Dehydrogenation dichlorodicyanoquinone

The yellow crystalline azaberbinone (388) has been prepared by several, closely related methods. Dehydrogenation of betaine 378 (Section III,E,2) with dichlorodicyanoquinone gives compound 388 directly. Alternatively, this compound (388) has been prepared in three ways from 6 -nitropapaverine (385) (Scheme 15) (i) Treatment of compound 385 with triethyl phosphite gives a low yield of betaine 388 directly (ii) compound 385 in methanolic KOH at reflux temperature gives the anthranil 386 which rearranges to the betaine 388 in hot triethyl phosphite and (iii) iodine oxidation of 6 -nitro-papaverine (385) generates the A-oxide 387 which is reduced to the betaine 388 by sodium bisulfite. The chemistry of compound 388 remains unexplored. 

That the chalcone units combined in a head-to-tail manner was conclusively decided by showing that the thiolane could be dehydrogenated by dichlorodicyanoquinone in boiling chlorobenzene to a dibenzoyldiphenylthiophene, 8, whose 13C NMR revealed the presence of two different benzoyl groups (18). It followed that each of the two possible modes of head-to-nead fusion of chalcone units in a thiolane could be rejected. 

The mechanism of dehydrogenation of aromatic compounds with dichlorodicyanoquinone (DDQ) is shown in Scheme 7,32. 

The cycloaddition of l,3,4-oxadiazin-6-ones 97 (R =Ph, Pr R = Ar, C02Me) to cyclopentadiene in the presence of TFA leads to the formation of regioisomeric dihydro-a-pyrones 106 and 107 <1998CC2387>. The dehydrogenation of 106 and 107 with dichlorodicyanoquinone (DDQ) affords the a-pyrones 108 and 109. The latter are converted to cyclopenta[f]pyrans 110 with diisobutylaluminium hydride (DIBAL-H) (Scheme 13). 

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