Ring expansions, four atom

The trimethylsilyl ethers 212 of four-membered 1-alkenyl-1-cyclobutanols rearrange to the ring-expanded 0-mercuriocyclopentanones 213. These can be converted into the a-methylenecyclopentanones 214 through elimination or further expanded by one-carbon atom into cyclohexanones 215 via the Bu3SnH-mediated free radical chain reactions [116]. A similar radical intermediate is suggested to be involved in the ring expansion of a-bromomethyl-fi-keto esters [117]. (Scheme 84)... 

Because of our interest in one-pot, multicomponent annulations,7 we envisioned a flexible and efficient protocol which would link the four different components via the formation of four new bonds (a-d, eq. 1) in one reaction vessel. The intermediate 7-hydroxystannanes thus formed in eq. 1 could be oxidatively fragmented8 to produce both ring enlargement and regiospecific formation of an alkenyl unit. This 4-atom ring expansion methodology of common sized a,p-unsaturated ketones has led to the syntheses of many mono- and disubstituted 9-, 10-, and 11-membered unsaturated macrolides (Table ). 

Ring expansion of cycloalkanones.1 The hypoiodite reaction can also be used for a four-atom ring expansion of cyclic ketones via the corresponding lactols (cf., 13, 150). 

In contrast, three- and four-carbon atom ring expansions proved more successful. Similar results were observed in case of cyclopentanones and cyclohepta-nones [31], and in various heterocyclic systems (N, O, S) [32]. 

It is well documented that carbenium ions undergo nucleophilic attack by the carbon atom of isocyanides [46]. Significantly, diphosphirenium 2 Pa reacts at -50°C with ferf-butyl isocyanide in a similar manner to give the transient ni-trilium salt 21, which undergoes a ring expansion reaction affording 22 [45] (Scheme 16). Note that 22 is one of the very rare four-membered heterocycles featuring a phosphaalkene moieties [47]. 

Suginome, H. and Yamada, S., Simple new synthesis of macrolides by a four atom ring expansion of cyclic ketones through a consecutive intramolecular homolytic addition-P-scission of alkoxyl radicals. A new entry to the synthesis of 15-pentadecanolides, Chem. Lett., 245, 1988. 

The zincblende (ZB), or sphalerite, structure is named after the mineral (Zn,Fe) S, and is related to the diamond structure in consisting entirely of tetrahedrally-bonded atoms. The sole difference is that, unlike diamond, the atoms each bond to four unlike atoms, with the result that the structure lacks an inversion center. This lack of an inversion center, also characteristic of the wurtzite structure (see below), means that the material may be piezoelectric, which can lead to spurious ringing in the free-induction decay (FID) when the electric fields from the rf coil excite mechanical resonances in the sample. (Such false signals can be identified by their strong temperature dependence due to thermal expansion effects, and by their lack of dependence on magnetic field strength). 

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