Ring expansion regioselective

Some straightforward, efficient cyclopentanellation procedures were developed recently. Addition of a malonic ester anion to a cyclopropane-1,1-dicarboxylic ester followed by a Dieckmann condensation (S. Danishefsky, 1974) or addition of iJ-ketoester anions to a (l-phenylthiocyclopropyl)phosphonium cation followed by intramolecular Wittig reaction (J.P, Marino. 1975) produced cyclopentanones. Another procedure starts with a (2 + 21-cycloaddition of dichloroketene to alkenes followed by regioselective ring expansion with diazomethane. The resulting 2,2-dichlorocyclopentanones can be converted to a large variety of cyclopentane derivatives (A.E. Greene. 1979 J.-P. Deprds, 1980). 

Perhydroazulenones.2 The key step in a synthesis of carpesiolin (3), a pseudoguaianolide lactone, is a regioselective ring expansion of a perhydroindanone (i) to 2 by dibromomethyllithium. 

In contrast, under identical conditions, benzosilacyclobutene reacted with aldehydes in a highly regioselective ring expansion to give oxasilacyclohexenes (50-75% yields) (Scheme 55). 

The rippertene diterpenes are of current interest due to the condensed tetracyclic 16-carbon atom core and the presence of several quaternary carbons. The following synthesis starts with isopulegol and passes through a silylated diazomethane-induced regioselective ring expansion (Scheme 50) [87]. 

The unsubstituted tetralone 66 undergoes regioselective ring expansion,but the regiospecificity can be compromised with substituted system as shown in the case of 68. Reaction of this compound with hydazoic acid gives lactam 70 as the major product due to alkyl migration. 

The Schmidt reaction of the tertiary alcohol 111 is used as a key step in the synthesis of nicotine. The reaction proceeds through 112, which undergoes a regioselective ring expansion to give imine 113. 

Regioselective ring expansion of alkynyl cyclopropanes to cyclobutenes has been reported via a copper-catalysed cycloaddition of an alkyne with an arylsulfonyl azide and a silver-catalysed carbene formation followed by ring expansion of a cyclopropyl carbene intermediate (Scheme 75)7° ... 

Synthesis of multifunctional 3-amino-4-phosphono-2-quinolinones (525) has been realised by one-pot nucleophilic addition of diallq diazomethylphosphonate (523) to imino isatins (524) followed by regioselective ring expansion promoted by salicylic acid (Scheme 154). 

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... 

Asymmetric hydroboration.1 The key step in a synthesis of natural (+ )-hir-sutic add-C (1), based on an earlier synthesis of racemic 1, is an efficient asymmetric hydroboration of the meso-alkene 2. Reaction of 2 with (+ )-diisopinocampheyl-borane (90% ee) followed by oxidation provides the exo-alcohol 3 in 73% yield and in 92% optical purity. Ring expansion of the corresponding ketone with ethyl diazoacetate is not regioselective even in the presence of BF3 etherate or (C2H5)30+ BF4, but does afford the desired a-keto ester in the presence of SbCl5 (8, 500-501). Decarboxylation of the crude product gives (— )-4 in 90% ee after chromatography. 

This regioselectivity has been useful in ring expansions (equation 110)178. 

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