Ring contraction furans

There are several examples of the formation of pyridazines from other heterocycles, such as azirines, furans, pyrroles, isoxazoles, pyrazoles or pyrans and by ring contraction of 1,2-diazepines. Their formation is mentioned in Section 2.12.6.3.2. 

Into Other Ring-Contracted Systems To Furan and Pyrrole Derivatives... 

Anew route to 2,5-dihydrofiirans and tetrahydro[3,2-b]furans via ring contraction of pyranoside C-glycosides was reported <96CC1663>. ( )-Homononactic acid and its 8-epimer were synthesized by using a c -selective iodoetherification as the key step (Scheme 21, <96SL777>). 

Ring contractions of pyran derivatives are occasionally valuable. The contraction of 3-halo-2-pyrones to 2-furoic acids under the influence of alkali has been studied and the conditions defined.58112113 The method is adaptable to the preparation of 3-furoic acid via furan-2,4-dicarboxylic acid58 and of 3,4,5-triphenylfuran-2-carboxylic acid.113 Another ring contraction involving halides is the conversion of 4-chloromethylpyrylium salts into furylmethyl ketones as indicated in Scheme 21.114 Pyridine oxides may be transformed with unexpected ease into furans through treatment with a thiol (Scheme 22).115... 

Unexpected transformations of 5-(phenylthio)pent-2-en-4-ynal and 5-(methylthio)pent-2-en-4-ynaI to 2-bis(phenylthio)- and 2-bis(methylthio)methylfurans respectively in the presence of a slight excess of HCl have been recorded. The transformations are expected to involve a pyran to furan ring-contraction sequence <00HCA1191>. 

Ring contraction forming the corresponding furan derivatives... 

A similar autooxidative ring contraction was recognized in the oxidation of 2-substituted 3,4,5,6-tetrachloro-2//-pyrans (144b) to furan derivatives 428, proceeding probably via peroxide 427.32... 

Condensed 4//-pyrans 76 (X = CN) undergo acidic hydrolysis by ring contraction to furan derivatives 498. The detailed mechanism of the process was discussed.417... 

Certain pyridazine iV-oxides are isomerized into the corresponding diazoketones, e.g. (994) — (995) ring contraction to the corresponding furan (996) can then occur. 

The furan (351) accompanied by the butenolide (352) is produced by the action of copper salts on the ester (350) (Scheme 95) (B-75MI31200). The lactides (353) undergo ring contraction to the furandiones (354) on treatment with sodium hydride (79AG(E)310). 

Ring contraction of coumarins is used for the preparation of benzo[Z>]furans. Alkaline degradation of the 3-halo-, 4-halo- or 3,4-dihalo-coumarins gives the coumarilic acids (Scheme 104). The coumarilic acids are decarboxylated to the corresponding benzo[Z> ]f urans. Basic mercury(II) oxide oxidation of 4-phenylcoumarin (neoflavanoid) yields the 3-phenyl-benzo[/>]furan (71IJC1316). 

Ring contraction was also noted on acetolysis of 3-tosyltetra-O- methyl-(+)-catechin the 2,3-dihydrobenzo[Z> ]furan formed can be oxidized to the benzo[f>]furan (Scheme 105) (66JCS(C)634). The 2-phenylbenzo[6]furan and its 2,3-dihydro derivative are degradation products of lignin and as such have been investigated extensively. 

One of the major areas of study in the photochemistry of heterocycles is the photorearrangement of five-membered heteroaromatic systems. Various mechanisms have been proposed to account for these transformations.123 A ring contraction-ring expansion pathway, via an intermediate cyclopropene, appears to be involved in the rearrangement of derivatives of furan. The isolation of an intermediate (140) of this type together with an allene (141) has been effected on irradiation of 2,4-di-tert-butylfuran (142), as shown in Scheme 9.124 Similar transformations have been reported in perfluorotri-... 

E is facile. Dehydrochlorination provides the aromatic products 233. An alternatively possible HC1 elimination/electrocyclization/decarboxylation pathway was excluded, since lactone 230B was thermally stable under the reaction conditions in the absence of the catalyst. (NHC)Cu(I) catalyst 232 gave comparable or better yields than 231 in these ATRC/ring contraction sequences, while other (NHC)Cu(I) complexes provided considerably lower yields [320]. Chlorinated cyclic compounds arising from ATRC can also be transformed to chlorinated furans [321]. 

Pyrylium salts are also suitable for the preparation of furans.7Z,7S Oxidation of these salts by aqueous hydrogen peroxide in the presence of perchloric acid leads to ring contraction, with the formation of furan derivatives. Nenitzescu and Balaban described the following example72 ... 

The preparation of furans from a-pyrones has been reported.74-76 For ring contraction to occur it is necessary that a suitable leaving group should be present in the 3-position. An example is given by Broy and Mayer,76 where ring opening on 3-hydroxy-a-pyrone occurred in alkaline solution, followed by ring closure to the 2-furoic acid. This reaction was also applied to 3-bromo-a-pyrone. 

Flavones have been shown to suffer from a stereospecific ring contraction to give rra r-2-aryl-2,3-dihydrobenzo[ ]-furan-3-carboxylates upon treatment with phenyliodonium acetate, trimethyl orthoformate, and sulfuric acid (Equation 151) <2002T4261>. 

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>. 

During the period of review, a book containing major sections dealing with the photochemistry of furan and related heterocycles appeared.1 Although well established, the photochemical ring contraction of furan (Scheme 1) could originate from the ground state as well as it could from... 

A - -Butenolides are valuable synthetic reagents which constitute the active functionality of many known natural products Recently, the preparation of spiro[cyclopropane-1,5 -furan-2 (5 //)-ones] 2 was reported from the thermal ring contraction of 2-(2-diazoacetyl)cy-clobutanones. 

The corresponding ring contractions of furans normally give cyclopropenyl products, but in some cases a cyclopropyl product is isolated. Hence, photolysis of 2-cyanofuran gave a mixture of stereoisomers of 3-alkoxy-2-formylcyclopropane-l-carbonitrile (14). ... 

Oxabicyclo[2.1. OJpentcne (6), prepared by flash-vacuum pyrolysis of 5 underwent either ring expansion to furan (60%), or ring contraction to cycloprop-2-enccarbaldehyde (7)." ... 

---