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References
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Scheme 1. Reagents and conditions: (a) H2 (1 atm), 10%
Pd/BaSO4 (cat.), pyridine (cat.), toluene, rt (94%); (b) H2 (1
atm), 10% Pd/BaSO4 (cat.), toluene, rt (91% for 9a; 71% for
9b).
2. Bellina, F.; Biagetti, M.; Carpita, A.; Rossi, R. Tetra-
halides are reacted with 1-(hetero)aryl-2-propyn-1-ols in
THF under reflux in the presence of Et3N and catalytic
quantities of PdCl2(PPh3)2 and CuI.16
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R. Tetrahedron 2002, 58, 5023–5028.
This last result, however, was not unexpected. In fact, it
has been reported that 12 is able to react with N- or
O-nucleophiles and that alcohols such as methanol
cause ring opening of 12.17 On the other hand, we
observed that compound 12 was completely consumed
in the Pd/Cu-catalyzed reaction with 14h, even though
we were unable to isolate and identify the product(s)
derived from the nucleophilic attack of 14h on 12.
7. Biagetti, M. Ph.D. Thesis; University of Pisa, 2002.
8. (a) Bohlmann, F.; Hopf, P.-D. Chem. Ber. 1973, 106,
3772–3774; (b) Bohlmann, F.; Burckardt, T.; Zdero, C.
Naturally Occuring Acetylenes; Academic Press: London,
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9. Pirkle, W. H.; Dines, M. J. Am. Chem. Soc. 1968, 90,
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11. For leading references on this reaction, see: (a) Sono-
gashira, M.; Tohda, Y.; Hagihara, N. Tetrahedron Lett.
1975, 4467–4470; (b) Rossi, R.; Carpita, A.; Bellina, F.
Org. Prep. Proced. Int. 1995, 129–160; (c) Sonogashira,
K. In Comprehensive Organic Synthesis; Trost, B. M.;
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12. Compound 12 was prepared according to this procedure:
Phosphorous pentachloride (32.0 g, 153.7 mmol) was
added portionwise to 11 (techn. 90%, 10.0 g, 76.9 mmol)
cooled in an ice bath. The mixture was heated to 100°C
for 10 min and then cooled to 0°C, poured into ice water
(100 ml) and extracted with CH2Cl2 (3×80 ml), dried over
Na2SO4, and concentrated under reduced pressure. The
residue was fractionally distilled to give 12 (8.2 g, 82%
yield) as a colourless solid. Mp 27°C (lit.8 mp 27°C).
13. It should be noted that, due to the high reactivity of
Et2NH towards 14, the Sonogashira reaction between 14
and 1-alkynes 16 could not be performed in the presence
of this secondary amine.
Having secured good access to compounds 8 we next
demonstrated their synthetic utility by their conversion
into 6-alkyl-2(2H)-pyranones 9 and 6-[(Z)-1-alkenyl]-
2(2H)-pyranones 10 (Scheme 1). Thus, hydrogenation
of 8a in toluene at room temperature using pyridine-
poisoned 10% palladium on BaSO4 selectively gave 10a
in 94% yield. Compound 10a is the (Z)-stereoisomer of
a substance extracted from Solenopsis invicta.1c
On the other hand, when toluene solutions of 8a and 8b
were hydrogenated at room temperature in the presence
of 10% palladium on BaSO4, chemically pure 9a and 9b
were selectively obtained in 91 and 71% yield, respec-
tively. Compound 9a is a fungal metabolite of Tricho-
derma viride18 and a pheromone component of ants.19
In conclusion, we developed an efficient and convenient
synthesis of 6-(1-ynyl)-2(2H)-pyranones 8 through Pd/
Cu catalyzed reaction of 1-alkynes with easily available
6-chloro-2(2H)-pyranone (12). Moreover, we demon-
strated the synthetic utility of compounds 8 by their
conversion into 6-alkyl-2(2H)-pyranones 9 and 6-[(Z)-
1-alkenyl]-2(2H)-pyranones 10. A study on the use of
12 and 6-(trimethylsilylethynyl)-2(2H)-pyranone (8e) as
direct precursors to 6-(hetero)aryl- and 6-(1,3-diynyl)-
2(2H)-pyranones, respectively, is in progress.
14. All new compounds were obtained in analytically pure
form if not otherwise noted. Selected spectral properties
of compounds 8a–g are as follows.
Compound 8a: IR (film): 2227, 1736, 1620, 1541, 1324,
1097, 801 cm−1. 1H NMR (200 MHz, CDCl3): l 1.03 (3H,
t, J=7.5 Hz), 1.62 (2H, pseudo-sext., J=7.5 Hz), 2.40
(2H, t, J=7.0 Hz), 6.29 (1H, dd, J=9.0 and 1.0 Hz), 6.30
(1H, dd, J=7.0 and 1.0 Hz), 7.28 ppm (1H, dd, J=9.0
and 7.0 Hz). 13C NMR (50 MHz, CDCl3): l 13.4, 21.2,
21.3, 73.7, 97.9, 109.1, 116.0, 142.9, 145.3, 161.3 ppm.
Acknowledgements
This work was supported by the Ministero del-
l’Istruzione, dell’Universita` e della Ricerca (MIUR)
and the University of Pisa.