endo-Adducts of cyclopenta-or cyclohexa-1, 3-dienes with cyclohexa-1,4-dienes, especially p-benzoquinones (Ia) , cyclise on ultraviolet irradiation to cage-like isomers (Ib) by formation of a cyclobutane ring from the two double bonds. The effects of interaction between the two carbonyl groups in the photoisomers (Ib) appear in their infrared and ultraviolet spectra. * After the appearance of our preliminary note 3 we learnt €hat the photoisomerisation of benzoquinone adducts had been independently observed in the laboratory of Professor C. H. De Puy (Iowa State College, Ames, U.S.A.), who very generously offered to leave the field to us.

Hydrogenation of the endo-cyclopentadiene-benzoquinone adduct yields the endo-tetrahydro-derivative, isomerised by alkali to the exo-derivative. The high-melting substance obtained as a by-product on hydrogenation under certain conditions and previously assumed to be another isomer is in fact dimeric .ONE of the first Diels-Alder reactions to be observed was that between cyclopentadiene and $-benzoquinone,l and the structure of the adduct was later established by Diels and his collaborators during their classical researches. The accepted endo-cis-stereochemistry (I) is confirmed by the conversion of the adduct into a saturated photo-isomer (II).4The stereochemistry of the adduct's hydrogenated derivatives , however, has never been established, and is indeed the subject of some confusion.Albrecht reduced the adduct with zinc and acetic acid to a dihydro-derivative (111), n. p. 35", that Diels and Alder5 hydrogenated further over palladium in ethanol to a tetrahydro-derivative (IV), m. p. 56". By similar hydrogenation of the adduct (I) itself Diels et aL2 obtained another dihydro-derivative, m. p. 190", which we have shown to be the diol (V). Hydrogenation of the adduct in chloroform over platinum oxide was reported by Bergel and Widman to yield a second tetrahydro-derivative (IV?), m. p, 246".Recently Wiley, Batchelder, and McRowe reduced the adduct with lithium aluminium

The endo-trans-endo-configuration of the biscyclopentadiene-benzoquinone adduct has been established by photochemical cyclisation, in agreement with Winstein's conclusions. p-Benzoquinones and ene-1,4-diones with olefinic double bonds overlapping the chromophore show a new band in the ultraviolet spectrum, not present in that of the parent chromophore, attributed to intramolecular charge-transfer. The medium-wavelength band in the spectrum of p-benzoquinones is very sensitive to the effects of strain.AN interest in their use for the study of photochemical reactions and electronic spectra led us to examine critically the evidence for the stereochemistry of the mono-and bisadducts of cyclopentadiene with fi-benzoquinone. The elzdo-configuration of the monoadduct (I) was simply confirmed by its isomerisation by light to the cage-isomer (11).

water and was dried in vacuo at 100°over phosphorus pentoxide; 1.1 g. (48.2% of the theoretical yield) of crystals was obtained which melted at 200-205°. Several recrystallizations from hot water to a constant microbiological activity gave silky needles which melted at 206-208°.

A solution containing 4.78 g. (5 ml.) of glycol in 100 ml. of water was refluxed for 5.75 hours. The characteristic odor of methyl isopropyl ketone became evident within one hour. A 25-ml. sample of the solution required 0.45 ml. of 0.141 N sodium hydroxide to neutralize the acid produced. Analysts for ketone indicated a yield of 1.2%. In another experiment where 1.2 g. of sodium carbonate was added at first, no ketone was formed.Refluxing and stirring 16.63 g. (10 ml.) of trimethylethylene bromide with 200 ml. of water and 10 g. of sodium carbonate for six hours gave no detectable amount of ketone. The dibromide phase, however, disappeared in two hours.