Pyran ring systems

As mentioned above, the main problem with the 2H-naphtho[l,2-fc]pyran ring system (1.20) is the very slow rate of fading back to the colourless state (1.20 R, ... 

Pyrans lack aromatic properties and behave as dienolic ethers showing the instability predicted by MO calculations. Hence, the parent compound proved elusive and was not synthesized until 1962. Although several routes are available for the synthesis of this ring system, such is the variation in stability of 4//-pyrans no one method can be described as of widespread application. Routes to the 4//-pyran ring system have been reviewed <80H(14)337>. 

The oxidative cyclization of the alcohol (214) with ceric ammonium nitrate gave a mixture of the two naphthopyrans (215) and (216) in a ratio of 3 1 (81CC534). The reaction is of interest because of the natural occurrence of quinones containing the naphtho[2,3-c]pyran ring system. 

The earliest studies centered on the cyclization of oi-hydroxyalkenes to furan and pyran ring systems (equations 4 and 5).10 The regiochemistry (attack at the more substituted position) was that expected and the yields were only modest. However, modem advances in catalysis should permit much more efficient processes and many older systems probably warrant re-examination. 

Several synthetic compounds containing the naphthoquinone skeleton, such as 2-(4-cyclohexylcyclohexyl)-3-hydroxy- 1,4-naphthoquinone [169], are effective in the treatment of coccidial infection. Two structurally related antibiotics WS-5995A (36) and WS-5995B (37a), isolated from S. auranticolor sp. nov. near Tokyo, efficiently protect Eimeria tenella (a species of coccidia) infection. An inactive component WS-5995C (37b) can be readily converted to the active (36) by simple dehydration [ 170-172]. Compound (36), the structure of which resembles the aforementioned benz[a]anthraquinone antibiotics, is the first example of a 5//-benzo[d ]naphtho[ 2,3-6 ]pyran ring system found in nature. 

Although the instability of the simple, unsaturated pyran ring systems seen in 2//-pyran 1 and 4//-pyran 2 prevents their production via biosynthetic pathways, six-membered oxygen heterocyclic ring derivatives are found in an abundant range of naturally occurring compounds. 

A Suzuki coupling of 2,6-dimethoxyiodobenzene with the dihydroaromatic boron compound 13 forms the basis of a synthesis of the dibenzo[6,t/]pyran ring system <03AG(E)2795> and naphtho[2,3-c]chromenes result from an intramolecular dehydro DA reaction on the diarylalkynes 14 <03SL1524>. 

The methods described thus far represent only a small subset of the available techniques for the formation of C-furanosides and C-pyranosides. Other useful methods include cyclizations of halo olefins and ene-ynes [224] and the direct modification of pyran ring systems [251]. Although not discussed further here, these examples are mentioned to illustrate the breadth of work accomplished in this area. 

Both Pd-catalysis and microwave irradiation promote a [2+2] cyclisation of 1,7-ynallene carboxylates 4 to the fused cyclobutena[c]pyran ring system <05CC5670>. 

This high efficiency and remarkable functional group tolerance makes the Prins cychzation a very attractive tool in the field of polycychc and macrocyclic compounds containing the pyran ring system. 

Furukawa, H., M. Yogo, C. Ito, T.S. Wu, and C. Kuoh New Carbazoloquinones Having Dimethyl Pyran Ring System. Chem. Pharm. Bull. 33, 1320 (1985). 

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