An artificial cytochrome P450 that hydroxylates unactivated carbons with regio- and stereoselectivity and useful catalytic turnovers

Abstract

A catalyst has been synthesized comprising a manganese porphyrin carrying four beta-cyclodextrin groups. It catalyzes the hydroxylation of substrates of appropriate size carrying tert-butylphenyl groups that can hydrophobically bind into the cyclodextrin cavities. In one example as many as 650 catalytic turnovers are seen before the catalyst is oxidatively destroyed, and with a rate comparable to that of typical cytochrome P450 enzymes. In another example, a steroid derivative is regio-and stereoselectively hydroxylated at a single unactivated carbon atom, but more slowly and with fewer turnovers. The carbon attacked is not the most chemically reactive, and the selectivity is determined by the geometry of the catalyst-substrate complex. Nonbinding substrates are not reactive under the conditions used, and substrates with more f lexible binding geometries give more than a single product.Enzymes are remarkably selective catalysts. They bind a particular substrate out of a sea of available compounds in solution, then they perform a reaction at a particular position of the bound substrate (thus showing regioselectivity), often with stereoselectivity as well. The geometric control in the enzyme-substrate complex can completely dominate the normal reactivity of the substrate. For example, enzymes in the class cytochrome P450 can hydroxylate unactivated carbons in steroids while leaving much more reactive substrate positions, such as those in or next to double bonds, untouched (1-3). In these enzymes an oxygen atom becomes attached to the iron atom in the metalloporphyrin, and is then transferred to the substrate within the enzyme-substrate complex.Although it is of interest to learn how to mimic the great rate accelerations achieved in enzymatic catalysis, imitating the selectivity is even more important. For this reason, we have carried out studies over many years to learn how to use the geometric control typical of enzyme reactions in selectively functionalizing steroids and other substrates. In the earliest work, a benzophenone attached to a steroid was shown to perform selective photochemical functionalization of the substrate (4-11). In later work, a template attached to the substrate was able to direct free radical reactions to specific carbons because of the geometry of the template-substrate species; see refs. 12-14 for reviews. However, there were limitations to these methods.For one, the reagent or template was covalently attached to the substrate, so catalytic turnover was not possible. As a corollary of this, relatively simple reagents or templates were used-attached by a single flexible link-so the geometric control was not perfect. have shown that organization in a bilayer can be used to achieve hydroxylation of a steroid by a metalloporphyrin catalyst, but catalytic turnover was blocked by strong binding of the product. Grieco and colleagues (18-20) have also shown that a metalloporphyrin can hydroxylate a steroid in an intramolecular reaction if it is covalently attached, but again, this is no...

Self‐Assembled Nanoreactors

“…[35][36][37][38] An artificial enzyme system which perfectly illustrates the possibility to achieve high selectivity is the manganese porphyrin 7, which possesses four covalently linked peripheral -CDs (Figure 1c). [39][40][41][42][43] In the presence of iodosyl benzene, this tetramer is capable of selectively epoxidizing or hydroxylating stilbene derivatives. Its superior selectivity is best illustrated by the fact that it can regio-and stereoselectively oxidize the unactivated 6-CH 2 position in the B-ring of steroid derivative 8.…”

Artificial enzymes, cancer chemotherapy, conjugation and nanoelectronics, and prebiotic chemistry

“…When we attached two or four cyclodextrins to a porphyrin, we were able to use the iron derivative or even better the manganese derivative to carry out directed hydroxylations of unactivated carbon-hydrogen bonds in a bound steroid using the binding of the steroid into the cyclodextrin units [23][24][25][26]. With binding into two cyclodextrins on opposite sides of the porphyrin we saw selective hydroxylation at C-6 in a cholestanol derivative, while with triple binding into three cyclodextrins we selectively hydroxylated C-9, giving an entry into corticosteroids.…”

Chemical Synthesis and Medicinal Applications of Glycoporphyrins

“…In the natural P-450 enzymes the iron porphyrin is axially coordinated by a cysteine thiolate sulfur and the oxygen adds to the sixth coordination position. Initially, imidazole had been incorporated as the fifth ligand in such P450 mimics [180,414,415,418]. Inclusion of thiolates in the P450 mimic either covalently attached (see 665) or via hydrogen bounding to the catalyst allowed the use of H 2 O 2 as oxidant [419] .…”

“…These compounds display significant inhibitory effects and offer potential for the treatment of herpes virus infections [30] . Another broad study compared the antiviral effect of neutral porphyrins (412,414,415) and their cationic analogues (416a-c) against HSV-1 [229] . With the exception of 416a all compounds gave IC 50 Earlier it had been shown that the fully deprotected derivative have the best activity [30] .…”

“…Studies with these and related catalysts (e.g., 662-665, 100) or P450 mimics [413] focused on several different substrates. For example, catalyst 662 was used for studies on steroid hydroxylation [414] and comparative reactions with stilbenes [179,415] . Pioneering studies by Breslow indicated that catalyst 662 provided the best results for hydroxylation at C-9 of steroids with 90 turnovers [416] .…”