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Hirofumi Ohmura

Publications and source records attributed to Hirofumi Ohmura.

4 recordsLinked to original sources
PubMed2006

Studies on oxidopyrylium [5+2] cycloadditions: toward a general synthetic route to the C12-hydroxydaphnetoxins.

[Structure: see text] 12-hydroxydaphnetoxins, members of the structurally fascinating daphnane diterpene family, exhibit a wide range of significant biological activities. A general route to the BC-ring system of 12-hydroxy daphnetoxins is reported based on D-ribose. Depending on the choice of protecting groups and solvent, the oxidopyrylium-alkene [5+2] cycloaddition originating from A provides cycloadduct diastereomer B or C with good to excellent selectivity.

Bridged-Ring Compounds↗
PubMed2003

(2,5)-Ene cyclization catalyzed by mesoporous solid acids: isotope labeling study and ab initio calculation for continuum from concerted to stepwise ene mechanism.

(2,5)-Ene reactions catalyzed by mesoporous solid acids are reported from the mechanistic point of view. The continuum (2,5)-ene mechanism from the concerted to the cationic cyclization followed by 1,2-hydride shift is evaluated. The solid-acid-catalyzed cyclization of the oxonium ion intermediate 4 derived from cyclic allylic lactol ether 3 bearing allylic hydroxy group affords the (2,5)-ene product as the enol form, eventually tautomerizing to the corresponding aldehyde 6. The continuum from the concerted to stepwise mechanism is experimentally and theoretically verified in the present ene cyclization of the oxonium ion intermediate such as 4. The stepwise cyclization leading to aldehyde 6 is thus shown to associate with the concerted version as a result of the stabilization of the beta-hydroxycarbenium ion intermediate.

Journal Article↗
PubMed2002

Palladium-catalyzed isobenzofuran generation under neutral conditions via oxidative addition to lactol methyl ether.

A novel method for generation of isobenzofuran is developed from lactol methyl ether using palladium catalysts. This reaction can be carried out under neutral conditions and hence improves on the precedent methods under acidic or basic conditions and at high temperatures. Furthermore, this Pd-catalyzed isobenzofuran generation suggests the involvement of oxidative addition of Pd catalyst into benzylic or allylic methyl ethers. [reaction: see text]

Journal Article↗