[3+2] Cycloaddition of Propargylic Alcohols and α‐Oxo Ketene Dithioacetals: Synthesis of Functionalized Cyclopentadienes and Further Application in a Diels–Alder Reaction

Abstract

Cyclopentadienes are valuable intermediates in organic synthesis and also ubiquitous as the Cp ligands in organometallic chemistry. As part of ongoing efforts to develop novel organic reactions that employ functionalized alkynes, a [3+2] cycloaddition of propargylic alcohols and ketene dithioacetals has been developed, which leads to fully substituted 2,5-dialkylthio cyclopentadienes in good to excellent yields. In an unusual dethiolating Diels-Alder reaction, the cyclopentadienes were further reacted with maleimides to afford a family of novel fluorescent polycyclic compounds.

Scope and advances in the catalytic propargylic substitution reaction

“…Regiospecic [3 + 2] cycloaddition of propargylic alcohols 39 and a-oxo ketene dithioacetals 43 in the presence of BF 3 $Et 2 O generates 2,5-dialkylthiocyclopentadienes 44 in good yields, Scheme 14. 46 As detailed in the mechanism, the catalyst BF 3 $Et 2 O initiates the formation of propargylic carbocation 39a by the loss of the OH species from the propargylic alcohol 39 which isomerizes to the less sterically hindered allenic carbocation 39b. The preferable nucleophilic attack on the allenic carbocation 39b by the a-oxo ketene dithioacetals 43, followed by the cyclization, affords ve-membered carbocycle, along with allylic carbocation.…”

“…Regiospecific [3 + 2] cycloaddition of propargylic alcohols 39 and α-oxo ketene dithioacetals 43 in the presence of BF 3 ·Et 2 O generates 2,5-dialkylthiocyclopentadienes 44 in good yields, Scheme 14 . 46 …”

[(NHC)NiIIH]‐Catalyzed Cross‐Hydroalkenylation of Cyclopropenes with Alkynes: Cyclopentadiene Synthesis by [(NHC)NiII]‐Assisted C−C Rearrangement

“…It provided a new general access to functionalized 3 in one step under mild reaction conditions (30 °C), low substrate ratio (as low as equimolar), and low catalyst loading (1–4 mol %). It complements a few shortcomings of the intramolecular approaches, tailormade ynamides with Au vinyl carbenoids, cycloaddition, and approaches that use stoichiometric amounts of organometallic reagents . High yield, functional‐group compatibility, and regioselectivity (up to 46:1, >20:1 in general) were noted in cases with either the commercially available or simple terminal alkynes 2 (Table , Sets 1–3).…”

Addition of dithi(ol)anylium tetrafluoroborates to α,β-unsaturated ketones

“…They also allow the formation of valuable building blocks that can be used for diverse transformations in organic chemistry (e.g., Umpolung) [3]. Ketene 1,3-dithioacetals [4] are of particular interest as attractive nucleophiles for the addition to halonium ions [5–6], acyl chlorides [7] and other electrophiles [8–10] and are broadly used as precursors for [2 + 2]-cycloaddition [11–12], (aza)-Diels–Alder reaction [13–14], and [3 + 2]-cycloaddition reactions [15–16]. The products of these reactions are diversely substituted (ketene) dithiolanes or dithianes which allow a wide range of transformations like oxidation [17], fluorination [18] and cyclative reaction with [19–20] and without conservation [21–22] of the dithioacetal functionality.…”