Investigation of the Cyclopentenone Formation via the α‐Alkynone Cyclisation: Synthesis of the Acorone Intermediate 8‐Methylspiro[4.5]deca‐3,7‐dien‐2‐one

Abstract

As a further application of the cyclopentenone formation A+C via the thermal a-alkynone cyclisation B-tC and in order to test the fate of an isolated C,C-double bond within a molecule under these conditions, we investigated the synthesis of the acorone intermediate 3 starting from the known carboxylic acid 1. The a-alkynone 2 was obtained from 1 uiu the acyl chloride 6 and a Pd(I1)-catalysed route (22%). The thermolysis of 2 at 550" provided the target molecule 3 (48%) together with the product 9 (20%) of a competing intramolecular ene reaction and its dimer 10 (4%). At a higher thermolysis temperature (650"), the spiro ketone 3 was found to be unstable, affording the retro-Diels-Alder fragments 4-methylidene-2-cyclopentenone (12) (33 YO) and isoprene (32%). A further example of the influence of an isolated double bond on the yield of the cyclopentenone-formation sequence A+C was provided by the comparison of the annelation 14+20 (5% overall with Pd(I1)-catalysed acylation) with that of its non-olefinic analogue 17+21 (53% overall with Friedel-Crafts acylation).1. Introduction. -The cyclopentenone-formation sequence A+C (Scheme I ) , which was made possible by the discovery [1] of the thermal a-alkynone cyclisation step B+C, has been applied to the synthesis of various cyclopentanoid systems [l] [2] including natural products [3] [4]. So far the acids A and the a-alkynones B used in this sequence contained no other functional groups. In order to establish the synthetic scope of this annelation, the compatibility with the reaction conditions of functional groups and other structural elements aside from the reacting substructures must be examined. We first chose to investigate the influence of an isolated C,C-double bond located several bonds away from the reactive moiety (COOH or COC=C) on the annelation sequence A K . To that purpose we considered the synthesis of 8-methylspiro[4.5]deca-3,7-dien-2-one (3),

Camphor-Derived, Chelating Auxiliaries for the Highly Diastereoselective Intermolecular Pauson−Khand Reaction: Experimental and Computational Studies

“…( − )-( S )-4-Butyl-2-cyclopentenone (26). In a round-bottomed flask under nitrogen were placed 23 (130 mg, 0.63 mmol), maleic anhydride (0.3 g, 3.1 mmol), and CH 2 Cl 2 (4 mL). To this solution was added MeAlCl 2 (1 M in hexane, 0.7 mL, 0.7 mmol) via syringe.…”

Corannulene. A Three-Step Synthesis1

“…Inspired by the reports of R. F. C. Brown et al that vinylidenes generated thermally from terminal acetylenes in the gas phase can be trapped intramolecularly to construct both five-membered rings and six-membered rings , (eq 1), we worked out a short synthesis of 7,10-diethynylfluoranthene ( 3 ) and found that it does indeed provide a convenient new route to corannulene (eq 2). 1a,b The utility of our new synthetic strategy for constructing curved PAHs was thereby established, and the success of the experiment provided validation for the concept on which the strategy was based.…”

Asymmetric Pauson−Khand Reactions Using Camphor-Derived Chelating Thiols as Chiral Controllers

“…The enantiomeric purity of (−)- 15 was measured by chiral GC (α-Dex). Samples of (−)- 15 arising either from diastereomerically enriched 11a or 11b showed high enantiomeric excesses (94−96% ee) in good agreement with the diasteriomeric ratio of the starting adducts, thus confirming that no significant racemization takes place in the retro-Diels−Alder step.The absolute configuration of adducts 11a and 11b (as depicted in Scheme ) could be established from the sign of the specific rotation of 14 , of known absolute configuration, , by assuming the universally observed exo configuration of the Pauson−Khand adducts involving norbornene and norbornadiene. The absolute configuration of the major diastereomer of adducts 12a , b was established by chemical correlation since hydrogenation of the major stereoisomer of 11a , b led to the major stereoisomer of 12a , b .…”