Syntheses of Alkylidene cyclopropanes

2 Syntheses of Alkylidene Cyclopropanes and Cyclobutanes by formal Elimination of a Hydroxyl Group and a Heteroatomic Moiety The methods discussed above, although efficient, possess important limitations. They do not permit, for example, the synthesis of tetrasubstituted alkylidene cyclopropanes due to the unavailability of the starting selenides, the alkylation of a-selenocyclopropyllithiums with sec-alkylhalides being not feasible at present. 

These 1-seleno-l-vinylcyclopropanes have proved especially useful, inter alia, for the synthesis of allyl alcohols where the carbon-carbon double bond is part of an alkylidene cyclopropane 4S) and for the preparation of allylidene cyclopropanes45, (see page 52). 

Synthesis of Alkylidene and Allylidene cyclopropanes from l-( 1-silyl) cyclopropyl carbinols 

Side reactions may also occur, due to the high reactivity of such strained olefins towards the species present in the reaction medium and also due to the well-known propensity of cyclopropylcarbinols to rearrange to cyclobutyl or/and to ho-moallyl derivatives when the hydroxyl group is transformed to a better 

The novel reagents which have been introduced for the synthesis of alkylidene cyclopropanes from (P-hydroxyalkyl) methylselenides proved particularly efficient for the synthesis of a large variety of olefins, including other alkylidene cycloalkanes . PI3 or PjI /NEtj are also to transform (p-hydroxyalkyl) sulfides to olefins but Instead lead to a stable l-(l-phenylthio)-l-(iodomethyl) cyclopropane starting from l-(l-phenylthio)-(hydroxymethyl)cyclopropane (scheme 46) The reaction of 

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Syntheses of Alkylidene cyclopropanes and Alkylidene cyclobutanes by Formal Elimination of a Selenenyl Moiety and a Hydrogen... 

Syntheses of Alkylidene cyclopropanes from 1 -A Ikyl-h seleno cyclopropanes... 

The first approach to the seleno series taken advantage of the wide availability of 1-(l-seleno)cyclopropyl carbinols and the small tendency (discussed in Section Syntheses of alkylidene cyclopropanes from 3-hydroxy alkyl selenides), of the phenylseleno derivatives to produce alkylidene cyclopropanes on reaction 37), with for example, thionyl chloride/triethylamine mixture (Scheme 51). However, the best results have... 

Syntheses of Alkylidene cyclopropanes Via the Selenonium route The selenonium route proved to be more valuable. It has been specifically designed by us to replace the deficient selenoxide route (Scheme 38). It was expected to produce alkylidene cyclopropanes by a mechanism which mimics the selenoxide elimination step but which involves a selenonium ylide in which a carbanion has replaced the oxide. Cyclopropyl selenides are readily transformed to the corresponding selenonium salts on reaction with methyl fluorosulfonate or methyl iodide in the presence of silver tetrafluoroborate in dichloromethane at 20 °C and, as expected, methylseleno derivatives are more reactive than phenyl-seleno analogs. Alkylidene cyclopropanes are, in turn, smoothly prepared on reaction of the selenium salts at 20 °C with potassium tert-butoxide in THF (Scheme 38). Mainly alkyl cyclopropenes form at the beginning of the reaction. They then slowly rearranges, in the basic medium, to the more stable alkylidene cyclopropanes( 6 kcal/mol). In some cases the complete isomerisation requires treatment of the mixture formed in the above reaction with potassium fcrt-butoxide in THF. The reaction seems to occur via a selenonium ylide rather than via a P-elimina-tion reaction promoted by the direct attack of the /crt-butoxide anion on the P-hydrogen of the selenonium salt, since it has been shown in a separate experiment that the reaction does not occur when a diphenylselenonium salt (imable to produce the expected intermediate) is used instead of the phenyl-methyl or dimethyl selenonium analogs. It has also been found that the elimination reaction is the slow step in the process, since styrene oxide is formed if the reaction is performed in the presence of benzaldehyde which traps the ylide intermediately formed... 

Synthese of Alkylidene cyclopropanes Via the Selenoxide Route The synthesis of olefins by oxidative elimination of selenides is one of the most versatile and useful methods in selenium chemistry reaction usually... 

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