The Substrate Specificity ofβ,β-Carotene 15,15′-Monooxygenase
摘要:
The synthesis of several substrate analogues of the enzyme beta,beta -carotene 15,15'-monooxygenase is reported. The substrate specificity of enriched enzyme fractions isolated from chicken intestinal mucosa was investigated. Regarding substrate binding/cleavage, these experiments demonstrate that i) any deviation from the 'rod-like' beta,beta -carotene structure is not tolerated, ii) one 'natural', unsubstituted beta -ionone ring is required, iii) the position and presence of the Me groups attached to the polyene chain is significant. These results suggest a hydrophobic barrel-like substrate binding site in which the protein's amino acid residues through interaction with the Me groups, direct the central C=C bond in binding distance to the active site's metal-oxo center, supporting the unique regiospecificity of cleavage to retinal (provitamin A).
The coupling of arylboronicacids with electron-rich allylicbromides is accomplished in the absence of any transition-metal catalyst through conventional heating. The reaction is completely regioselective, affording only the α-coupled product, and can be carried out under mild aerobic conditions in an organic solvent; the presence of a base is required.
The dissolution of a stainless steel anode provides catalytic nickel species which enable the efficient synthesis of ketones by electrolysis of organic halides in DMF in the presence of bipyridine and carbon monoxide.
to produce unisolable allylchromium species which add efficiently to aldehydes or ketones with high degree of stereo- and chemoselectivity. Particularly, high threo selectivity is observed in the reaction of aldehydes and 1-bromo-2-butene and is ascribed to a chair-like six-membered transition state. Simple reduction of allylic and benzylic halides produces biallyls and bibenzyls, while gem-dibromocyclopropanes