•Reductive debrominations compete with dehydrobrominations.•C–H acidity of the substrates determines selectivity.•Triethylamine is not an efficient base for reductive debromination.The interaction of various 1,2-dibromides with NEt3 under various conditions (THF and DMF, respectively) at different temperatures was investigated. Our results from these reactions show that substrate dependent dehydrobrominations compete with reductive debrominations. A comprehensive discussion of these competitive pathways is offered.Download high-res image (34KB)Download full-size image

•cis-β-Bromostyrene derivatives are synthesized from cinnamic acids.•The reactions are stereospecific and only cis-β-bromostyrenes are formed.•β-Lactone intermediates are stable but do not need to be isolated.cis-β-Bromostyrene derivatives were synthesized stereospecifically from cinnamic acids through β-lactone intermediates. The synthetic sequence did not require the purification of the β-lactone intermediates although they were found to be stable and readily purified in most cases.Download high-res image (26KB)Download full-size image

The title compound exhibits a number of modes of reactivity toward nucleophiles/bases owing to the presence of several electrophilic and potentially nucleophilic sites in the molecule. We explored the reactions of 6-(chloromethyl)-6-methylfulvene with oxygen and nitrogen nucleophiles and bases as well as a carbon-based nucleophile (an enamine) and realized all possible reactivity modes predicted on the basis of electrophilic and nucleophilic positions in this compound.

Aldonitrones derived from spiro[2.4]hepta-4,6-diene-1-carbaldehyde and its benzo analog undergo a tandem uncatalyzed intramolecular cyclopropane–nitrone cyclization-5,6-dihydro-1,2-oxazine cycloreversion to give cyclopentadienones. Similarly, the NH-nitrone generated in situ from spiro[cyclopropane-1,1′-indene]carbaldehyde oxime leads to benzocyclopentadienone (1H-inden-1-one) by the same mechanism. DFT calculations are in favor of a concerted yet highly asynchronous pathway for the cyclizations. Control experiments with the dihydro and tetrahydro derivatives show that the spirocyclopentadiene unit is essential for the success of the reaction, invoking spiroconjugative effects for increased cyclopropane reactivity.

Junionone is the first monocyclic cyclobutane monoterpenoid isolated from a plant. Of the existing four syntheses of this compound, none employs a ketene cycloaddition to construct the four-membered ring. Herein, we report the first total synthesis of junionone that features a ketene cycloaddition for the synthesis of this compound, starting from the commercially available 1,5-hexadiene.

•Fulvene endoperoxides ordinarily decompose via allene oxide intermediates.•A hydroxyl substituent has a significant effect on the decomposition mechanism.•In some cases the hydroxyl group causes intramolecular 1,7-H shifts.•Allene oxide/oxy-allyl species have been implicated in some of the isomerizations.•Molecular modeling studies support the proposed pathways.The thermal decomposition of fulvene endoperoxides ordinarily proceeds via an allene oxide intermediate affording oxepin-2(3H)-one derivatives. We have now uncovered new, unusual pathways in these decompositions where the presence of a hydroxyl group on the alkyl or aryl attached to the fulvene exocyclic double bond has a profound effect on the fate of the reactive intermediates derived from the unstable endoperoxides. Computational work supports the proposed mechanistic pathways.

vic-Dibromides containing the α-bromocarbonyl or α-bromoaromatic moieties were reductively debrominated to furnish alkenes in high yields. o- and m-anisidines but not p-anisidine were found to be effective debrominating agents. The reductive debrominations were found to be trans-stereospecific.

A new method for a one-step synthesis of imidazolidin-4-ones has been developed based on tandem nucleophilic additions-decarboxylation-intramolecular cyclizations of imines with a Leuchs’ anhydride. Depending on the reaction conditions (with or without base), one either obtains the title compounds, or α-amino acid amide-derived Schiff bases. The latter have also been shown as useful precursors of α-amino acid amides by selective reduction of the imine bond with NaBH4 in the presence of p-TsOH.

Acylketene, generated from 2,2,6-trimethyl-4H-1,3-dioxin-4-one reacts with aryl aldimines derived from 4-methyl-2-aminopyridine to give a variety of products, depending on the substituent on the C-aryl group, base used, and hydrolytic stability of the starting aldimine. Also the presence of the N-(2-pyridyl) group plays an important role in the fate of the reaction course, frequently participating in intramolecular conjugate additions, giving rise to interesting heterocycles.

A systematic study of the reactions of cyclopentadiene with α,β-unsaturated carbonyl compounds in the presence of catalytic pyrrolidine-H2O revealed that the reactions can either proceed with a Michael attack at the β-carbon of enone, or 1,2-addition to the carbonyl, leading either to 4-cyclopentadienyl-2-butanones or 6-vinylfulvenes. The former can be isolated and/or converted to the corresponding 1,2-dihydropentalenes with base (or in one-pot at longer reaction times). Substitution pattern on the enones on the competing pathways have been studied and consistent mechanisms are proposed.

The activating effects of the benzyl and allyl groups on SN2 reactivity are well-known. 6-Chloromethyl-6-methylfulvene, also a primary, allylic halide, reacts 30 times faster with KI/acetone than does benzyl chloride at room temperature. The latter result, as well as new experimental observations, suggests that the fulvenyl group is a particularly activating allylic group in SN2 reactions. Computational work on identity SN2 reactions, e.g., chloride– displacing chloride– and ammonia displacing ammonia, shows that negatively charged SN2 transition states (tss) are activated by allylic groups according to the Galabov–Allen–Wu electrostatic model but with the fulvenyl group especially effective at helping to delocalize negative charge due to some cyclopentadienide character in the transition state (ts). In contrast, the triafulvenyl group is deactivating. However, the positively charged SN2 transition states of the ammonia reactions are dramatically stabilized by the triafulvenyl group, which directly conjugates with a reaction center having SN1 character in the ts. Experiments and calculations on the acidities of a variety of allylic alcohols and carboxylic acids support the special nature of the fulvenyl group in stabilizing nearby negative charge and highlight the ability of fulvene species to dramatically alter the energetics of processes even in the absence of direct conjugation.

In situ generated acetone pyrrolidine enamine undergoes [6 + 2] cycloadditions with fulvenes to give 1,2-dihydropentalenes. This ring annulation method works particularly well with 6-monosubstituted fulvenes and is subject to steric hindrance at C-6 of the fulvene. On the basis of mechanistic studies, optimal conditions have been developed for a one-pot synthesis of 1,2-dihydropentalenes using catalytic amounts of pyrrolidine.

Triphenylphosphine reduction of saturated endoperoxides derived from 6,6-dimethylfulvene and spiro[2.4]hepta-4,6-diene in the presence of nucleophiles results in the formation of products that mainly stem from deoxygenation followed by carbocation formation. Nucleophilic attack by solvent proceeds by an SN1 like mechanism; allyl shifts and cyclopropylcarbinyl-cyclobutyl rearrangements also occur. With the systems lacking carbocation-stabilizing groups, the deoxygenation step is preceded by attack of H2O at the phosphorus.