing-incidence X-ray diffraction techniques. Ultraviolet photoelectron spectroscopy results indicated abrupt electronic energy levels and the occurrence of band bending across this quasi-homoepitaxial interface. This study verifies that the minimization of the lattice mismatch enhances the crystalline qualities at the heterojunctions even for van der Waals molecular condensates, potentially opening an untested route for the realization of high-mobility organic semiconductor optoelectronics.

The first example of intermolecular/intramolecular sequential aldol reaction of disilyl enol ethers is described. This strategy enables the formation of five-, six-, and seven-membered ring products. Four or more contiguous stereogenic centers are created with high levels of relative stereochemical control.