Organic & Biomolecular Chemistry (v.10, #22)

Front cover (4301-4301).

Inside front cover (4302-4302).

Contents list (4303-4310).

Diastereoselective synthesis of vicinal amino alcohols by Oskari K. Karjalainen; Ari M. P. Koskinen (4311-4326).
The vicinal amino alcohol is a common motif in natural products and pharmaceuticals. Amino acids constitute a natural, inexpensive, and enantiopure choice of starting material for the synthesis of such functionalities. However, the matters concerning diastereoselectivity are not obvious. This Perspective takes a look in the field of diastereoselective synthesis of vicinal amino alcohols starting from amino acids using various methods.

Sulfoxide-mediated Umpolung of alkali halide salts by Sebastian Klimczyk; Xueliang Huang; Christophe Farès; Nuno Maulide (4327-4329).
A new protocol for the direct two-electron oxidative Umpolung of alkali halide salts is reported. This procedure, relying on the use of a commercially available sulfoxide as the oxidant, allows the electrophilic halogenation of carbonyl compounds as well as halolactonisation reactions to proceed from the corresponding sodium salts, at room temperature and under mild conditions.

New insights into the water-solubilisation of fluorophores by post-synthetic “click” and Sonogashira reactions by Cédrik Massif; Sébastien Dautrey; Alexandre Haefele; Raymond Ziessel; Pierre-Yves Renard; Anthony Romieu (4330-4336).
New synthetic methodologies for the efficient chemical conversion of hydrophobic fluorescent dyes into bioconjugable and water-soluble derivatives are described. The combined use of an original sulfonated terminal alkyne and a metal-mediated reaction, namely the copper-catalysed Huisgen 1,3-dipolar cycloaddition (“click” reaction) or the Sonogashira cross-coupling, is the cornerstone of these novel post-synthetic sulfonation approaches.

A synthetic oxygen (O2) and carbon monoxide (CO) receptor (hemoCD) composed of 5,10,15,20-tetrakis(4-sulfonatophenyl)porphinatoiron(ii) and a per-O-methylated β-cyclodextrin dimer with a pyridine linker (Py3CD) was functionalised with poly(ethylene glycol) (PEG) to elongate the circulation time of the receptor in the bloodstream. α-PEG monocarboxylic acid (HOOC(CH2)3(CO)O-PEG(mw)-OCH3; mw = 750 or 5k) or α,ω-PEG dicarboxylic acid (HOOC(CH2)3(CO)O-PEG(mw)-O(CO)(CH2)3COOH; mw = 10k or 20k) was reacted with the amino group of 5-(4-aminophenyl)-10,15,20-tris(4-sulfonatophenyl)porphyrin to afford a porphyrin monomer having a PEG chain or a porphyrin dimer having a PEG linker, respectively. The ferrous complexes of these PEGylated porphyrins (PEG750-, PEG5k-, PEG10k- and PEG20k-hemoCDs) bound O2 in aqueous solution, P1/2 values being 6.5–8.1 Torr at pH 7.0 and 25 °C. Each PEG(mw)-hemoCD was infused into the femoral vein of a Wistar male rat. After 6 h of the infusions, 67, 82, 86 and 42% of PEG750-, PEG5k-, PEG10k- and PEG20k-hemoCD were excreted in the urine. PEG750-hemoCD with a hydrodynamic diameter (Dh) of 3.4 nm seemed to partly leak from the blood vessels (pore size: 2–6 nm) before renal filtration (pore size: 4–14 nm). PEG5k- (Dh = 6.2 nm) and PEG10k-hemoCDs (9.0 nm) hardly passed through the blood vessels but were fully filtered by the kidney, resulting in high excretion rates. A considerable amount of PEG20k-hemoCD (Dh = 12.0 nm) was retained in the blood even at 6 h after administration. The present study demonstrates that the behaviour of hemoCD in blood after administration can be controlled by modification of hemoCD with PEG having an appropriate molecular weight.

Applications of 3-aminolactams: design, synthesis, and biological evaluation of a library of potential dimerisation inhibitors of HIV1-protease by Eulàlia Pinyol; Silvia Frutos; Dolors Grillo-Bosch; Ernest Giralt; Bonaventura Clotet; Jose A. Esté; Anna Diez (4348-4354).
In the context of our studies on the applications of 3-aminolactams as conformationally restricted pseudodipeptides, we report here the synthesis of a library of potential dimerisation inhibitors of HIV1-protease. Two of the pseudopeptides were active on the wild type virus (HIV1) at micromolar levels (EC50). Although the peptides showed lower anti-viral activity than previously reported dimerisation inhibitors, our results demonstrate that the piperidone moiety does not prevent cell penetration, and hence that such derivatization is compatible with potential anti-HIV treatment.

β-Isocupreidine (β-ICD) catalyzed asymmetric [4 + 2] cycloaddition of β,γ-unsaturated α-ketoesters with allenic esters afforded ester-substituted functionalized dihydropyran derivatives in high yields along with high enantioselectivities under mild conditions.

Total synthesis of ent-calystegine B4 via nitro-Michael/aldol reaction by Akio Kamimura; Koichiro Miyazaki; Shuzo Suzuki; Shingo Ishikawa; Hidemitsu Uno (4362-4366).
Optically active ent-calystegine B4 was prepared in 13 steps from commercially available chiral l-dimethyl tartrate. The synthesis was achieved by the Michael addition and the aldol reaction of nitromethane to form cycloheptanone in a stereoselective manner. Reduction of the nitro group in the presence of Boc2O accomplished an efficient conversion to amino cycloheptanone, which readily afforded the desired ent-calystegine B4.

Regulation of NH-tautomerism in N-confused porphyrin by N-alkylation by Motoki Toganoh; Takaaki Yamamoto; Takayoshi Hihara; Hisanori Akimaru; Hiroyuki Furuta (4367-4374).
A variety of internally N-alkylated N-confused porphyrins were prepared in a stepwise manner through the protection of the reactive peripheral nitrogen atom. NH-Tautomerism in N-confused porphyrins was found to be regulated by N-alkylation, which enabled us to obtain discrete information on two important NH-tautomers of an N-confused porphyrin.

A receptor incorporating OH, NH and CH binding motifs for a fluoride selective chemosensor by Liang Xu; Yongjun Li; Yanwen Yu; Taifeng Liu; Songhua Cheng; Huibiao Liu; Yuliang Li (4375-4380).
An anion receptor combined different types of hydrogen bond donors such as OH, NH and CH groups has been synthesized. By rotation of the sub methyl group, this receptor showed evident 1H NMR response to both fluoride and sulfate, while colorimetric and fluorescent responses were only observed in the presence of fluoride.

Synthesis of novel enantiomerically pure tetra-carbohydrazide cyclophane macrocycles by Hany F. Nour; Nadim Hourani; Nikolai Kuhnert (4381-4389).
A total of twelve novel enantiomerically pure tetra-carbohydrazide cyclophane macrocycles have been synthesised in quantitative yields by reacting chiral (4R,5R)- and (4S,5S)-1,3-dioxolane-4,5-dicarbohydrazides with aromatic bis-aldehydes in a [2 + 2]-cyclocondensation reaction. The compounds show a dynamic behaviour in solution, which has been rationalized in terms of an unprecedented conformational interconversion between two conformers one stabilised by intramolecular hydrogen bonding and π–π stacking interactions.

The rearrangement of N-p-toluenesulfonyl 2-tert-butyldiphenylsilylmethyl-substituted azetidines into 3-tert-butyldiphenylsilyl-substituted pyrrolidines under Lewis acid conditions in dichloromethane involves 1,2-migration of silicon through a siliranium ion. The formation of siliranium ion was discovered not to be in concert with σC–N cleavage from stereochemical analysis of the pyrrolidine products formed from 3- and 4-substituted-2-tert-butyldiphenylsilylmethyl azetidines and also from the optical rotation data and chiral HPLC analysis of the pyrrolidine product formed from N-p-toluenesulfonyl 2(R)-tert-butyldiphenylsilylmethyl azetidine. The formation of sterically less hindered siliranium ion is followed by its SN2 opening by the internal nitrogen nucleophile. Oxidative cleavage of σC–Si bond leads to the formation of 3-hydroxypyrrolidines.

Solutions of chloranil (CA) in chlorobenzene were irradiated in the presence of cyclobutene and cyclopropene. Cyclobutene gave rise to two conventional 1 : 2 cycloadducts onto the dichloroethene subunits of CA and an α,β-unsaturated α,γ-dichloro-γ-lactone. Heating of the crude product in methanol converted the lactone into an α,β-unsaturated methyl γ-oxocarboxylate (25% yield) and a large amount of the major 1 : 2 cycloadduct, which contains chlorocyclobutane entities, into a cyclopropylcarbinyl chloride derivative (24% yield). An entirely new product type was the result in the case of cyclopropene. After treatment of the crude product with methanol a tetracyclic acetal containing a cyclopentanone and a dihydropyran subunit was isolated in 36% yield. Apparently, CA had taken up two molecules of cyclopropene. One of the resulting cyclopropane entities must have undergone a rearrangement en route to the final product.

Toluene dioxygenase mediated oxidation of halogen-substituted benzoate esters by Vladislav Semak; Thomas A. Metcalf; Mary Ann A. Endoma-Arias; Pavel Mach; Tomas Hudlicky (4407-4416).
A series of ortho-, meta-, and para- halogen-substituted methyl benzoate esters was subjected to enzymatic dihydroxylation via the whole-cell fermentation with E. coli JM109 (pDTG601A). Only ortho-substituted benzoates were metabolized. Methyl 2-fluorobenzoate yielded one diol regioselectively whereas methyl 2-chloro-, methyl 2-bromo- and methyl 2-iodobenzoates each yielded a mixture of regioisomers. Absolute stereochemistry was determined for all new metabolites. Computational analysis of these results and a possible rationale for the regioselectivity of the enzymatic dihydroxylation is advanced.

The mechanism of the gold-catalyzed annulations of 3-aryloxy alkynyl indoles developed by Tu et al. was studied by DFT calculations. It was found that both indole derivatives of electron-donating and electron-withdrawing protective groups would first undergo the 5-exo-dig cyclization simultaneously upon activation by cationic [PR3Au+] species. However, divergent reactivity of the resulting spirocyclic intermediate in competitive 1,2-alkenyl migration and nucleophilic water addition reactions towards C3 was predicted. When protected by electron-donating group, the 1,2-alkenyl migration occurs to generate a tricyclic intermediate, from which an aromatic Claisen rearrangement/nucleophilic addition sequence results in the observed 1,2-phenoxy migration. In case of electron-withdrawing group, the 1,2-alkenyl migration would be unfavorable. Instead, the nucleophilic addition of water oxygen to C3 is more facile, and leads to the hemiketal intermediate. The possible roles of water-cluster and OTf anion as proton shuttles in both reactions were also evaluated.

Asymmetric synthesis of 2-alkyl-substituted tetrahydroquinolines by an enantioselective aza-Michael reaction by Laura L. Taylor; Frederick W. Goldberg; King Kuok (Mimi) Hii (4424-4432).
An optically active tetrahydroquinoline intermediate (5) was prepared in 8 steps from monoprotected ethylene glycol, using a Pd-catalysed aza-Michael reaction to induce chirality. This can be transformed into three Galipea alkaloids (angustureine, galipeine and cuspareine). The proximity of a benzyloxy group is found to exert profound effects in several steps of the synthesis.

Computational studies on the mechanism of the gold(i)-catalysed rearrangement of cyclopropenes by Maximillian S. Hadfield; L. Jonas L. Häller; Ai-Lan Lee; Stuart A. Macgregor; James A. T. O'Neill; Ashley M. Watson (4433-4440).
Density functional theory calculations have been employed to investigate the mechanism of gold(i)-catalysed rearrangements of cyclopropenes. Product formation is controlled by the initial ring-opening step which results in the formation of a gold-stabilised carbocation/gold carbene intermediate. With 3-phenylcyclopropene-3-methylcarboxylate, the preferred intermediate allows cyclisation via nucleophilic attack of the carbonyl group and hence butenolide formation. Further calculations on simple model systems show that substituent effects can be rationalised by the charge distribution in the ring-opening transition state and, in particular, a loss of negative charge at what becomes the β-position of the intermediate. With 1-C3H3R cyclopropenes (R = Me, vinyl, Ph), ring-opening therefore places the substituent at the β-position.

Synthesis of amino-substituted indoles using the Bartoli reaction by Laura Wylie; Paolo Innocenti; Daniel K. Whelligan; Swen Hoelder (4441-4447).
We report herein the concise preparation of a range of functionalised aminoindoles via a new application of the Bartoli reaction. Scope and limitations of the methodology have been extensively studied to reveal the importance of protecting groups and substitution patterns. The use of amino substituted nitroanilines for the Bartoli reaction is to our knowledge unprecedented. Our work thus represents a novel entry into substituted aminoindoles which are relevant building blocks for both the fine chemical and pharmaceutical industry.

Preparation of modified peptides: direct conversion of α-amino acids into β-amino aldehydes by Carlos J. Saavedra; Alicia Boto; Rosendo Hernández (4448-4461).
A direct method for the transformation of α-amino acids into β-amino aldehydes was developed, and applied to the modification of the C-terminal residue of peptides. The method takes place in good yields and under mild conditions. The application of this methodology to the preparation of small peptides with γ-amino alcohol units, which are precursors of analogues of peptaibol antibiotics, is also described.

A stereoselective synthesis of (−)-1-epi-ventiloquinone L and (+)-ventiloquinone L, the monomeric unit of cardinalin 3 has been described. The synthesis is completed in 7 steps with 10.5% and 13% overall yields for (−)-1-epi-ventiloquinone L and (+)-ventiloquinone L respectively. The key steps involve Dötz benzannulation of carbene 5 with alkyne 6 to give a substituted naphthalene moiety and oxa-Pictet–Spengler reaction to install the 1,3-dimethylpyran moiety.

Highly enantioselective Biginelli reaction catalyzed by SPINOL-phosphoric acids by Fangxi Xu; Dan Huang; Xufeng Lin; Yanguang Wang (4467-4470).
A highly enantioselective Biginelli reaction promoted by chiral spirocyclic SPINOL-phosphoric acids has been developed. Under the optimized conditions with 5 mol% catalyst loading, a wide range of optically active dihydropyrimidinethiones (DHPMs) were obtained in high yields (up to 98%) with good to excellent enantioselectivities (up to 99% ee). The synthetic utility of this method was demonstrated by the synthesis of chiral precursors of three drugs, including (S)-Monastrol, (S)-L-771688 and (S)-SQ 32926.

Back cover (4471-4472).