Combined Addition of Alkenyl and Allenic Anions to Squarate Esters. Direct Competition between Six-Ring and Eight-Ring Electrocyclization of 1,2,4,6,8-Cumulenic Pentaenes

Preparation of Vinyl Allenes from 1-Lithio-1,3-dienyl Phosphine Oxides and Aldehydes by the Wittig−Horner Reaction

[reaction: see text] Vinyl allenes were prepared in high yields by the Wittig-Horner reaction of 1-lithio-1,3-dienyl phosphine oxides with aldehydes. 1-Lithio-1,3-dienyl phosphine oxides were generated in situ from lithiation of 1-iodo-1,3-dienyl phosphine oxides, which were obtained by iodination of alpha-phosphinozirconacyclopentadienes. As a whole, a vinyl allene is synthesized from two different alkynes and one aldehyde.

Palladium-Catalyzed Tandem Reaction of Yne-Propargylic Carbonates with Boronic Acids: A Simple Method for the Synthesis of Fused Aromatic Rings through Allene-Mediated Electrocyclization

The palladium-catalyzed tandem reactions of yne-propargylic carbonates with aryl boronic acids, 2-furyl boronic acid, and 2-thiopheneboronic acid, followed by 6pi-electrocyclization to give fused ring aromatic products such as naphthalene, benzofuran, and benzothiophene derivatives are realized. Screening of the reaction conditions revealed that the combination of [Pd(PPh3)4] in THF gave the best results in terms of reactivity and product yields in the reaction of yne-propargylic carbonates with phenylboronic acid. The reaction is sensitive toward steric hindrance when substituted phenylboronic acids are employed. However, when we take 2-furyl boronic acid as the organometallic reagent, most substrates perform very well to give benzofuran derivatives. In addition, 2-thiopheneboronic acid is also a very effective coupling reagent to give bezothiophenes in high yields. A mechanism is proposed that involves the formation of an allenylpalladium complex from Pd0 and propargylic carbonate, followed by insertion of an intramolecular triple bond and the Suzuki coupling reaction, and then electrocyclization.

Transition Structures, Energetics, and Nucleus-Independent Chemical Shifts for Divinylcyclobutene-to-Cyclooctatriene Rearrangement: A DFT Study

The minimum energy reaction paths and nucleus-independent chemical shifts (NICS) for the Cope rearrangement of cis-3,4-divinylcyclobutene, obtained by (U)B3LYP/6-31G calculations, are reported. Three transition structures (endo-boatlike, chairlike, and exo-boatlike) have been located, giving rise to formation of cis,cis,cis-, cis,cis,trans-, and trans,cis,trans-1,3,5-cyclooctatrienes, respectively. The minimum energy path proceeds through an endo-boatlike, aromatic transition structure. The reaction path of the rearrangement is intervened by enantiomerization saddle point of the product. NICS values calculated for transition structures agree qualitatively with their activation energy and reaction exothermicity orders. Cope rearrangement and electrocyclic ring-opening processes of cis-3,4-divinylcyclobutene are competitive, but the former is relatively more favored and exothermic than the latter.

Applications of the squarate ester cascade to the expeditious synthesis of hypnophilin, coriolin, and ceratopicanol

The first applications of the squarate ester cascade to natural products synthesis have been realized. Only 10 laboratory steps mediate the conversion of diisopropyl squarate to (+/-)-hypnophilin (8). Key reactions include a combination of chlorination, reduction, dehydration, and oxidation maneuvers in the proper sequence. A penultimate precursor to 8 has previously been converted into coriolin (9), thereby allowing a formal synthesis of racemic 9 also to be claimed. A rather different strategy was employed to arrive at (+/-)-ceratopicanol (10). Of the seven steps involved, three consisted of the use of lithium in liquid ammonia. The three divergent synthetic objectives realized in these experiments involved (a) generation of an extended enolate anion and its regioselective C-methylation at the gamma-carbon; (b) unprecedented reductive cleavage of a beta-isopropoxy group in a 2,3-diisopropoxy-2-cyclopentenone setting; and (c) conventional conversion of an alpha-alkoxy ketone to the parent carbonyl system. Thus, the appreciable enhancement in structural complexity offered by the squarate cascade holds considerable potential for the concise synthesis of constitutionally intricate targets.

Three-component coupling via the squarate ester cascade as a concise route to the bioactive triquinane sesquiterpene hypnophilin

[reaction: see text] A squarate ester cascade is used to provide in one step, via the coupling of three reactants, a highly oxygenated linear triquinane product. The latter is transformed in nine steps into hypnophilin. The access route involves a combination of chlorination, reduction, dehydration, and oxidation maneuvers in the proper sequence.

Ab initio studies of

Ab initio calculations of the [1,5]-H shift in (3Z)-penta-1,3-diene and other substituted pentadienes and heteroanalogues using the hybrid density functional Becke3LYP with the 6-31G basis set are presented. Electron-donating substituents, such as methoxy in (3Z)-3-methoxypenta-1,3-diene 1, or heteroatoms such as a nitrogen atom in (Z)-ethylidenevinylamine 2, (1Z)-buta-1,3-dienylamine 3, (2Z)-but-2-enylideneamine 4, (Z)-allylidenemethylamine 5, and methylene-(Z)-propenylamine 6 are introduced. The electron-withdrawing fluoride is substituted for the hydrogen atoms in (3Z)-3-fluoropenta-1,3-diene 7, (3Z)-2,4-difluoropenta-1,3-diene 8, (3Z)-1,1',2,3,4,5,5'-heptafluoropenta- 1,3-diene 10, (1E,3E)-1,3,5-trifluoropenta-1,3-diene 11, and (1Z,3E)-1,3,5- trifluoropenta-1,3-diene 13. A detailed analysis of the geometries, energies, and electronic characteristics of the sigmatropic transposition compared to those of the unsubstituted case provides insights into substituent effects of this prototype of pericyclic reaction. The inductive and mesomeric effects of heteroatoms or heterosubstituents are of a great importance and in a continuous balance in the energetics of the transformation. Sterics can also play an important role due to the geometrical constraints of the reaction. As a general trend, decreasing the electron density of the phi system destabilizes the aromatic transition structure and increases the activation energy, and vice versa.