768 



Colchicine 



Even before all of these facts were available, Dewar^^ suggested 

 that ring C of colchiceine was trojiolonoid and on this basis the struc- 

 ture of colchiceine is represented by the tautomeric system (XXI) ^ 

 (XXII) . The validity of this formulation is now generally accepted 

 and an earlier formida, proposed by Windaus,-- need not be dis- 

 cussed here. 



6.4: Comparison With Tropolones 



It is necessary, however, to refer briefly at this stage to some of 

 the more general featines of tropolone chemistry (for more compre- 



MeO 

 MeO 



NHAc 



MeO 



MeO 



MeO 



O 



OH 



NHAc 



(XXI) 



( XXII ) 



OH 



hensive treatment, see Cook and Loudon-''") . Tropolone (2-hydro- 

 xycyc/oheptatrienone) and its derivatives have aromatic properties, 

 the reactivity of the ethylenic and carbonyl functions being sup- 

 pressed. Thus the compoimds are substituted by electrophilic reagents 

 but do not react with carbonyl reagents. The hydroxyl group is 

 markedly acidic. Salt formation is accompanied by development or 

 intensification of color, and coordination complexes are produced 

 with ferric or cupric ions. Tropolone itself exhibits feebly basic 

 properties and yields a hydrochloride and a picrate. Tropolone 

 ethers resemble esters in their ready hydrolysis. With varying ease 

 individual tropolones (or their ethers) are isomerized by hot alkali, 

 the 7-membered ring luidergoing contraction to the benzenoid struc- 

 ture of an appropriately substituted benzoic acid (or ester) . Catalytic 

 hydrogenation of tropolones is seldom simple. When complete, it 

 yields octahydrides which are l:2-diols, but it may involve loss of 

 oxygen, and ketonic intermediates are frequently detectable. 



The general analogy with colchiceine, implicit in this account of 

 tropolone behavior, is borne out by more specihc comparison. Like 

 unsymmetrically substituted tropolones, colchiceine is known only as 

 a single substance which yields two isomeric methyl ethers, colchicine 

 and wocolchicine, corresponding to the tautomerides (XXI) and 



