Stereoselective Synthesis of (+)-Loline Alkaloid Skeleton

General Approach to Enantiopure 1-Aminopyrrolizidines: Application to the Asymmetric Synthesis of the Loline Alkaloids

The synthesis of a range of loline alkaloids is reported. The C(7) and C(7a) stereogenic centers for the targets were formed by the established conjugate addition of lithium (S)-N-benzyl-N-(α-methylbenzyl)amide to tert-butyl 5-benzyloxypent-2-enoate, ensuing enolate oxidation to give an α-hydroxy-β-amino ester, and then formal exchange of the resultant amino and hydroxyl functionalities (via the intermediacy of the corresponding aziridinium ion) to give an α-amino-β-hydroxy ester. Subsequent transformation gave a 3-hydroxyprolinal derivative which was converted to the corresponding N-tert-butylsulfinylimine. Mannich-type reaction with the enolate derived from O-Boc protected methyl glycolate then formed the remaining C(1) and C(2) stereogenic centers for the targets. The 2,7-ether bridge was formed by a displacement reaction, completing construction of the loline alkaloid core. Facile manipulations then gave a range of loline alkaloids, including loline itself.

Stereoselective synthesis and transformation of pinane-based 2-amino-1,3-diols

A library of pinane-based 2-amino-1,3-diols was synthesised in a stereoselective manner. Isopinocarveol prepared from (−)-α-pinene was converted into condensed oxazolidin-2-one in two steps by carbamate formation followed by a stereoselective aminohydroxylation process. The relative stereochemistry of the pinane-fused oxazolidin-2-one was determined by 2D NMR and X-ray spectroscopic techniques. The regioisomeric spiro-oxazolidin-2-one was prepared in a similar way starting from the commercially available (1R)-(−)-myrtenol (10). The reduction or alkaline hydrolysis of the oxazolidines, followed by reductive alkylation resulted in primary and secondary 2-amino-1,3-diols, which underwent a regioselective ring closure with formaldehyde or benzaldehyde delivering pinane-condensed oxazolidines. During the preparation of 2-phenyliminooxazolidine, an interesting ring–ring tautomerism was observed in CDCl₃.

Reactivity and Synthetic Applications of Multicomponent Petasis Reactions

The Petasis boron-Mannich reaction, simply referred to as the Petasis reaction, is a powerful multicomponent coupling reaction of a boronic acid, an amine, and a carbonyl derivative. Highly functionalized amines with multiple stereogenic centers can be efficiently accessed via the Petasis reaction with high levels of both diastereoselectivity and enantioselectivity. By drawing attention to examples reported in the past 8 years, this Review demonstrates the breadth of the reactivity and synthetic applications of Petasis reactions in several frontiers: the expansion of the substrate scope in the classic three-component process; nonclassic Petasis reactions with additional components; Petasis-type reactions with noncanonical substrates, mechanism, and products; new asymmetric versions assisted by chiral catalysts; combinations with a secondary or tertiary transformation in a cascade- or sequence-specific manner to access structurally complex, natural-product-like heterocycles; and the synthesis of polyhydroxy alkaloids and biologically interesting molecules.

Petasis three-component reactions for the synthesis of diverse heterocyclic scaffolds

The Petasis three-component reaction (PR) of hydroxy aldehydes, amines and boronic acids is an important multi-component reaction for the synthesis of structurally diverse scaffolds and biologically interesting small molecules. The reaction has been significantly explored in the past decade, and many new variants have emerged, such as asymmetric, traceless and four-component approaches. The excellent stereoselectivity, high yield and broad functional group tolerance altogether make this reaction ideal for fragment and compound collection synthesis, since orthogonal chemical handles can be incorporated for subsequent scaffold formation and appendage modification. Herein we present a selection of recent variations on the PR theme for the synthesis of scaffolds of relevance to medicinal chemistry.

Installation of the Ether Bridge of Lolines by the Iron- and 2-Oxoglutarate-Dependent Oxygenase, LolO: Regio- and Stereochemistry of Sequential Hydroxylation and Oxacyclization Reactions

The core of the loline family of insecticidal alkaloids is the bicyclic pyrrolizidine unit with an additional strained ether bridge between carbons 2 and 7. Previously reported genetic and in vivo biochemical analyses showed that the presumptive iron- and 2-oxoglutarate-dependent (Fe/2OG) oxygenase, LolO, is required for installation of the ether bridge upon the pathway intermediate, 1-exo-acetamidopyrrolizidine (AcAP). Here we show that LolO is, in fact, solely responsible for this biosynthetic four-electron oxidation. In sequential 2OG- and O₂-consuming steps, LolO removes hydrogens from C2 and C7 of AcAP to form both carbon–oxygen bonds in N-acetylnorloline (NANL), the precursor to all other lolines. When supplied with substoichiometric 2OG, LolO only hydroxylates AcAP. At higher 2OG:AcAP ratios, the enzyme further processes the alcohol to the tricyclic NANL. Characterization of the alcohol intermediate by mass spectrometry and nuclear magnetic resonance spectroscopy shows that it is 2-endo-hydroxy-1-exo-acetamidopyrrolizidine (2-endo-OH-AcAP). Kinetic and spectroscopic analyses of reactions with site-specifically deuteriated AcAP substrates confirm that the C2–H bond is cleaved first and that the responsible intermediate is, as expected, an Fe^IV–oxo (ferryl) complex. Analyses of the loline products from cultures fed with stereospecifically deuteriated AcAP precursors, proline and aspartic acid, establish that LolO removes the endo hydrogens from C2 and C7 and forms both new C–O bonds with retention of configuration. These findings delineate the pathway to an important class of natural insecticides and lay the foundation for mechanistic dissection of the chemically challenging oxacyclization reaction.

Pyrrolizidine alkaloids: occurrence, biology, and chemical synthesis

Covering: 2013 up to the end of 2015This review covers the isolation and structure of new pyrrolizidines; pyrrolizidine biosynthesis; biological activity, including the occurrence of pyrrolizidines as toxic components or contaminants in foods and beverages; and formal and total syntheses of naturally-occurring pyrrolizidine alkaloids and closely related non-natural analogues.

Synthesis of Peramine, an Anti-insect Defensive Alkaloid Produced by Endophytic Fungi of Cool Season Grasses

A seven-step synthesis of peramine, which required three chromatographic separations, is described. Key to the synthesis is an enolate alkylation of a pyrrole-fused diketopiperazine, reduction of the acyl pyrrole, and dehydration of the intermediate pyrrolyl carbinol to establish the pyrrolopyrazine core of peramine.

A versatile access to vicinal diamine motifs by highly anti-selective asymmetric vinylogous Mannich reactions: an efficient total synthesis of (+)-absouline

An anti-stereoselective vinylogous Mannich reaction between 2-TBSO-pyrrole and N-tert-butanesulfinyl imines and its application to the efficient synthesis of (+)-absouline were reported.

The asymmetric total synthesis of (+)-N-acetyl norloline

The asymmetric total synthesis of (+)-N-acetyl norloline, the putative biogenic precursor of all known loline alkaloids, has been achieved in 12 steps from a commercially available compound.