Ring-contracted methyl ester

In the laboratory of K. Fukumoto, the stereoselective total synthesis of (+)-A -capnellene was carried out using an intramolecular Diels-Alder reaction to obtain a tricyclic 5-5-6 system. Since the target molecule was a triquinane, the six-membered ring had to be converted to a five-membered one, a transformation achieved by a Wolff rearrangement. The required a-diazo ketone was prepared via a deformylative diazo transfer reaction and was photolyzed in methanol. The ring-contracted methyl ester was isolated as a 3 1 mixture of separable isomers favoring the a-isomer. 

Methyl trisnorshionanoate (126) has been synthesized from friedelan-19a-ol (127) via the ring-contracted intermediate (128) and the trisnor-ketone (129). Irradiation of (129) in methanol afforded the desired ester (126). An investigation of the effect of solvent on the backbone rearrangement of 3/8,4/3-epoxyshionane (130) by boron trifluoride etherate has shown that nucleophilic solvents tend to interrupt the rearrangement at an early stage (see Vol. 6, p. 130). 

The structure of factor III, the trimethyl isobacteriochlorin, is worthy of note. Preliminary studies had suggested the structure (81), which was in better accord with a subsequent ring contraction to the corrinoid skeleton involving oxidation at C-20 followed by extrusion of formaldehyde. However, the 13C NMR spectrum of factor III (as its octamethyl ester) enriched biosynthetically with [13CH3]methionine and [5-13C]ALA (Scheme 25) shows both the meso and meso- methyl carbons as doublets, a feature in accord with substitution of... 

Attempted displacement of the 2-O-imidazole-1 -sulfonyl ester of methyl 3,4,6-tri-O-rhethyl 3-D-galactopyranoside with azide ion led to the formation of the corresponding 2,5-arihydro sugar [86], as illustrated in Scheme 17. The predominance of a ring-contraction... 

Aminothiocarboxylic acids react with aldehydes and 3-dimethylamino-2-isocyanoacrylic acid methyl ester following the pathway described for /7-aminoacids, affording, after ring contraction of the seven-membered intermediate 87, Michael-type cyclization and /7-elimination, /7-lactam 88 equipped with a thiazole... 

Favorskii rearrangement of cyclic 2-bromoketones leads to ring contraction and this has become one of the most fruitful uses of the rearrangement in synthesis. Bromination of cyclohexanone is a simple reaction (Chapter 21) and treatment with methoxide gives the methyl ester of cyclopentane carboxylic acid in good yield. 

The rearrangement of cyclic ketones involves ring contraction. Thus, 2-chlorocyclobutanone (2.29) on treatment with sodium methoxide followed by hydrolysis is converted into cy-clopropanecarboxylic acid (2.30). In a similar manner, 2-bromocyclohexanone (2.31) on treatment with sodium methoxide gives methyl ester of cyclopentanecarboxylic acid (2.32). 

Certain a-halo ketones undergo rearrangement with sodium alkoxides in anhydrous ether to form esters. Methyl txrbromoisopropyl ketone and sodium ethoxide give ethyl trimethylacetate (6l%). Ring contraction occurs with a-chlorocyclohexanone to give cyclopentanecarboxylic ester (53%). ... 

Hydroxy-2-imino-6-phenyl[l,3,4]thiadiazinane-5-carboxylic acid methyl ester 255 underwent ring contraction in the presence of NaHCOs, acetic anhydride, or acetone, giving the 2-hydrazino-, the 2-(A -acetylhydrazino)-, and the 2-(A -isopropylidinehydrazino)-5-phenylthiazole-4-carboxylic acid methyl esters 256, 257, and 258, respectively (Scheme 103) <1997J(P1)2673>. 

In the first step of the reaction sequence a methyl ester is formed via a Wolff rearrangement. The Wolff rearrangem ent provides acids, esters or amides from a-diazo ketones and is often used in a ring contractive way to form strained ring systems, which are not accessible by other sequences.It also occurs as the key step in the Arndt-Eistert homologation of carboxylic acids. ... 

An ingenious procedure for the synthesis of 5H-pyrrolo(3,2-d)pyrimidines involves treatment of a 6-methyl-5-phenylazopyrimidine with t-butoxybls(di-methylamlno)methane or an orthoformate ester, followed by hydgogenolytic ring contraction of the resulting pyrlmido(4,3-c)pyridazlne. 

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