AN UNUSUAL EXAMPLE OF A 6-ENDO-DIG ADDITION TO AN UNACTIVATED CARBON-CARBON TRIPLE BOND

Methyl 2-[2-[2-(1-hydroxy-1-ethyl)phenyl]ethynyl]benzoate was prepared as an intermediate for subsequent conversion to an a-diazo ketone. Un der the basic conditions used to hydrolyze the methyl ester, the neigh boring hydroxyl functionality underwent reaction with the unactivated acetylenic group, producing a benzopyranyl-substituted alpha-diazoacet ophenone. Treatment of this diazocarbonyl compound with a catalytic qu antity of rhodium(II) mandelate afforded a novel dibenzo[a,e]cyclonone none derivative. The reaction proceeds via an initially formed oxonium ylide which rearranges further by means of a 1,2-alkyl shift. A prime factor in determining the direction of internal cyclization to the tr iple bond is the presence of the carbomethoxy group in the ortho posit ion of the beta-phenyl ring. Thus, in contrast with related systems wh ich exhibit a clear preference for 5-exo-dig cyclization at the acetyl enic center, the 6-endo-dig addition is the preferred pathway for the o-formyl- and o-carbomethoxy-substituted alkynyl alcohols. Careful mon itoring of the reaction actually showed that the reaction proceeds by initial formation of the 5-exo-dig product followed by a novel rearran gement to the 6-endo product.