Crotonyl chloride Nazarov cyclization

A complementary approach, developed by Paquette, - uses substituted acryloyl chlorides as addends in reaction with structii ly embellished vinylsilanes. A general route to the vinylsilanes (87) was found in the silylation of vinyllithiums generated by the Shapiro reaction.The acylation with acryloyl chlorides takes place readily with aluminum trichloride to afford the divinyl ketones which are subsequently cyclized with tin tetrachloride. The Nazarov cyclization products were formed as a mixture of double tend isomers (equation 47). The best results were obtained with p,p-dimethylacryloyl chloride. Crotonyl chloride could be employed, but acryloyl chloride proved impractical. This metl owes much of its utility to the regiocontroUed synthesis of Ae vinylsilmes, thereby clearly establishing the loci of cyclopentannulation. 

The ob tent ion of pentalenone may result from an acylation by crotonyl chloride of methylcyclopentene generated in situ. The first intermediate gives the divinvlke tone-aluminium chloride complex precursor of the Nazarov reaction followed by cyclization, then hydride and methyl migrations. In this process, the electrophilic and oxidizing reactivities of acyl chlorides are dissociated since acetyl chloride induces the hydride ion transfer and alkenoyl chloride performs the acylation reaction. 

The divinylketone 1 has been prepared and isolated by direct acylation of methylcyclopentene with crotonyl chloride at low temperature (-90 C). When a methylene chloride solution of 1 was treated by aluminium chloride, the pentalenone 6 has been obtained in low yield (40 %). Thus, the structure of the divinylketone 7 slows down the Nazarov cyclization (presence of two methyl on the C 3 -atoms and obtention of bicyclo 3.3.0. octane intermediate with strained structure). 

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See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ ]

See also in sourсe #XX -- [ , ]

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