Enantioselective ring construction through asymmetric olefin-ketene cycloaddition. A highly enantiocontrolled approach to (-)-.alpha.-cuparenone and (+)-.beta.-cuparenone

Abstract

amounts of p-benzoquinone), For example, the oxidation of 1,3-cycloheptadiene by O2 in acetic acid (2 M on LiOAc) at 40 OC catalyzed by 5 mol % each of Co(TPP), hydroquinone, and P~( O A C )~ afforded cis-1,4-diacetoxy-2-cycloheptene (>92% cis) in 65% isolated yield.The corresponding oxidation using Mn(TPP) in place of Co-(TPP) also worked but was considerably slower. For example, the oxidation of 1,3-cyclohexadiene gave only a 56% yield of 1,4-diacetoxy-2-cyclohexene after 48 h, and now the trans/cis ratio was only 72/28.The oxidation described here has similarities with biochemical processes where an oxidation becomes very mild and selective when several redox couples with falling redox potentials are interacting. In many biological systems metalloporphyrins and p-hydroquinoneslp-benzoquinones play important roles as electron carriers. For example, ubiquinones are known to mediate the electron-transfer processes involved in energy production in aerobic organisms. The model system described here is to our knowledge the first example of a selective oxidation using a triple catalysis with oxygen as the ultimate oxidant.

Cycloaddition and Electrocyclic Reactions of Vinylketenes, Allenylketenes, and Alkynylketenes

“…Intermolecular ketene-alkene cycloadditions require activation of one or both of the components to secure good yields, as exemplified by the addition of dichloroketene to a chiral, nonracemic enol ether (Scheme 183), 203 …”

Synthetic Organic Chemistry Based on Small Ring Compounds

“…These include [2ϩ2] cycloaddition reactions via photolytic [25][26][27][28][29][30][31][32] and thermolytic [33][34][35][36][37][38][39][40][41][42] processes, enantioselective alkylation, 43) ring contraction of cyclic acetals, 44) radical cyclization, 45) and chemical 46) and enzymatic 47) resolution of racemates or prochiral precursors. In contrast to these well documented methods described above, there have been few studies on the asymmetric induction of the ring expansion reaction of cyclopropane rings to form cyclobutanes.…”

Efficient Ether Cleavage in Lignins: The Derivatization Followed by Reductive Cleavage Procedure as a Basis for New Analytical Methods

“…The β-bromo ethers 7 that are produced are amenable to formal two-electron transformations ( Figure 6). Related ethers have been cleaved using Zn dust (31) or Cr(II)en (32,33), for example. Reaction of the lignin or lignin model products with Zn° in dioxane/acetic acid/water at room temperature produces the cleaved products within minutes.…”