Jr-facial selectivity

A certain jr-facial selectivity was achieved when MCpCl2 (M = Ti, Zr) fragments were coordinated to the optically active fused cyclopentadienyl ligands. For instance, reaction of ZrCpCl3 with the lithium derivative of 126 at —78 °C gave predominantly 133 which was characterized by X-ray structural analysis [152]. 

Klein showed that axial reaction of the parent methylenecyclohexane 37 is preferred in hydroboration [106], The experimental data on the parent methylenecyclohexanone 37a accumulated by Senda et al. [107] and the more recent systematic studies by Cieplak et al. [108, 109] on jr-facial selectivities of 3-substituted methylene-cyclohexanes 37 have characterized the intrinsic features of the facial selection of methylenecyclohexanes. That is, axial preference of unsubstituted and 3-substituted methylenecyclohexanes was observed in oxymercuration [107] and epoxidation reactions [110], There is also an increase in the proportion of axial attack with increase in the electronegativity of the remote 3-equatorial... 

The reactivity and jr-facial selectivity of CH3OH and H2O additions to the radical cations of 7-benzhydrylidenenorbornene derivatives (68), generated by photo-induced... 

Compound 13a exhibits high jr-facial selectivity in the re-gioselective [4 + 2] cycloaddition (promoted by SnCU) with 2,2-... 

Trisubstituted 2-azetidinones have been prepared by diastereoselective and jr-facially selective Lewis acid-catalyzed intermolecular Diels-Alder reactions of 3-butadienylazetidin-... 

A review of the cycloaddition reactions of o-benzoquinones as carbodiene, heterodiene, dienophile, or heterodienophile has been published. In the Diels-Alder reaction of furans with masked o-benzoquinones (145), the furans unexpectedly behaved as dienophiles to yield cycloadducts (146) (Scheme 56). Masked benzoquinones behave as dienes which undergo Diels-Alder reactions with electron-rich dienophiles such as enol ethers and thienol ethers.The asymmetric Diels-Alder reactions of 5-substituted and 5,6-disubstituted (S)-2-(p-tolylsulflnyl)-l,4-benzoquinones with cyclopentadiene and fran -piperylene show complete regio- and jr-facial selectivities. The hetero-Diels-Alder reactions of o-benzoquinones with tetracyclone produce cyclopenta[I ][l,4]benzodioxinone derivatives in high yield. 

In the addition of an achiral enolate to an aldehyde bearing a stereogenic center at Ca, the jr-facial selectivity can usually be predicted by the Felkin-Anh model (158 see also Chapter 2) [83]. However, simple 1,2-diastereoin-duction with enolates derived from typical metals such as lithium, boron, and titanium is often insufficient to be synthetically useful [16, 20). Higher levels of diastereoselectivity are generally obtained by use of enol silanes [34], as reported by Heathcock (Equation 14) [84]. 

Cyclopropanation and Aziridination (Alkene Three-Membered Cycloadduct) - Cyclopropanation of various trans-N-enoyl derivatives using diazomethane with Pd(OAc)2 as catalyst affords cyclopropyl products with good C(a)-re -facial control (eq 8). Similarly, aziridination with N-aminophthalimide-lead tetraacetate affords N-phthalimidoaziridines with variable but generally good jr-face selectivity (33-95% de). ... 

To achieve a high degree of facial selectivity in asymmetric cycloaddition, the employment of tt-shielding in combination with a chiral auxiliary has turned out to be an effective strategy [18]. The capability of asymmetric induction was applied to an immobilized system, designed on the basis of jr-shielding. For this purpose... 

Dipolar Cycloadditions with Nitrile Oxides (Alkene Isoxazoline) 1,3-Dipolar cycloaddition reactions of N-acryloyl-Q -f-butyltoluene-2,Q -sultam 6 with various nitrile oxides give isoxazolines with extremely high C(Q )-re jr-facial control (eq 3). The levels of selectivity exceed those obtainable with the 10,2-camphorsultam auxiliary and are comparable to the highest levels reported for such cycloadditions. The corresponding reactions of a-methyltoluene-2,Q -sultams are less selective. 

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