Natural products pyridine system

There appear to be no reports of direct radical attack on the pyridopyrimidine ring system, but radical bromination of methyl substituents in the 7-position of the pyridine ring has been utilized in the synthesis of deaza analogues of natural products (62JCS4678, 79JHC133). 

The nature of the nitrogen protecting group also played a significant role in the chemoenzymatic total synthesis ofepibatidine, which shall be outlined as an example for the synthetic elaboration of the regioselective biooxidation product of a nonnatural precursor. B. bassiana mediated hydroxylation of the aza-norbornane system enabled functionalization for the subsequent introduction of the pyridine system (Scheme 9.10) [82,83]. 

In contrast to the conjugated system, the reactivity of hexahydro-isoxazolo[2,3-tf] pyridines has been the subject of considerably more attention, which can most certainly be attributed to its greater synthetic potential, as demonstrated by the synthesis of many complex natural products. However, most of the reactions reported since 1996 have been known for many years and the last decade was in fact characterized by their use in syntheses or optimization. After a brief survey of the thermal reactions, procedures involving the reductive cleavage of the N-O bond will be detailed. 

Regioselective reactions in systems which contain multiple functional groups are an area ideally suited for biocatalysis. Linhardt and co-workers at the University of Iowa s Division of Medicinal and Natural Products Chemistry recently published the synthesis of a series of 1 -O-acyl sucrose derivatives [31], Using Chiro-CLEC -BL (the CLC of subtilisin) and vinyl esters of the acylating agent in pyridine as solvent, the authors prepared l -O-lauryl sucrose, l -O-myristyl sucrose, and l -O-stearyl sucrose in 80-90% yield (Fig. 9). Their method represents a green alternative to the tin chemistry previously used [32],... 

Oxazoles readily participate in cycloaddition reactions as dienophiles and as dienes in Diels-Alder reactions, and suitably substituted oxazoles participate in sigmatropic rearrangements (e.g., aza-Claisen rearrangements). In particular, the Diels-Alder reaction of oxazoles is one of the most widely explored and synthetically useful reactions, and as such, it has been used extensively both in natural product syntheses and to convert oxazoles to other heterocyclic ring systems. For example, a partial list of heterocyclic systems readily accessible from oxazoles via Diels-Alder reactions or other cycloadditions include pyridines hydroxy-pyridines isoindoles pyridazines tetrahydronaphthyridines benzo[h]-l,6-naphthyridines benzopyrano[3,4-b]pyridines 2-substituted, 2,4-disubstituted,... 

The pyridine system is found in natural products, for example, in nicotine (1) from tobacco, ricinine (2) fi-om castor bean, vitamins such as pyridoxine or vitamin Be (3) and vitamin P (4), and alkaloids such as coniine and piperine. Free pyridine is present in tobacco smoke. Diploclidine (5) and njdcinadine A (6) are two examples of recently isolated and structurally diverse natural products containing the pyridine core. ... 

Quinoline and isoquinoline are heterocycles in which a benzene ring and a pyridine ring are fused through carbon. The isomeric heterocycles 1- and 2-azanaphthalene, better known by their trivial names quinoline and isoquinoline, have been the subject of extensive studies since their discovery in the extracts of coal tar at the beginning of nineteenth century. Since then their heterocyclic ring systems were found incorporated in several hundreds of natural products and were used as pharmacophore units in dozens of pharmaceuticals especially anti-bacterials, better known under the general name of Quinolones . ... 

Piperidine and pyridine ring systems are the key structural elements in a vast array of natural products as well as in a large class of biologically active natural products. They are also often embedded within scaffolds as privileged structures by medicinal chemists. Indeed, over 12000 piperidine derivatives have been mentioned in clinical or preclinical studies during the last ten years. The development of new methods for the synthesis of multi-functionalized piperidines and pyridines is therefore of considerable importance. ... 

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