248
(2 H, s), 3.28 (2 H, s), 3.64 (3 H. s), 4.98 (1 H, m), and 5.04 (1 H, m). Treatment of this product with 1 drop of dilute HC1 in methanol (40 ml) overnight at room temperature gave the same mixture (nmr comparison) of esters (15-18) obtained by irradiation of 4,6-dimethyl-2-pyrone in methanol (see Scheme 11).
6 6 6
Scheme I1 13
+
14
-
' 0 C02CH3
H3C
\o
H,C
\O CO.CH
16
CH,
0
111
CO?CH,-t H3C%co-cH,
17
19
I1
HJC
15
HC
I
&C02CH3
HCI
CH OH
mechanism below, where the production of I11 is a minor photolytic pathway. The back reaction of the ketene to a-pyrone is fast except at cryogenic temperatures.
de Mayo112 first suggested a ketene as a primary product in the photolysis of a-pyrones to explain the production of CH30COCH=C(CH3)CH2COCH3 on photolysis of
18 HC1
i
CH,OH
CH;
1Hc-ocJ=)cHJH C
H ,cA O - C H
41CI C'
___f
20
It I S clear that the photochemical formation of ester products from 4,6-dimethyl-2-pyrone does not involve a ketene intermediate as has been generally assumed. This conclusion raises serious questions whether ketene intermediates are involved in esterforming reactions of other pyrones and coumarins. Acknowledgment. This research was supported by Grant No. AM-14624 from the National Institute of Arthritis and Metabolic Disease, U. S. Public Health Service. C. L. McIntosh, 0. L. Chapman* Department of Chemistry, Iowa Stare Uiziuersrty Ames, Iowa 50010 Receiied J d y 7 , 1972
Photochemistry of a-Pyrone in Solid Argon
Sir: The photochemistry of a-pyrones has been well studied. We present here direct evidence for the
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