Fused furans

Furan Fused to Another Five-Membered Heterocycle. 280... 

A further neat example of multicomponent reactions in heterocyclic synthesis was reported by Ma et al. <06AG(E)7793>. They prepared the furan-fused 1,4-thiazepine 140 in good yield using the three components 137, 138, and 139 in the one reaction. A range of other furan-fused analogues with different substituent groups in the thiazepine ring were also synthesised. 

The related furan-fused quinone 24 is produced using a similar approach from the furan 3-carboxamide using a strong base, such as LDA, in low yield (Equation 99) <1995MM9, 2000JOC2577>. 

The furan ring of the A-benzyl furan-fused tetrahydroquinolinequinone (18 X = 0) suffers reduction under debenzylation conditions (H2, Pd/C) affording compound (19) (89%) <9lJOC6379>. The thiophene derivative (18 X = S) gives a similar overreduction product, but in low yield only,... 

A simple procedure to prepare 5-aryl- and 5-pyridyl-2-furaldehydes from inexpensive, commercially available 2-furaldehyde diethyl acetal was reported. The reaction proceeded in a four-step, one-pot procedure and the yield of coupling step was usually between 58-91% <02OL375>. A facile route to 3,4-furandicarboxylic acids was developed. DDQ-oxidation of 2,5-dihydrofuran derivatives, which were produced from dimethyl maleic anhydride, furnished the desired esters of furan-3,4-dicarboxylic acid <02S1010>. The furan-fused tetracyclic core of halenaquinol and halenaquinone possessing antibiotic, cardiotonic, and protein tyrosine kinase inhibitory activities was synthesized. Intramolecular cycloaddition of an o-quinodimethane with furan gave the adduct as a single isomer via an enrfo-transition state, which was converted to trisubstituted furan by oxidation-elimination reactions <02T6097>. 

The furan-fused 1,2-oxazocine 126 cleaved the N-O bond when treated with NaBH4 catalyzed by Mo(CO)6 to give the furan 127 (Equation 6). It is noteworthy that hydrogenation conditions, such as activated zinc and samarium iodide, failed to cleave the N-O bond <2005JOC6995>. 

Alkyllithium compounds have been used to induce a new anionic ring contraction proceeding by a complex mechanism involving carbenes and illustrated by reaction of a furan-fused 1,3-dioxolane to form the oxetane product (Equation 22) <2004SL651>. 

Copper acetate and manganese acetate-mediated radical reactions of [60]fullerene with 1 gave dihydro furan-fused fullerene 139 via a radical mechanism (05OBC794) (Scheme 23). 

Selnick and Brookes prepared a series of thiopyrans as carbonic anhydrase 11 inhibitors via intramolecular oxazole-alkyne cycloadditions. The starting 4-methyl-2-thiooxazole-alkyne 215 was readily constructed from 2-mercapto-4-methyloxazole (Fig. 3.62). Refluxing 215 in toluene afforded an 88% yield of the furanothiopyran 216. Varying the length of the oxazole-acetylene linker provided furans fused to 5- and 7-membered rings as well. 

Furan-Fused Adler-Becker Oxidation Substrates... 

Furan fused carbohydrates 144 and 145 have been prepared by Grignard additions to C-4 aldehyde-bearing hexopyranosides, followed by intramolecular displacement of the triflate derivatized alchol by the 6-OBn goup. Similar cyclization chemistry was used to convert 146 to 147. ... 

The condensation of enaminoketones derived from acetophenones with 1,4-naphthoquinone resulted in the furan-fused (3-benzoylnaphtho[l,2-l ] furan-5-ol) (14T9352). 

An enantioselectlve redox-neutral domino reaction catalysed by gold(I) occurs allowing functionalization of unreactive sp C—H bonds and furan-fused azepines with high enantioselectivities (Scheme 124). ... 

Furan-fused corannulene 77 was efficiently prepared by a brief heating of 31 with 3,6-di(2-pyridyl)-l,2,4,5-tetrazine in chloroform (Scheme 22) [107]. Isocorannu-lenofuran 77 is a reactive diene for the Diels-Alder reaction (see Scheme 34) [108]. Difuran-fused corannulene 79 can be generated from synlanti-7S, which is prepared by treatment of a solution tetrabromocorannulene 18 in THF with ferf-butyl... 

Furthermore, highly substituted furan-fused 1,4-thiazepine derivatives 14 can be prepared by a three-component reaction (3CR) involving thiazoUum salts 12, DMAD, and ketenes or acid derivatives [15] (Scheme 5.9). 

SCHEME 5.9 Synthesis of polysubstituted furan-fused 1,4-thiazepine derivatives 14. 

D. Conreaux, S. Belot, P. Desbordes, N. Monteiro, G. Balme, EtsN-induced demethylation-annulation of 3-alkynyl-4-methoxy-2-pyridones and structurally related compounds in the synthesis of furan-fused heterocycles, J. Org. Chem. 73 (2008) 8619-8622. 

An enantioselective variant of this process was later accomplished by employing a catalytic amount of a chiral gold-bisphosphine complex. The resulting furan-fused azepines 154 could be obtained in good to excellent yields with satisfactory enantioselectivity (Scheme 12.67) [71]. 

Pyrrole- and Furan-fused Thio diens and Related Compounds.— The interest in the chemistry of thienopyrroles is still increasing. Starting from 2-amino-3-carbonyl-substituted thiophens obtained by the Gewald reaction, the A -ethoxy-carbonylmethyl derivative (245) was prepared either by direct alkylation with ethyl bromoacetate, or by reductive alkylation with methyl glyoxalate. The compounds (245) were then ring-closed directly, or after acetylation, to the... 

Pyrrole- aud Furan-fused Thiophens and Related Compounds.— During recent years there has been a great interest in the thiophen analogues of indole, the thienopyn oles, especially among Russian workers, and several... 

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