Structure of furan

Give the structure of the cycloaddition product formed when benzyne is generated in the presence of furan (See Section 11 22 if necessary to remind yourself of the structure of furan )... 

The potential of these transformations has not been fully exploited. The method is certainly valuable both for the purpose of studying the structure of furan polymers which might be of interest but cannot be prepared easily by conventional techniques and for preparing polymers in which a certain proportion of the hydroxyl groups of poly(vinylalcohol) are substituted with a furan moiety which gives the new product some special property. 

The electronic structures of furan, thiophene, and selenophene, their protonated complexes, and their anions have been calculated by the extended Hiickel method.6 The results of these calculations have been used to determine the influence of the heteroatom on the degree of aromaticity and electron density. 

The naming of endocyclic O-atom heterocycles is based on the same principles as for N-PACs e.g., the structures of furan (XVII) and dibenzofuran (XVIII) are... 

This technique links several others, especially mass spectrometry and UV absorption spectrophotometry, and its results often illuminate them. A relatively new technique, it has been used extensively to probe the electronic structure of furan and its allies a general review is available (80PAC1509). 

Fig. 6. (a) Cyclization of the open-chain form of D-fructose (b) the structure of furan. 

The basis and extent of their aromaticity is discussed in Chapter 1. In summary, the capacity for the lone pair on a particular heteroatom to be delocalised is inversely related to the electronegativity of the heteroatom. For instance, furan is the least aromatic of the trio because oxygen has the greatest electronegativity and hence mesomeric representations 2.4b-e make relatively less of a contribution to the electronic structure of furan than they do in the cases of pyrrole and thiophene. The order of aromaticity is furan < pyrrole < thiophene. We shall see later how this variation in aromaticity affects the reactivities of these three related heterocycles. 

This chapter, covering the literature of the period 1995-2006, surveys the structure of furans and their benzo derivatives in continuation of chapter 2.05 of CHEC-II(1996) <1996CHEC-II(2)259>. A fairly recent review of, mostly synthetic, furan chemistry also contains a useful bibliography of spectroscopic data <2001SOS(9)183>. Structural aspects are included in a recent review of aminofurans <2006AHC1>. 

When the resonance is between purely homopolar valence bond structures, the resonance energy is less than when ionic structures arc involved. This general rule may be illustrated by considering the resonance structures of furan and r/( pentadiene. In furan, /, the resonance involves... 

This chapter surveys the structure of furans and their benzofuran derivatives in order to complement the first edition of Comprehensive Heterocyclic Chemistry (CHEC-I) with materials published between 1983 and 1993. The determination of the structural features of the furan ring has received remarkable attention, since this five-membered heterocyclic ring is encountered as a building unit in a variety of natural and synthetic products. Besides terpenes, it suffices to mention ribose and deoxyribose, which play an important part in the vital function of nucleic acids. 

Figure 5.1 Structure of furan (bond lengths in pm, bond angles in degrees).

Figure 5.2 Electronic structure of furan (a) sp -hybridization of the ring atoms, (b) energy level scheme of the jt-MO (qualitative) and occupation of electrons, (c) jt-MO (the O-atom is situated at the lowermost corner of the pentagon), and (d) Ji-electron densities calculated by ab initio MO...

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