Cornforth reagent

These results may be explained either by Cram s cyclic model in the case of lithium alkyls or by Cornforth s dipolar model if copper-boron trifluoride reagents are used. Boron trifluoride causes double complexation of both nitrogen and oxygen atoms which results in the formation of an adduct with rigid antiperiplanar conformation due to electrostatic repulsion (see 4 and 5)9. 

Figures 3, 4, 5, 6 all locate the Cram and Cornforth transition states very near to a crossing of the solid and dashed curves. It follows that a small conformational change of the substrate may reverse the direction of the preferential attack by putting the dashed transition state below the solid one. The reagent then arrives from the apparently more hindered side, thus violating the Cram s (Cornforth s) ruie.

Phosphoryl chloride-Stannous chloride-Pyridine. The reagent is used in one step of the Cornforth steroselective synthesis of olefins as illustrated by the example formulated (Me= methyl, Et= ethyl). The steric course of the addition of Orignard o H HO H... 

COREY KIM Oxidizing reagent 79 COREY-WINTERAlkenesynthesis 80 CORNFORTH Rearrangement 81 Crafts 131... 

COREY Oxidizing reagents for alcohols 78 COREY Enantioselective borane reduction 77 COREY Homologatne epaxidation 78 COREY - KIM Oxidizing reagent 79 COREY-WINTERAlkenesynthesis 80 CORNFORTH Rearrangement 81 Crafts 131... 

Delphisine (43) was oxidized with Cornforth s reagent (Cr03-Py-H20) to yield 1-ketodelphisine (53) in 93% yield. Compound 53 was hydrolyzed with 5% methanolic potassium hydroxide solution at room temperature to give 1-ketoneoline (54) in 76% yield. The latter compound was converted into the corresponding benzoate (64) by treatment with benzoyl chloride and pyridine for 3 hr. Compound 64 was converted to 8-acetyl-14-benzoyl-l-ketoneoline (65) by treatment with acetic anhydride and catalytic amounts of p-toluenesulfonic acid on a steam bath for 1 hr. Sodium borohydride reduction of compound 65 yielded the desired product, 8-acetyl-14-benzoylneoline (63), and its C-l epimer (66). The structures of these compounds were confirmed by their H and 13C NMR analysis. 

The previously suggested structure (71) for isolongistrobine has been confirmed by synthesis (Scheme 4). TTie amino-alcohol (69) available from related synthetic work (see Vol. 4 of these Reports), was acylated with 4-pentenoyl chloride to give (70), which upon successive oxidation with Cornforth s reagent and oxidative double-bond cleavage with sodium periodate-osmium tetroxide gave isolongistrobine (71)-... 

Cornforth proposed a different explanation for the diastereoselective addition of Grignard reagents to a-chloro aldehydes and ketones. The underlying premise of this model is that electrostatic effects such as dipole-dipole interactions favor a reactant conformation in which the C=0 group and the C —Cl bonds are oriented anti-coplanar. The preferred path for approach of the nucleophile could then be predicted on the basis of the sizes of the other substituents on the a carbon (Figure 9.57). 

While it was previously believed that all enzymes were composed of protein, it appears that this view is currently undergoing some alteration, as we will see. But it can certainly be said that the vast majority of enzymes are proteins (there are over 2000 known), and each has its own specific three-dimensional structure that is the key to its functionality. In the late 1800s Emil Fischer expressed this as the lock and key model An enzyme has a particular shape so that reagent(s) for the reactions it will catalyze fit into it and are held there for reaction— as a key fits into a lock (see Fig. 16.2). John Cornforth, an Australian chemist, used this model to explain why natural molecules are formed in only one of two possible mirror images—z mystery since Pasteur s work with tartaric acid and tweezers. Cornforth saw that the enzyme acted as a three-dimensional template and only one shape would come... 

Without additional reagents Cornforth oxazole rearrangement... 

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