o-Quinone Methides from 4-Allenylcyclobutenones: Synthesis and Chemistry

CuI-Catalyzed Decarboxylative Thiolation of Propargylic Cyclic Carbonates/Carbamates to Access Allenyl Thioethers

The first CuI-catalyzed decarboxylative thiolation of terminal alkyne-substituted cyclic carbonates/carbamates to access allenes has been developed. A wide range of hydroxymethyl- and aminomethyl-containing allenyl thioethers were smoothly obtained in good to excellent yields under mild conditions. The copper-allenylidene intermediate among the process is crucial to the decarboxylative thiolation reaction. This method opens up a new channel to access allenyl thioether compounds.

Unravelling the configuration of transient ortho-quinone methides by combining microfluidics with gas phase vibrational spectroscopy

The alkylidene double bond configuration of transient ortho-quinone methides (o-QMs) is studied by cryogenic ion trap vibrational spectroscopy.

Cycloaromatization protocol for synthesis of polysubstituted phenol derivatives: method development and mechanistic studies

The scope of the cycloaromatization of propargylic ethers was explored using operationally simple air- and moisture-insensitive conditions. Highly substituted phenol derivatives were obtained in high yields. Mechanistic experiments indicate that the reaction occurs by an electrocyclization followed by 1,3-proton transfer.

How easy are the syntheses of allenes?

This short review highlights some of the recent important developments on the synthesis of allenes and its applications on the synthesis of some allenic natural products and allenic-based optoelectronic materials.

Reactive iron carbonyl reagents via reaction of metal alkoxides with Fe(CO)5 or Fe2(CO)9: synthesis of cyclobutenediones via double carbonylation of alkynes

Alkoxy bases such as t-BuOK react with Fe(CO)(5) to give reactive iron carbonyl intermediates that in turn react with alkynes at 70 °C in THF to give 1,2-cyclobutenediones in 70-93% yields after CuCl(2)·2H(2)O oxidation. A novel 1,2-diacyloxyferrole derivative was isolated in the reaction of diphenylacetylene with Fe(CO)(5)/t-BuOK in the presence of acetyl chloride in contrast to the formation of a 1,4-diacyloxyferrole complex formed in the reaction using Fe(CO)(5)/Me(3)NO. The Fe(2)(CO)(9)/t-BuOK reagent system also converts the alkynes to corresponding cyclobutenediones in 63-90% yields under similar reaction conditions.

Pd-catalyzed, Cu(I)-mediated cross-couplings of bisarylthiocyclobutenediones with boronic acids and organostannanes

Bisarylthiocyclobutenedione 7 reacted smoothly with a variety of both organostannanes and (hetero)arylboronic acids in the presence of a catalytic amount of Pd and a stoichiometric amount of CuTC to produce symmetrical disubstituted cyclobutenediones in yields that range from 37 to 94% (18 examples).

Selective Cross-Couplings. Sequential Stille−Liebeskind/Srogl Reactions of 3-Chloro-4-arylthiocyclobutene-1,2-dione

The synthesis and initial reactivity studies of 2 are described. It was found that it participates in Stille couplings exclusively at the C-Cl site with a number of organostannanes (58-71% yield) in the absence of Cu(I). Then, these new derivatives were functionalized at the C-S site with boronic acids by switching to the Liebeskind-Srogl reaction conditions (in the presence of a Cu(I) carboxylate) to yield the bifunctionalized cyclobutenediones (44-90% yield).

New Synthesis of Semisquaric Acid Derivatives via Chlorinated N-(Cyclobutylidene)amines

The synthesis and reactivity of new chlorinated N-(cyclobutylidene)amines leading to new synthetic pathways toward various substituted cyclobutenediones is described.

Studies on the Regio- and Stereoselectivity of Halohydroxylation of 1,2-Allenyl Sulfides or Selenides

It was observed that the halohydroxylation of 1,2-allenyl sulfides or selenides with Br2 (CuBr2 or NBS) or I2 and water demonstrated a fairly good regioselectivity (i.e., the C=C bond that is remote from the S or Se atom was halohydroxylated with the halogen atom connecting to the middle carbon atom and the hydroxyl group connecting to the non-S terminal carbon or Se-substituted terminal carbon atom of the allene moiety), leading to the synthesis of synthetically important 3-organosulfur or seleno-2-haloallylic alcohols. The stereoselectivity depends on the nature of X+ and S or Se, showing a Z-selectivity with the matched Lewis acid-base pair.

Leishmanicidal and antiplasmodial activity of constituents of Smirnowia iranica

Three unusual, highly oxygenated novel phenylpropanoids (1-3) and two novel isoflavans, 8-prenylmucronulatol (4) and smiranicin (6), were isolated from Smirnowia iranica together with a previously described isoflavan, glyasperin H (5). The structures were established using homo- and heteronuclear two-dimensional NMR experiments. The isoflavans significantly inhibited the growth of extracellular stages of three Leishmania species in vitro, their activity against the intracellular stages being considerably lower. 8-Prenylmucronulatol (4) showed moderate in vitro toxicity against Plasmodium falciparum, without noticeable erythrocyte membrane effects at the inhibitory concentration. Because of the structural relationship of isoflavans with chalcones and aurones, some of which are potent antiprotozoal agents, the isoflavan skeleton may be a template structure in search for new compounds with leishmanicidal and antiplasmodial activity.

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.

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

The sequential addition of 1 equiv each of a lithioallene and an alkenyllithium to diisopropyl squarate has been studied. The product compositions resulting from the ensuing reaction cascades were not at all comparable to those resulting from the reverse mode of nucleophile addition. These results require that different intermediates intervene. When the alkenyl anion is the lead reagent, the lithioallene enters predominantly trans and two sequential electrocyclic steps ensue. Initial introduction of methoxyallene causes the second nucleophile to enter preferably from the cis direction, presumably because of oxygen coordination to lithium. The proximal orientation of the unsaturated moieties leads to operation of the dianionic oxy-Cope rearrangement. The second electrocyclic event that transpires following formation of the trans dialkoxide is not of the customary 8pi conrotatory type. Rather, a 6pi disrotatory process occurs exclusively. This "allene effect" is shown to originate from strain, orbital alignment, and conjugative factors operating at the transition state.