Enzymatic chemistry of cyclopropane, epoxide, and aziridine biosynthesis.

Cyclopropane, epoxide, and aziridine groups are three-membered ring structural elements found in a wide variety of natural products, some of which are depicted in Figure 1.1 The antibiotic and antitumor properties of many of these compounds, including duocarmycin (1),2 dynemicin (2),3 epothilone (3),4 mitomycin (4),5–8 and azinomycin (5)9–11 are well known. The pharmacological activities of others are more diverse. For example, pentalenolactone P (6)12,13 has antineoplastic and antiviral activities,14 scopolamine (7) has a subduing effect on the central nervous system,15 azicemicin (8) is active against Gram-negative bacteria and mycobacteria,16 and ficellomycin (9) shows inhibitory activities towards Gram-positive bacteria.17,18 Figure 1 Representative three membered ring-containing natural products. The inherent ring strain present in the small ring moiety is frequently responsible for the biological activities of these compounds, many of which are potent alkylation agents.1,19,20 For example, upon sequence specific binding to the minor groove of double stranded DNA, a twist around the amide bond of duocarmycin (1) activates the cyclopropyl ring towards alkylation by a suitably positioned adenine residue to give 10 as an adduct (Scheme 1A). 21,22 Opening of the oxiranyl ring in the reduced dynemicin A (11) is known to trigger the rearrangement of the enediyne group to a 1,4-dehydrobenzene biradical (11→12) that initiates DNA degradation (Scheme 1B). 23,24 The reductive activation of mitomycin C (4) involves opening of the aziridine ring (13→14), which unmasks the electrophilic site at C-1 and results in DNA alkylation (14→15→16, Scheme 1C). 5–7 In other examples, such as the tubulin-binding cytotoxin epothilone (3, Figure 1), the epoxide ring introduces a rigid structural element into the parent compound. While derivatives lacking the epoxide ring exhibit similar activities as the parent compound,25 the epoxide moiety may be important for the directing/binding of epothilone to its biological target in the cell.4,26,27 Scheme 1 Mechanism of action of (A) duocarmycin, (B) dynemicin, and (C) mitomycin. Despite the recent advances in our understanding of the biosynthesis of many different types of natural products, the specific enzymes responsible for making these strained, 3-membered rings have only been identified in a few cases, and many of the mechanistic details regarding small ring closure remain obscure. Due to the impressive array of diverse biological activities exhibited by these small-ring containing compounds and their potential applications as therapeutic agents as well as mechanistic probes for studying enzyme catalysis,1,19,20,28–30 a thorough understanding of their biosynthesis is warranted. These studies are a crucial first step in maximizing the potential of these compounds as general mechanistic probes, or as specific tools in the rational design of drugs with optimal in vivo specificity. In this review, we will illustrate the established biosynthetic strategies for construction of cyclopropane, epoxide, and aziridine rings, focusing primarily on studies that were performed in the last decade. Only those pathways that have been genetically and/or biochemically verified will be discussed. Special attention will be directed to the prototypes of chemical transformations observed in small-ring biosynthesis and, when applicable, mechanistic details of the enzymes involved. In addition, we will highlight the occurrence of several poorly understood, but potentially novel modes of enzyme-catalyzed small-ring biosynthesis that merit further investigation.