Reaktionen CH-aktiver Verbindungen mit Aziden, XVIII. Synthese von 2-Oxo-1-diazo-cycloalkanen durch entformylierende Diazogruppen-Übertragung

Deaminative coupling of benzylamines and arylboronic acids

A metal-free deaminative coupling of non-prefunctionalised benzylamines and arylboronic acids is reported. In this operationally simple reaction, a primary amine in benzylamine is converted into a good leaving group in situ using inexpensive and commercially available isoamyl nitrite as a nitrosating reagent. Lewis-acidic arylboronic acids are shown to replace mineral acids such as HCl or HBF₄ that are conventionally used in the preparation of aryl diazonium salts. This unlocked the formation of the corresponding diarylmethanes by forging a new C-C bond in good yields.

Synthesis of α-Aryldiazophosphonates via a Diazo Transfer Reaction

The simple synthetic procedure for preparation of α-aryl-α-diazophosphonates via a diazo transfer reaction is proposed. Benzylphosphonates reacted with tosyl azide (TsN₃) in the presence of potassium tert-butoxide (KO^tBu) to afford diazophosphonates in a yield up to 79%. The proposed method is general. The reaction uses easily available starting materials, tolerates various functional groups, and may be applied for multi-gram scale synthesis.

Functionalization of 3-Iridacyclopentenes

Members of a series of iridacyclopentenes of composition [Tp^Me2 Ir(k² -C,C-CH₂ CR'=CRCH₂ )(CO)] (Tp^Me2 =hydrotris(3,5-dimethylpyrazolyl)borate; R=R'=H, 1; R=Me, R'=H, 2; R=R'=Me, 3) have been subjected to common organic chemistry procedures for hydrogenation, cyclopropanation, epoxidation, water addition through hydroboration, cis-dihydroxylation, and ozonolysis. The stability of metallacycles 1-3, imparted by the presence of the co-ligands Tp^Me2 and CO, directs the reactivity towards the C=C double bonds, and furthermore the stereochemistry of the products formed is strongly dictated by the steric demands of the Tp^Me2 ligand. While the products obtained in some of the above-mentioned reactions are the expected ones from an organic chemistry point of view, in other cases the results differ from the outcomes of similar reactions carried out with the all-carbon counterparts.

Galbulimima Alkaloids

This chapter provides a comprehensive overview of recent achievements in the area of Galbulimima alkaloids. Following a discussion of the isolation of Galbulimima alkaloids and structural features of members of this fascinating family of secondary metabolites, biological properties of selected compounds are briefly discussed. Furthermore, the proposed biosynthetic routes toward Galbulimima alkaloids are outlined. The main section of the chapter is devoted to a detailed discussion and comparison of all total syntheses of Galbulimima alkaloids published to date.

Total Synthesis of the Galbulimima Alkaloid (±)-GB 13

The synthesis of alkaloid GB 13 (4), isolated from the North Australian rain forest tree Galbulimima belgraveana is described. Birch reductive alkylation of 2,5-dimethoxybenzoic acid by 3-methoxybenzyl bromide, followed by an acid-catalyzed cyclization was used to synthesize the [3.3.1]bicyclononane 12. Ring contraction performed on the diazoketone 19 followed by a Diels–Alder reaction generated a pentacyclic intermediate 34 with a carbon skeleton closely resembling the target alkaloid. The surplus nitrile substituent, required for activation and regioselectivity in the Diels–Alder reaction, was removed by treatment with lithium and liquid ammonia. Birch reduction of the aromatic ring could be performed at the same time to give diene 38 and thence enone 41, which was cleaved by means of an Eschenmoser fragmentation. The piperidine ring found in the natural product was formed by reductive cyclization of bis-oxime 49 derived from the alkynyl ketone 48 and the resulting material further elaborated to GB 13.

The total synthesis of the Galbulimima alkaloid GB 13

This contribution describes a synthetic approach to alkaloid GB 13, previously isolated from the North Australian and Papua New Guinean rain forest tree Galbulimima belgraveana. A Birch reductive alkylation of 2,5-dimethoxybenzoic acid by 3-methoxybenzyl bromide, followed by an acid-catalyzed cyclization was used to synthesize the [3.3.1]bicyclononane 8. A ring contraction performed on the diazo derivative 9 of the [3.3.1]bicyclononane led to [3.2.1]bicyclooctane 10. This [3.2.1]bicyclooctane was converted into a dienophile and subjected to a Diels-Alder reaction to generate a pentacyclic intermediate 13 with a carbon skeleton closely resembling the target alkaloid. The surplus substituent, required for activation and regioselectivity in the Diels-Alder reaction, was removed using Birch reductive conditions to effect a decyanation. It was discovered that a Birch reduction of the aromatic ring also present in the molecule could be performed at the same time to give the enone 15, which was cleaved by means of an Eschenmoser fragmentation. The piperidine ring found in the natural product was formed by reductive cyclization of the bis-oxime 18 derived from the alkynyl ketone 17 and the resulting material further elaborated to GB 13 (1) via ketone 20.