Recent advances of various homologation reactions, especially the method of extension of carbon chains having terminal functional groups, are reviewed from the view point of organic synthetic methodology. In this review, homologation reactions are classified as follows : 1) C1 homologation, 2) C2 homologation, 3) C3 and higher homologation, and 4) C5 isoprenoid homologation in acyclic terpenoid synthesis.
The present review surveys some of the recent developments in the methodology for the transposition of carbonyl group including related hydroxy transposition. The chapter of “1, 2-Transposition” describes various methods which are classified according to the methods for α-functionalization of carbonyls. The methods concerned include benzylidenation, acetoxylation, bromination oximination, nitration, sulfenylation, and hydroboration. The chapter of “1, 3-Transposition” deals mainly with the 1, 3-transposition in α, β-unsaturated carbonyl systems. Among the methods described here are included ones which are based on the Wharton reaction, the oxidative rearrangement of allylic alcohols, the 1, 3-hydroxy transposition, the interconversion of sulfoxides and sulfenates via the [2, 3] sigmatropy, the [3, 3] sigmatropy of allylic thion-esters, and oxidative cyclization of enon-oximes to isoxazoles. Finally, the chapter of “1, n (n>3) -Transposition” describes the Willgerodt-Kindler methods and points out some possibilities of 1, 4-transposition via the Barton reaction and the cyclopropane ring-opening and of 1, n (n>4) -transposition based on the microbial transformations.
Syntheses of hardly available ring structures through ring enlargement reactions are illustrated with the emphasis on the methods of high utility from the synthetic point of view. I One-Carbon Ring Enlargement. The molecular rearrangements involve active species such as carbonyl-diazomethane adducts, β-hydroxycarbenium ions, β-oxidocarbenoids, and β-bromo alkoxides respectively. Oxidative rearrangement is performed by means of cyanogen azide or thallium (III) nitrate. Bicyclo [n. 1.0] alkanes also are key intermediates for the one-carbon homologation. Photolysis of bicyclo [n. 2.1] alkanones having a carbonyl bridge undergoes Norrish Type I reaction resulting in ring expansion. II Two-Carbon Ring Enlargement. This methodology consists in the reaction of enamines of cyclic ketones with ketenes or acetylenes, acyloin condensation of cycloalkane-1, 2-dicarboxylic acid esters followed by thermolysis, and [1, 3] sigmatropic rearrangement. III Three-Carbon Ring Enlargement. Photolysis of α-cyclopropyl ketones and β, γ-unsaturated ketones, and fragmentation of bicyclo [n. 3.0] alkenones are discussed. IV Four-Carbon Ring Enlargement. Typical examples are the oxy-Cope rearrangement and fragmentation of bicyclo [n. 4.0] alkanes. V Miscellanea
Preparations of alicyclie compounds by ring contraction are discussed from a standpoint of synthetic chemistry. About its mechanistic point of view, recent advances in Favorskii rearrangement and Ramberg-Backlund reaction are reviewed. Syntheses of three-, four-, five-, six-membered and other alicyclic compounds by ring contraction of larger alicyclic systems are documented from published papers mainly since 1970.
The recent methods for preparations of carbocyclic spiro compounds are reviewed. The synthetic methods are classified into the type of the reaction by which the spiroannelation occurs : intermolecular and intramolecular alkylations and Michael addition reactions, intramolecular condensation reactions, acid-catalyzed or thermal rearrangements, and acid-catalyzed or thermal cyclizations. The formations of carbocyclic spiro links with organosulfur or organophosphorus reagents are reviewed in the latter sections.
Reaction schemes which involve successive cleavage and formation, or vice versa, of carbocyclic and/or heterocyclic compounds, are reviewed. These processes are quite effective for constructing complex natural products. Ring transformation is chosen as an example for successive cleavage and formation of cyclic compound. Successive formation and cleavage of cyclic compound are discussed for stereospecific introduction of substituent (s) into monocyclic compound via bicyclic one. These two types of reaction schemes are illustrated by the use of recently reported syntheses of natural products including sesquiterpenes, prostaglandins, steroids, alkaloids, etc.
This review article treats the transition metal catalyzed metathesis of olefins which provides a useful method for synthesizing olefinic compounds. Contents of the present review are as follows : 1. The applicability of the olefin metathesis to the synthesis of acyclic and cyclic olefins, acetylenes, cyclopropane derivatives, perfectly alternating copolymers, catenanes, and sex attractant hormones is mentioned. 2. A general survey of the mechanistic considerations of the olefin metathesis is given. There is a growing consensus that the metal carbene and metallocyclobutane intermediates are involved in the reaction. 3. Some recent aspects of the stereochemical selectivity in the olefin metathesis reaction are also discussed in terms of the carbene complex mechanism.
The use of cyclopropane derivatives as C3 synthetic unit is now attracting much attention. The ringopening reactions of cyclopropylmethanol derivatives, cyclopropanols, and cyclopropanes activated by electron withdrawing substituents were reviewed with emphasis on their application to the synthesis of natural products.