Quantum chemical study of the formation of acetylenic alcohols and 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes from acetylene and carbonyl compounds

The geometric structures of the conformers of 2,3,4-trimethyl-7-methylidene-1,5-di-(thiophen-2-yl)-6,8-dioxabicyclo[3.2.1]octane were studied by the MP2/6-311++G**// B3LYP/6-31G* method. The average deviations of the calculated bond lengths from those determined by X-ray diffraction are no larger than 0.02 Å, and the deviations of the bond angles and dihedral angles are 0.7 and 3.5°, respectively. In the gas phase, the chair conformation is thermodynamically ～7.3 kcal mol^?1 more favorable than the boat conformation, and, taking into account the solvent effect of DMSO, the chair conformation is ～9.4 kcal mol^?1 more favorable. For formaldehyde and a series of ketones, including alkyl hetaryl ketones, the enthalpies of competitive formation of acetylenic alcohols and 7-methylidene-6,8-dioxabicyclo[3.2.1]octanes were calculated. The formation of the latter compounds is characterized by a considerable decrease in the enthalpy (to ?91 kcal mol^?1).     

A peculiar transition-metal-free cyclodimerization of propargylic alcohols to vinyl bicyclic ketals

Propargylic alcohols, alkylaryl(hetaryl)ethynyl carbinols, under transition-metal-free conditions (KOH/DMSO, 70–80 °C, 10–15 min) unexpectedly undergo stereoselective cyclodimerization to 7-methylene-6,8-dioxabicyclo[3.2.1]octanes having the scaffold of known insect pheromones and marine toxins. Under acetylenic pressure the yields of bicyclic ketals reach 80%. In all the cases, the cyclodimers are formed exclusively as one diastereomer.     

Quantum-chemical models of KOH(KOBut)/DMSO superbasic systems and mechanisms of base-promoted acetylene reactions

The mechanisms of fundamental base-promoted acetylene reactions, namely, nucleophilic addition to the triple C ≡ C bond (vinylation) and nucleophilic addition of acetylenic carbanion to a carbonyl group (ethynylation), are addressed using three models of different complexity—pentasolvate, monosolvate, and anionic—which describe the catalytic superbasic systems MOH(OBu^t)/DMSO (suspensions of alkali hydroxides or tert-butoxides in dimethyl sulfoxide). The above acetylene reactions and sequential transformations of reagents arranged by the superbasic center are modeled within the framework of the most complete pentasolvate model, in which the superbase is represented by the KOH·5DMSO (KOBu^t·5DMSO) complexes. We have developed approaches to the construction of simplified models (monosolvate and anionic) to describe transformations in complex systems. The mechanisms of cascade assemblies of 6,8-dioxabicyclo[3.2.1]octanes, cyclopentenones, and furan cycles from ketones and acetylenes in the superbasic environment are investigated using a uniform B2PLYP/6-311+G**//B3LYP/6-31+G* approach, and the energy profiles of these different carbo- and heterocycles are analyzed.     

Two classes of heterocycles—6,8‐dioxabicyclo[3.2.1]octanes and cyclopentenols from the same reagents: A quantum‐chemical comparison of mechanism

Most of the strategies for the synthesis of 6,8‐dioxabicyclo[3.2.1]octanes (BCO) and cyclopentenols (CP), which show high biological and pharmaceutical activity, are multistage and/or require hardly accessible starting materials and catalysts. The general method for BCO and CP preparation in a single preparative stage from two simple reagents, ketones and acetylene, under the action of an available super‐basic catalytic system KOH/DMSO becomes promising for the development of general approaches to the design of these classes of biologically active compounds. The mechanism of competing reactions yielding both 7‐methylene‐6.8‐dioxabicyclo[3.2.1]‐octanes and functionalized cyclopentenoles from their common intermediate 1,5‐diketones and acetylene in the KOH/DMSO superbasic media is investigated using the B2PLYP/6‐311++G**//B3LYP/6‐31+G* calculations with particular attention to the diastereoselectivity aspects of individual stages.     

Formal total synthesis of (+)-didemniserinolipid B

The formal total synthesis of (+)-didemniserinolipid B, a marine tunicate possessing a 6,8-dioxabicyclo[3.2.1]octane framework, was accomplished starting from l-(+)-tartaric acid. The key transformations in the synthesis include the elaboration of a γ-hydroxy-amide readily obtained by desymmetrization of tartaric acid bis-amide via the controlled addition of a Grignard reagent followed by stereoselective reduction of the resulting ketone.     

Stereoselective synthesis from D-glucose of (−)-∂-multistriatin (component of the European elm bark beetle, Scolytus multistriatus, aggregation pheromone).

A six step stereoselective synthesis of (−)-∂-multistriatin [(1S,2S,4S,5R)-5-ethyl-2,4-dimethyl-6,8-dioxabicyclo[3.2.1]octane, 1] is presented, starting from 2,4-O-ethylidene-D-erythrose (2), which is readily available from D-glucose.     

Formation mechanism and conformational structure of 2,3,4-trimethyl-1,5-di(thiophen-2-yl)pentane-1,5-dione: quantum chemical study

The mechanism of formation of 1,5-diketone, a key intermediate in the stereoselective assembly of 2,3,4-trimethyl-7-methylidene-1,5-di(thiophen-2-yl)-6,8-dioxabicyclo[3.2.1]octane, and the conformational structures of the reagents and products were studied by quantum chemical calculations at the MP2/6-311++G**//B3LYP/6-31G* level of theory with solvent effects treated within the polarizable continuum model. The activation barrier for the addition of the 1-oxo-1-(thiophen-2-yl)propan-2-ide ion (1), which is generated from 1-(thiophen-2-yl)propan-1-one, to acetylene is 15.2 kcal mol^–1. The reaction affords β,γ-unsaturated ketone. The subsequent addition of carbanion 1 to the C=C double bond of the resulting β,γ-unsaturated ketons yields 1,5-diketone and occurs with insignificant (less than 1 kcal mol^–1) activation barrier.     

TiCl4 catalyzed tandem construction of C-C and C-O bonds: a simple and one-pot atom-economical stereoselective synthesis of spiro-oxindoles

An atom-economical stereoselective synthesis of [{1-acetyl-5-methyl-6,8-dioxabicyclo(3.2.1)octane}-7-spiro-3'-(indolin-2'-one)] derivatives, containing both the oxindole and 6,8-dioxabicyclo(3.2.1)octane moieties via TiCl(4) catalyzed coupling of 2-acetyl-6-methyl-2,3-dihydro-4H-pyran with isatin derivatives is described.     

A novel bicyclic ketal fragmentation reaction

Bicyclic ketals of the 6,8-dioxabicyclo[3.2.1]octane series are specifically cleaved to give δ, ε-unsaturated ketones by treatment of the ketal with acetyl iodide.     

Novel octulosonic acid derivatives in the composite Smallanthus sonchifolius

Two novel octulosonic acid derivatives with a 6,8-dioxabicyclo[3.2.1]octane skeleton that are major water-soluble phenolic compounds were found in the roots of Smallanthus sonchifolius. The structures of these compounds were determined to be (1R,2S,3S,4R,5S,7R)-4-hydroxy-7-hydroxymethyl-3-[3-(3,4-dihydroxyphenyl)-1-oxo-2-propenyloxy]-6,8-dioxabicyclo[3.2.1]octan-5-carboxylic acid (4-O-caffeoyl-2,7-anhydro-d-glycero-β-d-galacto-oct-2-ulopyranosonic acid) and (1R,2S,3R,4R,5S,7R)-2,4-dihydroxy-7-hydroxymethyl-2,3-bis[3-(3,4-dihydroxyphenyl)-1-oxo-2-propenyloxy]-6,8-dioxabicyclo[3.2.1]octan-5-carboxylic acid (4,5-di-O-caffeoyl-2,7-anhydro-d-glycero-β-d-galacto-oct-2-ulopyranosonic acid) by MS, NMR and CD spectral analyses.     

Structures of the products of reaction of 1,5-diketones with hydroxylamine

Alkylidenebis-2,2′-cyclohexanones react with two molecules of hydroxylamine to give 8-hydroxy-1,2,4,5-bistetramethylene-7-oxa-6,8-diazabicyclo[3.2.1]octanes, whereas 5,5′-methyl ene-2,2-dimethyl-4-pyrone and “semicyclic” and aliphatic-aromatic 1,5-diketones form dioximes.     

Synthesis of a C(9)—C(13) fragment of acutiphycin from levoglucosan

Alkylation of (1R,2R,5R)-2-benzenesulfonyl-6,8-dioxa-bicyclo[3.2.1]octan-3-one, which is accessible from levoglucosan, afforded (1R,2R,5R)-2-benzenesylfonyl-2,4,4-trimethyl-6,8-dioxabicyclo[3.2.1]octan-3-one. This was further converted into (1S,2R,3S,5R)-2,4,4-trimethyl-6,8-dioxabicyclo[3.2.1]octan-3-ol representing the C9—C13 fragment of acutiphycin molecule.     

Photolysis of saturated bicyclic peroxides

Under direct photolysis, [n.2.1]-peroxides (n = 3–5) isomerise chiefly to epoxyaldehydes, [n.2.2]-peroxides (n = 2–4) mainly undergo dehydrogenation to cycloalkane-1,4-diones, and 2,3-dioxabicyclo[2.2.1]heptane exhibits both types of behaviour; photo-isomerisation of 6,7- into 6,8-dioxabicyclo[3.2.1]octane occurs when benzophenone is present as sensitizer.     

Synthesis of (±)-bicolorin, the aggregation pheromone of beech bark beetle Taphrorychus bicolor

A new procedure was proposed for the synthesis of (±)-bicolorin, the aggregation pheromone of beech bark beetle Taphrorychus bicolor. Following the proposed procedure, readily accessible ethyl levulinate ethylene acetal was converted in three preparative steps into 1,5-dimethyl-6,8-dioxabicyclo[3.2.1]-octan-2-one which was subjected to Wittig olefination and subsequent homogeneous hydrogenation.     

Stereoselective synthesis of optically active forms of δ-multistriatin,the attractant for european populations of the smaller european elm bark beet

A stereoselective synthesis of highly optically pure enantiomers of δ-multistriatin [(1S,2S,4S,5R)-2,4-dimethyl-5-ethyl-6, 8-dioxabicyclo[3.2.1)octane and its antipode] was accomplished starting from tartaric acid enantiomers.     

Ready access to the 6,8-dioxabicyclo[3.2.1]octane ring system using asymmetric heterocycloaddition induced by a chiral sulfoxide: application to the total synthesis of the Mus musculus pheromone

A new stereocontrolled total synthesis of the (1R,5S,7R)-exo-6,8-dioxabicyclo[3.2.1]oct-3-ene skeleton of the Mus musculus pheromone has been achieved via an asymmetric intermolecular Diels–Alder reaction and an intramolecular conjugated addition, controlled by a chiral auxiliary.     

Squarate desymmetrisation–ozonolysis as an approach to β-substituted-α-ketosuccinates and squalestatin synthesis

Silylated tertiary alcohols from 1,2-addition of alkyllithiums to dialkyl squarates undergo alkene ozonolysis to give β-substituted-α-keto-β-(silyloxy)succinates. With 3-(triethylsilyloxy)butyllithium the methodology was applied to the 2,8-dioxabicyclo[3.2.1]octane core of the squalestatins. Enantioselective 1,2-addition to di-tert-butyl squarate using butyllithium or diethylzinc/Ti(iPrO)₄ in the presence of chiral ligands (such as bisoxazolines or camphorsulfonamides, respectively) gave the corresponding tertiary alcohols in up to 67.5:32.5 er.     

Stereoselective Synthesis of Tropanes via a 6π‐Electrocyclic Ring‐Opening/ Huisgen [3+2]‐Cycloaddition Cascade of Monocyclopropanated Heterocycles

The synthesis of tropanes via a microwave‐assisted, stereoselective 6π‐electrocyclic ring‐opening/ Huisgen [3+2]‐cycloaddition cascade of cyclopropanated pyrrole and furan derivatives with electron‐deficient dipolarophiles is demonstrated. Starting from furans or pyrroles, 8‐aza‐ and 8‐oxabicyclo[3.2.1]octanes are accessible in two steps in dia‐ and enantioselective pure form, being versatile building blocks for the synthesis of pharmaceutically relevant targets, especially for new cocaine analogues bearing various substituents at the C‐6/C‐7 positions of the tropane ring system. Moreover, the 2‐azabicyclo[2.2.2]octane core (isoquinuclidines), being prominently represented in many natural and pharmaceutical products, is accessible via this approach.     

Total synthesis of thromboxane B2 starting from (R,R)-tartaric acid as a chiral pool

Optically active natural thromboxane B₂ (TXB₂) was synthesized from (R,R)-tartaric acid as only chiral source. The synthesis was achieved through regio- and stereoselective introduction of acetate moiety at the C2-position of the 6,8-dioxabicyclo[3.2.1]octene derivative (2) to provide an acetamide derivative (6), partial ring opening of 6 to give a pyranoid (10), and construction of the C15-hydroxyl group of TXB₂ by stereospecific allylic transposition of the inherent chirality of tartaric acid in the trans-allylic acetate (18).     

Studies toward the total synthesis of cyclodidemniserinol trisulfate. Part II: 3,5,7-Trisubstituted 6,8-dioxabicyclo [3.2.1] octane core structure construction via I2-mediated deprotection and ring closure tandem reaction

The 3,5,7-trisubstituted 6,8-dioxabicyclo [3.2.1] octane core structure was synthesized by employing a chiral pool convergent synthesis strategy and the I₂-mediated simultaneous deprotection and ring closure reaction as the key step, providing a practical and efficient synthetic approach applicable to the further total synthesis of the natural product cyclodidemniserinol trisulfate.