Regioselective cleavage

Regioselective cleavage of dibenzofuran derivatives has been achieved with lithium metal, as exemplified by the preparation of 3-methyl-2-phenylphenol (108) (80S634). 

Regioselective cleavage of dimethoxyaryl derivatives with methanesulfonic acid/ methionine has been reported. 

A facile synthesis of 49 was described as follows. When the bromolactone 34 reacted with hydrogen bromide in acetic acid, regioselective cleavage of... 

Lactones by ring expansion of lactols. Medium-sized lactones can be obtained in reasonable yield by photolysis of the hypoiodites of catacondensed lactols, which results in regioselective cleavage of the bridging bond. The substrates are available by the general route shown for the 6/6 lactol 1, which is cleaved to the 10-membered lactone 2. This cleavage can be used for preparation of 9-membered... 

This hypoiodite reaction can also be used for ring expansion of cyclic ketones.2 Thus Barbier cyclization of a-(w-iodopropyl)cyclododecanone (3) furnishes the bicyclic alcohol 4, which undergoes regioselective cleavage to a 15-membered iodo... 

A tricyclene has been converted in acetic acid/Et3N to Nojigiku alcohol with a 77% yield by regioselective cleavage at the external cyclopropane C—C bond (Fig. 14) [88]. 

The 1,2-carbonyl transposition takes place through the enJo-epoxide 18 easily prepared through the tosylhydrazone 16, followed by regioselective cleavage to the less substituted double bond (17) with 2 equivalents of methyllithium [4] and epoxidation with MCPBA in chloroform from the more accesible convex face of the decalin system. 

How would you explain this example of regioselective cleavage ... 

Regioselective Cleavage of O-Benzylidene Acetals to Benzyl Ethers... 

Treatment of a cyclic 23- or 3,4-ortbo-ester in a pyranoside under mild acidic conditions will cause regioselective cleavage so that a free equatorial OH is produced adjacent to an axial O-acyl group [14] (Scheme 1). The reasons for this have engendered considerable theoretical treatment based on considerations of stereoelectronic effects [IS]. 

I. Hanna, J. Pan, and J. Y. Lallemand, Optically active cyclobutanones from glycals Preparation and regioselective cleavage, Synlett p. 510 (1991). 

The most common procedure is ozonolysis at -78 °C (P.S. Bailey, 1978) in methanol or methylene chloride in the presence of dimethyl sulfide or pyridine, which reduce the intermediate ozonides to aldehydes. Unsubstituted cydohexene derivatives give 1,6-dialdehydes, enol ethers or esters yield carboxylic acid derivatives. Oxygen-substituted C—C bonds in cyclohexene derivatives, which may also be obtained by Birch reduction of alkoxyarenes (see p. 103f.), are often more rapidly oxidized than non-substituted bonds (E.J. Corey, 1968 D G. Stork, 1968 A,B). Catechol derivatives may also be directly cleaved to afford conjugated hexa-dienedioic acid derivatives (R.B. Woodward, 1963). Highly regioselective cleavage of the more electron-rich double bond is achieved in the ozonization of dienes (W. KnOll, 1975). 

Vinylnorcaradiene derivatives 1175 undergo a photochemical reaction resulting in the regioselective cleavage of one of the cyclopropyl a-bonds and the formation of isochroman-3-ones (Equation 457) <1997CC1973>. 

Katagiri, N. et al. A Highly Efficient Synthesis of the Antiviral Agent (+)-Cyclaradine Involving the Regioselective Cleavage of Epoxide by Neighboring Participation. 2.4 1997 [136]... 

Regioselective cleavage of gem-difluoropropanes. l,l-Difluoro-2-(bromo-methyl)cyclopropane is cleaved by Bu3SnH/AIBN to 3,3-difluoro-l-butene as the... 

The diaryl substituted systems (139) have been shown to undergo regioselective cleavage of the cyclopropene o-bond bearing the more electron donating substituent under direct photolysis 116). 

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