Alk-2-enoic Acid Esters

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    Total Synthesis of 5-Hydroxygoniothalamin

    Patpi, Santhosh Reddy; Jin, Guangyi; Kantevari, Srinivas, Synthesis 2019, 51, 780-786

    Keywords: C1 Wittig reaction · C2 Wittig reaction · Yamaguchi reaction · cross-metathesis · 5-hydroxygoniothalamin

    The total synthesis of 5-hydroxygoniothalamin is achieved from commercially available l-xylose. The α,β-unsaturated-δ-lactone core is constructed in very good yield by utilizing one-carbon and two-carbon cis-Wittig olefinations and δ-lactonization using Yamaguchi conditions. Subsequent Grubbs cross-metathesis followed by desilylation results in 5-hydroxygoniothalamin.

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    Asymmetric Allylic Amination of Morita–Baylis–Hillman Adducts with Simple Aromatic Amines by Nucleophilic Amine Catalysis

    Zhao, Shuai; Chen, Zhi-Li; Rui, Xue; Gao, Ming-Mei; Chen, Xin, Synlett 2019, 30, 703-708

    Keywords: MBH adducts · aromatic amines · asymmetric allylic amination · nucleophilic amine catalysis · inorganic fluorides

    Asymmetric allylic amination of Morita–Baylis–Hillman (MBH) adducts with simple aromatic amines is successfully realized by nucleophilic amine catalysis. A range of substituted α-methylene-β-arylamino esters is accessed in moderate to high yields (up to 88%) and with excellent enantioselectivities (up to 97% ee). Inorganic fluorides are found to be able to improve the enantioselectivity of the allylic amination reaction. A pyrrole-2-carboxylate and a cyclic imide are also compatible with this catalytic system. A chiral 2,3-dihydroquinolin-4-one derivative is easily obtained from the allylic amination product.

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    Total Synthesis of the Natural Pyridocoumarins Goniothaline A and B

    Ahn, Sungwan; Yoon, Jeong A; Han, Young Taek, Synthesis 2019, 51, 552-556

    Keywords: total synthesis · goniothaline · Perkin reaction · silver-catalyzed cycloisomerization · selective demethylation

    In this paper, we report the first total synthesis of goniothaline A and B, which are rare natural pyridocoumarins isolated from Goniothalamus australis. The key feature of the synthesis of goniothaline A is high-yielding silver-catalyzed cycloisomerization to afford the pyridine moiety. In addition, goniothaline B, a natural 8-hydroxyquinoline derivative, is readily synthesized by the regioselective demethylation of goniothaline A.

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    A Tandem Sulfonylation and Knoevenagel Condensation for the Preparation of Sulfocoumarin-3-carboxylates

    Dong, Ziyang; Chen, Yang; Yang, Zhiheng; Yang, Zhanhui; Xu, Jiaxi, Synthesis 2019, 51, 1809-1818

    Keywords: sulfocoumarin-3-carboxylates · tandem reactions · sulfonylation · Knoevenagel condensation · salicylaldehyde

    Sulfocoumarins are key structural motifs in several bioactive molecules. Herein, we describe a simple, one-pot procedure for the synthesis of structurally diverse sulfonocoumarin-3-carboxylates by heating 2-hydroxyaryl aldehydes with an active sulfonyl chloride in the presence of pyridine. The process tolerates numerous functional groups including alkoxy, alkyl, halogen, nitro, and even nucleophilic phenolic hydroxy. Additionally, reactions of 2-hydroxyaryl ketones and 2-methylaminoaryl aldehydes give 4-substituted sulfocoumarins and 1-aza-2-sulfocoumarins, respectively. A gram-scale synthesis and further derivatizations are also reported. The ester group is easily removed via ­Happer’s decarboxylation.

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    A Copper Halide Promoted Regioselective Halogenation of Coumarins Using N-Halosuccinimide as Halide Source

    Su, Jinling; Zhang, Yan; Chen, Mingren; Li, Weiming; Qin, Xuewei; Xie, Yanping; Qin, Lixiao; Huang, Shihua; Zhang, Min, Synlett 2019, 30, 630-634

    Keywords: coumarin · halogenation · aryl halide · electrophilic aromatic substitution · N-halosuccinimide

    A safe, convenient, and regioselective synthesis of 3-halo coumarins using a metal halide (CuX2 alone or with ZnX2) promoted halogenation with N-halosuccinimide (NXS) as halide source is reported. The synthesis involved the steady in situ generation of highly reactive positive halogen (X+) by the coordination of copper or zinc with the N-halosuccinimide and subsequent electrophilic aromatic substitution of the electron-deficient coumarins. This procedure works well also for the halogenation of less electron-rich naphthoquinones, flavones, and methoxypsoralen in moderate to quantitative yields. This protocol features simple experimental conditions using readily available inexpensive reagents and provides a convenient approach to the chlorination or bromination of some useful heteroaromatic compounds.

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    Synthesis of the Deacetoxytubuvaline Fragment of Pretubulysin and its Lipophilic Analogues for Enhanced Permeability in Cancer Cell Lines

    Reddy, Ramesh B.; Dudhe, Premansh; Chelvam, Venkatesh, Synlett 2019, 30, 77-81

    Keywords: amino acid · deacetoxytubuvaline · tubulysin · pretubulysin · medicinal chemistry

    In the last two decades, tubulysins have emerged as alternatives to microtubule depolymerizing agents such as colchicine and vinblastine, which are well-established anticancer agents. However, the complex structure of tubulysins has always posed a challenge for synthetic chemists to scale up the production of these compounds. We report a new strategy for the practical gram-scale synthesis of a (4R)-4-[(tert-butoxycarbonyl)amino]-5-methylhexanoic acid through regioselective cleavage of a chiral aziridine ring with a vinyl Grignard reagent to afford tert-butyl [(1R)-1-isopropylbut-3-en-1-yl]carbamate, which was subjected to regioselective hydroboration–oxidation with 9-BBN. The resulting (4R)-4-[(tert-butoxycarbonyl)amino]-5-methylhexanoic acid was successfully transformed into the deacetoxytubuvaline fragment of pretubulysin or its highly lipophilic methyl-substituted thiazole and oxazole analogues for incorporation into pretubulysins. Increasing the lipophilicity of tubulysin or pretubulysin molecules should enhance their cell permeability and cytotoxicity in cancer cell lines.

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    Gold-Catalyzed Cyclization/Intermolecular Methylene Transfer ­Sequence of O-Propargylic Oximes Derived from Glyoxylates

    Gima, Shinya; Shiga, Keigo; Terada, Masahiro; Nakamura, Itaru, Synlett 2019, 30, 393-396

    Keywords: gold catalysis · rearrangement · heterocycles · cyclization · progargylic oximes · isoxazolines

    We successfully extended our gold-catalyzed skeletal rearrangement reaction of O-propargylic oximes through C=N bond cleavage to include substrates having an ester group on the oxime moiety, affording the corresponding 2-isoxazolines having an alkoxycarbonylmethylene group at the 4-position in good to high yields. Our mechanistic studies indicated that the transfer of the alkoxycarbonylmethylene group proceeded in an intermolecular manner, confirming that the reaction proceeds through cyclization followed by intermolecular transfer of the alkoxycarbonylmethylene group.

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    One-Pot Approach to Pyrido-4-phenanthridinones by Palladium-Catalyzed Annulation of 4-Quinolones with 2-Bromobenzyl Bromides

    Arasakumar, Thangaraj; Shyamsivappan, Selvaraj; Gopalan, Subashini; Ata, Athar; Mohan, Palathurai Subramaniam, Synlett 2019, 30, 63-68

    Keywords: one-pot reaction · palladium catalysis · quinolones · phenanthridinones · polycyclic heterocycles · bromobenzyl bromides

    A straightforward approach toward the assembly of phenanthridinone heterocycles has been developed through the palladium-catalyzed N-benzylation/intramolecular coupling reactions of readily prepared 4-quinolones with commercially available 2-bromobenzyl bromide derivatives. The target products were prepared in moderate to good yields, with tolerance of various functional groups.

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    Palladium-Catalyzed Carbonylation of Coumarin C(sp2)–H Bonds: A New Entry to Arylcoumarin Ketones

    Mirzaei, Siyavash; Rajai-Daryasarei, Saideh; Soheilizad, Mehdi; Kabiri, Roya; Ansari, Samira; Shabanian, Meisam; Pashazadeh, Rahim, Synthesis 2019, 51, 1680-1688

    Keywords: Pd catalysis · carbonylation · arylcoumarin ketones · aryl halides · carbon monoxide.

    A facile and efficient palladium-catalyzed carbonylation of coumarins involving two C–C bond formations has been developed. The C–H bond oxidative functionalization proceeds through aroylation with insertion of carbon monoxide to give arylcoumarin ketones. The reaction conditions, employing ambient pressures of CO gas as C1 feedstock, dramatically improve the generality of the carbonylation of aryl halides.

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    Organocatalytic Allylic Amination of Morita–Baylis–Hillman Carbonates­

    Formánek, Bedřich; Šimek, Michal; Kamlar, Martin; Císařová, Ivana; Veselý, Jan, Synthesis 2019, 51, 907-920

    Keywords: alkaloids · allylic amination · asymmetric synthesis · Ezetimibe · Morita–Baylis–Hillman carbonates · organocatalysis

    An organocatalytic asymmetric allylic amination of Morita–Baylis–Hillman carbonates with aromatic amines in the presence of β-isocupreidine is described. Chiral allylic amines were obtained in almost quantitative yields (90–96%) with moderate enantioselectivity. Recrystallization afforded products in good yields (45–73%) and high optical purity (82–99% ee). This method provides a facile and efficient route to obtain optically active β-lactams, including the building block of the cholesterol-lowering drug Ezetimibe.