Crystal defects and spin tunneling in single crystals of Mn12 clusters

The question addressed in this paper is that of the influence of the density of dislocations on the spin tunneling in Mn12 clusters. We have determined the variation in the mosaicity of fresh and thermally treated single crystals of Mn12 by analyzing the widening of low angle x-ray diffraction peaks. It has also been well established from both isothermal magnetization and relaxation experiments that there is a broad distribution of tunneling rates which is shifted to higher rates when the density of dislocations increases

Aqua(1,10-phenanthroline)(l-serinato)copper(II) Nitrate

The unit cell of the title complez, [Cu(C3H6NO3)-(C12H8N2)(H20)]NO3, contains two nitrate anions and two complex cations, each with two chiral centers, one in the serine molecule and the other at the Cu uion. Each Cu ion displays slightly distorted square-pyramidal coordination, with the water molecule in the apical position and the base defined by the aliphatic N atom and one of the O atoms from the aminocarboxylate ligand and the two N atoms from the phenanthroline molecule. The relative position of the apical water molecule generates the chiral center at the Cu ion. In both molecules, the five-membered chelate ring defined by atoms N1, C12, C11, N2 and Cu is roughly planar, while the ring defined by atoms N3, C13, C14, O1 and Cu has a distorted half-chair conformation

Studies on the Enantioselective Synthesis of E-Ethylidene-bearing Spiro[indolizidine-1,3′-oxindole] Alkaloids

A synthetic route for the enantioselective construction of the tetracyclic spiro[indolizidine-1,3'-oxindole] framework present in a large number of oxindole alkaloids, with a cis H-3/H-15 stereochemistry, a functionalized two-carbon substituent at C-15, and an E-ethylidene substituent at C-20, is reported. The key steps of the synthesis are the generation of the tetracyclic spirooxindole ring system by stereoselective spirocyclization from a tryptophanol-derived oxazolopiperidone lactam, the removal of the hydroxymethyl group, and the stereoselective introduction of the E-ethylidene substituent by acetylation at the -position of the lactam carbonyl, followed by hydride reduction and elimination. Following this route, the 21-oxo derivative of the enantiomer of the alkaloid 7(S)-geissoschizol oxindole has been prepared

A Biomimetic Enantioselective Approach to the Decahydroquinoline Class of Dendrobatid Alkaloids

Frogs of the neotropical family Dendrobatidae produce a remarkably diverse array of biologically active alkaloids. One of the major classes of these amphibian alkaloids[1] are the decahydroquinolines, which have been isolated not only from skin extracts of dendrobatid and mantelline frogs,[2] but also from bufonid toads,[3] tunicates,[4] marine flatworms,[4b] and myrmicine ants.[5] They possess either a cis or trans decahydroquinoline ring fusion, with a side-chain substituent at both the C2 and C5 positions and, in the lepadin series,[4] an acylated hydroxy group at the C3 position. The most representative decahydroquinoline alkaloid is cis-195A (formerly called pumiliotoxin C), first isolated in 1969 from a Panamanian population of Dendrobates pumilio. [6] The source of amphibian alkaloids remains an unresolved and challenging question,[1] in particular after the discovery that some of these alkaloids also occur in ants, thus strengthening a dietary hypothesis for their origin in frogs.[5] Although there are no conclusive studies concerning the biosynthesis of these toxins and, consequently, little is known about the biosynthetic pathways, there has been speculation as to possible derivation from the polyketide route by aminocyclization of polycarbonyl intermediates (A), leading to either 2,5-disubstituted decahydroquinolines (C) or spiropiperidines (histrionicotoxins).[1a,b, 7] In accordance with this hypothesis, a plausible biosynthetic pathway to the decahydroquinoline class of dendrobatid alkaloids is depicted i

Preparation and Double Michael Addition Reactions of a Synthetic Equivalent of Nazarov Reagent

A synthetic equivalent of the Nazarov reagent, the silyl derivative 2, able to undergo base-catalyzed double Michael addition reactions with a,b-unsaturated carbonyl compounds, has been developed. The new reagent satisfactorily reacts with unsaturated indolo[2,3-a]quinolizidine lactams to give pentacyclic yohimbinone-type derivatives

Enantioselective protecting group-free synthesis of 1-S-ethyl-4-substituted quinolizidines

A practical enantioselective protecting group-free four-step route to the key quinolizidinone 6 from phenylglycinol-derived bicyclic lactam 1 is reported. The Grignard addition reaction to 6 takes place stereoselectively to give 1-ethyl-4-substituted quinolizidines 4-epi-207I and 7-9. Following a similar synthetic sequence, 9a-epi-6 is also accessed. However, the addition of Grignard reagents to 9a-epi-6 proceeds in a non-stereoselective manner. In order to gain insight into the different stereochemical outcome in the two series, theoretical calculations on the iminium salts A and B have been performed. The study concludes that the addition of the hydride, which is the step that determines the configuration of the final products, occurs in a stereoelectronic controlled manner. The theoretical study is in agreement with the experimental results

A synthetic approach to ervatamine-silicine alkaloids. Enantioselective total synthesis of (-)-episilicine

Starting from an appropriate unsaturated phenylglycinol-derived oxazolopiperidone lactam, the synthesis of (-)-16-episilicine is reported, the key steps being a stereoselective conjugate addition, a stereoselective alkylation, and a ring-closing metathesis reaction. This represents the first enantioselective total synthesis of an alkaloid of the silicine group

Studies on the Synthesis of Phlegmarine-Type Lycopodium Alkaloids: Enantioselective Synthesis of (−)-Cermizine B, (+)-Serratezomine E, and (+)-Luciduline

The synthesis of the Lycopodium alkaloids, (-)-cermizine B, (+)-serratezomine E, and (+)-luciduline using phenylglycinol-derived tricyclic lactams as chiral scaffolds, is reported. The requisite lactams are prepared by a cyclocondensation reaction between ( R)- or ( S)-phenylglycinol and the substituted δ-keto ester 11, easily accessible from ( R)-pulegone. The factors governing the stereoselectivity of these cyclocondensation reactions are discussed. Key steps of the synthesis from the stereochemical standpoint are the stereoselective elaboration of the allyl substituent to the ( S)-2-(piperidyl)methyl moiety and the stereoselective removal of the chiral inductor to give a cis-decahydroquinoline

A general methodology for the enantioselective synthesis of 1-substituted tetrahydroisoquinoline alkaloids

Starting from tricyclic lactam 2 , which is easily accessible by cyclocondensation of δ‐oxoester 1 with (R )‐phenylglycinol, a three‐step synthetic route to enantiopure 1‐substituted tetrahydroisoquinolines, including 1‐alkyl‐, 1‐aryl‐, and 1‐benzyltetrahydroisoquinoline alkaloids, as well as the tricyclic alkaloid (-)‐crispine A, has been developed. The key step is a stereoselective α‐amidoalkylation reaction using the appropriate Grignard reagent

Biomimetic Construction of the Hydroquinoline Ring System. Diastereodivergent Enantioselective Synthesis of 2,5-Disubstituted cis-Decahydroquinolines

The straightforward enantioselective construction of the hydroquinoline ring system from 1,5-polycarbonyl derivatives, using (R)-phenyglycinol as a chiral latent form of ammonia, is reported. The process mimics the key steps believed to occur in nature in the biosynthesis of amphibian decahydroquinoline alkaloids. Diastereodivergent routes to enantiopure cis-2,5-disubstituted decahydroquinolines, including the alkaloid pumiliotoxin C (cis-195A), are developed