Carbon−Carbon Bond Formation Reaction of Ethereal Oxonium Ylides via Metal−Enolate Intermediates

First, the carbon−carbon (C−C) bond-forming reaction of aldehydes with bicyclo[m.n.0]-1-oxonium
ylides was studied as the ylide was transiently formed in the Rh(II)-catalyzed reaction of a
nonenolizable diazoketone, namely, 2-(3-diazo-1,1-dimethyl-2-oxopropyl)-2-methyldioxolane (1). The
reaction of 1 with benzaldehyde in the presence of ClTi(Oi-Pr)3 gave the three-carbon, ring-enlarged,
and C−C-bonded product 2a (53%). Second, enolizable diazoketone 5 bearing no methyl substituents
at the α-position was studied under similar catalytic conditions, and the ring-enlarged and C−C-bonded products 19a and 20a were also formed (87%) when titanium compound ClTi(Oi-Pr)3 or
Ti(Oi-Pr)4 was used. Similar reactions of diazoketones 27, 29, and 31 bearing a cyclic acetal ring
and a longer tethering chain than 5 gave C−C-bonded products 28 (74%), 30 (8%), and 32 and 33
(overall 48%), respectively, albeit 28 and 30 possessed a spiro bisacetal structure. Thus, the hitherto
unclarified C−C bond formation of ethereal oxonium ylides with carbonyl electrophiles was realized
with the use of an appropriate Lewis acid, for example, ClTi(Oi-Pr)3.