Abstract
The use of biotransformations in organic chemistry is widespread, with highlights of interesting applications in the functionalization of natural products containing unactivated carbons, like the kaurane diterpenes. A number of compounds with kaurane skeletons can be isolated in large amounts from several plant species and a myriad of biological activities has been related to these compounds. Studies on structure versus activity have showed that, in most cases, in kaurane diterpenes, activity increases with the increase of functionalization. Since naturally occurring kaurane diterpenes usually have limited functional groups to be used as targets for semi-synthetic modifications, production of more polar derivatives from kaurane diterpenes have been achieved mostly through the use of fungal biotransformations. In this review, selected examples the wonderful chemical diversity produced by fungi in kaurane diterpenes is presented. This diversity includes mainly hydroxylation of nearly all carbon atoms of the kaurane molecule, many of them carried out stereoselectively, as well as ring rearrangements, among other chemical modifications. Sources of starting materials, general biotransformation protocols employed, fungi with most consistent regioselectivity towards kaurane skeleton, as well as biological activities associated with starting materials and products are also described.
Highlights
One of the major aims of natural products chemistry is the isolation of molecules with biological properties
In biotransformation of kaurenoic acid by A. blakesleeana hydroxylation of C-13 was accompanied by introduction of a 7α-OH
When a kaurane derivative hydroxylated at the C-18 methyl group was used as substrate in a biotransformations experiment using the same fungus, hydroxylation at C-19 was achieved, leading to the formation of ent-7α-acetoxy18,19-dihydroxy-kaur-16-ene (77), in 2.14% yield (Scheme 8) [10]
Summary
One of the major aims of natural products chemistry is the isolation of molecules with biological properties. NP: Natural Product isolated from plants; BP: Biotransformation Product obtained using fungi Among these compounds, ent-kaur-16-en-19-oic acid (kaurenoic acid, 1, Figure 1) is one of the most studied compounds, as it has several and varied biological activities, such as antimicrobial [29], inhibition of HIV replication [39], anti-inflammatory [40], trypanocidal [41] and cytotoxic properties [42], among others. The advancement of genetic sequencing techniques allowed gathering at least 50 different species of fungi of Fusarium genus including F. proliferatum, F. acutatum and F. fujikuroi within a group called G. fujikuroi complex [51] This group of fungi has the enzymatic system conducive to carry on the same type of biotransformation, and is reported as producers of diterpenes [52]. This inhibitor acts inhibiting ent-kaurene biosynthesis does not affect post-kaurene metabolism [58]
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