‘Locking’ dendrimer conformation through metal coordination

Chiral intramolecularly hydrogen-bonded dendrons that fold into a dynamically biased helical conformation can be ‘locked’ in kinetically controlled conformations by coordination to copper(II) metal centers. Covalent bonding of each pyridine-2,6-dicarboxamide repeat unit of the dendrons to a copper metal center forces this subunit to exist in only the syn-syn conformation and dramatically rigidifies the dendron structure.     

Stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol

A stereoselective total synthesis of 4-((3S,5R)-3,5-dihydroxynonadecyl)phenol has been accomplished in two different synthetic approaches. In the first approach, Prins cyclization has been successfully utilized to produce the anti-1,3-diol unit, which was further converted into a required syn-1,3-diol through Mitsunobu reaction. The side chain was constructed through cross metathesis and hydrogenation sequence. In the second approach, the chiral syn-1,3-diol was prepared by a sequence of reactions such as alkylation of 1,3-dithane with (R)-epichlorohydrin, ring opening of the epoxide with vinylmagnesium bromide, and 1,3-syn-reduction of the β-hydroxyketone with NaBH₄ in the presence of diethylmethoxyborane.     

Stereoselective synthesis of versatile 2-chloromercurium-3,5-syn-dihydroxy esters via intramolecular oxymercuration

Regio- and stereocontrolled alkoxy mercuration of α,β-unsaturated esters allows direct access to 1,3-syn diols in good yields. Demercuration of adducts leads to 1,3 skipped dihydroxy esters, alcohols, and α-halo esters. The deprotection of acetonide with Amberlyst 15 on 1,3-syn-dihydroxy esters gives the corresponding δ-lactones.     

First report of syn isomers in the diastereoselective synthesis of highly functionalized piperidines catalysed by wet picric acid: factors influencing the syn-anti ratios

This is the first report of the formation of a syn-diastereoisomer in the diastereoselective synthesis of highly functionalized piperidines catalysed by wet picric acid via a one-pot, step-economic condensation of aromatic aldehydes, 1,3-dicarbonyl compounds and aromatic amines. DFT calculation shows that anti isomers are more stable compared to the syn. The syn-anti ratio of the products depends on the temperature and the nature of substitution on both the aromatic aldehydes and aromatic amines. Conditions for getting pure anti and pure syn compounds have been studied.     

Development of a Double Allylboration Reagent Targeting 1,5-syn-(E)-Diols: Application to the Synthesis of the C(23)-C(40) Fragment of Tetrafibricin

Interest in the synthesis of the C(23)-C(40) fragment 2 of tetrafibricin prompted us to develop a new method for the synthesis of 1,5-syn-(E)-diols. Toward this end, the kinetically controlled hydroboration of allenes 6, 33, ent-39, 42, and 45 with the Soderquist borane 25R were studied. Tetrabutylammonium allenyltrifluoroborate 45 gave superior results and was utilized in a double allylboration sequence with two different aldehydes to provide the targeted 1,5-syn-(E)-diols in generally high yields (72-98%), and with high enantioselectivity (>95% ee), diastereoselectivity (dr >20:1), and (E)/(Z) selectivity (>20:1). This new method was applied to the synthesis of the C(23)-C(40) fragment 2 of tetrafibricin.     

Diastereoselective synthesis of syn-aminoalcohols via contributing CH-π interaction: simple synthesis of (−)-bestatin

¹H NMR and X-ray crystallography studies revealed that a CH-π and chelation control in aromatic aminoaldehydes (1-6) effects a highly diastereoselective addition to afford optically active syn-aminoalcohols (1a-6a). This methodology was applied to the synthesis of (−)-bestatin.     

Preparation of isoxazol(in)yl substituted selenides and their further deselenenylation reaction to synthesize 3,5-disubstituted isoxazoles

We report a mild 1,3-dipolar cycloaddition protocol for the preparation of 3-aryl-5-phenylselenomethyl isoxazoles and isoxazolines regioselectively. The former was further reacted with LDA and electrophilic substrates followed by selenoxide syn-elimination to afford 3-aryl-5-E-substituted-ethenyl isoxazoles stereoselectively and the latter was subjected to a ‘two-step’ elimination to afford 3-aryl-5-methyl isoxazoles.     

A new, enantioselective synthesis of (+)-isolaurepan

A highly enantioselective synthesis of 2,6-syn-disubstituted tetrahydropyrans from commercially available tri-O-acetyl-d-glucal, based on a thermal Claisen rearrangement, allows enantioselective synthesis of (+)-isolaurepan when combined with a ring expansion reaction using trimethylsilyldiazomethane.     

Cyclic-tri(N-methyl-meta-benzamide)s: substituent effects on the bowl-shaped conformation in the crystal and solution states

Cyclic trimers of 3-(N-alkylamino)benzoic acid (calix[3]amides) with various substituents at the meta position of the phenyl rings were synthesized and the effects of the substituents on the crystal structures and energy profiles in solution were examined. The calixamides existed in a syn conformation in the crystal state, and this was also the major conformation in solution, especially in polar solvents. The energy barrier between syn and anti conformers in the solution was not significantly affected by substituents (12.7-14.0 kcal/mol). The effect of the substituent on the temperature dependence of the syn/anti ratio are discussed.     

nπ-ct-π* Transitions: nπ* Transitions of the second kind

Two important classes of molecules, maleimides and syn-bimanes [syn-bicyclo[3.3.0]octadienediones], are shown to exhibit charge-transfer absorptions and emissions, mainly on the basis of solvent effects on emissions. The electronic excitation originates mainly in a “non-bonding” p-orbital substantially localized on N, and oriented parallel to the π-system, hence n_π. The label, n_π-ct→π*, is proposed for these transitions. For the first time, two n_π-ct→π* transitions for a molecule containing a pair of adjacent nitrogens adjacent to a π-acceptor system are reported.     

(S)-Threonine/α,α-(S)-diphenylvalinol-derived chiral ionic liquid: an immobilized organocatalyst for asymmetric syn-aldol reactions

Chiral ionic liquids containing (S)- or (R)-threonine amide and α,α-(S)-diphenylvalinol units were synthesized In the presence of the (S)-threonine-derived catalyst reactions between ketones with secondary carbon atom(s) at the α-position with respect to the carbonyl group and aromatic (heteroaromatic) aldehydes afforded the corresponding syn-aldols in high yields (up to 99%) and with high diastereo-(syn/anti up to 97:3) and enantioselectivity (up to 99% ee), which was maintained over three reaction cycles.     

Synthesis of enantiopure β-amino alcohols via AKR/ARO of epoxides using recyclable macrocyclic Cr(III) salen complexes

A series of chiral macrocyclic Cr(III) salen complexes 1-8 were synthesized and characterized. These complexes were found to be highly active, regio-, diastereo-, and enantioselective catalysts in aminolytic kinetic resolution (AKR) of racemic trans-epoxides as well as asymmetric ring opening (ARO) of prochiral meso-epoxides with various anilines as nucleophiles at room temperature in 18-24 h. Excellent yields (>99% with respect to the nucleophile) with high enantioselectivity (ee, >99%) of chiral anti-β-amino alcohols was achieved with concomitant recovery of corresponding epoxides in high ee (up to >99%). The complex 1 also catalyzed the ARO of meso-epoxides to provide corresponding syn-β-amino alcohols in high yield (99%) and ee (up to 91%). Due to built-in basic sites in the catalyst, no external base (as an additive) was required to promote AKR and ARO reactions. The catalyst 1 was conveniently recycled several times with retention of its performance. The AKR of trans-stilbene oxide with aniline was successfully demonstrated at relatively higher scale (10 mmol) using the catalyst 1.     

An efficient synthesis of semiplenamide C

Semiplenamides are anandamide-like fatty acid amide metabolites previously isolated from a marine cyanobacterium that display a range of biological activity. Novel syn-aldol/dehydration methodology has been developed for the stereoselective synthesis of the core (E)-α,β-unsaturated amide functionality of this class of natural product, and employed for the efficient synthesis of semiplenamide C.     

Probing the spatial requirements for [60]fullerene-[60]fullerene π-stacking and the syn addition of [60]fullerenes across acenes

The reaction between 2 equiv of [60]fullerene and 6,8,15,17-tetraphenylheptacene-7,16-quinone leads to both cis and trans-bis[60]fullerene adducts. This result contrasts sharply with the highly diastereoselective syn additions of [60]fullerenes across 6,13-diphenylpentacene and 6,8,15,17-tetraphenylheptacene. The importance of spatially dependent [60]fullerene-[60]fullerene π-stacking interactions in promoting a syn addition of [60]fullerenes is discussed.     

Synthetic studies on statins. Part 3: A facile synthesis of rosuvastatin calcium through catalytic enantioselective allylation strategy

A concise and stereocontrolled synthesis of rosuvastatin calcium has been accomplished, with the key steps including a Keck enantioselective allylation of chloroacetaldehyde with allyltributylstannane to install 5R-stereocenter and a VO(acac)₂-catalyzed syn-diastereoselective epoxidation of (S)-1-chloropent-4-en-2-ol to set the requisite 3R-chirality.     

Simple NMR method for assigning relative stereochemistry of bridged bicyclo[3.n.1]-2-enes and tricyclo[7.n.1.0]-2-enes

The stereochemistry of syn and anti-forms of bridged bicyclo[3.n.1]-2-ene, tricyclo[7.n.1.0]-2-ene (n=1-3) and bicyclo[4.3.1]dec-7-ene derivatives can be assigned from the ¹³C chemical shift difference of the double bond. Both syn-9-R-bicyclo[3.3.1]non-2-enes and syn-13-R-tricyclo[7.3.1.0^2,7]tridec-2(7)-enes have a large shielding difference between sp² carbons, while the corresponding anti-forms have a smaller one. In contrast, 8-R-bicyclo[3.2.1]oct-2-enes and 12-R-tricyclo[7.2.1.0^2,7]dodec-2(7)-enes have an inverse correlation. The reason of this specificity is the influence of the γ-gauche effect on the chemical shift of C(2) atom. The GIAO theory has been applied to investigate the ¹³C chemical shifts. The conformational equilibrium in the formamide group of 13-formylamino-tricyclo[7.3.1.0^2,7]tridec-2(7)-enes has been studied.     

Pd-catalyzed cycloisomerization-allylation of 4-alkynones: synthesis of 5-homoallylfuran derivatives

In the presence of Pd₂(dba)₃-tBu₃P catalyst and Cs₂CO₃, cycloisomerization-allylation reactions of 4-alkynone derivatives with allyl carbonates in MeCN proceed in 5-exo mode to give highly-substituted furans having homoallyl groups at 5-position.     

Reactivity of cyclopropanic δ-oxo-α,β-unsaturated esters towards SmI2: 3-exo-trig cyclisation versus cyclopropane ring opening

trans-(2′,2′-Diphenyl-bicyclopropyl-2-yl)-4,4-dimethyl-5-oxo-pent-2-enoic acid methyl ester 9, undergoes 3-exo-trig cyclisation in the presence of SmI₂ without competitive ring opening of Newcomb's bicyclopropylic probe next to the carbonyl group. From this result, it may be concluded that the absence of ring opening observed earlier in the case of 5-cyclopropyl-4,4-dimethyl-5-oxo-pent-2-enoate 7 is not due to the potentially reversible character of this process. Meanwhile, as deduced from kinetic considerations based on data of the literature, the absence of ring opening does not necessarily mean that formation of ketyl radicals is not involved in the 3-exo-trig cyclisations of δ-oxo-α,β-unsaturated esters.Compounds 7 and 9 cyclise with total syn selectivity, leading ultimately to lactones. This syn selectivity contrasts with that of other alkylic δ-oxo-α,β-unsaturated esters.     

Application of oxetane ring opening toward stereoselective synthesis of zincophorin fragment

A stereoselective synthesis of the C1–C11 fragment of zincophorin was achieved by using an intramolecular oxetane ring opening reaction as the key step. The oxetane moiety was synthesized by employing a desymmetrization protocol developed at our group and a non-Evans syn aldol as the key steps toward the synthesis.     

An efficient asymmetric synthesis of grenadamide

The cyclopropane containing natural product grenadamide has been prepared in six steps using (R)-5,5-dimethyl-oxazolidin-2-one as a chiral auxiliary for asymmetric synthesis. Key synthetic steps include the use of the β-hydroxyl group of a syn-aldol product as a ‘temporary’ stereocentre to control the facial selectivity of a directed cyclopropanation reaction, as well as the use of phenylethylamine as a nucleophile for the direct aminolysis of an N-acyl-oxazolidin-2-one intermediate.