Cyclopenta pyran

The cyclopenta[c]pyran system 6 is formed when allenyl propynyl ethers are heated in CO in the presence of Co/Rh nanoparticles a Pauson-Khand reaction is involved <07SL453>. An enantioselective Bronsted acid-catalysed Nazarov cyclisation of a dihydropyran-based divinylketone affords cyclopenta[/>]pyrans 7 <07AG(E)2097> and Sc and In triflates catalyse a similar reaction of 6-alkenoyl derivatives of both dihydropyran and thiopyran <07S1733>. 

Fischer-type chromium carbene complexes with 1-ethoxycyclopropylalkynyl substituents at the carbene carbon, e.g. 25, on reaction with dimethylamine and subsequent conversion of the resulting vinylcarbene with alkynes surprisingly did not give phenol derivatives, as would be expected from the known Dotz reaction, but gave cyclopenta[ )]pyrans, e.g. 26. The reaction is interpreted as a double alkyne-insertion/CO-insertion sequence with formation of a trienylketene intermediate, which undergoes intramolecular hetero-Diels-Alder cycloaddition and dimethylamine elimination. ... 

Synthesis and reactions of isoflavone analogs, 3-hetarylchromones 99KGS3. Synthesis of cyclopenta[ ]pyran skeleton of natural lactones 97T14507. Synthesis of isocoumarins 970PP631. 

Recently reported rearrangements of linear triquinanes (244) to their angularly fused isomers (247) have been envisaged to involve the formation and equilibration of the enolates (245) and (246) followed by an intramolecular Michael addition of the enolate ion in (246) to the cyclopentenone moiety see Scheme 62. It has been reported that treatment of l,10-dibromobicyclo[8.1.0]undecane-3,8-dione (248) with triethylamine in dichloromethane results in the formation of a novel cyclopenta[(>]pyran derivative (249). The mechanism outlined in Scheme 63 has been postulated to account for this unusual transformation. 

Recently, the cyclopenta[c]pyran (122) was obtained as a secondary product along with 1,2,3-tribenzoylcyclopropane from phenacyl-trimethylammonium hydroxide a complicated sequence was proposed involving a Michael addition to 1,2-dibenzoylethylene intermediately formed. 

Synthesis of cyclopenta[d]pyran skeleton of natural lactones 97T14507. 

The formation of the tricarbonylchromium-complexed fulvene 81 from the 3-dimethylamino-3-(2 -trimethylsilyloxy-2 -propyl)propenylidene complex 80 and 1-pentyne also constitutes a formal [3+2] cycloaddition, although the mechanism is still obscure (Scheme 17) [76]. The rf-complex 81 must arise after an initial alkyne insertion, followed by cyclization, 1,2-shift of the dimethylamino group, and subsequent elimination of the trimethylsilyloxy moiety. Particularly conspicuous here are the alkyne insertion with opposite regioselectivity as compared to that in the Dotz reaction, and the migration of the dimethylamino functionality, which must occur by an intra- or intermo-lecular process. The mode of formation of the cyclopenta[Z ]pyran by-product 82 will be discussed in the next section. 

Scheme 19 Formation of cyclopenta[b]pyrans 91 and 92 by a [3+2+2+1] cocyclization [41,80]. For further details see Table 3...

Table 3 Selected examples of cyclopenta[fr]pyrans 91 and 92 formed by [3+2+2+1] cocy-clizations (see Scheme 19)...

Systematic investigations on infrared spectra of pseudoazulenes have not been published. Data can be found only in the experimental sections of papers. Investigations on 2-phenyl-2H-pyrindine (29b),85 cyclopenta[c]thio-pyran (31a),85 4-methyl-4//-cyclopenta[6]quinoline (39a),11J-114 5-methyl-5H-cyclopenta[c]quinoline (42a),123 and oxalenes 27, 33, 40, 44231 show that the absorption frequencies fall into three main regions 600-1250 cm" 1 (in-plane and out-of-plane bonding vibrations of CH), 1300-1620 cm"1 (aromatic C—C bonds), and 2980-3120 cm" 1 (C H stretching vibrations). 

Many interesting photochemical reactions are possible with pyran-2-one and its various alkyl and benzo derivatives. For example, irradiation of 2-oxo-2/7-l-benzopyran 4-carbonitrile 170 in the presence of excess 2,3-dimethylbut-3-ene in Ar-degassed benzene provided imino-substituted cyclopenta[c]annelated product 171 (Scheme 19)... 

These are (1) The chiral substrate approach. This approach involves using chiral precursors that transfer their chirality to the final cyclopentenone. This implies the synthesis of chiral substrates, which has generally been made from classic chiral pools. Examples include carbohydrate derivatives like 40 that give 41 with variable yields depending on the substitution pattern. 41 is transformed into cyclopenta[c]pyrane 42, which is the skeleton of iri-doids [93]. In another example epichlorhydrin (43) is used to construct chiral enyne 44 which gives cyclopentenone 45 [94] (Scheme 14). 

Both a base and an Ir catalyst 6 are necessary to effect diastereoselective sequential Nazarov cyclisation and Michael addition between a pyran-based alkylidene (3-ketoester and a nitrostyrene. The reaction is diastereoselective, leading to substituted cyclopenta[b]pyrans with three stereocentres <06JA5312>. 

Formation of cyclopenta[6]pyrans (= oxapseudoazulenes) 102 is achieved by subsequent insertion of two molecules of alkyne and one carbon monoxide unit into the Cr=C bond of a (2-aminoalkenyl)carbene complex ( )-96 generated from compound lj (Scheme 36).9f... 

Reactions of 4-amino-l-metalla-l,3-dienes, generated from (1-alkynyl) carbene complexes 1 by addition of secondary amines (Scheme 32), with alkynes may lead to production of different compounds, depending on the substitution pattern as well as on the reaction conditions employed.3r Whereas successive addition of a secondary amine and one molecule of a terminal alkyne to a (l-alkynyl)carbene complex lj affords cyclopentadi-enes (Scheme 35), addition of two molecules of a terminal alkyne to complex lj yields cyclopenta[h]pyrans (Scheme 36). In an extension of this reaction sequence, (methylene)cyclopent-2-enones 103 are obtained from (1-alky-nyl)carbene chromium compounds 1 (R = rc-Pr, Ph, CH2CH2CH2OTBDS) by addition of secondary amines (NR) = NMe2, morpholino) and disubsti-tuted alkynes (Scheme 37).9e... 

Cyclobutanes, synthesis with phosphaal-kenes, 13 Cyclopentadienes annulation to enamines, 176-177 synthesis, 197-198 Cyclopenta[b]pyrans, synthesis, 199 Cyclopentenylidenes, synthesis, 192-193... 

For the preparation of the benzo[i>]cyclopropa[d]pyran 2-599, Ohta and coworkers have treated 3-ethoxycarbonylcoumarin (2-597) with dimethylsulfoxonium methylide (2-598), derived from the corresponding trimethylsulfoxonium iodide and NaH (Scheme 2.136) [307]. The isolated product was not 2-599, however, but an unexpected cyclopenta[i>]benzofuran derivative 2-600 probably formed through 2-599. 

A Nazarov cyclisation of l-(3,4-dihydro-2//-pyran-6-yl)-3-phenylpropenones affords cyclopenta[Z)]pyranones (Scheme 9) <03OL4931> and the conjugated ethoxytriene 7 derived from tetrahydropyran-2-one also yields a fused cyclopentenone <03JOC9728>. 

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