Ring contraction stereospecific

Ring contraction of 2-thiocephems has also been examined as a route to penems. Desulfurization of (82, n = 0) using triphenylphosphine gave mixtures of 5(R)- and 5(5)-penems (121). The stereochemical problem was neatiy overcome by regioselective oxidation to the thiosulfonate (82, n = 2) which underwent stereospecific thermal extmsion of sulfur dioxide (122) to give the S(R)-penem (83). 

The benzilic acid rearrangement of steroid a-diketones is often the preferred method for ring contraction. Yields are generally excellent, and in several instances the reaction has been shown to be stereospecific. Preparation of the starting a-diketones is discussed below and illustrated with several excellent experimental procedures. 

Stereospecific contraction of the seven-membered nucleus was observed on treatment of the dimesylated thiepanetetrol derivatives 67 (obtained in turn from d-sorbitol) with sodium azide to give a 5 1 mixture of the bis(azido)tetrahydrothiophenes 68 and 69. Intramolecular nucleophilic displacement of either mesylate group initiates this ring contraction <00TA1389>. 

Participation of the ring-oxygen atom has also been postulated,124 to account for the stereospecific formation of L-mannopyranoside (40) and D-allofuranoside (41) products in the deamination, by way of 39b, of methyl 4-amino-4,6-dideoxy-2,3-0-isopropylidene-a-L-man-nopyranoside (39a). Ring contraction to give furanoside products pre-... 

Pyrazolines, in general, undergo a photochemically induced ring contraction in solution to form a cyclopropane derivative and nitrogen. This process, unlike some equivalent thermal decompositions, is stereospecific, and methyI-as-3,4-dimethyl-l-pyrazoline-3-carboxyl-ate (88) is converted in high yield into methyl-as-1,2-dimethylcyclo-propane carboxylate (89).69 This route is of considerable preparative... 

Diphenyl-l,2-diaza-l-cycloheptene (98) undergoes a similar type of ring contraction, accompanied by the formation of nitrogen, on exposure to UV light.79 In the solid state, this photolysis is stereospecific and the product is cis-1,2-diphenylcyclopentane (99). Both thermal and photolytic decompositions in solution yield a mixture of cis- and trans-1,2-diphenylcyclopentane in addition to cis- and... 

Flavones have been shown to suffer from a stereospecific ring contraction to give rra r-2-aryl-2,3-dihydrobenzo[ ]-furan-3-carboxylates upon treatment with phenyliodonium acetate, trimethyl orthoformate, and sulfuric acid (Equation 151) <2002T4261>. 

In connection with his investigations of the importance of nitrogen nonbonding electron orientation in morphinans to their stereospecific binding to opioid receptors, Belleau(149) reported a D-ring contracted morphinan (142a), but without synthetic details (see Chapter 13 p. 465). 

Diastereoselective Favorskii ring contractionBicyclic a-chloro-8-lactams can undergo highly diastereoselective ring contraction to proline derivatives when treated with base, preferably barium hydroxide in aqueous solution. Thus 1 is converted into 2 and 3 in the ratio 9 1. The ring contraction of 4 to 5 is stereospecific. In contrast, rearrangement... 

Preparation of 2,3-m-2,5-trarts-trisubstituted tetrahydrofuranes by this methodology is possible, but presents some problems. Cyclization is not stereospecific or results only in tetrahydropyranes because of unfavorable 1,3-interactions. Moreover, ring contraction can result in the undesired 2,5-cw-diastereomer, even as the major product. 

In the second reaction the ring contraction is stereospecific. A similar contraction of a five-membered ring is not possible. In all cases the product oxime has the xyn-configuration. 

The first reaction reported in this category was the ring contraction of 2-bromocyclobu-tanone into cyclopropanecarboxylic acid or derivatives. It occurs with high yield and stereospecificity, under the action of aqueous sodium carbonate, sodium ethoxide in ethanol or ether, liquid ammonia or even boiling water (equation 158) . 

Useful applications of this ring contraction of a-halocyclobutanones have been reported, in particular in the field of pyrethrins and pyrethroids For instance, biologically interesting dihalovinylcyclopropanecarboxylic acids (242) have been obtained from optically active a-halocyclobutanones (241) by a stereospecific C4 - C3 ring contraction and subsequent elimination of HX (equation 161) (To date twelve patents are based on the ring contraction of a-halocyclobutanones.)... 

Fleet, G W, Se3miour, L C, Synthesis of intermediates for stereospecific synthesis of a- and P-C-nucleosides ring contraction of protected 2-O-trifluoromethanesulphonates of galacto- and altro-pyranosides. Tetrahedron Lett, 28, 3015-3018, 1987. 

Aromatic sulfonyl azides react with enol ethers of cyclic ketones to form arenesulfonyl imidate esters (96) with ring contraction, the addition-rearrangement process is very stereospecific (Scheme 63). 

Optically active (-h )-3-substituted 2,2-dimethylcyclobutanol and its tosylates undergo facile stereospecific ring contraction under different conditions to yield mainly optically active 1,2-cis-disubstituted cyclopropanes. 

The hydride reduction of tosyloxycyclobutanes also proceeds mainly with concomitant ring contraction providing cyclopropane derivatives related to the solvolysis products. The synthetic usefulness of this stereospecific route to three-membered rings has been illustrated by the preparation of a 1,2-disubstituted cyclopropane of known absolute configuration from an optically active tosyloxycyclobutane. 

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