Five-membered Ring Expansion

Undoubtedly, further work will be necessary in order to get a solid background for silyl radical cyclizations. In particular, further investigations on the nature of the pendant alkenyl substituent as well as on the ring size and substituent effects on the silicon in the ring expansion can be expected in the future. 

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The electrophilic addition to the double bond in the dioxolane 212, followed by five-membered ring expansion, resulted in the dioxocanes 213, as a mixture of four inseparable stereoisomers (Equation 46) <1996TL2245>. The relative stereochemistry of the major product 214 was determined by 111 NMR, nuclear Overhauser effect (NOE), and by converting 214 into the known 157 (cf. Section 14.06.6.1 and Equation (40)). 

Ring expansion of five- to six-membered rings such as oxazole —> pyridine derivatives via a Diels-Alder reaction is a well-established procedure. However, the conversion of a six-membered heterocycle into a five-membered ring system has not been exploited to any great extent, and those systems that have been studied usually involve a cationic species. 

The diaziridines arc somewhat more stable than the oxaziranes. The three-membered ring of the oxaziranes is decomposed in all of its reactions, but with the diaziridines substitution on the nitrogen atoms can be effected and reactions involving fission and expansion of the ring to a five-membered ring are i)ossiblc,... 

A sequential process of carbopalladation and ring expansion of allenylcyclobuta-nols provides five-membered rings (Scheme 16.19) [24], The intramolecular version of this approach offers a facile synthetic method for 5,7- and 5,8-fused ring frameworks (Scheme 16.20) [24]. 

Ring expansion of a four- or five-membered ring is a more common route for the preparation of 1,3-dioxanes, 1,3-dithianes, and 1,3-oxathianes and various methods have been developed. 

The Co-catalyzed carbonylative ring-expansion reaction has also been applied to other five-membered ring heterocycles such as oxazolidines/ oxazines/ oxazolines/ and thiazolidines. ... 

Lewis acid catalyzed ring expansion is found for 2-azabicyclo[3.2.0]heptane-5-carboxylates with an imidate group in the five-membered ring, e.g. 978 which rearranged to give 10.79,80... 

Ring expansions are generally not of synthetic value, primarily because it is usually easier to make a five-membered ring directly than to form a three- or four-membered ring and carry out the subsequent expansion. 

By formation of seven- from five-membered rings The formation of the oxepanes (160) and (161) by treatment of 2,2,5,5-tetramethyl-tetrahydrofuran-3-one with diazomethane is included in the present category although it is probable that the reaction proceeds by two separate ring expansion steps (cf. equation 47) as shown in Scheme 32 (72HC(26)l). Photolytic C—O bond cleavage of a dihydrofuran... 

Ring expansion by chemistry discussed in the workbook gives 82 whose enolate is sulfenylated 83 and cyclised in acid solution with hydrolysis of the acetal to give 84 and the five-membered rings in 73 start to appear. 

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