New Six- to Eight-Membered-Ring Formation Based on the lntramolecular Endo-Mode Ring Closure of Allenyl (Substituted Phenyl)alkyl Ketones

1994 ◽  
Vol 23 (3) ◽  
pp. 597-600 ◽  
Author(s):  
Yoshimitsu Nagao ◽  
Woo Song Lee ◽  
Yoichi Komaki ◽  
Shigeki Sano ◽  
Motoo Shiro
Keyword(s):  
Membered Ring ◽  
Ring Formation ◽  
Ring Closure

On the Concerted Ring Opening of Protonated Squalene Oxide and A-Ring Formation in the Biosynthesis of Lanosterol

2003 ◽  
Vol 68 (1) ◽  
pp. 202-210 ◽  
Author(s):  
B. Andes Hess
Keyword(s):  
Ring Opening ◽  
Density Functional ◽  
Reaction Pathway ◽  
Ring Expansion ◽  
Membered Ring ◽  
Ring Formation ◽  
Ring Closure ◽  
Primary Role ◽  
Transition Structure

Density functional calculations were performed on a model system of squalene oxide to study the mechanism of the formation of ring A in the biosynthesis of lanosterol from squalene. When (2Z)-6,7-epoxy-3,7-dimethyloct-2-ene was protonated, it was calculated to undergo a very facile ring opening of the oxirane in concert with the formation of the six-membered ring of the 4-(hydroxymethyl)-1,2,3,3-tetramethy1cyclohexyl cation. A study of the reaction pathway (IRC) indicates a very early transition structure in which the carbon- carbon double bond participates anchimerically in the ring-opening of the protonated oxirane. It is suggested that the primary role of the enzyme in this first step of the biosynthesis of lanosterol is protonation of the oxirane ring along with holding the substrate in the proper conformation for the concerted ring-closure to occur. The similarity between this mechanism and that recently proposed for concerted C-ring expansion and D-ring formation in the biosynthesis of lanosterol is discussed.

Ring formation via β-keto ester dianions

1977 ◽  
Vol 55 (6) ◽  
pp. 996-1000 ◽  
Author(s):  
Phaik-Eng Sum ◽  
Larry Weiler
Keyword(s):  
Membered Ring ◽  
Ring Formation ◽  
Methyl Acetoacetate ◽  
Keto Ester ◽  
Keto Esters

The reaction of α,ω-dihalides with the dianion of methyl acetoacetate gives a mixture of mono- and bisalkylated products. The monoalkylated products can be cyclized via the monoanion to cyclic β-keto esters with a seven- or eight-membered ring. Alternatively these monoalkylated products can be cyclized via the dianion to γ-cyclopentyl- or γ-cyclohexyl-β-keto esters.

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