The oxygen-containing heterocycles are an important class of compounds in organic chemistry. These compounds are used as drugs (coumarin and oxazole), solvent (tetrahydrofuran), flavors, and fragrances (lactones). The fusion of aromatic ring to the oxygen-heterocycle will change the electron density; thereby, the physical/chemical/biological properties will alter. Also, the preparation of these fused molecules will require a different strategy/method/reaction condition. The topics covered in this chapter are the general synthetic methods and uses of fused heterocyclic compounds containing oxygen as a heteroatom. The derivatization of the primary scaffold is excluded from this chapter. Some of the fused compounds are coumarin (benzopyrans) and piclozotan (benzoxazepines).
- oxygen heteroatom
- fused molecules
- a flavonoid
The oxygen-containing heterocycles are an important class of compounds in organic chemistry mainly because of their natural abundance and diverse biological functions. Natural and semi-synthetic oxygen heterocyclic compounds such as Taxol  (anticancer), Digoxin (CHF treatment), Cyclosporine-A (immunosuppressant) and Lovastatin (hypolipidemic) are well known used as promising therapeutic compounds . Kaur et al. reviewed the oxygen heterocycles wherein saturated and unsaturated compounds are considered. They discussed the classification and chemistry of each of those compounds . Reports are available wherein the synthesis of natural products containing oxygen as heteroatom is reviewed by Cossy and Guérinot . Also, Rowlands and Farley chaptered the book on the anion radicals from oxygen-containing heterocycles . None of these reports target the oxygen-containing heterocyclic compounds where fused rings are taken into consideration.
2. Classification of oxygen heterocycles
The oxygen-containing heterocycles can be classified in several ways like the classification based on (a) the number of oxygen atoms, (b) saturation level, (c) aromaticity or (d) abundance. For the clarity of the concept, the classification based on a number of oxygen atoms is used. The benzene fused furan and pyrans are listed in Figure 1, whereas Figure 2 represents the compounds with two oxygen atoms in the ring system.
3. Synthesis of benzofurans
3.1 Mono-substituted benzofurans
3.1.1 Substitution on furan ring
The ortho-hydroxystilbenes are cyclized in 70–90% yield in a metal-free environment using hypervalent iodine and 1 eq. of (diacetoxyiodo) benzene to get 2-substituted benzofurans . The acids are converted to 2,4,6- trichloro-1,3,5-triazine esters, which are subsequently added to toluene, 2-hydroxybenzyl triphenylphosphonium bromide (1 eq.), and NEt3 and irradiated at 110°C for two cycles of 30 min to get the 2-substituted benzofurans having a chiral stereocenter adjacent to the heterocycle in 60–80% yield . Pd-catalyzed cyclisation of 2-chloroaryl alkynes using KOH at 100°C for 8 h resulted in the formation of benzofurans . All three reactions are as shown in Figure 3.
3.1.2 Substitution on the benzene ring
Also, the substituted 1-allyl-2-allyloxybenzenes cyclizes to give substituted benzofurans by isomerization followed by ring-closure metathesis reaction using 5 mol.% catalyst . The homologous members of benzofurans can be prepared by 0.1 eq. of Ru-catalyzed cycloisomerization of homo- and bis-homopropargylic alcohols in presence of pyridine at 90°C for 1–6 h  as shown in Figure 4.
3.2 Di-substituted benzofurans
3.2.1 Substitution on furan and benzene ring of benzofuran
The reaction between 1 eq. of 2-(2-hydroxyphenyl)acetonitriles with 2 eq of aryl boronic acids along with Pd(OAc)2 (5 mol.%), bpy (10 mol.%), TFA (10 eq.), with TFA as solvent heated to 80°C for 36 h resulted in benzofuran derivatives with more than 80% yield . Sonogashira cross-coupling reaction of halide with terminal alkynes followed by cyclization of the resulting 2-alkynylphenols in one-pot method results in benzofuran. The method employs the t-BuOH, PdCl2-(CH3CN)2 (2 mol.%), t-BuOLi (3.6 eq.) and 2-chlorophenol (1 eq.), alkyne (1.5 eq.) were taken in a sealed tube and heated to 110°C for 22 h to get benzofuran . Reaction of N-tosylhydrazones and terminal alkyne in a ligand free environment, using 10 mol.% of CuBr and 3 eq. of Cs2CO3 at 100°C for 4 h resulted in the formation of benzofurans with 38–91% yield . Reductive cyclization of 1-(2-hydroxyphenyl)-propargyl alcohols in presence of Pd(OAc)2 (5 mol.%), t-BuNC (1.2 eq.), Cs2CO3 (1.2 eq.) and MeCN as solvent gives benzofurans . These reaction schemes are as shown in Figure 5.
3.2.2 2,3-Substituted benzofuran
Ring closure of 2-haloaromatic ketones (1 eq.) in presence of K3PO4 (1.5 eq.), CuI (10 mol.%) and DMF at 105°C for 12–16 h results in di-substituted benzofurans . The good yield coupled with atom economy was achieved, when o-alkynyl phenyl acetals are cyclized using PtCl2 (2 mol.%), olefin (8 mol.%) in tolene at 30°C . Cyclization of o-iodoanisoles and terminal alkynes under mild conditions using an electrophile (EX like PhSeCl or p-O2NC6H4SCl) (1.5 eq.), DCM at room temperature for 2–6 h yields 2,3-disubstituted benzofurans . These reaction schemes are as shown in Figure 6.
Selective dehydrative C—H alkylation reaction of alkenes with alcohols using [(C6H6)(PCy3)(CO)RuH]BF4, cyclopentene, toluene at 100°C for 6–12 h results in 2,3-substituted benzofurans . O-Arylhydroxylamine hydrochloride (1 eq.) with cyclic or acyclic ketones (1 eq.) in the presence of methanesulfonic acid (2 eq.), THF at 60°C for 2–24 h yields benzofurans in 40–90% yield . Ketones (1 eq.) on treatment with Grignard reagents (3 eq.), benzofurans are formed, in a regioselective manner via [1,2]-aryl migration . These reactions are depicted in Figure 7.
Base-catalyzed, the condensation of o-hydroxyphenones with 1,1-dichloroethylene, gives substituted benzofuran carbaldehydes .
4. Synthesis of benzofuranones
5. Synthesis of dibenzofurans
Iododiaryl ether cyclizes under mild conditions to yield dibenzofurans . Ortho-diazonium slats of diaryl ethers undergo intramolecular cyclisation, resulting in dibenzofurans . Cross-coupling of 6-diazo-2-cyclohexenones and ortho-haloiodobenzenes gives substituted dibenzofuran  (Figure 10).
6. Synthesis of coumarins
Phenols react with beta-keto esters to give coumarins . If phenolic acetates are used, then, acrylates are used . Aromatic alkynoate undergoes cyclisation with aldehydes to form 3-acyl-4-arylcoumarins  (Figure 11).
Substituted 2-hydroxybenzaldehydes react with phenylacetic acids resulting in substituted 3-aryl coumarins . With dialkyl acetylenedicarboxylate, 2-hydroxybenzaldehydes gives 4-carboxyalkyl-8-formyl coumarins . 2-Hydroxybenzaldehydes (or 2-hydroxybenzaldehydes) cyclizes with aryl acetic acids to give 3-aryl coumarins (or 3-aryl-4-methyl-coumarins)  (Figure 12).
7. Synthesis of isocoumarins
Bromoalkynes reacts with benzoic acid and produces 3-substituted isocoumarins . o-Halobenzoic acids and 1,3-diketones reacts to give 3-substituted isocoumarins . 2-Halobenzoates and ketones react to give the same product . o-Halobenzoic acids add to alkynes resulting in isocoumarin derivatives  (Figure 13).
8. Synthesis of flavones and flavonols
8.1 Baker-Venkataraman rearrangement
The chemical reaction between 2-hydroxyacetophenone and acid chloride in the presence of base yields 1,3-diketone which undergo rearrangement and concomitant cyclisation to produce flavone . This reaction is often used to synthesize chromones and flavones (Figure 14).
8.2 Algar-Flynn-Oyamada synthesis
In this method, chalcone (1,3-diaryl-2-propen-1-one) is produced by condensing 2-hydroxyacetophenone with an aryl aldehyde in alkaline medium (Claisen-Schmidt condensation) followed by oxidative cyclisation of chalcone to get flavone  (Figure 15).
A mixture of acetophenone and aromatic aldehyde when exposed to Microwave irradiation, 2-phenyl γ-benzopyrones are obtained . The same compound can be prepared by cyclizing 1,3-diaryl propanediones (Figure 16).
In this chapter, the synthesis of fused heterocyclic compounds having oxygen as heteroatom is considered. The care is taken not to consider the reactions, where the reactions of the compounds leading to derivatizations are not included. Benzofurans, benzofuranones, dibenzofurans, coumarins, isocoumarins, chromones, and flavones are the fused heterocyclic compounds considered in this chapter. Also, the reactions are indicative and not the detailed reaction conditions, and appropriate reagents are not included in this chapter.
Conflict of interest
No conflict of interest from both the authors.
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