In the oxygenation of 4-alkyl-2, 6-di-tert-butylphenols catalyzed with t-BuOK in aprotic solvents such as DMF, DMSO, and HMPT, O2 is incorporated into the para position of the phenols exclusively to give the corresponding epoxy-p-quinols produced by an intramolecular decomposition of p-per-oxide anion intermediates. 4-Aryl-2, 6-di-tert-butylphenols are not susceptible to the oxygenation under these conditions. In the t-BuOK-catalyzed oxygenation of 2, 4, 6-tri-tert-butylphenols and 4-aryl-2, 6-di- tert-butylphenols in t-BuOH, on the other hand, O2 is incorporated into the o-position of these phenols exclusively to give epoxy-o-quinols. With a large excess of t-BuOK in DMF the methyl group of p-cresols is selectively oxygenated to give the corresponding 4-hydroxybenzaldehydes. When 4-alkyl-2, 6-di-tert-butylphenols are oxygenated in HNEt2 containing NaNH2, the corresponding p-quinols are obtained in nearly quantitative yield. The p-quinols are converted quantitatively to hydroquinones with migration of the 4-alkyl group to the adjacent carbon on the ring. The base-catalyzed reaction of acetates of the p-quinols results in the selective formation of m-hydroxyphenylacetic acids. It has been newly found that the generation of carbanion adjacent to the carbonyl group in esters of hydroperoxides selectively formed from the t-butylated phenols extremely weakens the peroxy bond to give unique products even at -70°C. Epoxy-o-quinols are quantitatively converted to the corresponding cyclopentadienones by base catalysis at an elevated temperature. Cyclopenten-ones are also obtained in good yield from epoxyquinols. A non-radical mechanism for the incorporation of O2 into the phenolates is argued on the basis of the regioselectivity in the oxygenation and of results obtained in the reaction of phenoxy radicals with O2-.
In an abundance of the previous papers methylene compounds linking two heteroatoms have proved to furnish versatilities and conveniences for syntheses of varied organic compounds. This article is an attempt to review the reactions of the compounds possessing methylene linking two N-, O- and S-functional groups, which bring on substitution of one group and insertion into one side. Particularly, one of the purposes of this article is to explain comprehensively the nature of the hetero-functional groups of the methylene compounds engaging in the reactions.
This review involves the synthetic studies on the salamander alkaloids isolated from an animal venom in the skin gland of Salamandra maculosa Laurenti. Syntheses of three groups of the alkaloids, one with a bicyclo-oxazolidine system, one with a carbinolamine system, and another with a pure 3-aza-A-homo steroid system, are discussed separately. It will be emphasized that the synthetic works described herein made a considerable contribution to the structure analyses of this field of alkaloids and earlier proposed structures based on X-ray and spectroscopic analyses have been changed step by step. Regio and stereospecific syntheses of the alkaloids are mainly discussed in connection with the construction of the bicyclo-oxazolidine system of the major alkaloids and with Beckmann rearrangement of steroid 3-ketoximes for the syntheses of the other alkaloids.
Synthetic procedures of arabinofuranosyl nucleosides, especially, of arabinofuranosyl purine nucleosides are reviewed from the following points of view; i) methods which include nucleoside epoxide as an intermediate ii) direct condensation methods of arabinose derivatives and various purine bases iii) methods which include purine-8-cyclonucleosides as an intermediate and iv) other methods. Biological activities of arabinofuranosyl purine nucleosides are also summarized briefly.
This review is organized under the following headings with 132 references. 1. Introduction 2. Preparative methods. 2. 1. From t-phosphines. 2. 2. From t-phosphine oxides. 3. Reactions. 3. 1. Formation of phosphonium salts. 3. 2. Reactions via quasi-phosphonium salts. 3. 3. Reduction to t-phosphines. 3. 4. Other reactions. 4. Reactions using t-phosphines and carbon tetrahalides. 5. Polymeric t-phosphine dihalides.
Asymmetric hydrogenations of dehydroaminoacids with homogeneous rhodium catalysts are reviewed. High enantiomeric excess up to 100% is achieved with rhodium catalysts including chiral ligands such as phosphines, phosphinittes, and aminophosphines.