The reaction of α-diazo-β-hydroxy esters with boron trifluoride

Abstract

Exposure of a cyclic a-diazo-P-hydroxy ester to different concentrations of boron trifluoride in various solvents affords an interesting variety of products.Diazo-hydroxy-acylmethanes 3a,b, easily prepared by aldoltype condensation of lithio-diazo-acylmethanes 2 with aldehydes and ketones la,b, are valuable synthetic intermediates able to undergo a range of transformations.1-16 It was shown, for example, that 3a,b rearrange by proton acid catalysis or thermolysis with carbon or hydrogen migrati0n.2,~?5 The subsequent discovery that the transformation can be achieved smoothly in almost quantitative yield by exposing 3a,b to catalytic amounts of dirhodium(I1) tetraacetateg has been exploited to obtain synthetic intermediates which are difficult to obtain by other means.17-20 The possibility that 3a,b may be smoothly converted into the corresponding P-hydroxycarbony1 compounds by catalytic hydrogenation (PdC) has also been exploited synthetically. 16 The little studied Lewis-acid catalysed decomposition of a-diazo-6-hydroxy ketones or esters has been reported to follow a different path. Thus, when exposed to boron trifluoride-diethyl ether in a polar solvent such as acetonitrile, the acyclic compounds 3a (R' = H) give the corresponding acylacetylenes 6 as major products along with minor amounts of migration products.3.779J4 The

Homologation Reaction of Ketones with Diazo Compounds

“…A series of rearrangements leads to formation of a more stable allylic cation that can be trapped by the solvent of the reaction. 231,232 Concerning the ring expansion of four-membered derivatives, other chemicals were shown to be superior to BF 3 •OEt 2 . TFA, TMSOTf, Rh 2 (OAc) 4 , and photochemical activation were tested in the ring expansion of cyclobutenone 118 to provide the five-membered β-diketone 119 or 120 in reasonable yields together with 2(5H)-furanone 121 in most cases (Scheme 66).…”

“…Instead of following synchronous migration of one of the substituents together with molecular nitrogen extrusion, the Lewis acid complexed alcohol follows a neighboring-group participation in the diazo moiety to generate a cycloalkylidene diazonium salt that eventually results in a highly reactive linear vinyl cation after nitrogen extrusion. A series of rearrangements leads to formation of a more stable allylic cation that can be trapped by the solvent of the reaction. , …”

BF3·Et2O-Promoted Decomposition of Cyclic α-Diazo-β-Hydroxy Ketones: Novel Insights into Mechanistic Aspects

“…Herein, as a continuation of our interest in diazo chemistry [9,[22][23][24][25][26][27][28][29][30][31], we investigated the cascade rearrangement of destabilized vinyl cations deriving from the BF 3 •Et 2 O-induced decomposition of α-diazo-β-hydroxy ketones, prepared by aldol-type condensation of cyclic ketones with DAA (1). In particular, the effect of solvent and ring size on products distribution and mechanism paths were analyzed and discussed.…”

Remote C–H insertion of vinyl cations leading to cyclopentenones

“…2 ). 21 – 24 In that reaction, a vinyl cation, 25 , 26 formed from the diazo alcohol by the process described by Pellicciari 27 and Padwa, 28 underwent a Grob-type fragmentation that was facilitated by the γ-silyloxy group. We hypothesized that a β-hydroxy-α-diazo ketone that lacks a fragmentation-promoting group at the γ position might instead react by a C–H insertion pathway.…”