Pyridinium chlorochromate in the organic synthesis: a convenient oxidation of enol-ethers to esters and lactones

Piancatelli, Giovanni; Scettri, Arrigo; D’Auria, Maurizio

doi:10.1016/s0040-4039(01)83272-x

Cited by 58 publications

(28 citation statements)

References 2 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…This oxidation has been previously found [54] using chromic acid salts for the oxidation of enol-ethers to lactones. So, reaction (2) probably occurs with the formation of the enolate as intermediate.…”

Section: Reaction Pathwaysmentioning

confidence: 61%

FTIR studies of tyrosine oxidation at polycrystalline Pt and Pt(111) electrodes

Zinola

Rodrı́guez

Arévalo

et al. 2005

Journal of Electroanalytical Chemistry

View full text Add to dashboard Cite

Section: Reaction Pathwaysmentioning

confidence: 61%

FTIR studies of tyrosine oxidation at polycrystalline Pt and Pt(111) electrodes

Zinola

Rodrı́guez

Arévalo

et al. 2005

Journal of Electroanalytical Chemistry

View full text Add to dashboard Cite

“…Assuming that hydride shift and alkyl shift termination pathways are not readily available owing to geometric constraints, insertion into the proximal and wellaligned methylene CÀH bond to form cyclopropane 25 seems reasonable. 13 C NMR chemical shift predictions for this structure support this hypothesis. [6] The side product (26) of enol-ether oxidation by PCC presumably arises from a mechanism related to that proposed by Cossy et al, who found that PCC oxidation of tricyclic cyclopropylcarbinols often leads to rearrangement with cyclopropane cleavage.…”

mentioning

confidence: 64%

“…In later experiments, cycloisomerization was conducted on diastereomerically pure substrate, resulting in a yield of 56 % (46 % over two steps when 12 is not purified), along with quantities of a cyclopropane-containing side product, which was tentatively assigned as 25. Although formation of 24 completes a formal synthesis of both echinopines according to the synthesis of Nicolaou, Chen, and co-workers, [3] we opted to generate echinopine B in only one step by enol-ether oxidation as prescribed by Piancatelli et al [13] Thus, treatment of 24 with PCC provided echinopine B (2), along with unexpected side product 26. The synthesis of echinopine B was complete in only 13 steps.…”

mentioning

confidence: 99%

A Synthesis of Echinopine B

2012

View full text Add to dashboard Cite

The synthesis of natural products of unprecedented structure can drive innovation in strategy and provide opportunities to test the limits of new methodology; therefore, it remains a worthy venture even when the targets are not yet known to possess important biological activity. In that context, we felt that the synthesis of the structurally unusual echinopines (Scheme 1) was warranted for the opportunity to evaluate metal-catalyzed polycyclizations in complex settings, and to access these natural products and structural analogues for a broader evaluation of their biological properties.Echinopines A and B (1 and 2) are sesquiterpenes isolated by Kiyota and co-workers from the root of the plant Echinops spinosus [1] and they probably originate from biosynthetic modification of the guaiane framework (see 3). In spite of the lack of reported biological activity, their novel structure inspired several research groups to embark upon and complete syntheses of these compounds. First, in 2009, Magauer, Mulzer, and Tiefenbacher reported a clever enantioselective route beginning from 1,5-cyclooctadiene; this work served to determine the absolute configuration of these natural products. [2] Shortly thereafter in 2010, Nicolaou, Chen, and co-workers in Singapore reported an asymmetric synthesis of these targets. [3] A formal synthesis followed later that year from Chen and co-workers. [4] We felt that a strategy related to the presumed biogenesis of the echinopines might yield a more direct synthesis than these previous routes. We posited that a bioinspired conversion of a suitably functionalized cis-fused guaiane-like precursor into the tetracyclic core of the echinopines might take place through a transitionmetal catalyzed bicyclization reaction; the cyclopropane acetic acid/ester functional group would then be introduced directly, thus avoiding the multi-step homologation of cyclopropane carboxylate ester intermediates that was employed in previous syntheses. In a third-generation route to the echinopines, Chen and co-workers recently reported the successful execution of a related strategy, providing echinopine B in 25 steps. [5] Herein, we report our synthesis of echinopine B, which proceeds through a concise sequence featuring a one-step conversion of a guaiane-like intermediate to the natural product.Initially, much effort was put forth to generate the fused tricyclic motif, which consisted of the cyclopropane and two cyclopentane rings, and involved either a carbonylative Heck cascade (4!5, Scheme 2a) or a Heck bicyclization terminating in b-hydride elimination (6!7); unfortunately, these approaches were never successful. [6] In late 2009, we were inspired by a PtCl 2 -catalyzed reaction of alkene-tethered propargylic ethers that was disclosed by Michelet and coworkers. [7] In the simple enyne model system 8 (Scheme 2 b), Scheme 1. The echinopines and the related guaiane framework.Scheme 2. a) Attempted Heck cascades to access tricycles relevant to the echinopines. b) Successful enyne cycloisomerization. Bn = benzyl,...

show abstract

“…Therefore, the keto group in 20 was first masked as the ketal before effecting the t BuOK facilitated elimination reaction to arrive at the cyclic enol ether 22. Then, the PCC mediated oxidation [22] on 22 effortlessly furnished the lactone 23. Finally, the unmasking of the ketal group in 23 allowed us to smoothly access the bicyclic lactone 24 in an overall yield of~35% over 7 steps from 17.…”

Section: Resultsmentioning

confidence: 99%